Temperature, Heat Flux and Recession Sensing for Ablative Thermal Protection Systems

Although significant advances have been made in ground-based testing and simulations, it is still impossible to precisely replicate the diversity of in-flight conditions experienced by ablative thermal protection systems (TPS). This leads to uncertainty in the predictions of the magnitude and rate of TPS ablation. Because in-flight monitoring is difficult, the uncertainty in actual boundary conditions and models must be considered when designing a TPS. To reduce risk, designers must resort to trade-offs which often involve increasing heat shield mass. Direct ablator temperature, heat flux and recession measurements would allow engineers to reduce design uncertainty and improve modeling. These improvements will lead to decreased heat shield mass, enabling missions that are not otherwise feasible and directly increasing science payload and returns. Ultrasonic methods for real-time monitoring of ablator conditions including internal temperature distribution, heat flux and recession will be developed in this program. Internal localization methods of ultrasonic thermometry will be used to accurately measure temperature distribution to within close proximity of surface charring. Temperature compensation will be applied to ultrasonic thickness gauging techniques to estimate surface recession in real time. Heat flux can be extracted from the measured temperature distribution. Combined together, these ultrasonic techniques will form a sensor system capable of sensing and relating real-world ablator performance to computational models as well as qualifying ablator materials. When developed to maturation, such a sensor system even has applications for in-flight health monitoring.

Mass and heat dissipation performance are critical to thermal protection systems for many NASA objectives both in spaceflight and hypersonic flight. Low-density carbon phenolics perform well in both of these critical parameters. Phenolic Impregnated Ceramic Ablator (PICA) was developed by NASA/Ames in the mid-nineties and flown successfully in the Stardust mission. This particular material is of active interest to NASA, with its use in the upcoming Mars Science Laboratory (MSL) and the possibility of its selection for multiple future missions. Real-time recession and heat flux measurements will support continued development of this class of ablators as well as mission specific implementation. Ablator performance models can be enhanced with higher fidelity temperature data and used for faster development with decreasing cost. Future programs are also in need of PICA and PICA class ablators. Two of the New Frontiers Program proposals incorporate PICA for sample return missions including MoonRise, a Lunar South Pole-Aitken Basin Sample Return Mission which would place a lander in a broad basin near the moon's south pole and return approximately two pounds of lunar materials for study and Osiris-Rex which would rendezvous and orbit a primitive asteroid, returning more than two ounces of material from the asteroid's surface.

PICA and materials of its class are currently under development worldwide, including a Carbon-Resin by the European Space Agency (ESA). The largest current scheduled user of PICA-class ablators is SpaceX which utilizes a PICA-X variant in the Dragon spacecraft for earth re-entry. This environment is toward the lower end of heat fluxes and ablation to be encountered during a re-entry procedure. All private and public space endeavors that require re-entry heat shielding can benefit from the technology developed under this program which can augment and improve modeling, test ablators in real-world conditions and perform health monitoring roles in test articles.

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