Shuttle Damage/Repair from the Perspective of Hypersonic Boundary Layer Transition - Experimental Results

Abstract

An overview is provided of the experimental wind tunnel program conducted at the NASA Langley Research Center Aerothermodynamics Laboratory in support of an agency-wide effort to prepare the Shuttle Orbiter for Return-to-Flight. The effect of an isolated protuberance and an isolated rectangular cavity on hypersonic boundary layer transition onset on the windward surface of the Shuttle Orbiter has been experimentally characterized. These experimental studies were initiated to provide a protuberance and cavity effects database for developing hypersonic transition criteria to support on-orbit disposition of thermal protection system damage or repair. In addition, a synergistic experimental investigation was undertaken to assess the impact of an isolated mass-flow entrainment source (simulating pyrolysis/outgassing from a proposed tile repair material) on boundary layer transition. A brief review of the relevant literature regarding hypersonic boundary layer transition induced from cavities and localized mass addition from ablation is presented. Boundary layer transition results were obtained using 0.0075-scale Orbiter models with simulated tile damage (rectangular cavities) of varying length, width, and depth and simulated tile damage or repair (protuberances) of varying height. Cavity and mass addition effects were assessed at a fixed location (x/L = 0.3) along the model centerline in a region of near zero pressure gradient. Cavity length-to-depth ratio was systematically varied from 2.5 to 17.7 and length-to-width ratio of 1 to 8.5. Cavity depth-to-local boundary layer thickness ranged from 0.5 to 4.8. Protuberances were located at several sites along the centerline and port/starboard attachment lines along the chine and wing leading edge. Protuberance height-to-boundary layer thickness was varied from approximately 0.2 to 1.1. Global heat transfer images and heating distributions of the Orbiter windward surface using phosphor thermography were used to infer the state of the boundary layer (laminar, transitional, or turbulent). Test parametrics include angles-of-attack of 30 deg and 40 deg, sideslip angle of 0 deg, freestream Reynolds numbers from 0.02x10 6 to 7.3x10 6 per foot, edge-to-wall temperature ratio from 0.4 to 0.8, and normal shock density ratios of approximately 5.3, 6.0, and 12 in Mach 6 air, Mach 10 air, and Mach 6 CF 4 , respectively. Testing to simulate the effects of ablation from a proposed tile repair concept indicated that transition was not a concern. The experimental protuberance and cavity databases highlighted in this report were used to formulate boundary layer transition correlations that were an integral part of an analytical process to disposition observed Orbiter TPS damage during STS-114.

The Hythirm Project: Flight Thermography of the Space Shuttle During Hypersonic Re-Entry

“…The renewed interest in thermal imagery during Shuttle entry was initially motivated by the desire to reduce uncertainties associated with an empirical strategy to predict BLT onset. This empirical methodology, adopted to quickly assess thermal environments induced by damage to the Shuttle's TPS, is derived from ground-based measurements 16,17 that are extrapolated to flight using representative (and limited) flight data 18 . During the RTF BLT predictive tool development phase, it was recognized that the level of conservatism imposed by these extrapolation uncertainties could be more clearly established and/or reduced with quality data from a controlled roughness flight experiment.…”

Discrete Roughness Transition for Hypersonic Flight Vehicles

“…The experimental results for ablation (local blowing through a porous plug on the model surface, see Ref. 29) provided an initial assessment that while out-gassing of the repair material might lead to earlier transition onset times, the level of blowing required was over an order magnitude greater than was estimated for the STA-54 material under entry conditions. Also, just prior to the STS-114 mission, the STA-54 repair concept was shelved for large repairs, so no further attempt to provide a useable correlation approach for flight was made.…”

Supersonic/Hypersonic Laminar Heating Correlations for Rectangular and Impact-Induced Open and Closed Cavities

“…al. 34 . An example showing closed-cavity length (L/H) effects on heating augmentation for laminar entry conditions is presented in the heating images shown in Figure 5a for M e =2.25, Re θ =300, and H/δ≈1.35.…”