Remote Recession Measurements of Ablative Heat Shield Materials

Spacecraft entering into a planet's atmosphere experience significant heat loads while the kinetic energy is dissipated during slowing down the vehicle. To protect the spacecraft, thermal protection systems (TPS) are needed. For lower entry speeds (e.g. return from low earth orbit LEO), radiative cooling may be sufficient, for higher entry speeds as experienced for re-entries from interplanetary missions, ablative materials are needed. These materials thermally decompose during the hot re-entry phase therefore dissipating additional energy to the surrounding gas. Material recession and charring are two major processes determining the performance of ablative heat shield materials. In the proposed work, a remote sensing system for will be investigated. The general idea is to bury seeding materials at known depths and locations inside the ablator. Once recession reaches this depth, the seeding material will get in contact with the surrounding plasma and show up in the emission spectra of the post-shock plasma. If this plasma is detected either by external or internal instruments, the time trace of the seeding emission provides information about surface recession. If an external observation is conducted during a real re-entry flight, local recession could be measured without any instrumentation on board of the spacecraft. Initial feasibility studies were performed at NASA Ames but to place such a system in a NASA flight program, further characterization is needed, in particular in terms of candidate materials and necessary amounts of seeding material. This will increase the Technology Readiness Level (TRL) to a point where it would be suitable for qualification for upcoming mission designs. Therefore parametrized investigation of different materials in a micro-wave generated plasma will be performed to select the best seeding materials and quantify the observable effects. This project will offer a great opportunity for a graduate student to get acquainted with the general topic of ablative materials and get in touch with NASA. The work fits well into the current activities at the University of Kentucky which is already known as a center for ablation research. It would also support and extend activities in the frame current NASA EPSCoR work on investigation of micro spallation of ablative TPS materials. Once properly characterized the recession measurement method has a large potential to be incorporated in NASA missions involving high speed re-entry problems. (One initial target mission was OSIRIS REX but implementation in the baseline design failed due to low TRL level.) After proper characterization, a development of an in-flight system is a logical development. Such a program would support at least one PhD thesis and was already perceived with explicit interest by NASA officials in initial discussions. During the proposed project, samples of suitable materials (preferably not export restricted) will be prepared with seeding materials at different depths in collaboration with NASA Ames and exposed to a micro-wave generated plasma at elevated pressure levels up to one atmosphere in the Plasmadiagnostics Lab at the University of Kentucky. A data base will be generated for different materials to qualitatively show their feasibility for recession measurements and quantify the amounts of seeding material needed for successful recession depth and rate measurements. Finally, a design baseline for a suitable system for earth re-entry will be developed. An assessment of requirements for a possible in-flight system will be conducted.