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Transpiration Cooling as a Thermal Protection System for Extreme Environment Atmospheric Entry
Amit Mandalia
Georgia Institute of Technology

For future Venus, high mass Mars, and Giant Planet exploration missions, the extreme atmospheric entry environment demands a robust and capable thermal protection system. The carbon phenolics that have been used for past missions have become increasingly difficult to produce, and there exists a necessity for new thermal protection systems to be created to meet these intense flight regimes. Transpiration cooling is a technique in which a coolant fluid (such as water or air) is sent through a porous medium in order to protect that surface from heat. The project proposed here will determine if a transpiration cooling system would be a viable replacement to ablators on heatshields. This cooling technique has found success in jet engines and experimental hypersonic aircraft and may be capable of meeting the criteria of extreme environment entry.

In this project, the thermal performance and mass efficiency of transpiration cooling will be compared to carbon phenolic ablators, which is the heritage solution, and a low ballistic coefficient vehicle thermal protection system, which is a proposed alternate solution. Firstly, applicable extreme environment trajectories will be selected for analysis. Much of thermal performance will be tested experimentally in relevant arcjet, wind tunnel, and shock tube test facilities. Likewise, for consistency and for trajectories that cannot be experimentally evaluated, a computational fluid dynamics study will be conducted to assess thermal performance. Mass efficiencies will then be derived from the thermal performance and mission criteria to determine the feasibility of the cooling methods. Possible dual-use of coolants (like water) and reusability approaches will be explored as well.

As a technology, transpiration cooling is a rigid thermal protection system that may have the potential of protecting exploratory probes to Venus, Mars, and the Giant Planets. Exploration is important because one of the major goals of NASA is to extend human presence beyond Earth. Potential advantages in this cooling technique include production, control, and mass efficiency. Since this technology is already being used in jet engines, aerospace grade production lines exist and can be adapted for space use. Likewise, although an active thermal protection system is more complex, it allows for control of the coolant to compensate for different heating conditions throughout a trajectory. Also, as mass efficiency is an overall mission driver, coolants such as water may have multiple uses during a mission and perhaps be acquired in-situ at certain planets for return missions.