Wall Shear Measurement Technology for Plume Surface Interactions

Exploration of the Moon, or Mars, will require the development of landing vehicles, whose primary task is to decelerate its cargo and safely touch down on the surface. However, this is not a trivial task for these types of environments. As the jet/plume from the rocket engine, used to decelerate the vehicle, encounters the surface, the low atmospheric pressure, low gravity, and cohesive soil properties result in large dust clouds and even crater formation. These phenomena are called plume surface interactions (PSI). Incremental progress is being made to study PSI through a series of proposed test campaigns being conducted at multiple NASA Research Centers and utilizing large vacuum chamber facilities, such as the MSFC TS300 or LARC 60 Sphere, and tested under relevant conditions. One critical instrumentation need that NASA has identified is direct surface (wall) shear stress measurements. In the Lunar environment, surface shear stress is the primary driving force by which surface erosion occurs. To overcome this technology gap, Ahmic and our assembled team propose to adapt our state-of-the-art wall shear measurement technology to target surface erosion environments occurring in plume surface interactions. In Phase II, Ahmic will develop two wall shear sensor capabilities. The first will target wall shear measurements at the extremely high temperatures produced in rocket plumes exceeding 2000 K. The second will target wall shear measurements at the extremely low magnitudes produced in simulated Lunar and Martian environments where the ambient pressure is a low vacuum. The sensors will then be validated in clean-flow wind tunnel facilities before being tested in jet/plume impingement environments, including hot-flow and vacuum facilities. In addition, the sensors will also be used to expand a test methodology of reducing active surface erosion testing to a series of discrete static-surface contour geometries to be instrumented and tested in a PSI environment. Future exploration of the Moon or Mars will require the development of new landing vehicles. To safely touch down on the surface, the vehicle must overcome surface erosion and cratering challenges due to plume surface interactions (PSI). The primary challenges associated with studying PSIs in a laboratory setting come from creating a realistic testing environment and developing the diagnostic capabilities needed to characterize the complex domain. While full-scale testing of plume-soil erosion in a hot-fire environment has not been achieved due to the difficulty of simultaneously matching back pressure and gravity effects, subscale testing has recently been conducted, with more currently being planned. NASA has identified the need for surface shear measurements, which is the primary driving force by which surface erosion occurs in the case of the Lunar environment. Currently, no direct measurement technology exists for these types of environments. To overcome this capability gap, Ahmic proposes to adapt our state-of-the-art wall shear sensor technology for PSI environments. The Phase II effort will develop direct measurement wall shear sensor technology and demonstrate important test methodologies for relevant jet impingement environments. Executing these goals while reducing program risk will require the following Phase II technical objectives to be met: 1) design and develop a high-temperature wall shear (HTWS) sensor for hybrid rocket motor propellent sourced PSI ground testing, 2) design and develop an extremely low wall shear sensor for low vacuum cold flow PSI environments, 3) validate both sensors in clean-flow wind tunnel facilities and then demonstrate performance in jet/plume impingement environments, including hot-flow and vacuum facilities, 4) use the newly developed sensor technology to conduct a static-surface erosion study featuring multiple surface contours instrumented and tested in a PSI environment to demonstrate how the wall shear technology can be coupled with surface erosion measurements to understand viscous erosion models better. The proposed deliverables to NASA are 1) quarterly reports documenting the intermediate progress, 2) delivery of the final wall shear sensors to NASA personnel, and 3) a final report documenting both sensors’ design, validation, demonstration, and test methodology study.  

The proposed wall shear diagnostic techniques present a direct solution for NASA’s PSI ground testing needs. As part of the recent project focused on advancing modeling and simulation capabilities, PSI validation data was acquired in a campaign known as the Physics Focused Ground Test (PFGT). Much of this initial work was carried out in NASA’s Marshall Space Flight Center (MSFC) in the Test Stand 300. Ongoing test campaigns similar to this would directly benefit from Ahmic’s proposed instrumentation and methodology. The Air Force and SpaceX represent non-NASA markets for the proposed diagnostic techniques. These entities are currently investigating the technical feasibility of using rockets for point-to-point transport of high-value cargoes to remote landing sites. To do this safely will require knowledge of PSI, viscous erosion physics, and how the granular particles will be ejected from the surface.

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