Description: OKSI proposes a minimally invasive in-flight diagnostic to measure heat shield recession during flight tests. These measurements can be used to validate models and ultimately optimize heat shield design to reduce weight while maintaining sufficient safety margins. The concept has two components: 1) specially designed heat shield plug(s) and 2) a remote spectral sensor. First, the custom heat shield plug will have trace amounts of different indicator materials that are released as pyrolysis/ablation progresses into the heat shield. Using different indicator materials at different depths allows the spectral sensor to detect when each layer is releasing the materials, providing a measure of both total recession depth and ablation rates. Using multiple plugs (each with different indicator material layers) allows a single spectral sensor to detect when each trace material is released into the flowfield. In this way, the spectral signature uniquely identifies the level of recession at the different spatial locations. The spectral sensor can either be located on the capsule viewing aft into the wake or located on a remote airborne platform tracking the capsule reentry. Each sensor location has its benefits and drawbacks. If the proposed concept is proven successful, NASA will have a minimally invasive heat shield recession diagnostic using onboard or remote spectral sensors. The concept will allow NASA to measure heat recession rates during all phases of reentry which will be especially useful for multiple reentry (skip trajectories) and differentiating recession levels for each reentry. Phase-I will utilize existing high fidelity models to predict signature levels and signal to noise ratio (SNR). Phase-I will also involve limited arc jet tests of doped Avcoat heat shield samples. During Phase-II, we will conduct more extensive arc jet testing to correlate spectral signatures with actual recession/pyrolysis.

Benefits: NASA is in the process of improving heat shield design either through the use of new heat shield materials or by reducing the thickness of existing heat shields using conventional materials. Currently, NASA heat shield predictive models are not reliably consistent with observed measurements. The proposed concept will provide time-resolved recession measurements that can be used for model validation (both during arc jet test and flight tests). With validated predictive models, NASA can optimize the heat shield design for expected reentry conditions. Additionally, reliable onboard real-time recession measurements could possibly be used to identify localized excessive recession. This diagnostic could be integrated with capsule flight control system to orient the vehicle such as to reduce heating loads in the damaged area. Furthermore, this technology could be used to monitor re-entry events of planetary and lunar landers/probes whose heat shields cannot be physically inspected post-entry.

Commercial and DoD applications also exist. For instance, SpaceX and Blue Origin are pursuing capsule reentry capabilities. Currently, SpaceX is focusing on reentries from low earth orbits where heating rates are low. However, SpaceX is also investigating Mars missions where heat rates will require a much different heat shield design. SpaceX will likely conduct Mars-like entry conditions using Earth reentry missions. The use of non-intrusive heat shield recession diagnostics could be utilized to provide time-resolved heat ablation during all phases of reentry. This will allow SpaceX to validate their heat shield recession models under higher heating rates.