Source: http://news.yahoo.com/s/afp/20050818...ceshuttledelay

"From an overall standpoint we think really March 4 is the timeframe we are looking at," said NASA associate administrator for space operations Bill Gerstenmaier.

While the shuttle was still in orbit, I worte NASA, via website. Upon hearing no response, about three days after the shuttle landed, I wrote Mr. Gerstenmaier, via NASA, asking my letter be forwarded to him.

In it, I outlined two methods by which the foam on the orbiter's external tank could be stabilized enough to reduce or eliminate the problem of foam separation.

First, I proposed they use a hexagonal interlocking weave mesh which would be slipped over the external tank like a giant sock. It need not weight much, perhaps 50 lbs. Made out of a flexible carbon fiber string which is in turn made of hundreds of much thinner carbon fibers, this string would have an immense strength to weight ratio, five times stronger than steel. Because of the interlocking hexagonal weave, a break in any portion of the netting means that the rest of the netting would remain intact, resisting any tearing. An added benefit is that any fittings between the orbiter and the external tank can be made to fit through an ordinary pair of sheers. Because it's made of carbon fiber, any frictional heating due to its rapid assent would not affect it. Because it is a mesh, it's mostly open, but would provide enough pressure on the foam, not only to keep the foam in place, but also to keep the mesh netting in place. Finally, due to the fact that the boundary layer effect of the main tank is quite large, the any effects of the airstream flowing past the netting would be minimal.

The idea of using netting to protect foam from external stresses is nothing new, and we see it all the time in various applications raging from foam-insulated underwater lines to nylon-wrapped foam protection on children's playground equipment. It works very well.

In retrospect, I believe a more appropriate weave would be an interlocking, elongated diamond pattern, with the elongations parallelling the slipstream. However, to provide the necessary circumferantial compression, a horozontal cross-thread would be required across the middle of each diamond.

Second, I propose they imbed thin, long, multi-strand fibers in the foam as it's applied. The long fibers act as an anchoring system, much like embedding fiberglass in epoxy for superior strength, a technique long used in building boats and sailboats, surfboards, and composite aircraft, including Rutan's Starship 1. Because the foam is flexible, however, the fibers must be reasonably flexible as well, and the best fiber for the job is plain old very thin, braided nylon string, cut in lengths sufficient to provide the requisite holding power while short enough to allow many of them to be added to the foam, overlapping it in place. Monofiliment would not work, as it's too slippery. But a braided nylon might have sufficient surface area if the foam can penetrate deep enough into the fiber. Alternatively, pre-penetrating the foam via some form of foam "wash" might work as well.

A third technique I thought up in the meantime would be to use aerogel, covered with a high tear-resistance mono-coating polymer film. In it's least dense form, aerogel is the lightest material known on the planet, and excellent insulating qualities, but it's very fragile. Fortunately, it can also be made in densities sufficient to provide the requisite strength while maintaining very good insulative qualities, and the aerogel would not itself take much of the stress of the slipstream - that would be born by the mono-coating polymer film, which itself could be embedded with fibers to increase it's own strength.

A fourth technique I thought up involves a self-dissolving foam, one that, once it's no longer needed, such as during the ascent, would simply slew away from the orbiter in liquid form. There are several materials that can be foamed, but under stress revert nearly instantaneously to liquid form. There are many problems with this, however, such as how the super-cooled inner layer would responsd to the stress of flight through the air, how the liquid foam might affect the tiles if it ever touched them, and what effects strong wind would have on the foam.

It would be a sad day if 40-kt winds caused half the tanks foams to drip away!

Can anyone think of any other ways to improve the retainability of the external tank's foam?

Those are all nice ideas. I had also thought of some sort of mesh bag (I think I might have mentioned it around here?) but the glory rights go to first published, so more power to you. Let us know if you hear anything from NASA.

My only other thoughts are:
1) Early on the tanks were painted white, just to make them pretty I think. It was eliminated to save weight (yes, that much paint is a significant weight). But maybe a paint or polymer coating could help hold the foam together

2) I thought I had heard that early on the foaming gas was CFCs (chloro-fluorocarbons). Because of restrictions on those materials (ozone hole) they switched to something else, but had to also make modifications to the foam composition to do that. Maybe there is still some chemistry that could be played with.

(edited to add a thought and fix a typo)

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Great ideas indeed! Congrats.

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The Saturn V stages had the insulation on the inside of the tanks, but it was a difficult and expensive job to install it. That might be mitigated by spray application.

That still leaves various external plumbing that would need some other insulation technique.

You're correct - the foam composition was changed to protect the environment.

The main reason we didn't use internal insulation was two-fold. First, we had stronger foam - it could withstand the rigors of flight (well, at least the original formula). Second, in order for the tank volume to remain the same, an internal foam application would require both foam and an external skin and supporting structure, which weighs more than just the foam.

And, yes, they scrapped the white paint to save weight. It's unlikely that much additional strength would be achieved by putting a coat of paint over the foam, as it gives too much (cracking). A thin polymer sheeting, something akin to monokote, might work, but the weight builds up quick. The mesh netting with holes about 1" in diameter would fair much better in keeping the foam in place, I believe.

Here's a foolish question: can the foam simply be replaced with another insulating material?

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Sure, provided it's highly efficient (insulation per weight), relatively inexpensive, an outsanding vapor barrier (cannot be breathable), and able to withstand the rigors of flight.

Did you have anything in particular in mind?

Let's examine some possible insulators:

1. Asbestos: It has too low of an insulation to weight ratio, and it's definately not EPA-proof. Besides, it being a poor vapor barrier, it would allow the moisture to accumlate against the side of the tank.

2. Fiberglass: less weight, but still low insulation to weight ratio, and it would create an unbelievable mess immediately after liftoff. I'm sure the EPA wouldn't like all that fiberglass in the water, either.

3. Wool: Wool is actually quite durable, and would probably withstand the rigors of flight if interwoven with another fiber, like carbon or kevlar, but it's a poor insulator, and relatively heavy for it's insulative value.

4. Down: Excellent insulative value to weight ratio, but it quickly looses it's insulative value as ice accumlates in the fibers because it's too breathable.

5. Quallofil insulation delivers a balance of compressibility, warmth and durability--plus it continues to insulate when wet and dries quickly. Unfortunately, it, too, is too breathable, and the moisture that penetrates far enough to the tank would freeze into very heavy ice.

6. Styrofoam: Ever seen a cooler that actually lasts long enough to get it from the store to the car before a chunk breaks off? Me neither.

7. Blown shredded newspaper: While an innovative idea for attics, the major hurdle hear is keeping it appropriately distributed and close to the tank while providing protection against the slipstream. But not a great insulator for the purpose.

8. My dog, Skip: Skip's a furry beast, to be sure, and more than capable of standing up to 80 below temps, as are most Alaskan Klee Kais, but he's a miniature, so you'd need a lot of him to do the job. I think he's actually enjoy the ride, tongue a swinging in the breeze, at least until two or three hundred knots, then he'd probably pull his tongue back in and shut his eyes. But I love my dog, so I'll just keep him here with me for a spell.

I agree that those are great ideas. But, with the culture of safety and testing, how long do you think it would take for any of those to be implemented (not to mention beurocracy). Besides, I believe there would be some considerable development and engineering time for much of it (not that it would be hard, but that it would take time to engineer the "right" materials)
I dream of the day when NASA can lead in simple innovations.

I've thought of a netting or scrim also, so I bet it a common proposes solution. But the outside of launch vehicles is atricky business. Remember Skylab? The wanted to protect the Orbital Workshop (which was a refurbished Saturn 4B stage) from micrometeorites so they put a metal shield that laid flat against the outside of the vehicle and had deployment mechanism so that once it achieved orbit it would stand off 4 feet from the main structure. During launch trapped air under the micrometeorite shield caused the entire thing to peel off, taking one of the solar arrays with it and pinning down the other array. This shield also had the thermal paint on it , so when it went the workshop overheated.

The point is the aerodynamics and vibration of the launch is very tricky stuff and there must be care to make sure a netting doesn't disturb the flow or begin to rip and peel off even larger sections of the SOFI. The External Tank is huge and I'm sure that a netting would weigh more than 50 lbs. if it is to be strong enough while not disturbing the thermal protection function. Still, I'll bet it's being investigated.

Unfortunately, this is one of those areas where NASA really falls behind industry standards. The science of netting strength is very well known, as is the science behind carbon-fiber string. Knowing what angles are involved in the interlocking weave is about the only unknown. Solve and apply over the curved surface of the tank given the slipstream conditions and evaluate. If you're well within the ballpark with negligible aerodynamic effects or additional weight, then press on!

[edit] And also if it does the job of keeping the foam intact, of course...

Or, failing that, at least contains the chunks so they don't impact anything they shouldn't.

Does anyone (besides me) have this odd image of launching a shuttle that's wearing a hair net? Sort of like the space equivalent of a school cafeteria line? #-o

Tiny grooves could be etched into the skin for the strands to fit into if drag is a big factor.

Alternatively, you could use some sort of very light (and porous) film instead of a net, to reduce drag.

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That's an excellent idea, provided you glue the netting in place to prevent those grooves from catching a bit of the slipstream and becoming an entry point...

Alternatively, you could elevate the net away from the skin by the use of spacers, much like elevating rebar for a concrete slab. Then, simply spray on the foam until it barely covers the netting.