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Hypersonic Propulsion: Enhancing Endothermic Fuel Technology


OBJECTIVE: Enhance or increase the capabilities of scramjet-powered hypersonic systems through the development of advanced endothermic fuels, fuel additives, catalysts or fuel cooling concepts for high-speed propulsion systems. DESCRIPTION: Hydrocarbon-fueled supersonic combustion ramjets (scramjets) are expected to operate from Mach 3.5 (or lower) up to Mach 7 or 8. Scramjet engines are categorized into three general sizes: small-scale (nominal air flow of 10 lbm/s), mid-scale (nominal air flow of 100 lbm/s), and large-scale (nominal air flow of 1000 lbm/s). Recent efforts focus on mid-scale scramjets that need to operate over a broad Mach range, use minimal or no variable geometry, integrate with other propulsion cycles, and maintain thermal balance using only on-board fuel as a heat sink without significant losses to integrated system performance. The basic design strategy for cooling of scramjet combustors is fuel-cooling of metallic walls. The combustor walls are typically constructed of integrated heat exchanger (HEX) panels. Fuel is passed through coolant channels adjacent to the heated surfaces to absorb heat, prior to injection into the combustor. The X-51 program represents the current state of the art for scramjet engine technology and can be used as a reference point. The engine has a regeneratively cooled structure using endothermic JP-7 fuel to extract heat from the engine walls prior to injection of the heated fuel into the combustor. The upper Mach capability of a hypersonic propulsion system is currently limited by the ability to balance the structural cooling requirements with the available thermal capacity of the fuel. While the development of higher temperature materials, that will require less cooling, is already under way, it is important to acknowledge and pursue the opportunity for enhancement of the thermal capacity of fuels used in hypersonic propulsion systems. The high energy and volumetric density of liquid hydrocarbon fuels makes them attractive for high-speed propulsion applications. Furthermore, the endothermic reactions that occur during thermal decomposition of a hydrocarbon fuel can be exploited to provide heat sink in addition to what is available through the sensible heating of the fuel. The maximum fuel temperature is limited by the formation and accumulation of carbon deposits, often referred to as coking, which is detrimental to the performance of both thermal management and fuel injection systems. Methods to increase the temperature at which coke forms; to promote the onset of endothermic reactions at lower temperatures; and to minimize or eliminate the formation, deposition and accumulation of coke all have a positive influence on the total thermal capacity of the fuel. Some possible areas for proposal focus include: alternate fuels or fuel blends, tailoring chemical compositions for endothermic reactions, fuel processing technologies, catalyst development and application, additive development and application, innovative HEX design concepts, coke mitigation techniques, and so forth. Proposals may include any number of the areas identified and are by no means limited to the focus areas mentioned herein. Proposals in response to this topic may either be computational or experimentally focused. Note that there is not expected to be any government-furnished equipment (GFE) required to perform a Phase I effort and proposals should not assume equipment or facility is to be furnished at no cost. PHASE I: Demonstrate the feasibility of an advanced fuel concept to increase the capabilities of scramjet systems. This may be done via subscale testing, numerical analysis, simulations, or other means. Perform detailed numerical analysis or subscale testing of the proposed concepts and assess the impact of the concept and identify a path for application of the enhancing technology. PHASE II: Fully develop the concept developing during the Phase I effort. Develop a test plan and associated test rigs to demonstrate improved performance versus the baseline technology in a laboratory environment. Provide engineering systems analysis for developing larger and broader operating ranges for scramjet systems. Fabricate and evaluate prototypical devices, hardware or test rigs to confirm Phase I predictions at an acceptable scale. PHASE III: The advanced fuel will have applications in high-speed military systems, as well as future high-speed commercial applications. REFERENCES: 1. Jackson, K., Corporan, E., Buckley, P., Leingang, J., Karpuk, M., Dippo, J., Hitch, B., Wickham, D., and Yee, T.,"Test Results of an Endothermic Heat Exchanger,"AIAA Paper 95-6028, April 1995. 2. Wickham, D.T., Engel, J.R., and Hitch, B.D.,"Additives to Increase Fuel Heat Sink Capacity,"AIAA Paper 2002-3872, July 2002. 3. Edwards, T., DeWitt, M., Shafer, L., Brooks, D., Huang, H., Bagley, S., Ona, J., Wornat, M.,"Fuel Composition Influence on Decomposition in Endothermic Fuels,"AIAA Paper 2006-7973, 2006. 4. Castaldi, M., Leylegian, J., Chinitz, W., Modroukas, D.,"Development of an Effective Endothermic Fuel Platform for Regeneratively-Cooled Hypersonic Vehicles,"AIAA Paper 2006-4403, July 2006. 5. Billingsley, M.,"Thermal Stability and Heat Transfer Characteristics of RP-2,"AIAA Paper 2008-5126, July 2008.
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