Probably been asked and answered, but cripes this has been going on forever seems like, endangering shuttle launches. Is it just flat-out impossible to fix this? Or is there a good reason it keeps happening?
MOD, please repurpose to GQ, thanks!
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So repurposed.
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The External Tank (ET) is filled with liquid oxygen and liquid hydrogen, both of which are cyrogenic (i.e. only liquid at ?183°C and ?253°C, respectively). No amount of insulation is going to keep water from condensing on the outside of the tank and forming ice. During launch, the very aggressive acoustic and shock environments are naturally going to place a lot of stress on the vehicle, and the sprayed-on foam material with its low tensile strength is going to tend to come off in chunks.
Note that this occurs with all cryogeically-fueled launch vehicles, but with most vertical stack rockets it just isn’t much of an issue, even when they have strap-on boosters. (You can see this on gantry footage of Apollo/Saturn launches where ice and insulation falls away from the vehicle.) For instance, on the Delta II, steel-cased Castor 4s and GEM-40/-46 boosters can easily take the impact of a big chunk of ice without problem. However, because of the configuration of the STS, the Shuttle with its relatively delicate leading edges cannot withstand the impact of large chunks of ice and insulation without damage. Note that this problem was understood long before, and was actually a matter of concern on early Shuttle missions. However, when the first few missions showed no major impacts, NASA analysts took the rather blase attitude that past successes contribute to future failure, which was the same approach that led NASA engineers and managers conclude that o-ring erosion on the Solid Rocket Boosters that led to the failure of Challenger was a non-issue even though it was an out-of-design condition.
Would it be possible to design away the problem? To some extent, yes. Although ice will still form, it would, at least in theory, be possible to design an insulation system with some kind of structural overwrap that would prevent the insulation from breaking off. And some processes could be implemented to address the buildup of ice. However, this would probably require significant and costly redesign of the ET and support hardware. Given the nearly-retired status of the STS, there just isn’t justification for it, even given the risks. The change in design would carry its own set of unknown risks that would obviate the advantage of more resilient insulation.
Stranger
We’ve discussed this before and the idea of putting a mesh over the tank to which the ice would bind was mooted, but I’m wondering if, since the problem is the ice hitting the orbiter, they couldn’t put a launch-only protective shield between the tank and the orbiter?
It would be weight prohibitive to place a shield that would cover the leading edge areas of the Orbiter. There was an attempt to redesign a protective cable cover called the Protuberance Air Load (PAL) Ramp which was a major source of shedded ice and foam, but the value of the design was debatable and it was removed for later flights.
Stranger
Why not wrap the ET with a big sheet of Saran Wrap? The ice would build up on the Saran Wrap. Then, at T minus 10 seconds, you pull the whole sheet of Saran Wrap off and all the ice comes with it.
How long do you think it takes for ice to form again at -183º to -253º?
And at 10 seconds before those rockets fire, while I’m glad it’s “you pulling it off” and not “me” – I know I couldn’t run fast enough to get out of range in 10 seconds.
Unfortunately S.C. Johnson and Son no longer make Saran Wrap™ in the 160’ roll width.
Stranger
Basically, they are crossing their fingers and hoping that they don’t end up with another significant hole. If they do, they are counting on being able to diagnose the existence of the hole before the shuttle attempts a return through the atmosphere. After this mission to the ISS, all three remaining shuttle missions involve the ISS, so if a hole is found, they can simply leave the astronauts up at the ISS until a rescue mission can be sent up to collect them.
Shocking to me, since I wasn’t paying attention: the three missions next year, one by each of the shuttles, are the last in the program. I’m not sure I’m ready for that. 
I believe there are 7 future missions still planned, though your right they’ll all be to the ISS.
And there’s some talk of delaying the retirement as the new manned system is unlikely to be ready for several years and having to rely on the Russians to get to our brand-new multibillion dollar space station is a little embarassing. We’ll see.
Wikipedia says there are only 3 after the current one. Maybe Wiki is wrong…
Nope, maybe it’s listed differently somewhere else, but this wikipedia page lists the seven remaining shuttle missions (after the one that’s currently flying).
Delta hasn’t used the Castors in years. Also, I’m not sure if you meant it this way, but the GEMs aren’t steel-cased. In fact, GEM itself stands for Graphite-Epoxy Motor. But your point stands: the Shuttle wing leading edges and tiles are much more vulnerable to impact damage than typical rocket structures. Also, the Shuttle tank is enormous and has a lot of complicated flanges and stringers and stuff requiring manual applications of foam that are inherently hard to do in a consistent way. So it tends to generate a lot more debris.
This latest observation of debris was on the intertank, the corrugated section about two-thirds of the way up the ET. There is a long history of “popcorning” (small divots of foam that pop off due to air bubbles in the foam getting heated up and expanding due to the friction of the passing airstream) from the intertank. Just to be clear, the debris of concern from this flight was foam, not ice. If I were in charge of the investigation I’d probably first look for some kind of material or process change that led to an oriented line of air bubbles in the foam on the intertank stringers. They’re constantly having to change the foam insulation material due to environmental regulations and other reasons.
It’s my fault. I read this page as a list of all the missions. It actually lists only important milestones, including next year’s last missions.