Development and Modeling of Waterproofing Coatings or Surface Treatments for Reusable Thermal Protection Systems (TPS)

Reusable thermal protection system (TPS) tiles, originally developed for the Space Shuttle and now used in modern commercial spacecraft, are composed of Alumina Enhanced Thermal Barrier (AETB) a low-density, high-porosity ceramic made from a silica and alumina fiber network. This porosity provides excellent thermal insulation during atmospheric re-entry by minimizing thermal conductivity. However, it also makes AETB highly susceptible to water absorption, which can lead to several structural failure modes: internal pressure during extreme thermal transitions from orbit to re-entry, cracking caused by freezing under ambient launch pad conditions, cryogenic fueling, or evaporative cooling at high altitude, and unexpected increases in launch weight. To mitigate these risks, tiles are treated with waterproofing agents through a labor-intensive process involving injection of toxic silanes such as dimethoxysilane (DMS) and methyltrimethoxysilane (MTMS). These treatments require extended curing periods to form strong Si O Si or Si O Al bonds for adhesion to AETB fibers and must be reapplied after each re-entry due to degradation. Furthermore, the emissive glass coating on the tile exterior restricts flow paths to the insulating fiber network, compounding these challenges. These limitations hinder the frequent launch cadences anticipated for reusable spacecraft by extending turnaround times between flights. NASA seeks modern waterproofing solutions for AETB and similar materials that accelerate treatment through improved chemistries and faster processes, reduce or eliminate toxic materials, and enhance coating durability and survivability under re-entry conditions. Experimental development efforts are sought to identify modern waterproofing chemistries that meet all of the following criteria: Reduced moisture uptake of ceramic fibers at least commensurate to existing DMS and MTMS treatments Application timescales and thermal performance at least commensurate to existing DMS and MTMS treatments Ability to be injected or otherwise applied to tiles in situ on a spacecraft in open environments Additionally, compared to existing chemistries (DMS/MTMS) and application approaches (injection), successful solutions will meet one or more of the following criteria: Involve formulations that are less toxic and/or have less volatile organic compound content Leverage new chemistries, surface treatments, or catalysts that accelerate both initial application to tiles and re-application post-flight Exhibit improved thermal performance and reduced degradation to enhance reusability Utilize application processes that increase per-tile throughput Phase I shall demonstrate a waterproofing approach and quantify performance on porous silica or silica-alumina substrates akin to AETB tiles in laboratory environments. Performance quantification must include analysis of curing timescale, moisture absorption resistance of the substrate after application, and thermal degradation behavior.