Fist of Foam

Last weekend I was out riding my bike, pedaling pretty hard down a wooded gravel road, when a dried sycamore leaf drifted slowly down into my path. I brushed it on my forearm as I passed and to my surprise, it hurt! How could that be, I wondered. It was just a dried leaf, larger than most I suppose, but very light, no real sharp edges. I must have run into a thousand such leaves in my life, blown by the wind perhaps. There was not even any wind to speak of.

It dawned on me then that I was the wind. Here was an object, light though it may be, held in place by air that may have been still compared to the planet, but not compared to me. You may be aware that the energy involved in a collision increases as the square of the relative speed of the objects colliding. I had probably never experienced that particular flukey collision before. It makes it much easier for me to believe reports of straws sticking into telephone poles after hurricanes and tornadoes.

It also makes me feel sympathy for NASA. Years ago somebody had to make a decision about how to reduce the thermal contrast between the Shuttle and a silo filled with liquid hydrogen. We’re picking up too much ice; we’re having trouble keeping shuttle components at proper operating temperature; we’re seeing temperature stresses on connecting bolts. Who knows exactly? Maybe it was even during the early design phases, but somebody decided to use foam insulation for all sorts of good reasons. What could be the downside they asked, because they ask that about everything. Well, it could catch fire. Fumes from heated foam could build up in some cavity and cause an explosion if there were exposed wires. There might be a transfer effect, dampening vibration in one area only to increase it in another. What about the mass? Carefully calculated, deemed a good tradeoff. We can even make the metal thinner if we use the foam, actually saving considerable weight.

The one question they did not ask was, "What happens if it breaks off and damages a tile on the leading edge of a wing?" I don’t know. Maybe they did ask that question. Somebody surely responded helpfully that it wouldn’t matter because everything would be moving at the same speed, and besides the stuff has very little mass. After that, no one paid attention to it when they saw it breaking off repeatedly, causing no noticeable problems.

Well, as many have said, it’s not what you don’t know that gets you. It’s what you know that just ain’t so.

8/1/2005 11:52 PM