It is important to point out that the endothermic heat capacity of 500 Btu/lb is much less than the theoretical heat capacity achievable if the most desirable fuel reaction products could be obtained. Cracking JP-7 to 100 percent ethylene would absorb 1,500 Btu/lb, versus the 500 Btu/lb obtained from the product mix of methane, ethylene, ethane, etc. Thus, there is clearly room for increasing the endothermic capability of even our present candidate fuels. Because of the endothermic capability of our present hydrocarbon fuels, at the upper Mach number range for hypersonic air-breathing flight (approximately Mach >8), hydrogen is the fuel of choice; however, because of hydrogen’s difficulty of storage and its limited volumetric energy density, hydrocarbon fuels are more practical for lower Mach number applications. All of the hydrocarbon-based vehicles presently being contemplated anticipate using JP-7. As previously discussed, a robust fuels research and development (R&D) program could lead to higher Mach number limits for hydrocarbon fuels. The Russian AJAX (concept) vehicle, for example, increases heat sink capability by an interesting variation on conventional endothermic fuels. In the AJAX concept, water is added to the fuel to achieve steam reforming. In essence, steam reforming is fuel + water → CO + H2. This reaction absorbs (theoretically) 2,400 Btu/lb (versus 1,500 Btu/lb for cracking to ethylene). This is how the Russians theoretically obtain Mach 10 capability for AJAX, although the concept suffers reduced range owing to water consumption in the propulsion cycle.
In the 1960s and early 1970s, the Air Force funded a considerable effort at Shell Research to develop endothermic methylcyclohexane (MCH), based on studies that identified the heat sink required for hydrocarbon-fueled hypersonic vehicles (Churchill et al., 1965; Lander and Nixon, 1971). Endothermic MCH required a supported platinum catalyst for dehydrogenation (to toluene and hydrogen) that was developed in pellet form (as is used industrially in fluidized beds for petroleum processing). After a long period of dormancy in the 1970s and 1980s, the work was restarted by Allied Signal (now Honeywell) and culminated in an expendable turbine-engine test, where the fuel was used to cool a hot air stream and then burned in an engine (Lipinski et al., 1992). There were two drawbacks to this technology: (1) regenerative cooling is best accomplished through a wall-mounted catalyst rather than pellets and (2) MCH is a relatively expensive specialty fuel.
Extensive discussions inside the Air Force Propulsion Directorate in the mid-1980s led to an effort to develop the endothermic potential of thermally/catalytically cracked liquid hydrocarbons using commercially available, wall-mounted zeolite catalysts (Spadaccini et al., 1993a). This task at the United Technologies Research Center was funded under program element 62203F/Project 3048 of the initial contract, F33615-87-C-2744, managed by Charlotte Eigel. This effort first reported the endothermic potential of JP-7 and JP-10, as well as JP-8 (Sobel and Spadaccini, 1994). The resulting endothermic hydrocarbon fuel capability contributed to selecting such fuels for the HyTech engine development program, which began in the mid-1990s (Spadaccini et al., 1993b). This separate (from HyTech) but coordinated fuel development effort has continued sporadically since HyTech funding became available. Recent tasks include extending the endothermic heat sink database to higher flow rate conditions and to other fuels (e.g., RP-1), as well as looking at relative combustion performance of the various alternative fuels (Huang and Sobel, 2002). The fuels research effort has also looked at the applicability of endothermic fuels to reusable aircraft and other applications (Lehrach et al., 1995).
To provide some structure to a program that might be created by NAI, the committee consulted with Tim Edwards, a civilian fuels scientist at AFRL. Fuel is becoming the integrating factor of the