Time Resolved Heat-Flux Measurements on a CEV Candidate Shape at High Enthalpy

An experimental study of hypersonic flow over a candidate model for the CEV reentry capsule was conducted with the aim of observing the heat-flux at high enthalpy in T5, Caltech’s free-piston driven reflected shock tunnel ([1–6]). To supplement these internal reports additional data processing (the creation of heat-flux movies) is presented in this work. In total, there were six parts to the CEV study; heat-flux movies are made from the data reported in four of them ([2], [3], [5], [6]). This report describes the phenomena observed and the conclusions that are drawn from the heat-flux movies of hypervelocity flow over a CEV candidate shape. 1.0 INTRODUCTION Following the retirement of the Space Shuttle, NASA focused on developing a replacement, the Crew Exploration Vehicle (CEV). The CEV is an Apollo-like capsule, capable of return from low Earth orbit, lunar, and interplanetary missions. There is a window of time during reentry where the forebody is subjected to extreme heat-flux and skin friction. As a result, one of the main design concerns of the CEV is its thermal protection system (TPS). The TPS protects the CEV’s crew, cargo, and instrumentation during periods of extreme aerodynamic heating. For vehicles in hypersonic flight, transition to turbulence in the boundary layer leads to large increases in heat-flux and skin friction, which influences the design (primarily mass considerations [7]) of the TPS. Therefore, transition prediction in the boundary layer is important to overall mission design. As discussed in [8], laminar-turbulent transition in hypersonic boundary layers is not fully understood, wich led the CEV TPS to be designed conservatively assuming a fully turbulent boundary layer [9]. Two transition mechanisms are the crossflow instability and surface roughness [10–12]. Possible sources of roughness elements include misaligned TPS tiles, as well as spallation and ablation caused by the extreme surface temperatures present during reentry ([9]). Surface roughnesses fall into two categories ([8]): discrete, isolated roughness (such as the gaps in the TPS tiling), and distributed roughness (such as that caused by ablation). Roughness elements can be simulated through the use of “trips” on the surface of the experimental model. Surface roughness tends to promote transition to a degree that is proportional to the height of the roughness element ([8], [13]). Additional reviews on transition data for capsules and planetary probes can be found in [14] and [15]. RTO-MP-AVT-200 261 Proceedings of AVT-200 Specialists’ Meeting on Hypersonic Laminar-Turbulent Transition, San Diego, CA. April 16-19, 2012.