Hypersonic Vehicle Thermal Structure Test Challenges

Within the Air Force Research Laboratory, we are developing advanced thermal protection systems (TPS) and hot structures to meet Air Force requirements for hypersonic vehicles. Defense-related requirements include rapid sortie preparation, quick inspection turning, and low-maintenance, low-cost operations. To complete this task, analytical methods need to be developed and verified, and concepts must be experimentally validated, using ascent-cruise-descent thermal/acoustic/mechanical load profiles that sufficiently simulate flight environments. These loads can extend in temperature to 2,500oF, acoustic levels in excess of 165dB and mechanical loads to thousands of microstrain. Hypersonic flight structure requirements push AFRL’s instrumentation methods, test capabilities, and analytical models beyond the state-of-the-art. We must, therefore, develop test capabilities for hypervelocity vehicle structural concept validation. This paper details the difficulties associated with structures testing in extreme environments. Some of the challenges addressed include requirements definition, instrumentation design, loads application, specimen fixturing, boundary condition simulation, and cost/schedule management. The instrumentation system must be compatible with the extreme environment as well as the structural material and response. Simultaneous extreme-load application can be very difficult as one input type can adversely affect other load-introduction apparatus. Examples include the destructive effects of very high acoustic levels on quartz lamps and extreme heating on static load introduction hardware. High voltage heating methods commonly employed pose additional challenges in reduced pressures or in the presence of material outgassing. Requirements of heat flux versus temperature specifications must be analyzed and trade-offs made in the radiant simulation of convective heating environments. Further, these thermal requirements have to match a capable control and instrumentation system. Specimen fixtures must be able to withstand static and dynamic loads while allowing for thermal growth. They must also enable high-fidelity simulation of boundary conditions while maintaining a facility-compatible specimen size. Defining test requirements is complicated by the immature state of the vehicle configurations, flight trajectories, and long lead times associated with exotic hardware. In many cases, compromises must be made, or innovation employed, to remain within project fiscal constraints. The successful completion of these pursuits will enable the full validation of thermal structures concepts and analytical methods. Success, in turn, will also provide risk control for vehicle flight tests and reduction or elimination of the burdensome inspection and maintenance requirements associated with current state-of-the-art TPS. Validated extreme structures development methods will be enabling for hypersonic vehicles.