EXPERIMENTAL AND COMPUTATIONAL ANALYSIS OF SHUTTLE ORBITER HYPERSONICTRIM ANOMALY

During the high-Mach-number, high-altitude portion of the first entry of the Shuttle Orbiter, the vehicle exhibited a nose-up pitching momen t relative to preflight prediction of approximately Delta C-m = 0.03. This trim anomaly has been postulated to be due to compressibility, vi scous, and/or real-gas (lowered specific heat ratio gamma) effects on basic body pitching moment, body-flap effectiveness, or both. In order to assess the relative contribution of each of these effects, an expe rimental study was undertaken to examine the effects of Mach number, R eynolds number, and ratio of specific heats. Complementary computation al solutions were obtained for wind-tunnel and flight conditions. The primary cause of the anomaly was determined to be lower pressures on t he aft windward surface of the Orbiter than deduced from hypersonic wi nd-tunnel tests with ideal- or near-ideal-gas test flow The lower pres sure levels are a result of the lowering of the flowfield gamma due to high-temperature effects. This phenomenon was accurately simulated in a hypersonic wind tunnel using a heavy gas, which provided a lower ga mma, and was correctly predicted by Navier-Stokes computations using n onequilibrium chemistry.