Space shuttle fuel tanks, instead of insulation why don’t they use vacuum?

Question

Since the Columbia disaster NASA has spent a large amount of mission time surveying for tile damage. It appears now they’ve got a big ding.

My question is why do they use foam insulation? My lab typically stores these cryogenic fluids, and we do not use foam insulation for the storage and holding vessels. Our dewar has two walls and a vacuum between the layers. I’ve never seen any ice forming on my dewar, and the fluid lasts for days. Why don’t they use a double-walled evacuated external fuel tank?

I am not sure I totally buy the weight explanation. The foam is not the silicate ceramic tile that lines the shuttle hull. So I believe it must have appreciable density. It seems that they could make a honey comb support structure and then the outer wall could be quite thin, even Mylar. With a little engineering they might be able to have a double walled tank with less weight.

As far as re-entry goes - maybe, but I wonder about some things there too. How often do they actually reuse these tanks? I know the shuttle systems are supposed to be re-useable, but do they? And is that big tank jettisoned at a relatively low altitude? And last who cares if the tank is scraped? Isn’t the added risk and lost mission time more precious than a dumb tank?

Answer 1

Yes it would weigh twice as much if the tank were double walled like a thermos bottle and would be far to complicated and expensive to build. Also trying to mount an orbiter onto a double walled tank would be tricky because the tank is really the backbone of the stack when it's on the pad and at lift off. Industry really let NASA down with that tank design when they assured them that foam on the tank was not going to be a problem while the system was in development. The thermal protection system is another instance where NASA was sold a bill of goods. The tiles were a source of problems from the beginning and still are. The engines although they have worked almost flawlessly throughout the program are not as robust as was predicted and have never been able to be used the 100 times promised in the beginning. Failure to launch more than the record 6 times in one year doomed the shuttle to never have the cost reductions that the system was supposed to produce.

Answer 2

Weight. Turning the fuel tank (there is only one per launch) into a Thermos bottle would weigh more, because there would be a huge area subject to atmospheric pressure, so the tank would have to be very strong.

Answer 3

Because the external material would melt.

This is why the insulation is made of ceramics. At this time, it's the only material that can handle the extreme temperatures associated with re-entry into the atmosphere.

Answer 4

I haven't a clue but a word like implode is going through my mind. Like it might colaspe into it's self.

Answer 5

Raidiant heat would be a problem then.

Answer 6

weight?