Cruise Phase Velocity Study: The Driving Force

Defining the mission of SHAAFT was crucial to determining what type of vehicle was required. Similarly, defining the cruising velocities is vital to determining generic vehicle size, shape, performance, and supporting elements in the attack mission. This study considered two mach numbers (8.0 and 12.0) at which to fly. These two mach numbers represent the best means in which to achieve the desired survivability. mach 8.0 characterizes the highest velocity in which endothermic hydrocarbons can be effective in scramjet engines, while being used as a coolant for aircraft surface skins. mach 12.0 requires cryogenic fuels, such as liquid hydrogen, that can be used as an active coolant to accommodate extreme aircraft heating. However, active cooling requires a great deal of pipes, gasket, and seals which must be maintained. This report assumes that material strengths will be great enough by the year 2025 (as will be discussed later) such that this type of cooling will not be necessary. Therefore, mach 12.0 appeared to be the best design choice.

One advantage of mach 12 flight involves the usage of current technology. Although developing cryogenic facilities for the SHAAFT would cost money, much of the technology exists for handling mass quantities of liquid or even slush hydrogen. Furthermore, with slush hydrogen, SCREMAR deployment would occur at a higher velocity, increase satellite payload capability or increasing the orbital altitude.

Table 2 summarizes the positive and negative points of limiting the mach number to eight. Table 3 summarizes the same points for mach 12. Overall, the advantages of mach 12 flight appeared much greater than mach 8 and resulted in their incorporation into the SHAAFT. These key benefits include the reduction of the logistics arm required to support an allied attack on foreign land, the increased survivability, and expedient nature of attack. Also, a higher range results from higher velocity, thus conserving fuel.

Table 2 - Parameters Considered for the Supersonic/Hypersonic Attack Aircraft (SHAAFT) at Mach 8 Flight

PROS                                                                   CONS

Hypersonic vehicles powered by  Air-breathing propulsion systems with endothermic hydrocarbon fuels should be possible with reasonable advances in the technology of endothermic hydrocarbon fuels and in dual-mode ramjet/scramjet combustors.  * The increased density of endothermic hydrocarbons means that less volume is required for fuel. As a result, it is easier to generate aerodynamically efficient configurations. * Endothermic hydrocarbons are easier to store and easier to transfer. This simplifies base operations and preflight activities. It probably also saves on training of ground personnel relative to the safe handling of fuels. These features also simplify transporting personnel and supplies to a non-CONUS recovery base. * Since the SHMACs (the standoff weapons to be delivered by the SHAAFT) fly at mach 8, a flight mach number of eight for the SHAAFT presents no problems relative to the deployment of these weapons

* Endothermic hydrocarbons have lower specific impulse and lower cooling capacity than cryogenics (liquid hydrogen/liquid oxygen). As a result, if one uses endothermic hydrocarbons, the range is decreased and the time of flight to the target area is increased.  * Preliminary studies have shown that the mach number at which the SCREMAR (the TAV) is staged has a significant impact on the weight and the size of the TAV. This also affects the size and number of satellites that can be carried to orbit. Thus, it is possible that features which produce savings on the vehicle and on the infrastructure to support the SHAAFT may increase the cost of the SCREMAR and the cost of getting payloads to space. The trade studies conducted in support of the design of the integrated, multivehicle weapons system should consider the interdependence of such phenomena

Table 3 - Parameters Considered for the Supersonic/Hypersonic Attack Aircraft (SHAAFT) at Mach 12 Flight

PROS                                                                    CONS

* Aircraft is much more survivable  * Pilot fatigue is reduced by cutting the total amount of flight time-in the worst-case scenario, this could save the life of the SHAAFT * Decreases time to target (response time) * Increases range due to increased specific impulse of slush hydrogen versus endothermic hydrocarbons * Increased velocity results more design options for SCREMAR access-to-space vehicle * It is more advantageous to launch the SHMAC missile from a higher speed and decelerate rather than low speed and a need to accelerate (like an F-15 launch) * Technology already exists to handle mass quantities of cryogenic fuels

* Increased surface heating poses several problems. Material concerns, thermal expansion, and aero-acoustic problems all increase in magnitude. If active cooling is used, fuel pumps, gaps, and seals will drive up complexity and cost of the design.  * Base infrastructure, logistical support must be created at the SHAAFT base to support cryogenic fuels, which are inherently more expensive and complex * Low density of slush hydrogen means a larger fuel volume-this increases drag, which increases the required fuel, which drives up the size of vehicle even further

While some parts of the missile design already exist, much research and development is required in other areas. This is particularly true in the case of the scramjet propulsion system which allows the missile to sustain mach 8 flight. One design challenge is sizing the combustion chamber. It must be long enough to allow adequate air and fuel mixing and combustion within the engine. For example, flow going through a 15-foot-long missile at mach 2.0 (2,000 fps) will be contained within the scramjet chamber for approximately 0.007 seconds. This is an incredibly short time and does not allow for efficient mixing and combustion of all the fuel and air in the chamber of the scramjet using conventional fuel mixers and igniters.30

While new rocket fuels are not a must, it would certainly be desirable to have fuels available with higher specific impulses (ISP). These are particularly needed for the ground and sea-launched versions since they will have to be accelerated from a standstill at ground level and will therefore not have the speed and altitude advantages of the air-launched versions.

Structurally, the missile will have to withstand the high initial acceleration of the rocket boost phase and maneuvering en route to the target. The average load factor in the acceleration phase is nine Gs. This is a consideration since it is desirable to keep the overall weight of the missile as low as possible.

Better high-speed guidance, targeting, and control systems will also need to be developed if the SHMAC's capability is to be maximized. For example, it is believed that the SHMAC could be used in 2025 to intercept ballistic missiles in flight, although with current technology, this is not very feasible. However, with all of the research currently going on in this area, it is very possible that this mission will be one of the SHMAC's.

Thermal Protection Systems

The expected temperature extreme on the SHMAC is approximately 3,400 R for a leading edge radius of 1.0 in. This was based on calculations of the stagnation point heating rate as it varies with the nose radius and altitude of the vehicle.

The variant used in the shuttle is LI-900 (Lockheed Insulation, nine pounds per cubic foot) and LI-2200 (22 pounds per cubic foot) which are used to cover 50 percent of the exterior of the shuttle orbiter. They can withstand temperatures as high as 2,300 F. The black radiative coating applied to these silica tiles allows 90 percent of the heat generated upon reentry to be radiated back out into the atmosphere. The temperatures on the shuttle's aluminum skin never exceed 350 F.

FRCI-12 was used to replace LI-2200 and by so doing reduced the shuttle weight by 1,000 pounds. FRCI stands for Fibrous Refractory Composite Insulation and weighs 12 pounds per cubic foot. It is just as strong as LI-2200. It is tested up to 2,400 F with gradual reduction in strength beginning at approximately 1,600 F.

LI-900 has no organic constituents that will outgas to contaminate scramjet combustion chamber parts or equipment. It also does not weaken with increasing heat loads. It can withstand 2,500 F and does not degrade until 3,100 F. It is inert, therefore it does not react with most fluids and substances. Any of these variants will be acceptable for use on the SHMAC.

Flexible external insulation (FEI) was developed as an element for HERMES. Produced in blankets which bond to the primary structure. The bonding surface must not exceed 650 C during normal flight conditions, a maximum of 800 C is permitted for short periods of time in case of an emergency. FEI will be dimensioned such that its back surface does not normally exceed 200 C. It is sensitive to acoustic loads and tends to exhibit aerodynamic flutter. The density of it is 2,200 Kg/m3.

Honeycomb TPS is applied in panels. It is generally used for hot structures and heat shields which rely on thermally resistant materials and connections between core and cover sheets. Honeycomb TPS is attached by screw connections through the upper plate which have to be protected by ceramic plugs. This structure must be vented to allow for pressure equalization due to altitude and high speeds. The density of this material is 4.43g/m3.

Multiwall TPS is being developed at NASA. This consists of dimple foils made of superplastic forming and shear foils. Used for the heat shield and at the panel back face. Upper surface is coated with highly emissive Al2O3 . This construction principle can be used with different metallic alloys depending on the temperature range desired. Density: 4.43 g/m3 to 8.98 g/m3.

Ceramic shingles are associated with the HERMES program. This consists of intermediate multiscreen insulation with ceramic and coated screens separating individual layers from quartz silica fibers. Panel mass depends on material thickness which results from a tradeoff between manufacturing technology and mechanical panel design. Still under development in France and Germany. This material has a density of 2.2 g/cm3.

A new thermal protection technology currently under development by the Ames Research Center division of the National Aeronautics and Space Administration is ultrahigh temperature ceramics (UHTC). These ceramics are generally formulated from dibromide compounds. Experiments have validated these ceramics' ability to withstand temperatures up to 3822 oR.

In order to choose a proper thermal protection system, the tradeoff between cost for a UHTC against the effect an ablator will have on aerothermodynamic performance must be weighed. The advantage of the UHTC is that the shape of the leading edges of the missile will not change throughout the course of the flight. A disadvantage is its high cost due to its recent development as a revolutionary technology. Although the cost of ablators is attractive, the drawback is the changing shape of leading edges caused by the ablator burning off throughout the course of the flight and any possible effects this may have on the control and propulsion system of the missile.

Conclusion

For all of the possibilities described throughout this paper, the US needs a flexible, robust, easily planned and executed capability for global reach/global power and for access to space. The SHAAFT would serve as a mobile platform for deploying a widerange of UAV assets. The SHMACs would destroy key targets, including space ports, communications centers, computer centers, time critical targets, etc. The SCREMAR would serve the many war-time applications which require access to space. Thus, the integrated S3 (SHAAFT, SHMAC, SCREMAR) weapons system that has been described can perform Counterspace tasks for Aerospace Control, tasks of Strategic Attack, of C2 Attack, of Interdiction for Force Application, Aerospace Replenishment and Space Lift tasks for Force Enhancement, and On-Orbit Support for Force Support.

Furthermore, it is quite possible (perhaps, even likely) that, at the outset of hostilities, our adversary has created significant damage to our space launch complexes (just as we did to theirs with our SHAAFT mission), leaving the United States in an "Infrastructure Poor" situation (the term is attributed to Maj. (sel) M. B. Clapp). Thus, we need to be able to launch our global-range air and space missions from conventional military bases. The integrated, hypersonic weapons system described in this paper allows the US to accomplish a diverse set of missions, with a highly survivable, lethal weapon system capable of deterring and/or punishing adversaries anywhere in the world.

There is still room for further research and development. The first among these areas is the need for study on propulsion systems and the technology development for scramjet/rocket engines. Other areas to consider for further study include enhanced and improved thermal protection systems. Research developments are expected in finding ways to communicate through hot plasma boundary layers for continual data uplinks.

Also included in the need for further research are understanding shock/shock interactions at high speeds that the weapons systems would be operating at. Advances in the capabilities and accuracy of CFD are needed to explore the flight regimes that S3 will operate within.

It is of importance to note that most of these technologies have already been developed or are in the process of being developed. It is also important to realize that each advancement taken in a particular area aids in the development of not just one weapons unit, but to the entire S3 weapons platform, as well as other technology areas that will be important to the growth and survival of the US in the world of 2025.