The 5-mm Two-Stage Light-Gas Gun (mercifully called the "Light-Gas Gun" in everyday conversation) is the most complex and temperamental gun in the lab. It is also the most hazardous gun in the lab, in that it needs a diet of both gunpowder and gaseous hydrogen to work. Finally, at over 12 meters long, it's the biggest gun in our lab. (Another gun lab at JSC, the Hypervelocity Impact Test Facility, has a monster light-gas gun that's almost 29 meters long.) It's definitely not something you'd want to install in your garage.
We can accelerate projectiles in the light-gas gun to more than twice the velocity of those shot in the other two guns, but the price we pay for that capability is a relatively long time between shots, a lot of maintenance, stringent safety rules, and frequent equipment checks for fatigued metal, cracks, or other dangerous defects. While we can comfortably shoot the Vertical Gun eight or so times a day, we can get as many as two shots off per day with the light-gas gun only if everything goes right. Firing the light-gas gun is not something that can be rushed, no matter what the reason.
When the gun is fired, the sequence of events goes like this (Anything other than this sequence would constitute an unwelcome situation of one degree or another, all of which are basically evil.): The gunpowder is ignited and pushes the piston down the pump tube at high velocity. The piston, in turn, compresses the hydrogen in the pump tube, sending a shock wave down the tube at very high velocity. The shock wave hits the rupture disk, which tears along the arms of the "+", allowing the very hot, high-pressure hydrogen to escape faster than Gen-Xers from a Wayne Newton concert. Seriously fast. The hydrogen slams into the projectile, pushing it down the barrel and out toward the target, doing thought-provoking damage to whatever it hits. This can happen so fast that the sabot, if it has any defects such as bubbles or cracks or is poorly cast, can deform and ooze around the projectile itself, throwing everything off the intended trajectory, ruining the shot, and basically screwing everything up. (And this is a sabot made with a special plastic that's been chosen for its strength and rigidity. An easy calculation comes up with an amazing number here. The barrel is about a meter long, and a typical shot pushes the projectile from 0 to 6 km s-1 in that distance. Simple physics dictates that this is possible only if the acceleration is almost 2 million times the acceleration due to Earth's gravity -- that's almost 2,000,000 g's! No wonder the plastic can deform.) We don't like it when that happens. The high-pressure coupling is very massive for a good reason: the pump tube has a bore of 40 mm, while the inside of the barrel is only 5 mm in diameter. Forcing the high-pressure hydrogen from the 40-mm tube into a 5-mm hole over a distance of about 60 cm is not a gentle process.
We're presently using this gun to test the capabilities of aerogel as a target for trapping orbital-debris particles on collectors attached to the International Space Station. Aerogel is a very low density material, and the best way to describe it without being able to pass out samples is to call it "frozen smoke." That's a pretty accurate description, and it might be the weirdest stuff you've ever seen.
In a typical case, cosmic-dust particles would impact such a collector in low earth-orbit at an average velocity of 14 km s-1 -- a speed much greater than can be simulated appropriately in the laboratory. We're trying to find a material that will stop the particles as gently as possible so they don't vaporize when they hit the collector; it would be even better if they didn't melt, so their original structures and mineral assemblages could be examined. The only reasonable way to do this is to decelerate the very valuable (scientifically, that is), very small (much smaller than the diameter of one of the hairs on your head, if you have any) dust particle as gently as possible. For instance, imagine that you're walking on the railing of the observation deck of the Empire State Building in New York. A gust of wind pushes you off, and you're heading downward at over 150 km per hour in just a few seconds. One way that (very) quick-thinking rescuers could stop you without doing too much damage would be to unfold and stretch a few hundred bed sheets, one above the other, so you'd go through them one at a time. You probably wouldn't notice hitting each sheet, but every time you went through one, it'd slow you down a little. If there were enough sheets, you'd eventually stop, none the worse for wear and with one heck of a story for the police.
Well, the idea behind aerogel as a collector is very similar. If you think of the aerogel as a solidified foam (not exactly correct, but close enough) that can almost weigh less than an equivalent volume of air, you can imagine that the wall of each bubble making up the foam is very, very thin - hundreds of times thinner, in fact, than that same hair on your head. Each one of those walls to a cosmic-dust particle is equivalent to one of the imaginary sheets that saved your imaginary life when you fell off the imaginary skyscraper. Stopping a cosmic-dust particle moving at 14 km s-1 or so is a little different than keeping you from hitting the ground too fast, but the general idea is the same. We've been using the Light-Gas Gun to characterize the behavior of aerogels under such impact conditions, and the early results are promising. In fact, Fred has an aerogel experiment flying on the outside of the Mir space station right now; it was attached by Linda Godwin and Rich Clifford during the shuttle flight that took Shannon Lucid to Mir. We'll be able to check it out in about a year, after it's retrieved. Aerogels also will fly as the collectors on the Stardust mission to a comet at the end of the century; we'll be doing experiments in support of that mission, too.
More information on aerogels may be found at the following Web sites: