Freon-insulated ET Tanks and Foam-less ET Tanks

[dreada5]

I know there's another thread on NASA's continuing troubles with ET foam. RE: this story,<br />http://www.cnn.com/2005/TECH/space/11/22/space.shuttle/index.html<br /><br />But I just had a couple questions on a different note,<br /><br />1) How difficult would it be in terms of time/cost to redesign the ET internally so that it still contains the supercold fuels but by the use of some other type of active refridgeration system - without the need for foam?<br /><br />2) Why doesn't NASA go back to using freon to thermally insulate the ET? Afterall NASA would only operate with freon for 5 years to complete ISS. <br /><br /><i>Note: Prior to 1997... although NASA was exempted from [new] legislation that called for a reduction in freon use, and the amount of freon used in the tank contributed minimally to total freon use, the composition of the foam was changed [to this new problematic composition] in response. (Quote from Wikepedia)</i>

 

[CalliArcale]

1) Very expensive. An active refrigeration system is going to be complex, require very large amounts of plumbing, and will add an enormous amount of mass to the ET. I don't think it's practical, unfortunately.<br /><br />2) They did! <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[dreada5]

Oh ok, didn't know they're back using freon. So why is it still freezing over and falling off in big chunks... what's changed?

 

[kdavis007]

But after 1997 they had to change to the that they are using..

 

[henryhallam]

It's falling off less than before, because of the move back to freon-containing foam and because of other work. But some always did fall off, and some will continue to fall off. The idea is to reduce the maximum amount that falls off, and the size of the largest chunks, to below safe minimums.

 

[spacester]

I can see the confusion here I think. No doubt due to bad reporting somewhere along the line.<br /><br />Freon is a refrigerant, but it is also an "ingredient" in the foam. <br /><br />The ET foam issue has nothing to do with active refrigeration. The liquid hydrogen goes into the tank and then it's just a matter of "topping off" the tank - any heat transfer into the ET causes the hydrogen to boil off. There is no Freon-based system used to keep it cold.<br /><br />The problem with the foam is that you’re dealing with a HUGE temperature difference compared to normal engineering problems *and* it’s so cold that any moisture present will turn to ice *and* you’re sitting on a launch pad in high humidity air *and* that ice causes cracks in the foam. The inner surface of the foam is super-cold and the outer surface is in warm humid southern Florida, so it’s a heck of an engineering problem. <br /> <div class="Discussion_UserSignature"> </div>

 

[dreada5]

I think you misunderstand me. I understand the physics going on with the current ET sitting on the launch pad.<br /><br />What I was saying is that according to the above quote I got from wikipedia, when they were using freon, LESS chunks of foam was falling off, when they switched the problem appears to have got worst...so I was saying switch back. But CalliArcale pointed out that they have switched back to freon... so my question is why is the foam still falling off in BIG chunks instead of the smaller pieces as before 1997.<br /><br />As to "active refridgeration", I was enquiring if NASA could feasibly redesign the internals of the ET to incorporate a conventional refridgeration system that kept the cryogenic fuels happy - negating the need for foam insulation. But it'd probably be too costly etc...

 

[spacester]

Ah yes, I did misunderstand, sorry about that.<br /><br />I of course don't know the answer, but it occurs to me that it may be a simple matter of looking harder at the foam during ascent than before STS-107. IOW perhaps it's more a matter of perception than reality: big chunks fell off before but no one noticed. Just speculation . . . <div class="Discussion_UserSignature"> </div>

 

[tomnackid]

I think it was everybody's favorite loudmouth drug addict Rush Limbaugh who first started promoting the "Environmentalists destroyed Columbia" theory. Actually NASA was exempt from the EPA restrictions on freon-based foam. They did switch over much of the foam to a freon-free substitute because that was the way the industry was heading and and NASA didn't want to end up paying for a factory to specially make freon-based foam just for the shuttle ET. The new foam actually performed better than the old formula and, ironically, the chunk that damaged Columbia was from a section of tank that used the old freon-based foam.

 

[fkawi1]

Why can't they maybe coat the inside of the tank with something like the foam or a rubbery coating? Like you would the inside of a motorcycle gas tank? The weight would be the same if the stuff was on the inside or the outside? Would the Hydrogen or Oxygen react to or dissolve or something to a substance besides metal in there? Now as far as cracking wouldn't there be pressure inside a full tank that would be pushing against the walls of the tank and maybe hold the stuff? I'm not a engineer I'm just asking a dumb question?

 

[dobbins]

No the freon theory originated in a 1997 study by Greg Katnik, a KSC engineer, who was studying the damage done to STS-87. It was mentioned in a Florida Today News story as early as two days after the loss of Challenger. Lush Windbag deserves blame for a lot of BS, but the freon story isn't something he dreamed up.<br /><br />http://www.floridatoday.com/columbia/columbiastory2A42724A.htm<br /><br />

 

[tomnackid]

I know Limbo didn't originate the idea. I doubt if he even knows LOX as anything other than something to put on a bagel! I just said he started promoting the idea on the air. I'm sure he was far more concerned with how the "environmentalists killed our astronauts" angle would play with his audience than he was about the science involved.<br /><br />As far as I know the investigation showed that it was a section of old formula, freon-based foam that broke loose and damaged Columbia's wing.

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>Why can't they maybe coat the inside of the tank with something like the foam or a rubbery coating? Like you would the inside of a motorcycle gas tank? The weight would be the same if the stuff was on the inside or the outside?<p><hr /></p></p></blockquote><br /><br />That's a good question. I am an engineer, but unfortunately my realm is software, not materials science, so I'm not much help. <img src="/images/icons/wink.gif" /> I can hazard a guess; maybe it consumes too much space inside the tank, or is too difficult to apply evenly, or still doesn't have the right thermal effect. It seems to be a very common practise in the industry to put foam on the exterior, so there must be a reason. The question is what? Cost? Less efficient insulation (resulting in increased weight because you need more)? Reduced internal diameter of the tank's hull?<br /><br />Hey, I just had an idea. Maybe it's because it would mean increasing the diameter of the hull to make room for the interior foam. This would mean the hull would require more metal to fabricate, and would be more massive. So it might have a surprisingly large weight penalty even if the thermal properties are identical. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[tomnackid]

I've read that one problem with putting foam on the inside was that even tiny pieces that break off could be sucked into the fuel lines--not a god thing for high pressure liquid fueled rocket engines with delicate turbines running at thousands of RPMs.<br /><br />If the shuttle were to keep operating (which it isn't anyway) I guess the best solution would be to take the mass penalty and build a double walled tank with foam sandwiched in between. If we had an extensive orbital infrastructure (which hasn't happened yet) we could recoup some of the cost by taking the tanks into orbit and using them for building materials (Maybe NASA could have even sold them to a private company). A double walled tank would not only be safer during launch, but would also be safer in orbit--the foam will not be able to shed and caused the dreaded "orange cloud" of orbital debris some have feared.

 

[fkawi1]

I thought of the double wall thing but knew that wouldn't be cost effective if they trash the tank every time. That would be the ultimate fix though. Now maybe they could have stretched the tank length wise versus wider if internal foam took up to much volume? But I don't think that a rubbery type substance would flake like foam does?Whether or not they could get the same thermal value out of a thinner bladder type of lining I don't know? Something like even a woven type fiberous material? Carbon fiber even? They could have coated it when assembling the segments much like they load the solid fuel into the segments before it is put together right?<br /> Or maybe develope a alloy of metal that has the thermal property built right in to it now that would be the cats you know what huh? <br /> But being a one use product then none of that would be worth all the time and money needed to pursue such a high end unit product? Can't they just say spray a coating of paint type product over the exiting foam that had some type of epoxy in it that would kinda make the skin a sealed surface? I know that has weight but they used to paint the tanks in the beginning right?

 

[drwayne]

Keep in mind that a bonding agent, like that that is involved in securing the foam to the tank is designed to do just that - secure foam to metal. That does not mean that it will function in the role that you propose.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[tomnackid]

The problem with any kind of spay on "skin" or embedded reinforcing fibers or wire mesh is that it could cause the foam to peel off in large chunks rather than break off in small pieces. If the foam is going to fail (which it always will to some extent) better that it come off in tiny flakes rather than big hunks. The orbiter has been hit by foam pieces on every flight. Most impacts are on the underbelly and have caused minor denting and scraping of the tiles. Columbia was unfortunate enough to have a fairly sizable piece hit a particularly fragile area. It might be easier to accept being hit by foam and reinforce the orbiter. The orbiter has been lightened considerably over the years do to lighter, stronger materials. Maybe it could be beefed up in the more vulnerable areas and still stay within its original specs. Its all moot anyway since there won't be any more development on the shuttle and any future versions will probably be considerably different.

 

[dobbins]

I have an idea.<br /><br />Most people have see security glass that has chicken wire embedded in it. The wire holds the glass together even if it's broken to prevent access through the opening by breaking the glass. Why not put a mesh on the ET before the foam is applied so that the mesh is contained inside the foam? Then the foam would be held in place by the mesh instead of just falling away. It wouldn't totally eliminate all problems a piece could still gall off above the mesh, and if a piece did try to come off and was being held in place by the mesh the aerodynamic loads would likely cause the foam to break away from the mesh in small pieces. What it should do is decrease the size of any foam shedding to smaller and less dangerous pieces.<br /><br />

 

[najab]

See the post above yours.

 

[dreada5]

Talking about reinforcing the shuttle. What about covering the underside of the shuttle with a shell that can be jettisoned on orbit. But the shell doesn't make contact with the ceramic heat-shield tiles...probably a separation gap of about 2 inches. The gap would allow the shell to dent absorbing ET debris impacts, but without touching the tiles. The shell would cover the entire underside of the shuttle and such would have the orbiter's delta-shape profile and could be made from a variety of lightweight "launch-resistant" materials.<br /><br />Ok, you can pull it apart now <img src="/images/icons/smile.gif" /> I know I've probably forgot something obvious...

 

[drwayne]

Weight and behavior in a high mach number environment leap to mind as issues.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[dreada5]

Well I could see a significant increase in weight, but surely the total thrust from the STS system is sufficient. It'd probably be equivalent to carrying a @500kg (a guess) cargo in the shuttle payload bay...<br /><br />Why would the aerodynamic behaviour change that much, if the shell has the same shape profile as that of current tiled surface and is very close fitting to the orbiter's tiled surface?

 

[tomnackid]

Here's an idea. Leave the ET bare and coat the orbiter with foam! Actually I'm only half joking with this. What if the most vulnerable parts of the orbiter were covered in a layer of protective foam?

 

[dreada5]

Are you serious? Well if we're talking aerodynamics, I think that idea would be a problem! <img src="/images/icons/smile.gif" /><br /><br />Better off with my idea for a disposable shell for underside of the orbiter.

 

[space_dreamer]

Well, if we're half joking...<br />Why not attach the orbiter so the payload bay faces the ET and the tail is under the ET. Then the underside is kept away from any falling foam!