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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.
Everything flying around in space is potrentially dangerous. A fleck of paint hit one of the space shuttle's windows once and caused a surprising about of damage. Based on momentum, it was the equivalent of a bowling ball hitting the shuttle at 60 MPH. Yeah, that's definately dangerous.
Did they print the wrong picture? The article implies a great deal of damage but all I can see in the picture is the foam object getting destroyed. The wing itself looks completely fine.
I've been wondering this from the beginning of the foam investigations and tests...
They've talked about firing foam samples at wing mockups at hundreds/thousands of miles an hour, 'cause (I think) the Shuttle was flying at that speed when it was hit. But wasn't the foam also flying at that speed? Shouldn't the actual velocity of the foam hitting the wing edge be fairly minimal?
Or are they assuming that the wind drag on the foam chunk would reduce its absoute speed significantly, thus increasing the relative speed with which it hit the wing?
In other words, did the foam fall off and drop, low speed, into the wing, or did the foam flake off and stop dead in the air, then the shuttle ran into it at a huge velocity?
I think when people talk about the foam insulation hitting the leading edge of the left wing of Columbia during the launch phase, they have to consider the following:
1) When the insulation piece fell off it was essentially encased in solid ice, which is a pretty hard material to start with.
2) 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.
NASA should have tested the insulation foam encased in ice fired at physical simulation of the shuttle leading edge, in my opinion.
It's frightening that such a light-weight piece of foam can doom a fantastically complex and brilliant piece of machinery like the shuttle, not to mention the crew on board who are far more complex and brilliant - and the loss of whom is so much more painful. But it's not really a surprise. I mean, if a penny can kill - and it certainly can - then so can a big block of foam, even if it doesn't weigh much.
Unfortunately, dangers such as these are just a part of space flight. It's never going to go away: as someone else posted earlier, birds can bring down planes and that's a mature technology. If space flight ever becomes routine, it will still be filled with dangers - the question is whether or not people are willing to take the risk. From a scientific perspective, we're very, very lucky that so many astronauts are willing to take it to advance our understanding of the world and the universe.
Still, it's really hard to see that shuttle crew lost to a piece of foam. Or a piece of rubber (Challenger). It strikes me as odd that on something as monstrously complicated as the shuttle, the only two complete failures were due to relatively simple components. It also strikes me as a major accomplishment. Anyway...
If a chunk of foam can cause this much damage, what happens if a bird gets in the way?
I know they test-fire birds at the fuselage, but if a bird hits the wing (or rather, if the wing hits a bird) it could cause problems.
They can find ways to ensure that foam doesn't come loose like this in the future, but I don't think they can eliminate the possibility of overflying birds.
Tarsnap: Online backups for the truly paranoid
"He invoked the physics equation that describes the amount of kinetic energy in a moving object, saying, "That's when it came home to me what 1/2mv2 means." The simple equation says that kinetic energy is one-half times an object's mass times the object's velocity squared, so that even something very light can carry a great deal of force if it is moving fast enough. In fact, he said, the force was equivalent to catching a basketball thrown at 500 miles per hour."
If it's such a simple formula and the facts where known after the shuttle launch, how can the responsible people rely on intuition rather than getting out a 1$ pocket calculator and determine the force of impact? Something's pretty fishy here..
Well, there are a couple things at play here:
1) Materials are stronger at higher strain rates; essentially, the foam can probably remain elastic to much higher stresses when it is being deformed quickly, in a case such as this. To know more, you would want to do a series of high-strain rate tests on the foam to measure it's basic properties. In hindsight, choosing a foam with poor high-strain-rate performance should have been a requirement.
2) The piece of foam they fired was so big that it probably acted as a constriant; essentially, a piece of foam being confined laaterally will have greater apparent strength than one that is not. When a very small piece of foam is fired, this effect would not be present. Scale is important, beyond just increased mass causing increased damage.
It seems so obvious now, but I hadn't thought of these things before. Ideally, NASA would've conducted tests long ago with many sizes of foam hitting many parts of the shuttle, instead of abandoning the tests after seemingly benign results, in addition to basic experiments-- tests of the confined and unconfined foam.
I think the problem was that the foam had hit the shuttle before and it hadn't caused any damage.
Because of this, I think the engineers were lulled into thinking "Well, it happened before and it didn't cause any damage, why whould anything change now"
Tragically, their apathy about the whole situation cost the lives of 7 really smart and talented people.
In the future when we build the next generation shuttle, they integrate some better sensors that would detect that kind of damage.
Yes Francis, the world has gone crazy.
can jam pieces of straw into telephone poles and brick one would say "ummm ... Duh".
And even then an F6 on the Fujita scale which is completely inconceivable (if the F5 is the finger of God, this is the "2-stroke 250cc dirt bike of God") would have wind speeds of 319+ MPH
-- Knowing too much can get you killed, but knowing who knows too much can make you rich.
Tufte also examined the Challenger evidence in his excellent book Visual Explanations.
AlpineR
Something keeps bothering me about this foam story -
a piece falls off the tank and slams into the wing causing damage. OK, I can see that.
But what is that hundreds of miles per hour speed we are talking about here?
At the moment, the tank and the wing are BOTH traveling at 500 miles per hour so the RELATIVE speed of a piece (and its kinetic energy) should not be more than few FEET per second!
Anybody who has seen a piece of straw pushed through a tree or other tornado damage wouldn't be a bit surprised by the damage a bit of foam travelling at 500mph would do. I'm surprised the engineers could have missed this. They should know this kind of thing cold.
So a 1.7lb chunk of foam going 500 mph would do SERIOUS damage. Come on! I mean, what kind of physicists are they hiring that can't wrap their brains around this?
500mph = 804,672 m/h = 224 m/s
1.7lb = 0.77kg
from 1/2mv^2, we get...
0.5*0.77kg*(224 m/s)^2 = 19,000 joules of energy!
From a website on the power of explosives...
TNT releases 2.72x10^6 J/kg
So...
g of TNT = (19,000 J/ (2.72x10^6 J/kg) )*1000g/kg = 7g ~ 0.25 Oz
The size of a large blasting cap.
Now, if you asked Nasa if setting off a blasting cap on the shuttle wing would be good or bad, well, I'd think they'd give you an incredulous look and call the FBI on you for being a terrorist and asking suspicious questions.
This back of the envelop calculation MAY be off somewhat. But any engineer who sat down and said "Does this make sense" could have done it on an envelop as a sanity check.
Now, knowing that foam hitting the wing is like setting off a blasting cap on it, perhaps people will realize the dangers of light things traveling very fast...
Hmmm, I wonder how much energy a feather traveling at 0.5C would release...
What bothers me is the 500 miles per hour number. It's irrelevant how fast the foam was moving relative to the ground, only how fast it was going relative to the shuttle wing. And since this liftoff was very non-realativistic, we can use classical kinematics:
This foam was attached to the tank at lift-off, right? That means it was going the same speed as the shuttle at that instant it broke off. THAT means that RELATIVE TO THE SHUTTLE, it accelerated from zero to 500 mph (AGAIN, ELATIVE TO THE SHUTTLE) in the space of 200 ft or so. Well, using the kinematics equation:
Vf^2 - Vi^2 = 2ad
with:
d = (worst case for most acceleration) = shuttle length = 200 ft,
Vi=0
Vf = 500 mph = 733 ft/sec
gives
acceleration = a = (Vf^2 - Vi^2)/2d = 733^2/(.0379 * 2)= 7088245 ft/sec^2 = 220000 times the acceleration due to gravity!
Check my numbers, please, but that seems a little high to be caused by braking due to air resistance.
"Lawyers are for sucks."
- Doug McKenzie
You're right about that. At university I wrote a technical report about the failure of Arianne 5 - a rocket with complicated control systems. On the maiden flight the entire rocket was destroyed by a chain of events that started with an overflow error (well, really it started with sloppy application of a formal specification). Give me a russian ICBM any day of the week.
The tank is coated in a hard foam similar to the polyurethane foam used in insulation.
Do a little experiment yourself here (warning: not for little children : ) Go to the hardware store and find a can of "Great Stuff" foam insulation spray. It's used to fill the holes in walls around pipes.
Now, lay out a plastic trash bag, and empty the entire can onto the bag -- (warning: the stuff expands as it hardens; so, start in the middle of the bag).
Once it hardens, take a look at the result. This is similar stuff, not quite as nice as what they use on the shuttle of course... Also, keep in mind that an entire can of "Good Stuff" is only 12oz. (3/4 lb). You'd need over two cans of the stuff to make a piece the size they're talking about.
Think about that hitting you doing 500 mph...
I just did some rough math and guesswork. It seems the wing took about as much force as if I had driven my Ford Escort into it at 15MPH. That's quite a bit of force!
I wonder how many Volkswagen Beetles that is?
"Sometimes, I think Trent just needs a cup of hot chocolate and a blankie." -Tori Amos on Nine Inch Nails
First, RTFA...
/. before about the idea of ALL factors involved.
"In fact, he said, the force was equivalent to catching a basketball thrown at 500 miles per hour."
THEN
although the experiment "moves us a lot closer to saying that foam can do this kind of damage," it did not rule out other possible causes of the hole in the wing, including small meteorites and debris in space.
What is it about this being a peice of foam that they still can't cop to this being most likely.
If I saw you throw a basketball at my car at 500mph, I would likely stop looking for the "real" cause of the dent!
Even after the experiment and the basic lesson in physics, they still won't say "Yeah, we are keeping our minds open to any new evidence, but right now it appears that this foam strike was the a major factor in the accident."
THERE IS NOTHING WRONG WITH SAYING "Yes, we think this is what caused the damage" If it was instead the basketball that indeed hit the shuttle, the debate about what caused the damage would be wrapping up.
I say major factor, becuase I personally have theorized on
Below is the text of my former post on this idea-
'Under the conditions of a normal return to earth, the shuttle flies on autopilot until it is traveling more slowly than the speed of sound. But pilots train to take the shuttle all the way down in case the autopilot malfunctions, and so it is possible one of the pilots was trying to take control of the yawing craft in its final moments. 'It is relatively easy for the autopilot to be turned off by accident, which in fact happened just minutes before the problems with the Columbia started to become apparent. In the recovered segment of flight deck video of the waning minutes of the flight released by NASA, Colonel Husband is heard to exclaim, "Oh, shoot," and to tell mission control that "we bumped the stick earlier," briefly disengaging the autopilot. He quickly and calmly corrected the error'
What this all leads me to is this, and I have not seen this suggested in anything I have read as an important concern: Is it possible that this accidental disengaging of the autopilot CONTRIBUTED to the loss of the Shuttle? Although the pilots are trained to fly the Shuttle without the Autopilot, if they were unaware that it was turned off then the "minute" adjustments that either one would make would be missed. All accounts I have seen suggest that the slightest details on the approach make HUGE differences in the results. Add to this the fact that it has been reported that the Autopilot, when on, was acting to correct the flight path anomalies caused by the damage outside. If the autopilot is off, then what other consequences were being experienced?
Is it possible that this with the likely outside damage and other factors may have COMBINED have caused the loss of the Shuttle where any issue ALONE would have not? With all the speculation I have seen in the media, I am not sure this is any less of a possibility...
BTW, I personally am not trying to lay blame on the astronauts themselves. Much like a Cruise Control that starts to mysteriously disengage on a vehicle, I would not be surprised if the Autopilot may have "sensed" a disengage as simple as moving the stick, and the pilots assumed that one of them must have done it."
---"What did I say that sounded like 'Tell me about your day?'"---
Why does the old joke about "which is heavier, a pound of feathers or a pound of lead?" keep coming to mind?
Folks, a small weight moving fast packs a lot of punch. Even foam/feathers/pillows.
And actually, this is something that has been confusing me about the foam argument since day one. I know you lot are just joking around, but this seems an important point that I, for one, don't quite get.
Even though the foam was traveling at an extremely high velocity, wouldn't the relative velocity between it and the shuttle wing been quite low? Because, after all, until a few seconds before the strike, they'd been accelerating in the same direction as part of the same vehicle. Unless the acceleration rate was continuing at a substantial measure, I don't see how the total velocity of the foam off the tank would matter any more than the total velocity of the foam off your shelf.
Definitely not a physics major, am I? But could someone with a talent for dumbing things down explain this so I can understand it?
No relation to Happy Monkey
Nope - as discussed many times before they were in different orbit without anywhere near enough fuel to get to ISS.
There are a lot of other things they might have been able to do though. Stuff I've read included dumping excess weight/cargo, changing reentry profiles to reduce left wing heating, spacewalk inspect/repair, unmanned resupply rocket, shuttle rescue mission - all _possible_, but maybe not practical. Reaching ISS was not possible.
First of all the foam piece was part of the shuttle. So it broke off and hit the shuttle a little further down. In a "near vacuum" (sp?). So if the piece was traveling as fast as the shuttle before it broke off, didn't have much air resistance to slow it down and hit the shuttle a few tens of feet away how did it accelerate (well, decellerate, same difference, opposite vector direction) to 500 mph less than the velocity of the shuttle?!
Also why are NASA engineers surprised about Ek=1/2mv^2?! I think the reporter meant he didn't think the foam would do much damage, and the NASA guy couldn't think of anything simpler than Ek=1/2mv^2 to explain it to his dumb reporter ass. (sorry, small rant)
I haven't posted in so long, my sig is out of date.
It's the 1000 MPH relative wind that produced the huge change in speed between the wing and the foam. The shuttle was accelerating upward somewhere around 3-4 G, and there's the 1 G due to gravity, but those are small accelerations compared to the wind resistance.
The shuttle was still in the atmosphere. Drive down the road at 60 mph and throw styrofoam peanuts out your car window for a demonstration.
Whenever a plane "hits paydirt" as a result of hitting something else first (excepting other planes, of course), it seems the universal constant is a jet engine's intake manifold. A flock of birds, a weather balloon, a lost Australian Shepherd asking for directions, they all get sucked into the engine and wind up tearing it apart from the inside. IANA air traffic controller, but in my experience, that's the only reason a plane goes down as a result of an unexpected collision (excepting the ground of course).
The shuttle has no jet engines, and there's not really a way to damage the rockets that propel the vehicle except creating a breach of some kind.
With that in mind, I'd imagine the damage likelihood would be extremely low (famous last words?) if a bird hit the shuttle during liftoff. Beyond the already high unlikelihood of an actual collision (NASA has played the percentage game quite a few times now), their small bodies are padded by a relatively huge volume of feathers, which would absorb most of the impact. I won't stray far enough offtopic to divulge details, but four ingredients: younger days, an old Victorian house with an open-air attic (or whatever construction guys would call that), pigeons pooping all over the place, and a bb gun. That thing could embed small metal spheres half an inch into hardwood stairs (won't tell you who still doesn't know about that, either), but when pigeon hunting, if it wasn't a head- or neck-shot, the bb would just bounce off, sometimes not even scaring away the damn pigeon. Now don't call the ASPCA, because that would be too far offtopic.
So anyway, to bring down the shuttle, the bird would have to try to impale the craft with its beak, and that's only possible if the bird is looking down (it it a faux pas to say FTSOA that the launch is vertical?). Now, I won't pretend to know that a bird definitely could/couldn't get out of the path of a large object moving at mach three, but I think it's fair to assume that most birds would see it in time to at least attempt flight (sorry, bad pun), and therefore be facing away from the shuttle, enabling the rocketing craft to hit the soft part of the bird first. So, of the multitudes of birds that hit the shuttle during each liftoff, only an infinitesimal percentage would collide beak-first. I doubt birds are of much concern to the shuttle crew.
AFA the rail gun analogy, it's actually like electromagnetic charges that repel, not opposites, which attract. Other than that, your statement is very true.
I probably shouldn't mention these small details, but without the existence of small details, this entire thread would likewise not exist, right?
All right, I just couldn't resist the correction. So I qualify.
- zedmelon
Mom says my
a bay-to-bay transfer in EVA suits seems to be something that any astronaut should be able to accomplish
come to think of it, it might be worthwhile to provide special suits for this kind of thing. EVA suits are big, bulky things designed for extended work outside the vehicle, and are therefore expensive to launch and require lots of space to store.
Since human skin is plenty strong enough to hold people together for short periods under vaccum conditions (sci-fi movies not withstanding), I'll bet it would be fairly easy to design a simple, lightwight EV suit for just this kind of thing.
A half hour air tank, scuba-style full face mask, and a heavy-duty spandex suit (mechanical counter pressure prevents problems with vaccum, and is necessary to prevent the bends, which would really suck in an emergency situation) with built in saftey harness would be all thats required for an emergency vehicle transfer (gecko-tape style pads on the suit would rock). You'd sure get your lifetime quota of radiation in a hurry, but thats better than hyperventilating, jumping out and hoping the Heart of Gold will scoop you up. The whole package would be under 20 pounds