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NASA Finds 4-5" Crack in Shuttle Insulation
PresidentKang writes "Spaceflight Now is reporting that a large crack has been found in an external tank foam of Space Shuttle Discovery on the launch pad. According to the article: "Engineers inspecting the shuttle Discovery's external tank following Sunday's launch scrub found a crack in the tank's foam insulation near a bracket holding a 17-inch oxygen feed line in place. Some engineers believe the crack must be repaired but senior managers say a variety of options are on the table, from fly as is to making repairs.""
The only reason why the foam is a problem is because the orbiter hangs from the side of the tank and can be hit by the foam when it falls off. In future generations of manned spacecraft, the crew vehicle will be on the top again, like in Apollo, Gemini and Mercury. In that case, it really doesn't matter what falls off.
The way I understood the replacement is that the module will be sitting on top of the tank instead of piggybacking as the current shuttle is. If it does, do we really care if insulation falls off during launch? It won't hit the part that comes back into the atmosphere anyways. I remember those old Apollo films where the chunks of ice were just dropping off in huge chunks.
Because of the volume of fuel that would have be transferred to the shuttle - compare filling a moped (the Atlas) and a Peterbuilt tractor unit (the Shuttle). It is not hard to fill a moped in a few minutes, filling the Peterbuilt takes quite a bit longer. Meanwhile, fuel is evaporating, leaking, and filling the launch area with a flammable mix of hydrogen and oxygen, AND water is condensing on the fuel tanks, freezing, and turning into nice hard chunks suitable for breaking things, like, say, fragile heat-resistant tiles.
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"I wouldn't be suprised if the external tank is insulated just because of how the shuttle is mounted on the assembly."
It's more complicated than that. It is needed to stop ice forming that would trash the shuttle, but it also reduces fuel boiloff, protects the tank from aerodynamic heating, and keeps the metal cold... the metal in the tank gets stronger as it cools down, and that means they've been able to cut back on the amount they use. Since the tank goes most of the way to orbit, saving a pound of mass in the tank gives you close to a pound of extra payload in the shuttle.
The insulation is only there to stop ice forming, there was no insulation on the Apollo series boosters, you can quite plainly see massive chunks of ice falling off on launch. This wasnt a problem because the manned capsule was ontop, well away from potential danger zones.
The space shuttle runs on cryogenic fuel and oxidizer (liquid hydrogen and liquid oxygen) in the main tank. If the tank were not insulated, water vapor in the air would condense forming a think layer of ice which would fall off during the vibrations of launch. If foam striking the orbiter caused a loss of vehicle on reentry, just think what ice could do. If I remember correctly liquid oxygen boils at between 70 and 80 Kelvins. Liquid Hydrogen is even colder, so cold that the nitrogen and oxygen in the air would condense on the hydrogen tank if it were not insulated.
It's not as easy as it seems to insulate the shuttle from inside. Remember, the insulation is really just a tweaked version of that "Great Stuff" foam in a can you can pick up at any home improvement store. The stuff is a great insulator but it's not that resilient to a beating and I'm not sure it could stand the force of thousands of gallons per minute of fuel swirling across it. Sure they put the baffles near the opening inside the tank to cut down on the swirling effect, but it still gets pretty violent in there.Perhaps an insulated layer between two metal skins would work but then you're adding weight which is already at a premium.
The foam was never seen as a major problem until a shuttle didn't come home. Also probably less foam came off that ET than a lot of missions prior to that. The first ETs were notorious for their foam shedding when the foam was hand brushed on. Going to an automated spraying technique solved a lot of the problem as it avoided air pockets present in uneven applications. Still, there were a few places on the ET which still had/have the foam manually applied due to the curvature of certain areas. Lockheed Martin's Michoud Operation has done a lot to refine the process, but as long as there are manually applied sections of foam, there will be air pockets which explode during sudden temperature changes experienced during liftoff which cause parts of the foam to "pop" off.
Because in Florida (I live here) it doesn't matter what the weather report says, there's always a 50% chance of rain. I gave up listening to the weather reports long ago.
The only time they are right is when they say 'It's raining right now' or 'It's sunny outside.' We don't even need dark clouds for rain, lightning, or both. Sunny showers are not that uncommon.
In short, 30% is just as good as 80% here.
Oh, and btw, if the weather report says 'in 12 hours, a hurricane will hit your town' you can safely sit at home and eat popcorn. It's not going to hit you.
"If you make people think they're thinking, they'll love you; But if you really make them think, they'll hate you." - DM
Here are the slides and video from the talk. It was one of the good ones.
GP makes sense, but is not right. Note: I'm not an authority on the Shuttle program by any means, but this is just basic science. What I'm saying below is even more true for the hydrogen, but I used the oxygen as an example. There are two basic reasons it would be quite cold.
1. To make Liquid Oxygen at room temperature does require extremely low temperatures. But they aren't keeping it THAT cold, because it would be prohibitively difficult.
It doesn't need to be that cold, because under pressure the temperature where it stays liquid goes up. You can keep a strong tank of O at room temperature and it will still be liquid. So they likely keep it cold to reduce the pressures they have to maintain somewhat, but it's not required.
2. Whenever you reduce the pressure on a gas (or especially when a liquid becomes a gas) this is a very endothermic (heat absorbing) process. So a home oxygen tank IN USE is cold, but one being stored isn't. (Random metal at room temp will FEEL cold, but the tank isn't _especially_ cold)
When you pour the liquid oxygen into a large temperature tank - even if you do it really fast and in a sealed way - any room left in the tank instantly becomes filled with much higher-pressure gaseous Oxygen. This expansion makes it very cold. And there is a lot of room in the tank when you START filling, even if by the end you filled it all.
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The problem is the engineers had supported the position that "even though the primary o-ring is burning - the secondary is holding, so were are OK to fly". (Despite the fact that the spec said "there shall be no blow by, period".) It wasn't until the eleventh hour that they changed their stance and became concerned about the secondary O-ring - without being able to (in managements eyes) justify and articulate that concern. The key insight to understanding the attitude of management is to remember the evidence as presented by the engineers prior to the Challenger's launch campaign *wasn't* as clear as it is presented ex post facto with 20-20 hindsight. (Edward Tufte and the Rogers Commission examine this failure of presentation, communication, and understanding at some length.)
That's the key to understanding (not condoning!) the whole decision process - first, the engineers failed to clearly communicate the issue (contrary to urban legend version that has arisen over the last twenty plus years); and second, that management had become conditioned to thinking of the Shuttle as an operational vehicle vice an experimental one. This lead the managers to believe that since they had flown with this problem, and that since the problem was understood by the engineers (which it wasn't[1]), that it was an acceptable risk to continue to fly.
[1] The cause of the failure isn't clearly articulated even today. The cause of the failure was joint rotation - there was blowby even at temperatures that were within the nominal spec, not faulty O-rings.
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Sorry to piggyback on a joke. This is the actual status, copied from NASA's shuttle page
It sounds like "crack" isn't a very descriptive term. A small piece broke off, probably due to the weather-related causes. They're trying to decide if they need to erect a platform to do a hands-on inspection.
On the source page there's a picture that took me a minute to figure out, so I'll explain it. The solid orange to the left is the main body of the external tank. The round orange vertical item in the center of the picture is the LOX feed line. The big metal piece is probably part of the tripod mount. Just below and to the right of the mount is one of the brackets that holds the LOX line in place, covered in foam. Looking closely, you can see a little triangle of white where the little chunk (about 4 long) of foam spalled off. I assume the T-shaped feature to the right of that is the so-called ice/frost ramp, which they're concerned may be at risk for falling off in flight. It's much clearer in the high res picture linked on that page.
At first glance, it doesn't look like a concern at all, but obviously NASA wants to make sure of that. If anybody has serious reservations, they'll setup a platform to get somebody who knows what to look for up close to it and look for evidence that this may increase chances that the ice/frost ramp or other pieces of foam may fall (smaller cracks, loosening of the frost ramp, etc).
For comparison, this piece of foam is estimated at 0.0057 pounds and may be as large as a cellphone. The piece that struck Columbia's wing weighed an estimated 1.67 pounds and was described as "briefcase-sized"
(Also, the joke is 2nd Tuesday of next week. Your rendition makes about as much sense as a screen door on a battleship)Soyuz has an ablative heat shield. It's probably made of some sort of ceramic, formed into 8 layers called "blankets" (I think they're only flexible enough to allow for expansion and contraction of the spacecraft, not like a quilt). The capsule is single use, so the heat shield only needs to survive one re-entry, and the exposed surface area is small, reducing overall weight. The ablation (burning away of small particles) carries heat away from the capsule. A similar system has been used on pretty much every entry system except for the shuttles, even the Mars rovers. The Russians did use a system nearly identical to the space shuttles for their Buran shuttle, which was abandoned after one flight due to budget problems.
The weight and maintenance of an ablative heat shield were prohibitive in a very large reusable system like the shuttle. Instead, the leading wing edges and nose of the shuttle are protected by reinforced carbon-carbon tiles, the underside by ceramic tiles that are in some ways similar to aerogel, and other sensitive areas by a heat resistant cloth. The carbon-carbon is the most effective and actually pretty strong, but about 6 times more dense than the other options. Read more.
In short, no the Russians haven't had fatal problems with their thermal protection, but it's not feasible for a vehicle like the shuttle.