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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.
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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"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.
The instant the foam was no longer in contact with the rest of the shuttle, it would no longer have rocket thrust acting on it, only drag from the air, so it would have slowed down quite quickly.
"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???)
I'm sure most of the people here reading this article are very respectful of the situation.
Our hearts go out to the families and friends of those who lost their lives.
Yet, that does not negate the human need to use humor (however distasteful) to help overcome the emotions encountered when dealing with tragic situations. [Note that I'm not agreeing with some of the comments here, merely stating opinion.]
I'm sorry that you will be angered by those people whose postings are unfortunately offensive; I can only imagine there will be far more offensive things posted at 0 or below.
Sorry.
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
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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
I can't stand when the media sells disaster, cable programs like "What Went Wrong?"....
Leave the engineering issues to engineers and scientists. The general public doesn't give a rat's ass about kinetic energy or materials science, they just use it as an excuse to re-live the tragedy over and over.
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Considering how much damage something as small as a paint fleck can do, at high speeds, a 1.5 pound chunk of anything can be dangerous.
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Your calculation is completely off, I think you translated 500km/h into 500000 m/h, but did not realize that you have to change them into m/s.
Anyway, the acceleration is really 192 m/s^2, or 20g, which is totally possible.
Sigh
The speed of the foam relative to the shuttle can easily be determined by measuring frame-to-frame motion in the video given some point of reference, like, the shuttle wing.
:)
Argh, of course. Yeah. So if the "white blur" moves 10 feet in a single 1/60 second frame, then it's moving, what, 600 feet per second (or something around 400 MPH). Factor in uncertainty for the size of the blur (because, after all, it's blurred), and you get a nice clean velocity range.
I shoulda thought of this, too.
Could we please decide on a measurement system and stick with it?
This illustrates why it may be a good idea to put some money into research of an alternative to the shuttle program. The shuttle program will always face dangers of this type, considering the speeds/forces involved in getting the shuttle into orbit.
Perhaps a program where a spacecraft could actually take off like an airplane and be piloted out of the atmosphere. Even if a large burst of propulsion was needed to get it out of the atmosphere: it would be pulling less G's since it would already be moving with good speed, it would have to do so for less time, and there possibly wouldn't be external systems needed to do it (booster rocket and foam...).
If the official consensus ends up being that the foam caused this, perhaps it will be an impetus for change.
That was the stupidest analysis posted here yet. There are so many more factors than KE when determining damage. If I throw a five pound pillow and a five pound lead weight at you, which will hurt you more? What if I throw five pounds of balsa wood at you that happens to be shaped so that it's 100 feet long, and you get hit by the center, with the two ends to either side? You'd barely even feel it break when it hit you. But it weighs five pounds, and has the same KE of the five pound chunk of lead.
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Vf^2 - Vi^2 = 2ad
This formula assumes constant deceleration. However, aerodynamic drag (and hence deceleration) is proportional to the square of the velocity.
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.
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exactly, and since they are both on earth, they have the smae relative speed compared to the earth's frame of reference. Adding in the velocity of the earth's rotation is not needed.
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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.
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Of course you can't test everything, that's not what I proposed. The fact is, NASA knew that foam was falling off the external tank; they knew that it hit tiles, and it seems fair to analyze the possibilites of it hitting other leading edges. NASA had originally planned to do tests of larger chunks of foam, but decided they were not necessary.
You propose tests based on your knowledge of what events might occur. That's why they shoot birds into jet engines and cockpit windows. I guarantee that they also either a) shoot birds into the wing and nose, or b) have done analyses that show these are much less critical than a bird hitting the cockpit window. You can rule out a bird hitting a passenger window head-on-- but I bet they've done some glancing blow tests.
So, where is the analysis that shows hitting the tile is worse than hitting the leading edge, which the engineers at NASA knew was a more critical area? If they didn't have that analysis (maybe they did), then they should have done the tests.
And my guess is, someone has figured out what objects/ animals could possibly fly into the engines, and they have done tests or anaylsis which addresses that.
I don't know how fast the air friction melts this, but wouldn't foam laden with ice be even worse?
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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.
"Depends. Was your shelf headed upward at a thousand miles an hour?"
Choose the appropriate place to measure it from, and just about anything can be moving upwards at a thousand miles an hour.
Doesn't mean it's going to hit anything though...
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.
Try this... drive down the street at 1000 mph and stick your arm out the window. That force ripping your arm off is called drag. Now imagine another scenario where you are stationary in your car in a wind tunnel with a wind velocity of 1000 mph. Again stick your arm out in the freestream. How fast is your arm travelling about 30 feet after it has been detached from your shoulder?
"It takes considerable knowledge just to realize the extent of your own ignorance." - Thomas Sowell