Space Shuttle Debris
Date: 06/24/2003 at 18:13:30 From: Scott Subject: Space Shuttle Debris Why are they shooting it at the wing at 550 mph to assess damage? How do you get anywhere near 550 mph? If the space shuttle is lifting off, how fast and with how much actual force f = m x v can a 1.68 lb of foam hit the shuttle? The length from the tip of where the fuel tank attaches to the wing is only around 50 ft. They are "firing" this piece of foam at around 550 mph. If the shuttle is traveling 500 mph (I don't know the actual speed straight up, but I'm giving huge leeway here), the foam is travelling at 550 mph at the time of separation. It has 50 feet to fall prior to striking the craft. Gravity plays a part, but we all know about terminal velocity and there is no way it gets there - coefficient of drag plays a part - but to accelerate to 550 mph seems mind boggling.
Date: 06/25/2003 at 13:57:58 From: Doctor Rick Subject: Re: Space Shuttle Debris Hi, Scott. That's an intriguing question! I can't answer everything definitively, but I can make some rough calculations that might tell us something. First, I found a transcript from the Challenger explosion, for comparison, indicating that at 75 seconds after liftoff, that vehicle had a velocity of 2900 feet/sec = 1980 miles/hour. I assume that at 81 seconds, when the foam broke off, Columbia was traveling at least that fast. It's faster than you thought, isn't it? There are at least three factors that would accelerate the foam relative to the wing once it broke off. One is gravity acting on the foam. Another is the acceleration of the shuttle due to the rocket engines, in which the foam no longer shared. The third factor is the force of the air on the foam, which was traveling some 2000 miles/hour relative to the air. This is equivalent to a stationary object being hit by a blast of wind going 200 miles per hour. I have no experience with such winds, but let's just see what acceleration would be required to bring the foam to 550 miles/ hour in 50 feet. We have the formulas v = at s = (1/2)at^2 which we can solve to find a when we know v (550 mi/hr = 800 ft/sec) and s (50 ft). s = (1/2)a(v/a)^2 s = v^2/(2a) a = v^2/(2s) = 800^2 / (2*50) = 6400 ft/sec^2 which is 200 G's, so it does sound like a lot - but then it's a strong wind! One thing this number makes clear is that the first two factors in accelerating the foam relative to the wing are insignificant. The acceleration of the shuttle must be on the order of 1 G, and of course the acceleration of gravity is 1 G. The force required to cause this acceleration is found from the equation F = ma = (1.68 lb)(6400 ft/sec^2) The English units get very confusing, but to put it in terms of "pounds force," we just note that we have 200 times the acceleration of gravity, so it is equivalent to gravity acting on a mass 200 times as great. The force is 200 * 1.68 lbf = 336 lbf. One formula I found on the Web may help us see if this magnitude is reasonable. The wind pressure on an object is P = (1/2)(density of air)(v^2)(shape factor) where the shape factor depends on the shape and orientation of the object but is of order of magnitude 1. The density of air (at sea level) is listed as 1.25 kg/m^3, which comes out to about 0.078 lb/ft^3. With these figures, the wind pressure on the foam would be something like P = (1/2)(0.078 lb/ft^3)(2900 ft/sec)^2 = 327990 lb-ft/sec^2/ft^2 which is about 10,000 lbf/ft^2. How big was this piece of foam? If it's a square foot in cross-section area, we'd have 10,000 lbf, considerably more than the 336 lbf we needed to account for. Of course, I have ignored the fact that the shuttle was maybe 7 miles up at the time, so the air density was considerably less. But I'll leave that calculation to the reader, okay? My calculations may be way off; I won't guarantee that I didn't make a major error somewhere. One last point: The force of impact on the shuttle is not figured by the equation F = mv. As I said above, the correct equation is F = ma, but here the acceleration is actually the DECELERATION by which the wing stops the debris. We don't know enough about the details of the impact to make this meaningful. What I can do is compare it to more familiar collisions. One relevant factor is the kinetic energy of the foam: KE = (1/2)mv^2 = 0.5(1.68 lb)(800 ft/sec)^2 = 537600 lb-ft^2/sec^2 We can compare this with the kinetic energy of a typical car hitting a tree. Let's say the car has a mass of 2000 lb (I'm not sure how typical this is). To get the same kinetic energy as the foam, the car would have to be going at a speed given by 537600 = (1/2)(2000 lb)v^2 v = sqrt(537600*2/2000) = 23 mi/hr We can at least say that the impact of the foam would be greater than what automobile bumpers are designed to withstand. What the shuttle wing should be able to withstand, I don't know! - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
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