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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.
... could be possible terorist weapons :)
Fry: heh, Yakov Smirnoff said it
Leela: No he didn't.
Foam fell off my shelf the other week.
Should I be worried?
foam and southern florida to science. I tend to get flashbacks of spring break.
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.
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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.
It is pretty obvious that these guys are geeks yes?
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Look to the left -- it looks a bit like a shadow. You can also see where the foam embedded itself in a T-seal.
Toronto-area transit rider? Rate your ride.
"That's when it came home to me what 1/2mv2 means"
This guy is a rocket scientist? I guess that's one stereotype debunked.
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.
"people's intuitive sense of physics is sometimes way off."
No kidding. How could they think a piece of foam shot at over 500 mph would bounce off harmlessly? Nearly everyone knows a penny dropped off the Empire State Building can kill someone- this foam (which is heavier, and is going faster than the penny would be going) would most certainly do damage.
When you don't have a leg to stand on, don't even get up.
...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.
"That's when it came home to me what 1/2mv2 means...the force was equivalent to catching a basketball thrown at 500 miles per hour."
is he serious?? performing a 5 second equation before telling the shuttle to come back could have predicted and prevented this tragedy. i'm glad it's hitting home for him now...too bad he completley forgot his rudimentary physics a few months ago. this is just another in a long line of examples of NASA engineers not being up to par with basic math. (what...yards != meters???)
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...
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
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.
First there's L. Ron Hubbard, now G. Scott Hubbard? Maybe the problem is that they were using Scientology for the first mid-mission damage assessment instead of science? It's all becoming clear now...
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.
Tufte also examined the Challenger evidence in his excellent book Visual Explanations.
AlpineR
The videos are here (where the panel visibly ripples after the impact) and here.
The accompanying slide presentation has the details: the 1.7 pound foam block was fired at 531 mph and, where it struck a T-seal between two panels, displaced them and caused a 4/10 inch gap. This fake wing was made of fiberglass, but given the results, a test with actual shuttle wing material from the Space Shuttle Discovery is planned for today.
Here are some of the headlines from news.google.com:
Shuttle Wing Under Gun
Investigator Amazed by Shuttle Foam Force
Foam theory faces pivotal test
Tests Show Foam Causing Wing of Shuttle to Deform
Foam chunk was shuttle's undoing, tests indicate
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
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
These stories of ice covered foam remind me of something...
In one of the NRC labs in Ottawa, they have a "chicken gun" that fires broiler chickens at high velocity into mock ups of aircraft windshields. It is probably an urban legend, but I heard a story that some British engineers decided to duplicate the experiment, and were horrified to find that the chicken smashed a hole clear through the windshield mockup and buried itself in the far wall. They emailed their Canadian colleages to ask what they were doing wrong. The reply was simple: "thaw the chickens first."
But seriously, as the velocities increase, so does the danger. I once saw a picture of the windshield on another orbiter that had been struck by a tiny fleck of paint from an old booster. It looked like it had been struck with a bullet, and had the paint fleck been slightly larger, NASA would have had yet another catastrophic end to a shuttle mission.
If we ever develop a really good propulsion system that can approach light speed, we had better invent deflector shields along with it. As you hit relativisitic speeds, anything you collide with releases energy proportional to an equivalent sized hydrogen bomb. Even molecules become dangerous, and a dust speck would blow a good sized hole in your spacecraft.
My rights don't need management.
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?'"---
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
At the time the foam fell off, the shuttle was still accelerating at full power. The piece of foam also entered the slipstream between the shuttle and the external tank, which is where most of the acceleration came from.
And that was merely to protect the weaker heat shielding on the conical part of the spacecraft. The blunt end heat shield was protected by the Service Module until shortly before re-entry. The ice generating fuel tanks were also below the space craft, instead of partially above it. One of the problems with the Shuttle design is that much of the heat shielding is exposed during lifoff. I suppose some variation of the BPC could have been designed for the leading edges of the wings, but there certainly would have been a weight penalty, and the Shuttle is a heavy bird to begin with.
This is a problem that is going to have to be solved if a production re-usable spacecraft is ever to happen. We might not be at the stage of materials science to do it yet, though.
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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.
The insulating foam for the space shuttle that broke off and possibly destroyed the shuttle was a new formula since 1997 that has been problematic since it replaced a freon based foam. Although the freon-based foam worked better, the new foam was used instead rather than getting an exemption. So, if the foam is the root cause, it appears political correctness was more of a concern than using the best material for the job, possibly costing the lives in the process. Here's an article on the subject
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It's looking more and more like there was a management decision to accept foam impacts despite the engineers of the shuttle specifying that nothing should impact the shuttle.
Previous reports indicated NASA management argued that the impacts were OK since nothing bad happend from past impacts,and because it was "just foam". Some of the same articles stated that the engineering design docs stated no impacts were acceptable.
The challenger disaster was for sure due to managers deciding to launch against the strong advice of the engineers not to launch.
This current article's quote of the NASA Ames person (who has been in management for awhile now as someone has already pointed out) surely is suggestive of the problem. It indicates his surprise that the physics don't match his inuitive expectation. Maybe that's a root of the problem. People with some science background in a non-relevent field who move on to a management role are relying on their own intuition over that of those that are doing the actual engineering in the relevent field.
For sure if they were going to accept the impacts then they had a responsibility to put the resources into experts carefully analyzing what the outcome would be for all the possible impact area's and times. That would allow a scientifically informed decision.
Instead there was an intuitivly informed decision.