This isn’t my story, but it’s a good one so I’m going to write it down, since I can’t find any version of it online.
The Hubble Telescope is necessarily mostly hollow, made of big tubes as you’d expect. Inside these tubes are not only extremely delicate optics but sensitive instrumentation.
While inside the Space Shuttle that launched it, the cargo bay and therefore the tubes are full of air. The shuttle crossed the Karman line two minutes and thirty seconds after ignition, so by that point essentially all of that air must have gone somewhere else, and violently.
The Shuttles not only were to launch the telescope, but to service it, for repairs and upgrades, which later amounted to seven missions after its launch by Discovery in 1990.
Initially the Hubble was designed with a gap between tubes to allow the air to leave evenly. It was well into its many-year construction when someone noticed that the Shuttles use a Ku-band radar for on-orbit navigation in relation to other spacecraft, and this powerful signal risked destroying the instruments.
So, the gap needed to be sealed, and the requirements were many: controlled release of air, dimensional stability both in space and on the ground, flexibility, and most importantly, radar absorption. Also, probably plenty of other things I’m not aware of.
A rubber ring was discussed as meeting most of the needs, except the last one… Until Chief Engineer Bob Alexander suggested doping the rubber with graphite.
Here’s the drawing he showed me of the final design which still flies today.
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(story from memory, mostly mine. Don’t blame Bob. Corrections welcome.)