Profile picture
Dr. Phil Metzger @DrPhiltill
, 9 tweets, 2 min read Read on Twitter
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. 🤦‍♂️
Missing some Tweet in this thread?
You can try to force a refresh.

Like this thread? Get email updates or save it to PDF!

Subscribe to Dr. Phil Metzger
Profile picture

Get real-time email alerts when new unrolls are available from this author!

This content may be removed anytime!

Twitter may remove this content at anytime, convert it as a PDF, save and print for later use!

Try unrolling a thread yourself!

how to unroll video

1) Follow Thread Reader App on Twitter so you can easily mention us!

2) Go to a Twitter thread (series of Tweets by the same owner) and mention us with a keyword "unroll" @threadreaderapp unroll

You can practice here first or read more on our help page!

Did Thread Reader help you today?

Support us! We are indie developers!

This site is made by just three indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member and get exclusive features!

Premium member ($3.00/month or $30.00/year)

Too expensive? Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal Become our Patreon

Thank you for your support!