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I'm especially thinking of the loss of the Columbia due to damage from chunks of insulation that came off. I got to wondering, why is it there in the first place?

Of course it's needed while you are preparing for launch or you would get insane boil-off. However, why does it fly? Why isn't it designed to peel off as the rocket lifts off the pad? For the few minutes of flight the boil-off wouldn't be a big loss, especially since a lot of fuel is going to the engines which will negate at least some of the boil-off. I have a hard time picturing the boil-off being anywhere near as much of a penalty as the cost of lifting that insulation.

Loren Pechtel
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  • It would likely be more weigh complexity and cost to shed than it would save. – GdD Sep 11 '17 at 10:06
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    For a better flow of cryogenic propellants through the tubes, pumps and fuel injectors to the combustion chamber, there should not be too much gas bubbles in the propellants. Liquid cooling of the walls of the combustion chamber and the nozzle works better if there is mostly fluid in the cooling channels. – Uwe Sep 11 '17 at 11:11
  • @GdD Of course it would weigh more but it wouldn't be flying at all. The weight wouldn't matter. – Loren Pechtel Sep 11 '17 at 13:17
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    It's not the weight of the insulation itself, you have to keep the insulation on and have a mechanism to shed it, which adds weight and complexity. – GdD Sep 11 '17 at 14:06
  • The structural mass of a rocket is a small fraction of the total start mass of the rocket and the isolation is again a small fraction of the structural mass. Therefore the isolation is a tiny fraction of the start mass of the whole rocket. – Uwe Sep 11 '17 at 15:57
  • To design the optimal insulation thickness is a difficult problem, but no insulation at all is not the best solution. – Uwe Sep 14 '17 at 09:37

2 Answers2

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The insulation's job is also to prevent ice from forming on the rocket's skin (and breaking off during ascent). Shards of ice are more dangerous than chunks of foam.

Even if ice were no problem, for the Space Shuttle it'd be very difficult to design insulation that comes away cleanly from the tank without hitting the orbiter.

For classic rocket designs, this is much easier. The second stage of Ariane 4, for example, used insulation that's rigged to fall away at liftoff. This stage had precooled, but non-cryogenic propellants, so no icing risk but they wanted to avoid the propellants warming up (and expanding) before launch.

enter image description here

The third stage (LOX/LH) used non-removable insulation tiles, though (the dark-gray/brown area).

Given that this rocket used both solutions, the tradeoff of permanent insulation for the third stage vs. removal on liftoff and accepting the ice buildup is likely that permanent insulation gives better performance.

Hobbes
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  • If a cryogenic rocket launched out of an ICBM silo that had its volume cryocooled and dried without physical insulation on the flight hardware, ice formation wouldn't be a problem once the roof is removed and it ignites, right? – LocalFluff Sep 11 '17 at 13:04
  • You still have several minutes of atmospheric flight through an atmosphere that contains water which will readily condense and freeze. And since you've cooled the entire rocket including stage separation seams and RCS openings, ice will form everywhere and cause trouble. – Hobbes Sep 11 '17 at 13:13
  • It's going to develop ice during it's short passage through the atmosphere while going supersonic/hypersonic?? – Loren Pechtel Sep 11 '17 at 13:19
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    If you've ever seen instant ice - absolutely – Weckar E. Sep 11 '17 at 13:31
  • There is significant aeroheating during ascent. I'm not sure this is an "absolutely". – Organic Marble Sep 11 '17 at 14:40
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    Given that Columbia was damaged by insulation that came off, I think it's pretty self-evident why it might be risky for insulation to be designed to come off. There aren't many points during the flight where it can be jettisoned in such a way that you can guarantee that it won't hit the craft, and those parts are generally at points in the flight where there would be little benefit in it (eg. you could let the insulation drift away more slowly once you've reached orbit, but then you've already spent the fuel to lift it to orbit...) – anaximander Sep 11 '17 at 15:27
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    @Loren Pechtel: the rocket needs some time to accelerate to supersonic/hypersonic speed, enough time to build up ice. A thin layer of ice could be heavier than a thick layer of isolation material. The density of a good thermal insulator is lower than that of water ice. – Uwe Sep 12 '17 at 21:13
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Supplemental answer: I ran across another vehicle that jettisoned insulation in flight: some of the early Centaurs.

enter image description here

This 1968 mission jettisoned four insulation panels from the Centaur ~200 seconds into flight.

The mission was a failure for unrelated reasons: a Centaur engine failure.

Source: NASA TM X-2525

uhoh
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Organic Marble
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