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Completed TRL 4 (started at 2, targeting 4)
This project's plan was to determine if shear stress (skin friction) gauges could be employed in the Improved Hot Gas Facility (HGF) at NASA Marshall Space Flight Center.
The primary technical challenge and risk of this project was to obtain a method to accurately measure shear stress in the high enthalpy environment of the HGF. Traditionally measuring shear stress accurately has proven to be extremely challenging and when combined with the high enthalpy conditions of the HGF it becomes significantly more difficult. Many past attempts for measuring shear have been inconclusive or otherwise unreliable.
The research plan involved was to (1) design and acquire new shear stress gauges, (2) integration into the existing HGF calibration plate and data acquisition system; (3) conduct calibration runs and (4) reduce and analyze the data to determine gauge performance.
The primary results are raw data outputs from the sensors, which can then be analyzed, filtered, and compared to the predictions made based on Reynold's Analogy and oblique shock theory. This new data provided (1) a more complete HGF environment (shear & heating rates) envelope and (2) performance of the gauges. This could be of value to other organizations that desire to better understand the flow and shear conditions inside their test facility or to compare with their CFD models using the HGF. In continuing this research the intent is to acquire more shear stress gauges and perform exhaustive analysis of the HGF and to use the same gauges in the Hyperthermal Test Facility to determine how the gauges perform in much lower shear environments. This near-term follow-on work and a more extensive TPS recession characterization test program will allow to populate recession data in high and low shear environments in which more accurate correlation models can be developed and representative ascent environments obtained. This will significantly move the State of the Art (SOA) forward within the launch vehicle TPS discipline.
The work performed through this project will have a large anticipated impact for future missions because previous hot gas facility (HGF) tests have indicated that ignoring the impact of shear stress leads to thick and heavy thermal protection system (TPS) designs for launch vehicles. TPS recession rates observed in the HGF are significantly over-estimated compared to the recession rates observed in flight (Space Shuttle flights). This results in additional thermal protection being required to meet safety standards and reduces the potential available payload mass. The overarching goal is to more accurately predict TPS recession for launch vehicle ascent environments, adequately define the environments, and to minimize conservatism. More specifically, this is to adequately inform the Space Launch System (SLS) where vehicle performance may be enhanced.
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