Air Force technology roadmaps for Trans-Atmospheric Air Vehicles

Abstract

The Air Force research Laboratory’s Air Vehicles Directorate is engaged in planning Air Force technology investments for both spaceaccess and hypersonic cruise vehicles (together referred to as “Trans-Atmospheric Vehicles”). The technologies for both classes of vehicles are essentially alike, with limits on performance being the primary distinguishing characteristic. The current top-level roadmap being proposed calls for a hypersonic air breathing demonstrator vehicle that would operate at Mach 8. TECHNOLOGY ROADMAPPING The Air Force Research Laboratory’s Air Vehicles Directorate is tasked to plan and develop technologies that support Air Force missions for affordablespace access. In developing such technolosll y $1-5 million) which “ride” on a flight The five current hypersonic and space-access programs shown on Figure 2 will .be maturing in the next two to three years. However, all of these programs lack adequate flight demonstration of air-breathing propulsion systems. The author proposes that the next NASA Pathfinder vehicle should be a versatile air-breathing demonstrator. One possible means of obtaining such a demonstrator would be a follow-on version of the current Hyper-X design. ‘Such an “X-43B” would be at least twice the scale, fully recoverable, and able to test various propulsion system designs up to at least Mach 7. The AFRL Air Vehicles Directorate currently does not plan to develop any trans-atmospheric In the cruise mode, the vehicle could reach any location on earth in three hours, even with CONUS basing, and reach any trouble spot in less than two hours if a few no&ONUS bases were used. This offers many options to military planners. Such military missions likely will require diverting to friendly fields where only *Associate Fellow, AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. (c)l999 American Institute of Aeronautics & Astronautics or publist@ with permission of author(s) and/or author(s)’ sponsoring organization. demonstration vehicles independently, but will instead participate fully in the NASA Future-X program as the primary means of demonstrating advanced technologies. The Directorate will assist NASA in defining the future Pathfinder and Trailblazer vehicles, and will propose experiments with will be incorporated into such vehicles or comparable demonstration vehicles. The Air Force will work closely with NASA on demonstrations of key trans-atmospheric and space-access technologies by incorporating experiments onto Pathfinder or Trailblazer vehicles until at least FY 08, at which time the differing mission requirements would tend to separate Air Force and NASA vehicle development. NASA will likely seek concepts with high volumetric efficiency, whereas the Air Force will seek higher aerodynamic efficiency in order to achieve crossrange. Anticipating future selection of military missions and vehicle configurations, the Air Vehicles Directorate is concentrating on pervasive technologies covering aerodynamic configurations, structures, sub-systems, and plasma physics as listed in Figure 3. The Directorate is not concentrating on any specific concept. On going configuration analyses will explore many aerodynamic shapes for various staging scenarios or hypersonic cruise. Integration of propulsion systems will be key to successful hypersonic vehicles. For military vehicles the integration and release of weapons at hypersonic speeds will also be critical. The primary challenge for trans-atmospheric vehicles appears to-be structures that can survive high temperatures and pressures, yet remain durable for minimum maintenance. Structural development will yield hot and actively cooled structures, although the Air Force would prefer to avoid the complexity, cost, and maintenance associated with actively cooled systems. Integration of fuel/oxidizer tanks with the basic vehicle structure and the thermal protection system (TPS) will be required to significantly improve (decrease) the structural mass-fraction of future launch vehicles. The affordability of manufacturing airframe structures using hightemperature composites and metals will likely determine the success or failure of such vehicles. Complex health monitoring and prognostic systems will be incorporated into both structures and sub-systems permitting rapid turnaround and 2 aircraft-like operations. This is essential to achieve the turnaround (in hours) required for military operations. Also required will be advanced actuation systems will permit control at hypersonic speeds with robust, lowmaintenance actuators. Autonomous guidance and control technology, although already demonstrated, will have to be refined well beyond present day levels. Plasma technology will be developed in order to realize lower drag and heat transfer rates at a given Mach number. It may also provide a means of controlling the vehicle without the need of moveable control surfaces, thus eliminating weight and complexity. Plasma technologies will supplement he performance of hypersonic vehicles if they prove to be energy efficient. However, the performance of such vehicles, if not supplemented by plasma technology, will still be sufficient for military missions.