shuttle external tank

August 1, 2005 6:17 AM

Space Shuttle Filter: external tank - why is the insulating foam on the outside??

Why is the insulating foam on the outside of the tank? Wouldn't it be safer to have the foam sandwiched between 2 aluminum skins?
The tank is only moving through the air at mach 4+, so keeping it from peeling off is a big deal.

I've asked this of everyone I know, and the only response is: " I think NASA engineers would have thought of that"

Can't you make the external tank into a kind of thermos bottle?

posted by stevejensen to science & nature (15 comments total)

To save weight, the cliche is $10,000 a pound to put something in orbit by shuttle. I don't know if that is true but the increase in size needed to make space for the foam would mean a lot of added weight.
posted by 517 at 6:27 AM on August 1, 2005

To "insulate". The explanation I heard on CNN is that it helps keep the fuel in a liquid state and prevents the build up of ice on the outside of the tank. I'm sure Google could supply a more complete answer.
posted by Carbolic at 6:53 AM on August 1, 2005

I neglected to read any of the "more" part of the question. Ignore me, it's early.
posted by Carbolic at 6:55 AM on August 1, 2005

I second 517, it's the weight. Remember back to the first shuttle launch and the external tank was painted white same as the shuttle? It was only later that they eventually realised they could save themselves a huge amount of weight by leaving it unpainted. If they're removing paint to save weight then there's no way they're going to add 2 aluminium skins.
posted by oh pollo! at 7:17 AM on August 1, 2005

I had a question along the same lines: why not put the foam on the inside either with no skin or with a thin plastic skin?
posted by TedW at 7:22 AM on August 1, 2005

If they moved the foam to the inside, that would take up space that had been devoted to fuel.
posted by ROU_Xenophobe at 7:27 AM on August 1, 2005

These are all only partial answers. Why not make the external tank a little bit bigger and have the foam inside the skin, instead of outside it? You could do it without shrinking the tanks, and you wouldn't be adding a significant amount of weight, and you'd improve safety a lot.

Some things that might help to remember are:
1) When the shuttle was first designed, the foam contained freon. Eliminating this freon increased foam-brittleness by leaps and bounds.
2) Until the Columbia disaster, foam-hits were fairly common (at least a couple every launch), and the belief that they were a problem was a minority opinion.
3) They can't redesign the whole damn ET now. It'd be far more cost-effective to finish an Orbital Space Plane.
posted by Plutor at 7:41 AM on August 1, 2005

I always wondered if the paint from the old days may have helped the foam not break off. It would add to the weight, but a coat of paint might stop the foam from falling off?
posted by birdherder at 7:56 AM on August 1, 2005

I asked a foam question in a thread but don't think anyone answered, maybe I should get my own question, but:

I saw that NASA in order to remove some of the larger pieces of foam had built some very expensive heaters to go where thicker pieces had gone. Why, if the foam only has to last on the ground and part of the 8 minutes of launch, don't they just score the foam so that when it does fall off it falls off in smaller chunks that would have far less inertial force when hitting the craft?
posted by Pollomacho at 8:00 AM on August 1, 2005

It's a weight decision. A second aluminum skin would weigh quite a lot, and the shuttle is already laboring under a number of design compromises...

You can second-guess now that the foam has caused so much trouble recently, but they flew over 100 missions without major foam problems. I would hazard a very strong guess that someone at NASA is even now looking for a better type of foam.
posted by jellicle at 8:30 AM on August 1, 2005

Pollomacho, scoring the foam as you describe would dramatically increase the amount of debris flying around at launch time. It's hard to imagine that anyone at NASA would be comfortable with that idea, even if the individual pieces of debris are less dangerous. Debris flies around randomly, and engineers generally frown on random processes simply because they are not completely predictable.
posted by Galvatron at 9:58 AM on August 1, 2005

That makes sense, but if they are falling off randomly anyway, why not have it fall off randomly in a much less harmful form?
posted by Pollomacho at 11:32 AM on August 1, 2005

If you score the insulation, you break its structural integrity and simultaneously increase friction with the surrounding air. As a result, I expect that a much, much greater volume of insulation will shear off of the tank. The difference might be two or three orders of magnitude. Even if the pieces are smaller, I think it's a mistake to believe that it would be safer.
posted by Galvatron at 12:10 PM on August 1, 2005

Plutor, Reagan's Orient Express was cancelled a long time ago. They're calling the shuttle's replacement the CEV (wikipedia).

birdherder , I wonder about this, myself.
posted by Rash at 1:19 PM on August 1, 2005

I read the scoring idea in the other thread, I was kind of hoping someone else would have something to say first...

It sounds like a very reasonable idea. However, during development of such an idea the chances are high that you would run into insurmountable problems. That doesn't mean it shouldn't be looked into, but lots of things that seem like good ideas don't turn out to be effective under close scrutiny (e.g. the entire Space Shuttle system)

Questions I can think of:

• Is the insulation safe to distribute into the environment in that quantity.
  
• Do you compromise the insulative properties too much when you score it.
  
• Can you get it to fall off predictably at all. If so, can you get it to fall off at the right moment.
  
• Will the chunks gunk up the works, on the control surfaces of the orbiter, on the door mechanism, air pressure sensors maybe. (this is basically Galvatron's point)

Verifying the idea's effectiveness would be a daunting task. Others have pointed out the truly pertinent question, is it worth redesigning any one system on the Shuttle so thoroughly when the whole program is probably due for the scrap heap soon enough.

(This post sounds very negative about the Shuttle program in general, that isn't really intended.)
posted by Chuckles at 3:14 PM on August 1, 2005