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Completed TRL 2 (started at 2, targeting 3)
The oncoming era of Space Exploration presents need for control over entry vehicles to reach our neighbor Mars, and return safely to Earth. In addition to withstanding the intense heat seen in atmospheric entry, the vehicle also needs improved means of trajectory control to reach designated landing site as close as possible. A method of a transforming or morphing vehicle is desired to change the lift and drag characteristics, and thus change the vehicle's trajectory as needed. This concept has been explored recently by NASA in terms of the Inflatable Reentry Vehicle-3, which inflates concentric rings on entry to increase surface area of the capsule. The NASA Technology Roadmaps, defining the current challenges in development for upcoming missions, defines this system in chapter 9.1.4.3 for planned missions to Mars including the eventual manned surface mission. This proposed research plan focuses on altering local pressure fields to an entry capsule by means of actively controlling the porosity of outer protection layers that are already present in current entry systems, rather than devising an additional system for inflation or rigid deployment. Low-weight and highly thermal resistant materials such as Ultra High Temperature Ceramics (UHTC) are ideal for coatings to protect capsule payload from the extremely high heat load generated at atmospheric entry speeds. These materials are also highly porous, and thus have pockets that the hot and high-pressure gas at vehicle surface diffuse to, effectively lowering local pressure and dispersing heat. The use of porous regions have been explored in research as means of lowering pressure gradients in supersonic engine cowls, and studied in effect of local effects for ablative protection systems that decompose in high heat. The ceramic porosity can be changed by flooding the pockets with a higher density coolant, as done in method of transpiration cooling, where coolant pumped through the porous layers absorbs heat transferred to the ceramic layer. Transpiration cooling has been explored since 1940's as means of reducing heat loads within rocket engine nozzles and more recently re-evaluated for protection systems for atmospheric entry. The coolant behavior in supersonic and hypersonic conditions requires further investigation to compression effects, and exposure to temperatures that may superheat to coolant to critical state. A numerical model is used to solve compressible fluid dynamics equations of mass, momentum and energy across a mesh domain based on the Mars Science Laboratory, the largest Mars capsule thus far that landed 2012. The student's prior experience in simulating hypersonic flow fields is used in obtaining a steady-state solution. Porous regions on far radial locations on the capsule surface are activated in the solution, and further iterations run to observe local field effects and resulting pressure force creating a moment on the capsule, thus its pitch. The properties are dependent on ceramic geometry but approximated via factors of inertial, viscous and thermal resistance measured of UHTC zirconium diboride. To closer observe thermal response of ceramic and cavity regions, these local effects are transposed to a one-dimensional numerical solution of the hot gas and ceramic. Initial results are obtained for baseline comparison to adding transpiration coolant, and progression to direct numerical solutions that can address gas ionizing within porous solid cavities. Local thermal effects are required to translation to material response prediction of inner stresses, strains, expansion and integrity. Based on results of pressure force generation and thermal response, initial evaluation of method feasibility is drawn. The development of transpiration-cooled systems as dual use in heat mitigation and trajectory control would provide an effective and cost-effective means for upcoming and future missions, as this work aims to show.
The development of transpiration-cooled systems as dual use in heat mitigation and trajectory control would provide an effective and cost-effective means for upcoming and future missions.
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