I am puzzled as to how the shuttle was damaged after being struck by a piece of foam. No matter how fast it is travelling it seems bizarre that foam could do any damage to something that is solid.
The only explanation I can think of is that the foam is nothing like the stuff used to provide padding in cardboard boxes. Does anyone know?
Well, sorry, but when foam is moving really, really, really fast, it has a lot, a lot, a lot of kinetic energy. It is an insulating foam that sort of depends upon being light-weight, although maybe somebody else can come along and give a more exact demonstration of what type of foam was used.
The math.
Impact velocity was estimated at between 500 and 800 feet per second, which when you square is a lot of KE per unit mass.
It wasn’t necessarily just foam, either. Atmospheric condensation, and perhaps rain and salt spray, may have soaked into it and froze into ice.
Finally, the wing leading edge piece was made from a fairly brittle material, chosen for its heat resistance more than for the impact resistance it wasn’t thought to need. The part did not carry structural load and was fairly thin, as well. Even so, all it had to do was fracture enough to provide a path for extremely hot air to get by it and into the aluminum and titanium wing structure, acting as a blowtorch on low-melt alloy.
Kinetic energy is mv[sup]2[/sup] / 2 where m = mass and v = velocity.
The shuttle would have been traveling at roughly 840km/h at the time it was damaged. A small piece of foam used in one of the tests weighed about half a kilogram. That gives a kinetic energy of (0.5 * 844[sup]2[/sup]) / 2 = 178,084 joules. That’s about the same kinetic energy as a car travling 60mph. But a half-kilo piece of foam is much smaller than a car, concentrating the same impact energy on a much smaller area.
So imagine that the shuttle wing got crashed into by a drunken imbecile in a Ford Taurus speeding down the highway, but concentrated in a much smaller spot.
That should say, “the shuttle would have been traveling at roughly 840km/h at the time it was damaged relative to the falling block of foam.”
I understand the kinetic energy component of this event. But it would seem to me (incorrectly, I guess) that the energy would have been wholly dispersed by disintegration of the foam. That’s what seems to be intuitively wrong here.
But obviously they reproduced it in a conservative experiment on the ground, so…
All the foam had to do was knock of some of the heat shield tiles in a crucial place.
NASA fell back into the same mistake as was made with Challenger. Previous to that disaster the o-ring in the motor joints had burned part way through on most flights. However it had never burned all the way through and so NASA managers assumed a safety margin that didn’t exist. According to Richard Feynman in his book What Do You Care What Other People Think, NASA management assumed a safety factor of 3 because the most an o-ring had burned through was 1/3 of the wayl. However, the design was such that there should have been no o-ring burning and so there was no safety factor at all.
All the while that was going on, pieces of foam insulation were falling off on flights and in some cases causing a little damage but never in a critical place. NASA management seems to have assumed that would always be the case because there was little effort to analyze and fix the problem. If a design isn’t working in the intended fashion and you refuse to see that as a problem it is difficult to fix it.
Kind of off at a tangent, but just to illustrate that kinetic energy is kinetic energy and if there’s a lot of it, it doesn’t really matter whether it’s delivered by a hammer or a banana… Remember that case where one of the shuttle windows was cracked by an impact while in orbit? The cause was a fleck of paint about 0.2mm in diameter which was travelling at a few Km per second relative to the shuttle. More details here.
I suggest a simple practical experiment, in which kpinocchio and NattoGuy remove their pants, bend over (no, stay with me here) and have a block of foam fired at their buttocks at 840 km/h. Then we’ll see who’s plastically deformed.
No further explanations will be required, I predict.
Good one.
Only days ago I saw a documentary about this very accident. They made reference to the monitoring of 10,000 objects in earth orbit including “paint particles” and I had thought “paint particles?” Surprisingly in the whole one hour program they never mentioned the significance of the vehicles velocity in relation to the damage caused. The previous night the same program had been about the crash of the Concorde and had stressed that the problem had been caused because the hunk of tyre that hit the wing was several times larger than anything they had simulated. It created a wave in the fuel tank that they had not forseen.
A minor point is that the tiles on the Shuttle leading edges are rather “foam-like” themselves - you could easily poke your finger into one.
[QUOTE=Colophon]
I suggest a simple practical experiment, in which kpinocchio and NattoGuy remove their pants, bend over (no, stay with me here) and have a block of foam fired at their buttocks at 840 km/h. Then we’ll see who’s plastically deformed.
QUOTE]
There’s no need to get abusive. I wasn’t having a go at you or anyone else.
I was under the impression that the space shuttle was covered high-spec heat-resistant tiles. Apparently the buttock-covered version of the shuttle didn’t make it much past the prototype phase.
Whereas the covered-buttock approach to Shuttle engineering management seems to have caught on.