Slashdot Mirror
NASA's Foam Test Offers Lesson in Kinetic Energy
Puneet submitted a followup story on the foam test that NASA conducted to get an idea of what sort of damage could be caused by foam falling off the shuttle fuel tank at launch. As it turns out: a lot.
I'm surprised that the impact was ever taken so lightly. Paint chips drill holes into satellites and birds take down planes, any impact, given the forces involved with such vehicles has the potential to be catastrophic.
Help me become a Porn Star Guru
...that they've only just performed this experiment. They claimed earlier that foam falling off the fuel tank not extraordinary, and hadn't been a problem in the past. You'd think with the risks involved it might be worth checking out - just in case. The whole point of engineering is that we don't rely on intuition.
If I seem short sighted, it is because I stand on the shoulders of midgets
...than Columbia's as well.
From the article:
The next round of tests in Texas could add weight to the growing consensus about the cause of the accident. Last week's tests used wing panels from the Enterprise, a test vehicle that never flew in space. That craft's leading edge panels were made from fiberglass because the Enterprise never had to face the heat of re-entry.
Foam testing will resume on Thursday with the first effort to fire a chunk of foam at the actual material used on the leading edge of the shuttle's wing. The material, reinforced carbon-carbon taken from the shuttle Discovery, is substantially weaker and less flexible than fiberglass.
A lesson in kinetics indeed. Perhaps it was a micro-meteorite or junk, but based on this data I'd say they've solved it.
The most frightening part of this whole story is that the people expressing shock (SHOCK!) at the amount of damage a piece of foam can do at 500+ MPH are actual Rocket Scientists. Is a basic grasp of physics not required for an advanced degree in Aerospace Engineering?
The second most frightening part of the story is that these tests were performed on a mock-up wing taken from the Enterprise (which has never flown) and is made out of fiberglass, a stronger (but much more heat labile) material than the carbon-carbon stuff the leading edge of the actual wing was made from. I wonder how nasty the results will be once they use the real material that failed.
BFL
There's one thing computing teaches you, and that's that there's no point to remembering everything.
--Doug Copland
I don't have the numbers right here, and I'm too hungover to crunch them out, but I remember a few years back being told by a professor that a penny can't kill someone. It's too light, and the air resistance creates a terminal velocity that prevents it from becoming all that dangerous.
And the empire state building is wedge shaped, with ledges ever couple of stories. There's no way for a penny to even make it to the ground.
Also, it's not the fact that the foam was going 500 mph hour, it's the fact that the shuttle was.
How much ice exactly? There's no way of knowing. They do know how much foam fell off. If they test using just foam, they know the minimum amount of damage done for sure. If they add a guestimated amount of ice, they haven't proven anything.
At the time the insulation fell off, the space shuttle was travelling a couple thousand miles per hour already. That could (in theory) add to the impact force on the wing.
It's irrelevant how fast the shuttle was travelling. Only the speed of the foam relative to the wing matters (i.e. when bloan by a thousand mph wind). Presumably they measured this from the video they had.
If I seem short sighted, it is because I stand on the shoulders of midgets
From day one, they have danced around the subject of ice. They just won't talk about it.
The Shuttle's main tank is a huge cryogenic storage cylinder. It is cold, very cold. So cold that they have to insulate it. So cold that atmospheric air will form a sheet of ice on its outsides. So cold that ice formation is monitored before launch. Why won't they talk about this?
The leading portion of an aircraft body and wing is where ice will accumulate in flight. It can collect in amounts large enough to make the aircraft unaerodynamic. Amounts large enough to fall off in chunks. Why won't they talk about this?
The material seen impacting the Shuttle wing has been described as "grayish-white". Ice just happens to be this same color. What color was the insulation? Was it grayish-white too? I doubt it! If the insulation were the same color, how could they visually check against ice formation before launch?
The impression that I am getting (from this article as well as others) is that intuitively the engineers didn't think the foam collision could cause any damage. I haven't seen anything written indicating that there was any past history with pieces of foam striking the leading edge of the orbiter's wings. I have seen articles indicating that foam has struck the underside tiles and damaged one of the landing gear doors and while the tiles were damaged, none in such a way that the shuttle was ever in danger.
I would think that the line of thinking was, when the foam separated, it was moving at the same speed as the shuttle itself. Since the shuttle, at time of impact was at 50,000+ feet, the force of air drag on the foam would be negligible and the piece of foam would approximately maintain its speed.
I seem to remember that it is about 30 feet from the bipod to where the foam struck the orbiter's leading edge, so assuming that the foam travels at approximately the same velocity as when it came off and the shuttle was accellerating at 2.5 Gs, it would take about 1.4 seconds for the foam to hit the leading edge. Using these assumptions, the velocity of the foam at impact, relative to the leading edge, would be 110 ft/sec or roughly 75 mph.
This doesn't sound too bad - after all, it's foam. Getting hit by a Nerf football that has been thrown hard by somebody close by stings, but it won't break bones or even come close to breaking the skin. If you don't think it could do more than bruise you, then it would be hard to accept that the carbon-carbon leading edge of the orbiter could be damaged.
I think that this was the level of intuitive analysis that was done. Unfortunately, it wasn't backed up by any kind of quantitative analysis using known facts (such as estimating the speed of the impact from the film and checking it against the intuitive speed of impact) to test whether or not there were grounds for concern.
myke
Mimetics Inc. Twitter
You say "I would think". Well, if I was a responsible engineer at the time and place, I would have thought of a lot more than either you or they seem to have.
1) Aerodynamic drag at 50,000 feet is hardly "negligible". Drag is proportional to local atmospheric density times the square of the velocity. Atmospheric density at 50,000 feet is 15% that at sea level. Therefore the drag at that altitude is equal to the drag at sea level at 39% of the speed. In other words (pick a number) 500 mph at 50,000 feet causes the same drag as 195 mph at sea level.
2) Therefore, not only was the space shuttle ACcelerating, but the foam was DEcelerating - probably a LOT - but the point is, it needs to be taken into consideration.
3) The foam coating the fuel tank is HARDLY the same as that a nerf ball is made of. It is much more substantial.
4) As I understand it, the piece of foam that broke off was very likely coated with ice. I think if you got hit by a piece of ice travelling 75 mph (much less at an even higher speed), you would most certainly be injured, and so would the leading edge.
5) Prior strikes were grazing blows on the surface of the wing. We are postulating a direct hit on the leading edge of the wing, made up of very brittle carbon fiber composite.
All that said, in the end I don't blame those on the scene as much as those responsible for the crappy concept as a whole. Hopefully I would have thought of the case of a direct strike on the leading edge, and hopefully I would have woken up to danger (albeit maybe too late) when a piece of the shuttle was OBSERVED to part company while in orbit, but my true ire is reserved for whoever is responsible for the design concept as a whole. If the fuel tank was coated with crappy insulation that frequently broke off in chunks during launch, that in itself doesn't constitute a hazard. But as soon as you mount a manned space vehicle directly in the path of the debris, that is just unforgiveably negligible.