Welcome to the second installment in a series detailing the world of my upcoming novel No Frills. I have a bad habit of drifting between the real and fake worlds, but to a first approximation, everything here is in-universe (circa 1976), unless otherwise noted. Something is definitely wrong with me… I think I might have a nuclear weapons complex.
The United States Navy operates a fleet of Franklin D. Roosevelt-class nuclear-powered ballistic missile submarines equipped with UGM-96A Poseidon III submarine-launched ballistic missiles. These missiles are, in turn, equipped with a mix of mostly ballistic reentry vehicles, and some Mark 5 Hermes maneuverable reentry vehicles.
Geopolitics and Technopolitics
The MaRVed SLBM has the potential to seriously undermine strategic stability. In particular, the UGM-96A and Mark 5 are the ne plus ultra of counterforce first-strike weapon systems. First, the UGM-96A is an SLBM that can be launched “close to the adversary’s borders,” as little as 200nm offshore, and on a depressed trajectory. Second, the Mark 5 can evade even those anti-ballistic missiles with “fast response control not achievable with conventional aerodynamic fins.” Third, the Mark 5 can fly through “heavy weather” and dust “raised by nuclear detonations” with little to no nose tip recession, and may even be able to penetrate some dust defenses by deliberately allowing some “turbulent erosion” of the nose tip, seeing as how the “biconic shape” of the eroded refractory metal nose tip causes relatively little “center of pressure travel,” the “biconic configuration” of the overall vehicle is relatively insensitive to “nose-tip asymmetries,” and the flight control system can make “what corrections, if any, are necessary to hit the target.” Fourth, the Mark 5 “can achieve sufficient accuracy to destroy even the hardest targets.” This certainly could degrade crisis stability — giving the attacker an incentive to strike first, and giving the defender an incentive to launch even those ICBMs with “conventional interceptor-based” and dust defenses under attack, and “use them or lose them” with or without “highly reliable and accurate assessment of the attack.”
Why would you do this? The idea is that by appropriating or assimilating defensive nuclear missile technologies and “providing a capability to penetrate endoatmospheric ABM systems, evading MaRVs provide both a hedge for [United States] security and a deterrent against deployment of such systems, since they would negate any marginal benefits that might otherwise be had,” and thereby obviate the “high degree of cooperation between the two parties” necessary to program the “buildup of hardpoint defense… so as to maintain the confrontation in the zone of mutual deterrence at every stage,” and start down “crisis-stable paths leading from Mutually Assured Destruction to Mutually Assured Survival.”
This is not as provocative as it might seem, for two reasons. First, the United States and China (the Republic of China, 1912 to the present) each deploy “thousands of small, single-warhead missiles” in 23 or so times as many silos “located in granite outcroppings,” with “the highest possible hardness level for a surface-flush silo launcher,” in order to “proliferate targets as a way of diluting the effectiveness of a… counterforce attack.” The cost-exchange ratio of a ballistic RV attack on this deceptive, “multiple aimpoint system” basing scheme (with its “preservation of location uncertainty”) would be bad. The cost-exchange ratio of a MaRV attack would be atrocious. Second, the “non-alert SSBN force” “kept in port for routine maintenance and for other reasons” is sheltered from counterforce first strikes inside underground naval facilities with “underwater access points.” Obviously, these are both megaprojects, and no photoreconnaissance satellites or other “national technical means of verification” can verify the numbers of deployed ICBMs, SLBMs, or RVs.
There is no strategic arms control agreement between the United States and China. This state of affairs is perfectly “robust against cheating” in the trivial sense that there is simply no “formal treaty” to violate, and all strategic nuclear weapons are “produced and sequestered clandestinely” (in spite of “advanced national and multinational capabilities for verification”) by default. The superpowers have policies of deliberate ambiguity, eschewing the relaxation of secrecy necessary to accommodate intrusive inspection regimes, obfuscating their numbers of deployed strategic nuclear weapons in order to confront one another’s attack planners with additional “strategic uncertainty” and more difficult “information-gap robust-satisfying decisions,” and reducing the likelihoods that either side will become confident that they can achieve their political objectives by moving first. In other words, the policies of deliberate ambiguity “inject an element of uncertainty in the mind of a would-be button pusher, compounding the uncertainties already inherent in a nuclear attack.”
As the alternate history has it, strategic stability is considered to be quite high, despite the total lack of restraint in the development and operational deployment of offensive nuclear missile technologies. It is further enhanced by extensive pre-delegation, super-hard deep underground command and control facilities, a fiber-optic network of proliferated and buried VLF antennas for the detection, geolocation and characterization of nuclear ground bursts by their telltale electromagnetic pulses across the continental United States, and ELF ion-acoustic plasma antennas so much smaller than ground dipoles as to boggle the mind. “Acoustic waves have the same physical properties (for the same frequencies) as EM waves, but they have much shorter wavelengths. Acoustic oscillations, therefore, are able to drive the charge carriers in a plasma at ELFs over a shorter distance than are EM oscillations… the plasma can be contained in a structure. The ionization can be controlled by three methods: electrodes at each end, microwave heating, and laser ionizing.” These small, self-contained ELF transmitters are proliferated and based deep underground, with little to no sensitivity to the electrical properties of the surrounding rock, and perform the TACAMO mission with exceptionally high survivability, high availability, and low lifecycle costs.
Packaging Density
Although the terminal phase of the Mark 5’s flight can be shorter or longer than that of a ballistic reentry vehicle, it is generally shorter, in order to minimize the “loiter time,” and the probability of intercept in turn. The Mark 5 is an “evasion MaRV” rather than a “glide MaRV,” and as such, it does not need an especially high fineness ratio (and lift-to-drag ratio) in order to maintain a decisive energy advantage over ABMs (and stay well above the “plasma shielding velocity” below which “homing ABMs” are more effective) throughout the short terminal phase (as short as ten seconds, if the Mark 5 pulls down and “arrives on target vertically”). The Mark 5 will nevertheless be considerably longer than a ballistic reentry vehicle, not because it is “very slender,” but rather simply because it is large, in order to accommodate the avionics, propellants, transpiration cooling system, and divert and attitude control system.
This raises a serious, first-order issue. SLBMs are, of course, subject to severe length and volume constraints, making it difficult to accommodate MaRVs without hemorrhaging some combination of range — reducing the ocean area available for deterrent patrols, and reducing the survivability of the SSBN — and throw weight. The Poseidon III is intended as a solution to precisely this problem.
The UGM-96A is perhaps as close to being a flying propellant tank as is possible. It is interesting how the convex closures of propellant tanks (and some missile tubes) resemble the blunt bases of space capsules, and how integrating a rocket engine into the base of a propellant tank is not at all unlike integrating it into a heat shield. The “supersonic baffles” pull double duty, not only stiffening the consolidated nozzle and tank wall, but also providing surface normals with tangential components (i.e. breaking the circular symmetry of the nozzle) so that fluidic manipulation of the surface pressure distribution can generate roll moments. It is not the supersonic baffles that are deflected, but rather the surrounding flowfields. With the high dynamic pressures in the “high Mach region,” the required roll control authority can be provided by supersonic baffles of a “very reasonable” size, with low thrust losses due to shocks and skin friction. The consolidation of nozzle and tank walls places conflicting demands on the profiles or contours of the plug nozzles, but these might be resolved by the addition of stiffening supersonic baffles and perhaps even “secondary fluid injectors… to form an exhaust plume that is similar in geometry to the exhaust plume from a full-length plug nozzle.” I am indifferent as to what combination of primary and secondary injection (if any) provides the pitch, yaw, and roll control. The inflatable nose cone is much like a telescoping drag-reducing aerospike. The submersion of the engines in the “propellant of the stages below” is reminiscent of the Makeyev R-29, as well as how “the volume associated with inefficient packing of propulsion system components” in a torpedo can be used as an ersatz ballast or trim tank.
To my great discredit, I must do an embarrassingly crude calculation here. The Trident II third-stage motor case is 30in in diameter, and I’ve lowballed the length at 85in. I’ve also assumed that H₂O₂/RJ-5 delivers the same impulse in 80% of the volume, and that H₂O₂ occupies 75% of that volume in turn. If this volume were then deformed into a cylindrical shell with a length of 75in (in tandem with a 25in long RJ-5 tank, for an overall length of 100in) and outer diameter of 83in, the ratio of surface area to volume would be just under 0.43cm⁻¹, with at least some margin to the 0.5cm⁻¹ upper limit available for the addition of stiffening stringers, plumbing, and perhaps even anti-slosh baffles. It looks ridiculous, as though the sandwich structure of a payload fairing were filled with oxidizer. Assuming that I’m not insane, the moral of the story seems to be that the square-cube scaling law permits long-term storage of H₂O₂ in even elongated toroidal tanks, as long as they are on the scale of SLBMs. The heterogeneous catalysis of H₂O₂ decomposition by tank walls seems to be more of a problem for the satellites (and especially smallsats) for which green propellants are a killer application. At any rate, small amounts of stabilizing additives could be added, assuming that they don’t poison the catalyst in the engines.
The anhydrous H₂O₂ has a shelf life of 15 years. The inner surfaces of the “ampulized” anhydrous H₂O₂ tanks are polished to a mirror finish comparable to that of optical reflecting telescopes and high-gradient RF accelerator cavities. For the record, I do not believe that the real-world Navy would ever field a liquid-propellant SLBM in a million years. But if it did, this would not be a bad option at all. It’s not just the excellent impulse density, it’s the “semi-hypergolic” propellant combination with both the “greatly reduced handling and safety requirements” of non-toxic and non-hypergolic propellants, and the highly “reliable and repeatable ignition” of hypergolic propellants (catalytic decomposition of H₂O₂ generates highly reactive atomic oxygen radicals that spontaneously ignite hydrocarbons on contact). This is a little-known fact about solid propellants: they have high theoretical impulse densities, but are penalized at the system level by the need for “the large empty core of the propellant grain.” Anhydrous H₂O₂ and RJ-5 are so dense at the system level that the missile might not have adequate buoyancy without first inflating the nose cone.
The RVs are interdigitated and closely packed, not unlike the F-14s shown above. There will be weight and volume penalties with two PBVs (with their two sets of propellant tanks and so on), but the opposed configuration allows the RVs to be efficiently packaged and then dispensed without Chevaline-style “toe-in and tilt-out” mechanisms. In addition, the deployment of RVs and penaids by two PBVs working in parallel will compress the post-boost-phase “intercept window,” and the weight penalty is alleviated somewhat by integrating the PBVs into the airframe, so that they route thrust load paths around the payload bay. The concentration of axial compressive stress in the inner cylindrical wall of the toroidal tank is very important, because at any rate, it must be thick and stiff to resist buckling under tangential compression when the tank is pressurized — unless, perhaps, it is relieved by a tensile prestress.
The most objectionable, fantastical and futuristic aspect of the design concept is perhaps the complete and total lack of access to the interior of the missile for “inspection and maintenance… when loaded in the launching tubes and while the submarine is underway at sea.” It is very much a “store and forget,” wooden, all-up-round. While this would certainly be a major technical challenge, it would have the very real benefits of maximizing availability and minimizing labor costs, and perhaps even reducing the crew complement and “overall size of the submarine,” making it a smaller active sonar target. The (diffusion bonded titanium alloy) TWIN TUBE pressure hull of the Franklin D. Roosevelt-class, after all, provides “access trunks to the missiles within the tubes” from only one platform, rather than three as in the Ohio-class pressure hull. While a “through-deck motor” would provide access to at least the lower PBV, the annular third stage of the Poseidon III both allows the payload to be telescoped inside and “in parallel,” and allows the third-stage plug nozzle to fully utilize “the true diameter limit for the engine installation, i.e., the vehicle diameter,” and thereby maximize the total impulse, as “an increase in propulsion system vacuum performance occurs primarily by an increase in nozzle area ratio.” The plug nozzles also have “thrust loss due to flow divergence… less than that in a conical nozzle of the same area ratio and length,” and the first-stage nozzle has a “thrust advantage over a conventional nozzle” due to altitude compensation.
It must be said, however, that the engines and control effectors of the UGM-96A and Mark 5 involve large arrays of miniaturized and loosely-coupled modules, and this “offers fail-operational potential for module-out… modes” in which the system “continues to perform even if individual components fail,” with the same redundancy, fault tolerance, and “extremely high mission reliability” that makes the plug nozzle the key ingredient in “the ultimate customer-driven propulsion system.” This “graceful degradation” of performance as modules fail is not unlike that of an AESA radar. The “numerous valves” may not be directly accessible to technicians aboard the SSBN, but it should still be possible to periodically actuate and check them out, by driving them through the umbilical.
As depicted in the cutaway, it may be necessary to defuel the second stage prior to removal of the third stage and PBVs as a unit. It may be possible to change out the payload of the lower PBV through the third stage, after only the upper PBV is removed. If the third-stage engine were a detachable module, it would then be possible to remove the PBVs as a unit with only the toroidal propellant tanks, leaving the submerged third-stage engine on top of the stack in order to cap off the second-stage fuel tank in the manner of a reverse forward dome.
Nose Response Sensor
While the nose response sensor is much like a flight test instrument used to aid the design of nose tips, here it is installed in a serially produced and operationally deployed vehicle. I knew that it would be necessary for the piezoelectric elements to function not only at high temperatures, but also under gamma and neutron irradiation (to help reduce the kill radii of ABMs), and I was very fortunate to find the “under-sodium viewing” literature. In a sodium-cooled fast reactor, imaging and inspection of structures submerged in opaque, metallic sodium must be done acoustically, rather than optically. There has been quite a lot of research into phased-array sonars for USV, and it turns out that inorganic piezoelectrics have excellent radiation hardness. It became clear that I would have to pull out all the stops to get this technique to work even at the very high background noise level of “weather flights,” and because its much larger dielectric constant yields a “relative sensitivity” of +24dB in an “equivalent circuit,” I therefore chose PZT over lithium niobate, helped along by the fact that PZT is the piezoelectric most commonly used for USV. I am certainly open, however, to the possibility that the much higher temperature capability of lithium niobate will ultimately become necessary, if the SAW nose response sensor can in fact be made to work without PZT (or at all).
If the Mark 5 does penetrate a dust defense, I can imagine two ways in which holographic reconstruction of the surface temperature distribution could become impossible. First, the deafening background noise could simply swamp the signals. Second, shape change and surface roughening could render the SAW medium prohibitively lossy and heterogeneous. I am curious to know just how gracefully the temperature feedback control will fail during blunting of the nose tip. It may be the case that failing nose tips proceed through more than one distinct surface acoustic wave phenomenology, and that the nose response sensor could therefore continue to collect some useful information (and limit overcooling) in a reversionary mode.
Reactive Jet Interaction
Each additional fluid carries a fixed weight and volume cost. It really is a good sign that hydrazine is pulling triple duty here, generating gas for tank pressurization, generating a reducing and relatively cool gas for active thermochemical protection of the refractory metal nose tip, and acting as both a fuel and monopropellant. In particular, hydrazine is a monopropellant that decomposes into a fuel, and therefore lends itself to supersonic combustion — it can be decomposed and expanded through a nozzle to form a supersonic transverse jet of hydrogen-rich gas that will penetrate deeply into and rapidly mix with the freestream, rather than “simply follow the stream of supersonic airflow without significant mixing.”
It is a miraculous coincidence that iridium is at once the go-to catalyst for hydrazine decomposition and the go-to environmental barrier coating for radiatively-cooled rhenium rocket nozzles. The microthrusters are regeneratively cooled, but the mass flows of propellant are intermittent and mismatched with the rates of aeroheating. By fabricating the microthrusters from rhenium, and using low-pressure MOCVD to coat their passages with iridium and Ir-Pt alloy thin films, active cooling may not be required to keep them under 2200°C, even on the windward side at high angles of attack. The nose tip and DACS thrusters require penetrations through the ablative thermal protection system, and might benefit from a notional high-temperature intumescent ablator with closely matched rates of recession and swelling through much of the terminal phase, arresting shape change such that the surfaces of the nose tip, DACS, and TPS are nearly flush for much of the flight. The smaller protuberances may even delay laminar-to-turbulent transitions, reducing aeroheating and, in turn, further reducing recession and shape change.
The ejection orifices are much smaller than the wavelength of electromagnetic radiation emitted even by microwave target tracking radars, and scatter it only very weakly. If the minimum feature size reproducible in platelet technology is indeed two hundredths of an inch, then the resonance region would seem to be somewhere in the millimeter wave band. The “cavity flow” in the “thruster openings on the body” will probably not be an issue at such a small scale, especially if the divergent supersonic jets really are purely external, and the nozzle throats lie flush with the outer mold line.
The importance of the ‘flat-panel’ microthruster arrays to reducing the weight and volume of the MaRV cannot be overstated. It is interesting how a microthruster array ‘synthesizes’ much the same flowfield as a far longer and larger single thruster, similar to how an AESA synthesizes the same wavefield as a parabolic antenna, but in a “more desirable form factor” with far less depth and bulk. More fancifully, a microthruster array can be thought of as a quasi-two-dimensional ‘hologram’ of a virtual three-dimensional thruster. The divert microthruster arrays conform to the outer mold line, much like a conformal AESA. It is also interesting how, in order to defeat exoatmospheric kill vehicles, the MaRV has divert thrusters exactly like those of an EKV.
Guidance Accuracy
The linear optical accelerometer literature is actually quite contentious, and there is apparently quite a lot of junk. To avoid pitfalls, one must grasp certain subtleties of the Sagnac effect, special relativity, and so on. I trust the late, great Marvin Biren to have figured it out, however. The Mark 5 therefore has radiation-hard, strapdown optical accelerometers, with no moving parts, for all six degrees of freedom, rotational and translational alike. This technology was originally developed for the single-warhead Aerojet LGM-70A Nemesis ICBM, whose small size, “low lifecycle costs,” and “dormant operation permitted proliferation of the force in large numbers… to maintain a similar number of warheads as the large MIRV missile options.” The LGM-70A represents an application of SLBM technologies to the single-warhead ICBM, providing ICBM total impulse in an IRBM package, and making road mobility a truly practical proposition.
The Franklin D. Roosevelt-class ballistic missile submarine has an inertial measurement unit based on cold atom interferometry that drifts only one or two meters per hour, and of course, the Poseidon III has an astro-inertial guidance system with a star tracker. The gravity anomaly maps may have been obtained using cold atom interferometry.
It will certainly be a challenge for the adaptive model-predictive flight controller to achieve the required “high control update rate” within the severe size, weight, and power constraints of a reentry vehicle, but what’s even harder is radar signal processing in the highly compressed endgame phase of an active radar homing missile, especially in the presence of jammers. When I detail the Argus Combat System later in the series, it will be clear how three-dimensional integration of bare dies along the lines of the Hughes Systolic Cellular Array Processor makes this possible.
The construction of the aerodynamic database relating the “amplification factors and the JI environmental factors” is greatly aided by Castor and Pollux. Castor is a hypersonic test track with electrodynamic suspension, allowing it to “appropriately simulate” the “typical hypersonic missile vibration environment” of 4g RMS, and in turn, “boundary layer transition.” The variable-geometry ‘wings’ of the rocket sleds have “variable wedge” compression and expansion surfaces that alleviate “high drag coefficients and… flow hysteresis” throughout the Mach number range, and perhaps even cancel much of the wave drag, much like a “slender ship moving steadily at supercritical speed in a shallow water channel.” Pollux is a hypersonic wind tunnel with a magnetohydrodynamic motor, allowing it to simulate “the environment of hypersonic flight… in terms of velocity, temperature, pressure, air chemistry, and structural-thermal equilibrium.” The broad-beam, plasma-cathode electron guns have plasma windows, so there are no foil windows or hibachis to cool. Pollux can also abrasively blast test articles with “sand and glass beads” to simulate “the combined effects of erosion and ablation… in an environment closely simulating the important parameters of shear, heat flux and surface temperature.” The extensive full-scale physical simulation conducted by both superpowers enhances the credibilities of their strategic deterrents by simplifying the processing and interpretation of test data, so that it can better support strategic nuclear weapon system design decisions. I will detail these facilities later in the series.
Aerojet UGM-96A Poseidon III (submarine-launched ballistic missile) [anhydrous H₂O₂ oxidizer] [RJ-5 fuel] [platelet catalyst beds] [three pressure-fed rocket engines] [three ablatively-cooled plug nozzles] [two “independent post-boost vehicles”] [14 Mark 5 reentry vehicles] [penetration aids] [star tracker] [inflatable aerodynamic shroud]
It can be “launched while the submarine is traveling near or at patrol speeds without slowing down much if at all,” as in United States Patent 5425301, and has a 2.4m diameter. The titanium alloy propellant tanks have common bulkheads and graphite fiber-reinforced epoxy “filament-wound overwraps.” The plug nozzles 1.) have “supersonic baffles” that “provide circumferential stability of the rocket motor,” as in United States Patent 3733828, and 2.) generate roll moments by “valving flow to various segments of the plug” such that the fixed “supersonic baffles” act as “jet vanes.” The first- and second-stage plug nozzles are formed by the shaped “lower… ends of the tanks,” “expanding exhaust gases… impinge directly on the aft pressurized membranes of the tanks,” and “both the engines and the thrust structures” are “tightly integrated with the bases of the propellant tanks.” The first-stage engine is “submerged into” the concave “aft closure” of the missile tube. The second- and third-stage engines are “submerged into” the “full-diameter open ends” and “propellant of the stages below,” like the second-stage engine of the Makeyev R-29. The third-stage propellant tanks have a “tandem/toroidal configuration.” The post-boost vehicles are in tandem. The upper post-boost vehicle is “oriented in the reverse of the fight direction,” like the post-boost vehicle of the Makeyev R-39. The payload bay is formed by the “inner cylindrical wall” of the third-stage propellant tanks, and the opposed “payload decks” of the post-boost vehicles. The “ultra-high purity,” “100ppt grade” H₂O₂ can be stored for “15 years or more.” The idea is that it has a significantly higher total impulse than a conventional SLBM subject to the same length and volume constraints “imposed by the launch tube,” and can throw eight or more “evasion MaRVs” 12,000km downrange, because it efficiently fills the cylindrical “pressure vessel shell” with elliptical end-caps, the “overall vehicle packaging efficiency” is high, the liquid propellants have a high impulse density and no “empty core,” all three nozzles have low divergence losses and exit areas “maximized to the outside diameter of the stage,” and the altitude-compensating first-stage nozzle has “a thrust advantage over a conventional nozzle.”
“Believing that a simpler missile was better suited to a mobile role, [USAFBMD] contended that a missile with pressure-fed, storable liquid-fuel propulsion… lent itself to the mobile environment. Pressure-fed systems were not difficult to build; in fact, for manned spaceflight applications they were preferred because of their reliable simplicity. It would be lighter and smaller than [a solid-fueled] or a pump-fed, liquid-fueled missile of comparable capability.” “If H₂O₂/Syntin is used, the increase in range amounts to approximately 10% in relation to [N₂O₄/hydrazine]… H₂O₂/Syntin is attractive because of its low toxicity level… Therefore, it is advisable to consider H₂O₂/Syntin for use in sea-based complexes in the future…” “The bulk density of APCP is about 1.75kg/L but the large empty core of the propellant grain reduces the average density to about 1.3kg/L… H₂O₂/RP-1 has about the same density as solid propellant, but with much higher [exhaust velocity]. It also has the best performance in terms of [impulse density]… Most metallic materials will need cleaning and degreasing with detergent, pickling to remove metal impurities, passivating and conditioning… No ignition source is required since the very hot decomposed H₂O₂ will spontaneously combust with kerosene.” “Due to the tight confines and limited onboard support equipment associated with combatant vessels, the [United States] Navy has always been concerned about the shipboard use of toxic hypergolic liquid propellant systems. In addition… there is concern for the potential for fire caused by the natural hypergolic behavior of bipropellants in the event of a leak… H₂O₂/JP-10 has… relatively high performance… H₂O₂ was used in torpedo propulsion systems prior to Otto fuel II… pressure buildup during long-term storage of [H₂O₂] may require venting procedures… adding operational complexity… A catalyst may be required for ignition of the… non-toxic bipropellant…” “H₂O₂ is commonly believed to be unstable such that it is unsuitable to be stored for any extended period of time. This assertion is incorrect and H₂O₂ has in fact demonstrated successful long term storage… zirconium alloys [are suitable] for H₂O₂ usage… water actually acts as a destabilizing contaminant… anhydrous H₂O₂ could potentially show significant improvements in storability as well as offer system performance improvements… larger systems… with more spherical shaped tanks have better storability than smaller systems or tanks with longer aspect ratios… smoother surfaces have less effective surface area and would have better compatibility… surface finish processes such as electropolishing can be especially helpful…” “Zirconium… does not initiate the decomposition of [H₂O₂]. This… makes it ideally suited for… equipment with a high ratio of metal surface area to solution volume, such as piping systems, pumps, valves and heat exchangers…” “Aerojet… devised a heterogeneous catalyst design that is monolithic (single-piece), extremely compact, and has pressure drops equal to or less than traditional screen beds. The design consists of a bonded stack of very thin, photo-etched metal plates, silver coated. This… leads to a high surface area per unit volume and precise flow area, resulting in high, stable, and repeatable performance… with no flooding of the catalyst bed… The monolithic design also exhibits good starting performance, short break-in periods, and will easily scale to various sizes.” “The most important requirement by far for a missile fuel is to put more available energy in each unit of volume… the most effective means [by which to do so] is to use a liquid hydrocarbon fuel which has a specific gravity greater than conventional jet fuels… low-temperature performance is not as [important] for the Navy as for the Air Force, and a higher fuel freezing point and viscosity are permitted… the conventional jet fuels JP-4 and JP-5 are petroleum distillates. The high-energy fuels, however, are chemically specific structures. Although the average molecular weights of JP-9 and JP-10 are similar to JP-4 and JP-5, the higher fuel density results in the larger volumetric heating value… Bridged ring hydrocarbon structures (which contain high C:H ratios) increase both density and volumetric heat of combustion by the greatest amount… RJ-5 [offers] substantial gains in missile range over all other [logistically acceptable] fuels. With the increased heating value we pay the penalty in increased viscosity and freezing point, however.” “Many [rockets are] volume sensitive and require that wasted space within the vehicle be minimized… Tanks… are often shaped with symmetrical, convex walls. Consequently, space between independent tanks and requisite intertank structure is typical… the engine itself may be integrated with the tank, such that the lower (aft) end of the tank is shaped to form a nozzle. In such a configuration, the pressure of expanding exhaust gases in the nozzle would impinge directly on the aft pressurized membrane of the tank… the tank may also transmit the force of the engine directly, thus enabling integration of both the engine and the thrust structure into the tank. The nozzle… may have an annular, ‘aerospike’ design… Incorporating the engine’s chamber and nozzle into the tankage may be especially beneficial for upper stages and spacecraft that may use large engine nozzles, but requires that the engine be fixed (non-vectorable) with respect to the tank.” “The [Isayev 4D10] is a submerged engine, tightly integrated with the base of the propellant tank. This allows a much greater packaging density, which is advantageous for a mobile missile…” “A plug nozzle [differs from a conventional nozzle in] that the supersonic expansion, which is generally not confined within solid walls, is continuously ‘redirected’ by the ambient pressure to produce an essentially axial velocity vector… If the gas stream expanding through the design pressure ratio is to have an axial direction, the initial flow direction at the throat must be inclined at the Prandtl-Meyer angle from the vertical… The outer boundary of the jet along a plug nozzle is a free surface which can adjust itself continuously to the existing ambient conditions… It is significant that the jet cross-sectional area at the plug apex, which is the effective nozzle exit area, adjusts itself as a function of the pressure ratio. Below the design pressure ratio, the variation is approximately such that the exit to throat area ratios correspond to the ideal value required by the existing pressure ratio; i.e., the plug acts as a pseudo-variable area ratio nozzle. As a result, the performance of a plug nozzle operating below the design point is higher than that of the conventional nozzle. Above the design point, the theoretical performances are identical… This can be easily understood when one considers that at operation above the design point, the pressure distribution along the plug remains undisturbed, whereas at operation below the design point, the local pressure downstream of the plug location where the ambient pressure level is first reached adjusts itself to ambient conditions by a series of weak compressions and expansion… A significant [advantage] of plug nozzles is that they provide a relatively simple technique to produce adequate side forces for thrust vector control. If, for example, one divides the annular combustor around the base of the plug into independent units so that the chamber pressure can be raised in one sector and lowered in another, then the gas stream will undergo a rotation and translation with respect to the axis of symmetry of the plug… In most applications… roll control is also required of the main engine. In the case of the single plug, the use of some form of jet vanes can serve this purpose. If located just below and between segments of a segmented annular throat, drag losses can be minimized and vane sizes can become very reasonable, since they are working primarily in a high Mach region… The possibility of multiple chambers, clustered and exhausting on one large plug has some interesting ramifications, particularly for applications where very large missile control forces are required… Reasonable manipulation of pressure on the quadrants of a liquid propellant plug nozzle can produce thrust vectoring forces equivalent to a gimballed nozzle and with considerable decrease in complexity and response time.” “The baffle plates extend beyond the throat of the thrust chamber and into the supersonic region in the nozzle area and, thereby, provide circumferential stability of the rocket motor… The [plug nozzle] is attractive for space vehicle application, because shortened nozzles reduce interstage structure weight and permit an increase in payload through increased performance for a given length… essential and critical tolerances are, at best, difficult to maintain throughout the entire length of the toroidal thrust chamber, and… toroidal-type thrust chambers exhibit undesired and detrimental circumferential modes of instability which, at the very least, have some adverse effect upon the operation of the rocket engine as a whole.” “…the volume available for upper stages is usually critical… bell nozzles force severe volume, weight, and complexity problems on… an upper stage… Properly designed plug nozzles can attain very high performance… nominally equivalent to conventional contoured bell nozzles… the modules controlling the internal expansion are ‘tilted’ inward at the proper angle. The Prandtl-Meyer expansion remaining at the exit of the internal expansion expands or turns to the axial direction under the control of the plug contour, designed… to produce shock-free expansion and turning. Thus the control of the expansion by the plug is on the inside of the flow field as opposed to the outside… as in the case of a conventional bell nozzle. In a vacuum, the resulting exhaust is a fully-developed flow field and the expansion surface has no way of communicating with the ambient surroundings. As a result, some liberties can be taken with the design of the plug. One of the most useful [modifications] is to truncate the plug and allow internally recirculated gases to pressurize the captured base region… There are two distinct applications for external expanding nozzles: for first stage vehicles, where the external expansion allows an increase in effective area ratio as the vehicle moves up through the atmosphere, commonly referred to as ‘altitude compensation,’ and for space vehicles, where the vacuum environment allows a truncation of the ideal isentropic plug contour to a very short length with a minimum [effect on performance].” “The thrust loss due to flow divergence in an annular nozzle… would be less than that in a conical nozzle of the same area ratio and length… Due to the self-adjusting nature of the flow, a plug nozzle operating at below design pressure ratio shows a thrust advantage over a conventional nozzle… the amount of recompression occurring on the plug surface is intimately connected with the plug wall contour and consequently affects the off-design performance of the plug nozzle.” “The performance of a multistage missile can be highly improved by reducing the useless interstage volume… the forward dome concavity of the lowest stage of the missile is reversed. The propellant mass saving is [nearly] 10% of the loaded first stage propellant mass, so that the range of the missile would be highly increased… For a missile with a very scarce specified volume, [for example a strategic missile shipped aboard a submarine, increasing] the total impulse delivered by the launcher by filling the useless interstage volume [increases range]… a reverse dome on a missile case is costly in terms of initial inert mass. The dome has a rolling up and a buckling tendency under the effect of an internal pressure of about 10MPa.” “…integrated stage technology eliminates the missile interstage area by nesting the forward dome of the first-stage motor into the nozzle of the second-stage motor.” “The Integrated Stage Concept entails the integration of the rocket motor nozzle into the motor case, thus removing the need for a heavy interstage structure… The nozzle exit area is maximized to the outside diameter of the stage… A short nozzle is highly desirable to keep the overall stage length to a minimum and to maximize the amount of propellant carried in the stage… A conventional gimballed nozzle cannot be fully submerged into a case.” “The Integrated Stage Concept… results in a more efficient use of available propulsion system volume and… stage performance improvements approaching 30% for volume-limited systems… short nozzles provide increased propellant loading while maintaining structural efficiency… Because the ISC does not have a conventional interstage, the staging event is greatly simplified and [the] booster has increased stiffness. These… advantages make it attractive for the fast burn mission… For constant-range and -payload systems, the ISC system will have… up to a 20% reduction in length… In a first-stage ICBM booster, the pressure ratio varies over a considerable range (i.e. near sea level to near vacuum), and for a conventional fixed nozzle significant losses occur through over- or under-expansion of the nozzle flow depending on the design point.” “Toroidal tank arrangements [are used in the Khrunichev] Briz-M upper stage. The [Yuzhnoye] Zenit-3SL second and third stages also use toroidal kerosene fuel tanks… the inner cylindrical wall of an elongated toroidal tank will be in radial compression when pressurized… Being primarily in compression, the inner cylindrical wall must be designed for high strength. However, the mass and strength required of such a wall, as part of the pressure vessel, can also double as the means of carrying… weight and thrust loads.”
General Electric Mark 5 Hermes (maneuverable reentry vehicle) [“controlled atmosphere protected” W-5Re alloy platelet transpiration-cooled nose tip] [hydrazine fuel] [N₂O₄ oxidizer] [“multi-toroidal” piston tanks] [“bootstrap pressurization system”] [platelet catalyst beds] [“combined endo-exoatmospheric reaction control system”] [ablative thermal protection system] [inertial guidance] [adaptive model-predictive flight controller] [W76 physics package] [gravity anomaly map]
It has 1.) a slender, “sharp biconic configuration,” and 2.) “neutral pitch and yaw stability” over a wide “range of angles of attack, Mach numbers, and roll angles.” The “partitioned” nose tip is “dry coupled” to a circular phased array of PZT “spring-loaded transducers” by a “soft gold foil.” The “temperature-feedback, closed-loop control system” 1.) uses “cross-correlation processing” to “remove background noise” from “received ultrasonic signals,” 2.) uses “real-time holographic imaging methods” to reconstruct the nose tip “surface temperature distribution,” and 3.) drives electromechanical valves that meter coolant flows to the “independent hydraulic sections” of the nose tip, in order to minimize overcooling. The regeneratively-cooled, platelet rhenium “divert and attitude thrusters” have 1.) “fully variable mixture ratios,” and “can be operated either in monopropellant mode or in bipropellant mode,” 2.) catalyst beds with LPCVD “platinum-group metal and or alloy” thin-film coatings, and 3.) “zero-length” plug nozzles with bases that lie “flush with the aeroshell mold line.” The “attitude thrusters” are in a “clocked configuration,” “in the 45° planes with respect to the lateral thrusters.” The strapdown IMU has a triad of “vertical slab multi-pass optical accelerometers.” The ideas are that 1.) the “multi-toroidal” piston tanks prevent “sloshing… and its attendant dynamic effect,” are “efficiently packaged within the lifting body OML,” and allow high “overall vehicle packaging efficiency” that “reduces vehicle weight” and accommodates the “weight associated with internal tension membranes and lobe skin joints,” 2.) the “nose response sensor” has a reasonable SNR during “weather flights,” despite the “exceptionally high” background noise level, 3.) the weight and volume of the “transpiration system” is reasonable, because the “bootstrap pressurization system” has “no separate pressurization system or stored high-pressure gas,” the platelet nose tip has a very high “internal heat transfer coefficient” such that “the local coolant temperature is equal to the local matrix temperature” across the surface and “the cooling capability of the coolant is completely exploited,” and the “temperature-feedback, closed-loop control system” operates the nose tip “right on the threshold of failure,” “at the highest surface temperatures consistent with structural and oxidation limits,” 4.) the impulse bits are very small, because the very short “characteristic lengths” of the micronozzles and the “close proximity of the very fine,” “vaporized and partially dissociated,” hypergolic “fuel and oxidizer streams as they leave” the platelet injectors “facilitate the intermittent duty ignition requirement,” 5.) the micronozzles have reasonable performance despite their small “characteristic throat dimensions” and low Reynolds numbers, because their “completely external supersonic expansion” and relatively small wetted areas alleviate viscous dissipation and heat losses, 6.) the microthrusters have reasonable performance, because “selective on/off operation, rather than throttling of individual thrusters” avoids “the significant drop in performance at the lower power levels,” 7.) the “multi-jet control system” seeks the increasingly rich “optimum mixture ratios” at which “side-force specific impulses” are maximized as “the dynamic pressure increases during the descent,” 8.) the “fuel-rich jet interaction” attitude control system “can sustain high Damköhler number chemical activity at high strain rates,” because the gaseous, supersonic, hydrogen-rich “transverse slot jets” have high “jet-to-freestream dynamic pressure ratios,” fast “turbulent mixing and combustion,” “deep… jet penetration into the freestream, and very large recirculation regions” that “act as flameholders without the use of steps in the flowpath” and provide “additional residence time for combustion to take place,” 9.) the weight and volume of the “divert and attitude control system” is reasonable, because it “does not require excessive force to change the angle of attack” and can “hold an angle of attack” with very low “propellant consumption,” the “bootstrap pressurization system… stores all of the required pressurant as a liquid, but does not require a separate helium bottle for monopropellant pressurization,” the impulse bits are small, and the microthrusters have high thrust-to-weight ratios, 10.) the artificial stability “may be used to reduce ballast” and or the drag penalty associated with “departure from conical geometry,” because the “warhead forward diameter” and “biconic break station” can be positioned such that the “aerodynamic center of pressure is… at the center of gravity” rather than “aft of the center of gravity,” 11.) it can “maneuver effectively” even in the “several seconds” after it becomes distinguishable from “most penetration aids,” but before “aerodynamic forces” and moments “have built up to large amplitudes,” 12.) it has an “all-weather operational capability” and high “mission flexibility,” 13.) it is “specifically designed to penetrate dust clouds,” with “inherently high resistance to impact damage” and a “damage-adaptive flight controller,” 14.) the “observable signatures” are relatively low, because of the sharp nose tip and plasma quenchants, and 15.) it is hard to “distinguish from small, low-radar observable conic ballistic RVs” during the midcourse phase, because the attitude control system can not only generate moments without “flaps, flares, or fins,” but also “achieve robust ignition and complete combustion” without “steps, ramps, cavities,” or “pylons.”
“There are two major types of hostile atmospheric conditions than an RV may have to pass through. The first is heavy weather, such as thick clouds, snow, rain, or hail. The second is the dust clouds raised by nuclear detonations. Both… have large and unpredictable effects on the winds and density of the atmosphere, which degrade the accuracy of entering RVs. Their most significant effect, however, is the erosion of the RV’s nose tip caused by the particles in the clouds… In severe cases, the nose tip can erode away completely, exposing the RV to high temperatures which may cause it to burn up in the atmosphere… Tungsten is much more resistant to erosion than most other materials… A liquid (or a gas) is carried inside the RV to be forced out through the nose tip at high pressure; as a result, the temperature at the nose tip never gets high enough to cause the nose tip to burn away… Liquid-cooled nose tips have a number of distinct advantages. First, much larger quantities of heat can be dissipated without degrading the nose tip, which is important [in] maneuvering reentry vehicles… because their high speed maneuvers within the atmosphere result in increased heating rates… A second advantage… is that, since the nose tip does not ablate, the problem of asymmetric nose tip ablation is essentially eliminated… Finally… active nose tips will be significantly less susceptible to erosive environments than are even tungsten-backed ablative nose tips… the discrete matrix nose tip… rather than simply being porous, has a large number of tiny ‘canals’ for the fluid to exit from. Using this method, not only can the rate of flow be controlled, but the coolant can be directed to specific areas of the nose tip that sensors show to be experiencing higher rates of heating… Maneuvering reentry vehicles can be both more accurate and better able to penetrate ABM systems than ballistic RVs… MaRVs are significantly heavier, more complex, and more expensive than ballistic RVs… since MaRVs are guided through the atmosphere, such effects as unexpected atmospheric winds or unexpected lateral forces can be measured and compensated for by the guidance system… Exoatmospheric ABM systems can be confused by chaff and decoys. However, because they weigh less, these penetration aids will become distinguishable from the RV as they pass through the upper atmosphere. In the case of ballistic RVs, with their predictable trajectories, it is theoretically a simple matter for the ABM computer to dispatch interceptors to meet and destroy them… Evading MaRVs, by contrast, are able to swerve and duck at extremely high speed, making it impossible for ABM computers to predict their course and direct interceptors to meet them. There are, of course, some limits as to how sharply the MaRV can turn and to how fast it can go; thus, the ABM computer will know that the MaRV will be somewhere within a given ‘area of uncertainty’ when the interceptors… arrive. If enough interceptors are sent to destroy any vehicle within the entire area of uncertainty, it would… destroy the MaRV. One of the primary development goals for evasion MaRVs is to make the number of interceptors required to do this very large, thus exhausting the defense… Thus, the most important features of a MaRV intended for ABM evasion are its nuclear hardness, the sharpness of the maneuvers it can execute, and its ability to maintain high speed throughout its reentry. A high-performance MaRV could make the task of an endoatmospheric ABM essentially impossible… To execute sharp turns at high speeds, the MaRV’s control system should be able to provide accelerations of tens of gravities in the desired direction. To perform such maneuvers would require large amounts of fuel on the RV, posing an unacceptable cost in RV weight. The solution has been for MaRVs to rely on aerodynamics to provide the necessary forces… however… relying on aerodynamic maneuvers means that the RV cannot begin to maneuver before it is well into the atmosphere, when the aerodynamic forces will have built up to large magnitudes. MaRVs cannot maneuver effectively, therefore, at altitudes above 60km. Thus, like ballistic RVs, MaRVs require penetration aids in order to overcome exoatmospheric ABM systems. In the case of MaRVs, the complication is that most penetration aids become distinguishable from the RV at an altitude higher than that at which the MaRV can begin to maneuver effectively; thus, there will be a time period of the order of several seconds during which the MaRV, like a ballistic RV, is essentially undefended. If this ‘window of vulnerability’ is too large, it is at least conceivable that an ABM system could… destroy the incoming RV during this interval… As a result, the development of decoys capable of penetrating deeply into the atmosphere could potentially be a significant issue, even when MaRVs are available… the guidance components for an RV need not measure with extreme accuracy. Since the entire reentry process takes only [one to three] minutes, small errors in measuring the acceleration of the vehicle do not have enough time to propagate to significant impact errors. But equally, since there is so little time for guidance, the mathematical formulations used by the guidance computer (known as the guidance laws) must be able to correct for any errors the guidance components detect extremely rapidly. As a result of the extreme time constraints involved, the accuracy of an evasive MaRV is often more sensitive to the formulation of the guidance laws (the software of the guidance system) than to the accuracy of the guidance components (the hardware)… when formulating the guidance and control equations, the designer must have detailed knowledge of the aerodynamic properties of the particular vehicle in question… evasive MaRVs also require much better heat shield technology than do their ballistic counterparts. The high-g maneuvers and extended times spent in the atmosphere mean both higher heating rates and more total heat to be dissipated… if an ablative nose tip is used, and if it ablates unpredictably, then the aerodynamic properties of the vehicle will be different during flight from the expectations of the guidance computer, resulting in some inaccuracy… The guidance system of the [McDonnell Douglas] AMaRV is… extraordinarily small and light, partly as a result of substituting laser gyro technology for more conventional inertial measurement devices… This allows more room and weight to be devoted to the warhead… In addition, laser gyros are much better able to withstand high acceleration and nuclear effects than are conventional inertial technologies… It has been considered unwise to increase the vulnerability of RVs by having their guidance systems rely on satellites that may not be available when needed… Any significant maneuvers by the RV could be detected, as could the the flaps or vanes necessary for executing such maneuvers… if the technology of fuzing and arming mechanisms can be miniaturized, it is possible that large increases in yield could be accomplished without increasing the weight of the vehicle.” “The [General Electric] MBRV was… an evasion vehicle that during reentry could ‘out-duel’ enemy interceptor missiles by performing ‘jinking maneuvers’ (i.e. abrupt turns or changes in direction within the atmosphere) to ‘fake’ interceptors out of position, and thereby prevent engagement… to evade enemy anti-ballistic missile interceptors [capable of 50g maneuvers], it was necessary that MBRV survive 100g maneuvers, far in excess of strength required for ballistic RVs to survive reentry decelerations… MBRV… could reenter the atmosphere away from defense sites, maneuver to destroy either a target or defensive facilities, such as command and control centers or radars… The vehicle would… roll slowly to even out heating and ablation weight loss during reentry… Flat elliptical areas were cut in the vehicle’s heat shield forward of each flap to permit effective airflow for steering control… four small gas jet nozzles were positioned on the base between the flaps… These jets would provide exoatmospheric alignment of the vehicle prior to reentry… a conic vehicle is difficult to package because its CG approaches the base, somewhat behind its aerodynamic CP. Unlike a ballistic arrow, which has a heavy mass near the tip to keep the CG forward and feathers at the rear to keep the CP far back, a cone filled with uniform density material tends to be unstable and can flip around or tumble… A small positive stability margin (distance between the CG and CP) is desired for good maneuverability, since it does not require excessive force to change the angle of attack. A large stability margin (e.g. >10% of the body length) makes it difficult to change the vehicle’s direction, which follows its CG and resists attitude-changing commands due to strong aerodynamic pressure or forces at the rear (acting like a ballistic projectile)… The heat shield would be ablative, 1in-thick carbon-phenolic [thicker than on the Mark 12, because MBRV flights were more severe and lasted nearly twice as long as a ballistic reentry]. The heat shield alone would weigh nearly 800lb, most of which was in the aft third of the vehicle… Depending on flight range (i.e. reentry velocity and angle conditions), MBRV would have sufficient kinetic energy and aerodynamic lift to fly roughly 250–300mi downrange and over 150mi crossrange in either direction from its ballistic aimpoint. This led to a heart-shaped footprint capability dominated by downrange maneuvers. A sharp [80g] pulldown maneuver was environmentally most stressful, and its time of flight from [an altitude of 100,000ft, i.e. entering the sensible atmosphere where steering became effective] to impact was barely ten seconds… Douglas proposed to employ ‘jet reaction’ or ‘external burning’ control to maneuver a reentry body, instead of using aerodynamic flaps, flares, or fins, which increased the radar cross-section of the vehicle and made it easy to distinguish from small, low radar observable conic ballistic RVs… The jet reaction concept involved using a slot nozzle near the base of the RV to inject inert gas into the airstream, changing the vehicle angle of attack and… using the inherent body lift to turn. External burning was more exotic in that a hypergolic fuel (e.g. gasoline or hydrazine) was squirted into the airstream to ignite spontaneously and [raise surface] pressures… Reaction controls would avoid drawbacks of rear-mounted airfoils, which required high pressures to offset hypersonic airflow. In addition, jets were ten times faster in response to steering commands. Finally, they would work at higher altitudes (above 50,000ft) where airfoils were not overly effective, thereby permitting maneuvers earlier in reentry flight… ballistic RVs depended on penetration aids, including chaff puff packages in space, multiple decoys (some capable of high-altitude reentry before burning up) and suppression of observable signatures by sharp conic nose tips, smooth low radar [observable] surfaces and no ablation chemicals or ionization trails… The majority of maneuvering ballistic vehicles are biconic configurations, with a sharp cone forward and small-slope aft frustum… This is done to permit packaging of [the heavy and bulky] warhead as far forward as possible… vehicles designed to perform evasion of enemy defensive missiles require ballistic coefficients of [1500–2000psf] to permit maneuvers of [80–100g] and coverage footprints of a few hundred miles. Ballistic coefficients greater than about [2500psf] are difficult to achieve, because reentry velocities deep in the atmosphere cause thermal environments that exceed the strength of available heat-protection materials.” “The trend in strategic reentry vehicles has been toward higher velocities (less deceleration) during atmospheric reentry. Reentry vehicle configurations have thus evolved into… low-drag configurations, with a slender cone and small nose tip bluntness being one logical choice. A slender cone, however, may present stability, structural, and storage problems due to its long length and internal volume limitations. A biconic configuration, [i.e.] replacing the front portion of a cone with a larger-angle cone, is a possible alternate configuration… For the biconic configuration… nose-tip bluntness effects play a lesser role, and the main body produced the dominant moment characteristics (it can be inferred that the biconic configuration would also be less sensitive to nose-tip asymmetries such as might result from reentry ablation)… viscous effects are not large for the sharp biconic configuration for angles of attack [of 3–25°].” “Interest in biconic vehicle geometry is a consequence of… aerodynamic and vehicle design improvements which may be realized by a modest departure from conical geometry… For a given cone angle and base diameter, warhead forward diameter establishes the most aft biconic break station which can be accommodated without shifting the warhead aft… The planform area thus removed from the forward portion of the cone results in a more aft CP at angle of attack. CG is shifted aft a relatively small amount by this modification. Consequently a net increase in stability results which may be used to reduce ballast or shorten the vehicle. The latter results in a decrease in vehicle diameter for a given cone angle. Thus… the biconic geometry offers a more packageable, shorter length, lighter vehicle.” “The cone and conical ellipse present the smallest surface area consistent with high aerodynamic performance and especially if ablative material is used as the outer surface, surface area means weight in the thermally protected reentry body… To minimize ballast required to trim for the desired CG, the conical shape of the vehicle dictated that, wherever possible, heavy components be packaged forward, with light components toward the rear of the vehicle. Major consumables, such as… propellant, were balanced about the CG in such a way that their consumption during the mission would result in minimal shift in overall vehicle CG. Volume for ballast was located at the vehicle nose tip, where it would have the longest lever arm to the vehicle CG… The… bootstrap pressurization subsystem is potentially the lightest of all pressurization subsystems because it stores all of the required pressurant as a liquid, but does not require a separate helium bottle for monopropellant pressurization…” “…the ability to maneuver in the atmosphere, similar to [AMaRV] would allow… a reduction in ballast.” “Refractory metals characteristically exhibit high resistance to impact damage… Refractory metals recede more rapidly at temperatures above the melting point than at lower temperatures, where oxidation is the prime cause of recession. Therefore, a high melting point… minimizes surface recession.” “Erosion-resistant materials, such as tungsten, develop a biconic shape during turbulent erosion. As a consequence, the resultant CP travel is much less than for erosion-dominated nose tip materials.” “Keeping the aerodynamic drag low… allows for quicker penetration of the atmosphere which reduces the… ‘loiter’ time (the time for retaliation), and reduces susceptibility to influence from course-altering winds… The disadvantages of the transpiration method of heat protection are the high gross weight of the fluid to be carried and the weight of the delivery components that are necessary for the system to operate… In order to minimize the coolant flow rate and thus the overall vehicle weight, it is desirable to operate the tip at the highest surface temperatures consistent with structural and oxidation limits… Even with the most advanced materials such as [C/C], nose tip recession as much as several inches can occur… with sharp nose designs employing a passive nose tip cooling system… This nose tip recession seriously affects the impact accuracy and control of a reentry vehicle… the use of the [transpiration cooling] system in conjunction with a refractory tip should make possible zero or near-zero recession of a nose tip in a thermal and/or particle environment.” “Active cooling of the forecone… affords significant advantages over passive ablative cooled materials where… no in-flight shape change can be tolerated… Platelet devices have been fabricated from a variety of metals and their alloys including aluminum, beryllium, copper, nickel, stainless steel, molybdenum, tungsten, titanium, [Re-50Mo alloy], and [Zr-Cu alloy]. The platelet fabrication process enables the construction of hardware representing precisely the configuration desired by the designer. In other words, ‘you can put the amount of coolant you want, exactly where you want it.’ We know of no other process that offers this degree of design flexibility.” “Transpiration cooling is… capable of… surviving any reentry environment [ever] envisioned… This capability results in… retention of the nose shape, increased mission flexibility, and all-weather operational capability. Unfortunately, transpiration is not a complete nose tip panacea. The coolant and expulsion system storage requires a significant volume and weight… In order to deliver coolant flow to a partitioned nose tip, it is necessary to split the main flow such that each section of the nose tip receives the proper amount of coolant to reflect angle-of-attack effects… Electromechanical flow control valves [are fast] and can make use of environmental feedback to reduce excessive coolant usage… The ultimate in flow control concepts would be to sense the response of the nose tip to the aerothermodynamic environment and meter the flow in a manner which allowed the tip to operate right on the threshold of failure… the use of acoustic sensors to determine temperature change at the surface of the nose tip… involves the evaluation of surface wave velocity, which is affected by temperature. This is done by placing a piezoelectric transducer on the aft face of the nose tip and propagating a wave around the surface of the tip. The wave would be picked up by either a second receiving transducer or by the first transducer using a pulse echo technique (the transducer alternately transmits and receives). By measuring the time between transmission and receiving, wave velocity is determined. Changes in wave velocity (due to changes in nose tip temperature) would be monitored. By selecting a wave frequency in the ultrasonic region, wave propagation would be confined to a surface layer a few thousandths of an inch deep… There are three obvious problems. First of all… the [SNR] is low. Secondly… the [SNR] should be much lower for a nose tip that is experiencing hot spots over small portions of its surface. Finally, suitable insulation techniques would be required to keep the piezoelectric elements from going higher than [74°C and losing] sensitivity and output power… Shell 405 catalyst… can be used to spontaneously and repeatedly initiate decomposition of hydrazine for a range of exhaust gas conditions such that the temperature of the pressurizing gas can be controlled to reasonably low levels. This is done by controlling the amount of… ammonia dissociation… Hydrazine also offers the advantage of efficient, vapor-pressure storage in a liquid form, and simple conventional methods of fluid control… The bootstrap system… allows the pressurant to be stored at its own vapor pressure. This system may be activated by firing a squib in the forward end of the pressure amplification device (piston) which, in turn, pressurizes the fuel. When the control device opens, hydrazine is forced into the reactor, thus generating gas which is fed back to the top of the pressure amplifier, bootstrapping the system… The bootstrap concept combines the inherent simplicity of a solid-propellant system (no separate pressurization system or stored high-pressure gas) and the flexibility of a liquid system (greater versatility as a result of the capability of demand operation)… a short catalyst bed (low ammonia dissociation) would not cause a severe penalty in fuel weight.” “Coolant flow is controlled by a servo valve, which is positioned based on a computer flow control algorithm using vehicle altitude, velocity, and angle-of-attack conditions in order to optimize coolant flow…” “The preferred piezoelectric material for… temperatures up to 260°C is [PZT]… PZT has… excellent damage resistance both to neutrons and gamma radiation… real-time… digitization of the received waveform… permits… [SNR] improvement of 20dB by cross-correlating the detected ultrasound signal with a pure sine wave at the resonant frequency of the transducer… In general, inorganic piezoelectric materials… show minimal permanent damage due to gamma irradiation… PZT shows no significant degradation due to neutron exposure for fluences at or below 10¹⁵n/cm²… Received ultrasonic signals should be digitized at ten times the maximum anticipated frequency.” “The Controlled Atmosphere Protection System… utilizes a reactive gas, ammonia, as the coolant and a refractory metal as the matrix. The injection of ammonia into the boundary layer medium cools the matrix and reacts with the boundary layer oxygen… this control of oxygen allows operation at high surface temperature, which greatly lowers the coolant flow requirements while limiting the oxidation of the hot surface materials to acceptable values… the CONAP system using a porous tungsten matrix and ammonia coolant is 50% lighter than a water system… Excessive nose tip recession will create a vehicle CP shift which can result in increased control system weights… Transpiration cooled metal surfaces offer the advantages of reduced erosion because of their inherently high resistance to impact damage… transpiration cooling has been used… to provide zero-recession nose tips under the most severe ICBM reentry conditions. Although water has a relatively high efficiency as a coolant and has a good storage density… a reactive gas that is used with a high surface temperature matrix [is still more efficient]. A material that can operate at high surface temperature is an ideal matrix material since high surface temperatures greatly reduce the net incoming heat flux [and] local hot spots are not catastrophic. A porous refractory metal such as tungsten would be an excellent material for this application except that at high surface temperatures tungsten must be protected from attack by the boundary layer oxygen.” “The discrete water injection platelet transpiration-cooled nose tip [enhances] the reliability of the reentry vehicle under conditions of high aeroheating and adverse weather encounter… Although primarily related to reentry nose cones the [platelet TCNT] has application to… wake seeding and/or electronic attenuation/clarification… The nose tips were fabricated from thin sheets of 347 stainless steel which were diffusion bonded to form a monolithic structure with the mechanical properties of the parent material. The flow passages needed for flow metering and surface distribution were chemically etched in the individual sheets or platelets prior to diffusion bonding.” “…the mechanical radius of curvature is the smallest allowed by the fabrication method, [e.g.] less than about 0.02in… By adjusting blowing rate to the minimum necessary at any given velocity, transpiration consumption [and] drag are reduced or eliminated… At a low [blowing rate], the transpirant fluid dilutes the hot boundary layer, reducing the driving enthalpy and (normally) the heat flux. This… is referred to as ‘partial blockage.’ At higher blowing rates, the hot boundary layer is pushed completely away from the surface. Under these conditions, the surfaces are exposed only to the coolant temperature and the heat flux (ignoring radiation) is reduced to zero (providing full blockage)… orifices control the flow to each individual pore to match the local coolant flow to the local heat flux which can vary greatly across the surface… The metering of the coolant flow through the individual pores has a strong influence on the shape of the boundary.” “A peroxide catalyst bed typically uses silver screen packs, while a [hydrazine] system typically consists of iridium deposited on alumina granules… These materials are subject to wear and are relatively fragile (the alumina granules), and do not possess a consistent flow resistance (stacked screens)… Since decomposition will most likely not happen in a totally uniform manner, the [platelet catalyst bed] could become prone to the same problems as a screen pack, namely, recirculation of the decomposed products and the resulting hot spots. The intermittent metering plates… have a lower fraction open area than the surface enhancement plates that precede and follow. This results in a restriction that will isolate each group of surface enhancement plates from the others. Any hot spot/recirculation zone will be unable to propagate throughout the stack. The metering platelets will also act as flow distributors to keep the flow uniform across the bed which will avoid the initiation of the recirculation zones… Through-etched axial holes are uniformly spaced across the entire platelet but are offset in alternate platelets to preclude pure axial flow. By offsetting the holes, the fluid is forced to impinge on the catalyst of each platelet before traversing 360° sideways to exit through the next platelet holes where the process repeats. This continuous turning of the fluid promotes turbulence… and assures that the monopropellant makes continuous contact with the catalyst.” “A spherical tank provides minimum weight… when the vehicle centerline is unavailable for tankage, multiple spherical or cylindrical tanks are required. The most efficient configuration then becomes a single torus. A torus gains structural benefit from the fact that it can be thought of as a cylindrical tank doubled back on itself minimizing the need for, and weight of, end closures. The single torus eliminates the need for additional fill ports, isolation valves, burst discs and pressure switches/sensors associated with a multiple tank design… A piston in each compartment provides positive expulsion… the pistons move in the same direction around the torus, maintaining a 180° angular separation to provide balance control. Each piston is a section of a torus with sufficient arc length to resist applied moments induced by a hydrostatic pressure gradient across the piston face due to the imposed acceleration environment. Each piston is statically balanced, before assembly, to minimize rotation due to vehicle axial or lateral acceleration. In order to minimize piston ΔP and eliminate galling, the outside diameter of the titanium piston is anodized and [PTFE] coated. This also prevents diffusion bonding between the piston and bore during long term storage… A small positive pressure ullage gas bubble is maintained on the propellant side of the piston during storage. The bubble pressure preloads the piston to prevent bellows cycling while also compensating for thermal expansion of the propellant… The gas side of each compartment is insulated to minimize the temperature rise of the propellant and tank structural elements, and also to minimize the heat loss of the pressurizing gas… Three [PTFE] seals were used to provide the gas-to-liquid seal at the piston to bore interface. The seal configuration is a teflon ‘C’ cross-section with an internal stainless steel spring that loads the seal during zero ΔP operation. Differential pressure in the proper direction loads the seal into the piston and bore to promote sealing… Production of the toroidal tank will utilize a superplastic forming process to minimize unit cost… [PTFE] teflon lip seals are required to maintain good seal contact with the toroidal bore under environmentally induced loads.” “The tank may be of any desired cross-section… wherein the tank is of toroidal shape… To prevent leakage of the primary fluid from between the piston and tank walls during long term storage, the [tank] is provided with seals permanently affixed to the pistons and tank walls… Pressurization of the piston interiors ruptures seals thereby allowing the pistons to traverse the interior of the tank… where the need exists for enhanced tank capacity [multiple tanks] may be employed… the single toroidal tank… may be divided into four or more compartments, each compartment including a piston…” “There are many unique challenges associated with the control of… reentry vehicles, including: nonlinear and highly uncertain dynamics; time-varying plant parameters; and fast (and sometimes unstable) open-loop state dynamics, requiring a high control update rate… One [limitation] of all model predictive control schemes is their reliance on an accurate model, which determines the quality of the predictions and, therefore, the optimal control input computed by the algorithm. The presence of unmodeled nonlinear dynamics, noisy data, as well as aerodynamic and gravimetric uncertainties all contribute to model inaccuracies.” “Adaptive control approaches aim to preserve the nominal performance of a controller despite model uncertainties… Therefore, an adaptive control method may be combined with MPC to improve the overall performance… The adaptive MPC approach produces consistently superior performance to non-adaptive and non-predictive approaches… The key idea behind MPC is that the controller utilizes a model of the system to predict the future states of a system given a hypothesized sequence of control inputs. The predicted states are then used by a cost function to penalize tracking errors as well as control activity, leading to an optimal control solution… the main issue with MPC is that it typically requires the numerical solution of an optimization problem, which can be more computationally expensive than what is acceptable on resource constrained hardware for a given desired loop rate… A fundamental property of adaptive control is an adaptation law, which defines a performance metric as a function of the system response to various control inputs. The performance metric is then used to adapt the control law to compensate for uncertainties and disturbances… Aerospace vehicles often operate with fast dynamics and time-varying model parameters. Therefore, adaptive control architectures which are used in aerospace applications must allow for fast adaptation.” “The… self-learning network will learn and process the incremental changes to the aircraft plant that may occur under failure or battle damage conditions… Thus, non-linear aerodynamic properties can be accommodated to provide optimum control throughout the maneuver flight envelope… These neural networks model the data tables found in large simulations at a fraction of the software space… thus permitting flight control use of the complete aerodynamic database of the aircraft… all of the flight control gains are calculated in real time using the measured stability and control derivatives… Under major damage conditions, controllability of the aircraft may be compromised… Changes to the stability properties must be immediately determined… This will enable the flight controller to maximize the flight path stability…” “The [McDonnell Douglas] UpSTAGE, a maneuvering [Boeing] HiBEX upper stage, demonstrated over 300g lateral acceleration and a side-force specific impulse [over 1000s]… UpSTAGE… had a very ambitious objective of… chasing MaRVs… and possibly coming close enough for non-nuclear kill.” “The external burning control concept uses a highly pyrophoric pentaborane fuel metered to burn near the outside of the vehicle. The jet interaction concept… uses exhaust from a gas generator metered to the outside of the vehicle. The attitude of [UpSTAGE] is monitored by a laser triad rate gyro, a strapdown component.” “…external burning, in which a liquid pyrophoric fuel mixes with the air and burns producing a region of high pressure; jet interaction, in which a gaseous jet separates the boundary layer, producing a high pressure region; and reactive jet interaction, in which a fuel-rich gaseous jet separates the boundary layer, mixes with the air, and burns, thereby producing a high pressure region by combining both of the [former] concepts.” “The discharge of a high-speed gas from a control nozzle mounted on a maneuverable vehicle into a hypersonic external flow creates the phenomenon known as jet interaction… The pressure flowfield on the vehicle surface is complicated and the resulting forces and moments must be predicted for a successful mission… High-performance BMD interceptors require the use of reaction jets for fast response control not achievable with conventional aerodynamic fins. Attitude control jets, mounted at the vehicle’s tail, produce control moments, thus inducing an angle of attack which creates aerodynamic lift to maneuver the vehicle… advances in jet interaction technology have been in the direction of a multi-jet system which combines a large lateral thruster located at the vehicle midsection and the tail-mounted JI thrusters… The lateral thrusters give an ultra-fast response since lateral acceleration occurs directly from the thruster operation, without delay in airframe rotation to angle of attack. The lateral thrusters also provide maneuver capability at high altitudes when aerodynamic lift is diminished. Such a multi-jet control system can be mechanized to operate efficiently by using the attitude control jets at low altitudes where the dynamic pressure is sufficient to create the desired lateral acceleration and augmenting the attitude control system with the lateral thruster operation at the higher altitudes. Proper mechanization and blending of the control thruster operation… requires that the interaction performance of the lateral and attitude jets operating alone and in combination, be predictable… Forces and moments on the vehicle arise from the nozzle reaction forces as well as the flowfield interaction forces which result from the disturbance of the external flow. The interaction forces and moments may or may not act in the same direction as the reaction forces and moments and thus may augment or diminish the control system effectiveness, depending upon the vehicle angle of attack… The dominant factor influencing the multi-jet performance is the vehicle angle of attack because it determines the local external flow conditions into which the jet exhausts. The vehicle shape is also important because it has a pronounced effect on the flow conditions along the body surface… The biconic junction is an important feature of the body since it induces an abrupt expansion of the external flow along the body and effectively limits the extent of the separation region on the afterbody… the simultaneous operation of both lateral and attitude jets causes the attitude control jet directly behind the lateral thruster to become less effective… The attitude control jet behind the lateral thruster discharges into a region where the boundary layer is already separated, eliminating its effectiveness… Because of significant loss in performance of the attitude jet located directly behind the lateral thruster it is preferable to rotate the position of the attitude jets so that they are in the 45° planes with respect to the lateral thruster. This takes them out of the low pressure wake region and puts them in the high-pressure ridge that forms on either side of the lateral thruster… This reduces the performance degradation of the pitch plane attitude jet and also lessens the increase seen in the yaw plane attitude jet performance. Thus, the clocked configuration for the attitude jets is a much more ‘flyable’ design because the required nozzle thrust levels are minimized and guidance and autopilot compensation difficulties are mitigated.” “The interaction of the divert jet with the oncoming supersonic… flowfield creates a complicated three-dimensional flow structure which includes flow separation as well as a number of strong shocks and expansions… Changes in the surface pressure distribution about the interceptor due to jet interaction can cause either amplification or de-amplification of the basic jet thrust. In some extreme cases, complete reversal of the intended control force has been observed. A thorough understanding of the force and moment amplification/deamplification resulting from jet interaction can be of extreme importance for accurate [correlation] of interceptor response to control inputs, especially at low altitudes… the amount of influence the jet has on the flowfield (i.e. the extent of the upstream separated region or of changes in the surface pressure distribution) is directly tied to the size of the jet disturbance… It is for this reason that quantities such as jet to freestream mass and momentum flux ratios, kinetic energy (or velocity) ratio, or static and stagnation pressure ratios have often been used to characterize jet interaction flowfields. All of these quantities provide different [perspectives on] the size of the jet disturbance.” “…jet thruster control [is] capable of delivering several thousand pounds of thrust in less than 10ms. When used at very high altitudes the forces involved are fundamentally of the action-reaction type in the usual Newtonian sense. However, at the lower altitudes the jet interacts with the atmosphere flowing past the missile and can produce additional forces on the missile that are very different from their high altitude counterparts… Extensive testing… eliminated external burning as a candidate system for endo-homing interceptors because control performance was found to decrease rapidly with decreasing dynamic pressure, i.e. a serious altitude and/or Mach number limitation… Thruster control effectiveness can be thought of as the accuracy with which the autopilot knows the relationship between the amplification factors and the JI environmental factors, i.e., how the missile will respond when the thruster control is activated… A jet canted upstream produces larger amplification factors than one exiting normal to the surface… External ignition [was found] to be feasible for systems operating at Mach 4 or greater. An extra [20–30%] performance increment was achieved over non-reacting jets, everything else being constant. The altitude limit for the combusting jets was… 110,000ft or about 1psi local surface pressure… tests have shown [JI] to be an effective control which should be less vulnerable to the blast environment of MX defense and less sensitive than aerodynamic controls to the lower dynamic pressures expected for higher altitude defense scenarios.” “Exoatmospheric interceptors have the advantage of not having to consider the aerodynamic controllability, aero-optical distortion, aerothermal heating, thrust amplification, and other effects on the interceptor caused by the atmosphere. Engagement outside the atmosphere also makes interceptor and target performance significantly more predictable… Post-boost, exoatmospheric targets are usually ballistic and not maneuvering, so the line-of-sight ranges from the interceptor to the target do not fluctuate rapidly during engagement. Therefore, standard proportional navigation-type guidance algorithms are sufficient to engage and intercept them. Thus, [interception] is generally easier in the exoatmospheric regime.” “The divert and attitude control system accounts for a significant portion of the total vehicle weight… Blended control systems [make] the most effective use of divert and aero controls in the transition region [25–40km]… Attitude thrusters are generally sized by the low endo force requirements and may have too large an impulse for high endo control. However, by decreasing the static margin and using fast valves, the high and low endoatmospheric requirements can be fulfilled with common thruster sizes… [Biconic] and triconic shapes are often used to add stability, but… tend to increase drag.” “Even when the divert jets are turned off, there is an influence on the missile body forces due to cavity flow [in] the thruster openings on the body… The large circulation bubble upstream created by the free-stream interaction with the divert jet plume is evident in the region upstream of the divert nozzle exit slot. A secondary separation region is also apparent.” “…the use of a fuel-rich jet to provide a control force, or moment, on the vehicle flying in the atmosphere… typically involves a sonic or supersonic jet of fairly high pressure and temperature injected into an airstream of high Mach number and, at low altitudes, of high dynamic pressure… The separated region can be considered as an obstacle to the flow, as no substantial amount of mass flow exhausts from its downstream end.” “Bluff bodies and rearward-facing step flows stabilize the flame by introducing a low-velocity recirculation zone containing combustion products that act as a continuous ignition source… An alternative fluidic-based approach using a transverse slot jet would reduce the thrust penalties associated with the sudden expansion/bluff body geometry while producing a flowfield with flameholding potential… The recirculation zone formed by the transverse slot jet has strong qualitative similarities with the flow established downstream of a rearward-facing step.” “The extremely high T/W ratios achievable with micro-miniaturization of liquid bipropellant engines are a result of the ‘cube-square law.’ As the engine is scaled down linearly, the [thrust] decreases with chamber cross-sectional area (the square of the linear size) while the weight decreases with volume of the engine (the cube of the linear size)… The AFRL fabrication approach is applicable to [refractory metals] typically used in radiation-cooled [thruster] nozzles such as rhenium, or tungsten. The primary advantage… is the capability of surviving the corrosive environments of storable hypergolic propellants, such as hydrazine… By increasing the chamber combustion pressure, and adjusting the [O/F] ratio to fuel-rich… the combustion [timescale] can be shortened… below the chamber residence time… At the high [P/W] ratios associated with the small engine characteristic lengths utilizing MEMS fabrication techniques, regenerative cooling would be required to recover the high heat losses during combustion. Therefore… high specific impulse operation can be achieved with very small engine characteristic lengths… on the order of [0.1–1in]… For RCS thrusters of very small size, the jet rows would turn into thruster arrays which would cover the conic slice flat sections near the RV frustum… Because of the impossibility of imparting an [arbitrarily small impulse] to the RV, it is virtually impossible to obtain a given attitude with zero angular rate. As a result, the RV is kept… within a dead-band region established by a limit-cycle amplitude… To minimize [propellant] consumption, the aerodynamic CP is positioned at the CG. The RV is not aerodynamically stabilized, but is stabilized with reaction control jets… The impulse bit wants to be as small as possible, especially for very small dead-band angular tolerances… jet interaction increases the effectiveness of the control jet. This increased effectiveness is usually referred to as the thrust amplification factor… for smaller values of the minimum impulse bit, [there are lower] mission impulse requirements to maintain a given dead-band pointing tolerance… For tactical warhead RV applications, the RV has to have [CEP] of 15ft to be effective. Having RCS thrusters with a very small minimum impulse bit goes a long way to achieving this accuracy requirement… where a small minimum impulse bit is desired, hypergolic or catalyzed monopropellants would be the preferred propellant to facilitate the intermittent duty ignition requirement… Gaseous bipropellant injection may be also required for hypergolic ignition to be successful… [The T/W] ratio may be enhanced dramatically at reduced bipropellant engine characteristic lengths, but the achievement of a small minimum impulse bit as a result of reduced engine characteristic length is not to be ignored in reducing [the propellant] consumption rate.” “As the characteristic throat dimension of micronozzles continues to decrease… large Reynolds numbers can be maintained by increasing the stagnation pressure of the thruster… The performance penalty incurred [when the thruster is throttled, i.e. run at lower stagnation pressures, to achieve lower thrust levels] is substantial… Shorter expansion lengths are therefore advantageous at very low [Reynolds numbers] in improving performance and reducing thruster size and weight.” “The impulse ‘bit’ represents the minimum amount of impulse that can be delivered each time the thruster is fired; its magnitude is dependent upon both the level of thrust and the actuation speed of the thruster… it can be interpreted as a measure of the limiting resolution of spacecraft control capability… In traditional [nozzles], the combination of high speeds and moderate to large length scales result in very high Reynolds numbers — sufficiently large that inviscid analyses are employed as a first approximation… With the characteristic length scales… on the order of microns to millimeters, the… Reynolds numbers within [micronozzles are typically less than 500] and hence viscous effects can no longer be ignored… the viscous layer can occupy a sizable fraction of the divergent nozzle cross-section and, as a consequence, [reduce performance due to increased friction losses]… The distinguishing feature of the aerospike design is its ability to perform efficiently for a wide range of ambient backpressures whereas traditional nozzles are only designed for a single ambient pressure. Backpressure compensation is accomplished by utilizing the… free boundary… of the exiting jet flow to act as a virtual nozzle wall with a variable exit area ratio which adjusts with the ambient backpressure… For [micronozzle] applications, there is little need for the pressure compensation feature of the aerospike since the ambient conditions are those of either space or near-space. However… a virtual (free) boundary can [reduce the surface area of the nozzle and thus reduce viscous losses]… the total specific impulse of a thruster array can increase by as much as 16% compared to a single thruster.” “Full gasification… leads to faster chemical reactions in the combustion chamber, allowing a smaller combustion chamber.” “Hydrazine… has… been used as the primary fuel in supersonic combustion (the hydrazine was passed through a catalyst prior to injection).” “Water [cools] the plasma, causing ions and electrons to recombine to form a neutral air atom or molecule… electrophilic chemicals are more effective than water at reducing the electron density [and] plasma blackout effects. However… water is actually more effective at penetrating and mixing further into the plasma layer. Water is also more effective in reducing thermal loads to spacecraft… sublimating [WO₃], a metal oxide powder similar to [Al₂O₃], to quench the plasma… is more similar to the liquid quenchant approach because of the chemical reactivity of the quenchant material, which is in the gas phase when injected… alkali metal impurities in heat shields add significantly to the electron density of the reentry plasma sheath as they burn away into the plasma flow. It is possible to reduce alkali metal impurities and concurrently impregnate metal oxide particles such as [Al₂O₃] into the heat shield. In this case, heat shield ablation will naturally carry the [Al₂O₃] particles into the plasma sheath, thereby allowing depletion of the sheath plasma density via metal oxide particle charging.”
Los Alamos National Laboratory W76 (physics package)
The hollow pit is cast or hot-pressed.
