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  Thermal Protection System
  - Less Toxic Waterproofing Agent (TPS)

Problem: The Orbiter TPS (thermal protection system) consists of FIB (flexible insulation blankets), FRSI (felt reusable surface insulation), and rigid tiles. The TPS is waterproofed at the factory during fabrication before it is installed on the Orbiter. The waterproofing thermally decomposes in the insulation where temperatures exceed 1050 degrees F. The FIB and tile experience temperatures above 1050 degrees F, therefore they must be rewaterproofed in situ on the vehicle after every flight. If rewaterproofing is not performed, the insulation could absorb hundreds of pounds of water. This could result in damage to the rigid tiles, compromising safety.

It could also be an unacceptable weight penalty. The current TPS rewaterproofing process uses an agent known as DMES (dimethylethoxysilane). Due to the physical irritations DMES causes personnel, a TLV-TWA (threshold limit value-time weighted average over 8 hours) of 0.5 PPM is established. The bay must be cleared of all non-essential personnel, and the technicians must wear specialized breathing equipment and clothing to protect them from the DMES vapors during the rewaterproofing process. Rewaterproofing must be performed on third shift and on the weekends.

Solution: The objective of this project is to develop a less toxic TPS rewaterproofing agent with a more compatible TLV-TWA. This could decrease physical irritations to bay personnel, in turn reducing protective equipment and cost of operation. It could also minimize high bay clears, thus reducing rewaterproofing flow impacts to long term Program manifest requirements.
Initial Operation Date: FY99
Phase II Upgrade
Technology Goal: The test plan consisted of choosing candidate agents, screening them for rewaterproofing performance, conducting TPS material compatibility studies, optimize rewaterproofing variables (injection/spray concentration and volume, solvent composition, and cure time), and conducting rewaterproofing performance testing on tile and blanket materials. Some candidate agents have been studied by development of analytical methods and discussions with other NASA centers and contractors who have experience in this area.

The chosen agents to date are MDES (methyldiethoxysilane), DMDES (dimethyldiethoxysilane) and TMCTS (tetramethylcyclotetrasiloxane). These agents will be tested full strength, combined, and diluted with ethanol/water. Analytical methods development resulted in 3 chemical structures that could be successful rewaterproofing agents. Research into any existing agent compounds that may contain any of the 3 structures is currently being performed.

To date, screening for rewaterproofing performance is in work. DMES was used as a control agent to validate the procedure. Material compatibility studies and optimization of rewaterproofing variables will occur once the agent screening is complete. Throughout the test plan, information on the toxicology of the candidate agents is being gathered. A final rewaterproofing performance test will be performed on both tile and blanket materials once the best candidate agent is chosen.

Goals Supported: 1) Fly Safely
2) Meet the Manifest
3) Improve Supportability
4) Reduce Cost
5) Support HEDS

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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