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The STS Orbiter was infamous for losing tiles. This occurred at a higher rate than expected.

I understand the epoxy used to attach the tiles to the shuttle cured extremely quicky, so a fresh batch would have to be made every couple of tiles. It appears the enterprising workers discovered that adding saliva to the epoxy as they worked would allow them to attach more tiles before having to break off to mix another batch.

I though everything related to spacecraft construction was logged and double checked, Why wasn't this modification to procedure noticed for so long?

Bonus internet points for stats on the difference in tile shedding rates after the practice was stopped.

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    $\begingroup$ I'm unable to find any online references to the practice of adding saliva to epoxy. Can you provide a reference? $\endgroup$ Feb 2, 2021 at 19:02
  • $\begingroup$ @RussellBorogove I found the practice mentioned in "Safety of the Thermal Protection System of the Space Shuttle" spaceflight.nasa.gov/shuttle/archives/sts-107/investigation/… But they explictly state that "The only black tile that has been lost due to debonding not caused by debris occurred when the first internal waterproofing agent, HMDS, reacted chemically with the screed causing it to soften and revert back to its more viscous form." $\endgroup$ Feb 2, 2021 at 20:47
  • $\begingroup$ "In several cases, the curing time of the RTV has been reduced by the installers using water (or saliva). Such a procedure, which is explicitly forbidden, is not believed to affect the immediate strength of the bond, but may reduce its life." $\endgroup$ Feb 2, 2021 at 20:48
  • $\begingroup$ Not a tile-ologist, but my impression is that most of the tile damage incurred during actual space mission over the life of the program was due to debris impacts, not due to incorrectly-stuck-on tiles falling off. $\endgroup$ Feb 5, 2021 at 21:30

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Some of your information is a little off. The reason why water (or saliva) was allegedly sometimes added to the RTV used to attach the tiles was to speed up the curing time, not lengthen it. The reason why the person attaching the tile would be motivated to add water or spit to the RTV is to increase the amount of work they could complete in a day since normal application of the RTV is time-consuming and must be carefully monitored. I do not believe this was a common practice, and there is no evidence of this practice causing a failure of any tile. 99.999% of tile damage occurs from debris. The only known tile loss due to any means other than debris damage is attributed to an internal waterproofing agent (HMDS) they once used that had a undesirable effect on the RTV it came in contact with.

Update for reference: https://spaceflight.nasa.gov/shuttle/archives/sts-107/investigation/tps_safety.pdf Page 35/36

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    $\begingroup$ This is great info, and I believe it's accurate, but it sorely needs a reference. $\endgroup$ Feb 4, 2021 at 19:36
  • $\begingroup$ Great reference! The HMDS is discussed on page 28 as well. $\endgroup$ Feb 4, 2021 at 21:29

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