Development of X-33/X-34 Aerothermodynamic Data Bases: Lessons Learned and Future Enhancements

A synoptic of programmatic and technical lessons learned in the development of aerothermodynamic data bases for the X-33 and X-34 programs is presented in general terms and from the perspective of the NASA Langley Research Center Aerothermodynamics Branch. The format used is that of the Òaerothermodynamic chain,Ó the links of which are personnel, facilities, models/test articles, instrumentation, test techniques, and computational fluid dynamics (CFD). Because the aerodynamic data bases upon which the X-33 and X-34 vehicles will fly are almost exclusively from wind tunnel testing, as opposed to CFD, the primary focus of the lessons learned is on ground-based testing. The period corresponding to the development of X-33 and X-34 aerothermodynamic data bases was challenging, since a number of other such programs (e.g., X-38, X-43) competed for resources at a time of downsizing of personnel, facilities, etc., outsourcing, and role changes as NASA Centers served as subcontractors to industry. The impact of this changing environment is embedded in the lessons learned. From a technical perspective, the relatively long times to design and fabricate metallic force and moment models, delays in delivery of models, and a lack of quality assurance to determine the fidelity of model outer mold lines (OML) prior to wind tunnel testing had a major negative impact on the programs. On the positive side, the application of phosphor thermography to obtain global, quantitative heating distributions on rapidly fabricated ceramic models revolutionized the aerothermodynamic optimization of vehicle OMLs, control surfaces, etc. Vehicle designers were provided with aeroheating information prior to, or in conjunction with, aerodynamic information early in the program, thereby allowing trades to be made with both sets of input; in the past only aerodynamic data were available as input. Programmatically, failure to include transonic aerodynamic wind tunnel tests early in the assessment phase led to delays in the optimization phase, as OMLs required modification to provide adequate transonic aerodynamic performance without sacrificing subsonic and hypersonic performance. Funding schedules for industry, based on technical milestones, also presented challenges to aerothermodynamicists seeking optimum flying characteristics across the subsonic to hypersonic speed regimes and minimum aeroheating. This paper is concluded with a brief discussion of enhancements in ground-based testing/CFD capabilities necessary to partially/fully satisfy future requirements. Introduction: Aerothermodynamics, defined herein as encompassing aerodynamics, aeroheating, and fluid dynamics and physical processes, is the genesis for the design, development, and flight of space transportation vehicles and is in the critical path to success for such vehicles. The aerothermodynamic challenge is to provide the optimum design (i.e., outer mold lines (OML)) to safely satisfy mission requirements including abort scenarios and to reduce design conservatism, risk, and cost; i.e., the optimum flying vehicle with minimum structural (i.e., surface pressure/shear) and heating loads, translating into reduced weight and reduced operation costs. Aerothermodynamics provides crucial information to other key disciplines such as structures, materials, including thermal protection systems (TPS), avionics, guidance, navigation and control, propulsion, etc. The three sources of aerothermodynamic information are: (1) ground-based facilities, (2) computational fluid dynamics (CFD) and/or engineering computer codes, and (3) flight experiments.