Hypervelocity Expansion Tube Studies of Blunt Body Aerothermodynamics in CO2 and Air

To achieve higher-mass vehicle launches for robotic sample return and human exploration to Mars, improved predictions of aerodynamic and thermal loads are required (Wright et al. (2010)). Problematically, further development of current state-of-the-art simulations are limited by a lack of experimental validation data. The goal of this research project is to improve the database of surface and flow field measurements for hypervelocity carbon dioxide flows over blunt bodies such as the Mars Science Laboratory (MSL) heat shield. The experiments are completed in two Caltech hypersonic facilities; the T5 Reflected Shock Tunnel and the Hypervelocity Expansion Tube (HET). These two facilities can meet a range of complementary conditions such that facility independence of our results can be assessed. The following three tasks summarize the research performed. Task 1: Investigate anomalous shock layer results in CO2 ground testing. Background: Previous high enthalpy CO2 ground testing campaigns performed by MacLean and Holden (2006) in the LENS I facility found a discrepancy in the high-enthalpy (5 MJ/kg) CO2 runs where the experimental shock standoff distance was more than two times the simulated shock standoff distance predicted using the NASA Ames DPLR code. The increase in the experimental standoff distance was attributed to vibrational freezing in the nozzle. Additionally, experiments showed good agreement between computations and interferometric data for spheres and cylinders from the T3 Reflected Shock Tunnel in the 1970’s (Rock et al. (1993)) and T5 Reflected Shock Tunnel (Wen and Hornung (1995)). More recently, measured standoff distance from the T5 and HET facilities was similarly overpredicted by LAURA simulations (Hollis and Prabhu (2013)). However, US3D simulations matched the HET standoff distance with a state-specific vibrational model (Sharma et al. (2010)). Technical approach: The present study varied the nozzle area ratio and binary scaling parameter to examine the issue of vibrational freezing in a CO2 high enthalpy reflected shock tunnel nozzle, suspected to be responsible for the increased shock standoff distance in CUBRC LENS 1 Run 8. Sphere and sphere-cone (Mars Science Laboratory heat shield) geometry models were tested in T5 and HET at different binary scaling parameters to span the conditions of the LENS-I Run 8. High-speed videos were used to examine shock steadiness during test time. A high resolution schlieren image was acquired with the freestream and binary scaling closest to the LENS-I run 8 condition. Measurements in T5 and HET show that the standoff distance agrees with the theoretical predictions bounded by the frozen and equilibrium limits. The experimental standoff distance was underpredicted by the NASA Langley CFD LAURA simulation by 36%. This difference can be explained by varying kinetic rates. Scientific Merit: High enthalpy ground tests simulating Martian entry can be carried out in reflected shock tunnels and expansion tubes. The two types of facilities use different methods of gas acceleration and have advantages and disadvantages. Access to complementary facilities provides the flexibility to access a larger range of test conditions as well as test for facility independence of the results. Task 2: Extend the database of heat flux and shock shape measurements in CO2 flow over MSL scaled-model as well as fully characterize each test condition. Background: A previous study of laminar, reacting CO2 flow over the MSL in the HET (Sharma et al. (2010)) found a good match in heat flux and shock shape for one test condition with freestream velocity of 3036 m/s. Technical Approach: In collaboration with NASA researchers, an expanded envelope of CO2 test conditions in the HET was identified. The four test conditions were characterized for Task 3 and future tasks. Single shot, schlieren images were taken to obtain shock shapes and the model was instrumented with 11 type E fast response thermocouples to measure the wall heat flux. The thermocouples were benchmarked against previous results from Sharma et al. (2010). The heat flux results were compared with good agreement to previous measurements and then extended to higher velocity conditions. The results were then compared with the CFD LAURA simulations assuming a fully catalytic boundary layer. In total, 60 experiments in the HET were performed. Scientific Merit: Currently, validation-quality surface heating data in laminar, reacting CO2 gas environments in expansion tubes is limited to the LENS XX facility. In addition to providing another validation quality database, four test conditions are characterized for Task 3 and future tasks, increasing the envelope of CO2 test conditions available in the HET. Task 3: Measure mid-wave infrared line-of-sight radiative heating in the forebody and afterbody of an MSL model using fiber optic cables coupled to a spectrometer and detector. Background: Past predictions of heat flux for Mars missions such as the Mars Science Laboratory (MSL) mission have calculated negligible heating loads due to radiation. It was only recently proposed that heating due to mid-wave infrared (MWIR) CO2 and CO, bands that were previously neglected could be a significant cause of heating (Edquist et al. 2007). Below 4 km/s, radiation from the IR CO2 bands, specifically at 4.3 µm and 2.7 µm, are the dominant sources of radiation. Both wavelengths require improved predictive capabilities as the current uncertainty bounds predict a 30% uncertainty for the 4.3 µm band and a 60% uncertainty for the 2.7 µm band (Cruden et al. 2014). Furthermore, CO2 radiation can be a dominant source of heating in the afterbody, particularly later in the trajectory than anticipated at lower velocities (Brandis et al. 2015). Technical approach: Typical emission measurements collect light through test section windows depending on the line-of-sight calculations across the windows. However, when simulating the radiative heating on the surface of flight vehicles, the radiance at the surface is calculated based on using rays emanating from a point on the surface of the model outwards through the shock layer. By using a diagnostic where a fiber optic is embedded behind a window on the surface of the model, surface point measurements can be directly compared to radiation computations from simulations, calculating the radiation spectrum in the same manner used when calculating radiative heating on entry vehicles. Comparisons to measurements made in the HET while running the facility as a shock tube show good agreement with the exception of an absorption feature not accounted for in the simulation. This absorption feature has been shown to strengthen with increasing optical depth. By adding a cold layer at the wall to the simulation, the absorption feature in the experiment is reproduced. Tests in the HET while running as an expansion tube are underpredicted by the HARA radiation code. An additional measurement was made in the T5 Reflected Shock Tunnel at 0o angle of attack on a 7” diameter MSL model where the measurement was slightly overpredicted. Further analysis is underway to understand the disagreement. Next steps are also to make radiation measurements in the afterbody of the MSL vehicle in the T5 Reflected Shock Tunnel.