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Thread by @DrPhiltill: "Strange but true story: the giant round propellant tanks at the Apollo/Space Shuttle launch pads were certified to be emptied & refilled […]"

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Strange but true story: the giant round propellant tanks at the Apollo/Space Shuttle launch pads were certified to be emptied & refilled only 6 times. Nobody could remember why the number of refills was limited, but they were dutifully kept full of liquid oxygen for 40 years. 1/n
2/n. Nobody knew where the requirement originally came from, way back in the 1960’s. Was it a structural limit, the steel would get too weak if warmed then re-chilled to liquid oxygen temp more than 6 times? Nobody still living knew.
3/n The requirement itself was documented since the 1960s, but the reason behind the requirement was as lost as ancient texts in the Library of Alexandria. So we dutifully obeyed for DECADES keeping those tanks full without ever knowing why.
4/n. I was in a physics lab trying to figure out why such a requirement would exist. We worked on lots of problems like that. We considered ourselves the Myth Busters of NASA KSC.
5/n. We never totally figured it out, but the leading theory was this. The tanks have inner and outer concentric spheres. Between them is a powder material, perlite, for insulation. Draining/refilling the inner tank makes it expand/contract, which compacts the perlite.
6/n. If the perlite compacts then the inner tank is refilled, chills down and contracts so the perlite can settle. This lowers the height of the perlite, like cereal settling in a box so the box looks not completely filled.
7/n. If the perlite lowers down enough, the inner sphere can become uncovered - no perlite sitting on its top surface - so it is not as insulated. We calculated how many drain/refill cycles it would take for that to happen. Answer: 6.
8/n. But that is easily fixable. You just open the hatch on the top of the outer sphere and pour in some more perlite. Nobody knew that fixing the problem was so easy, because no living person actually knew why it was a problem. So we blindly topped off the tank for 40 years!
9/9. (I.e., topped it off with liquid oxygen as it kept boiling away, all those decades.) So we had a good laugh after we realized there was probably no good reason to keep the tanks filled all that time. Too bad we waited 40 years before asking the right question. 🤦‍♂️
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