A baseball-sized chunk of foam dislodged from the space shuttle Endeavour's fuel tank during last week's launch, gouging a 3.5-inch-by-2-inch hole in the belly of the shuttle. NASA has been under pressure to solve its foam problems ever since the Columbia accident in 2003. Why is it so hard to keep foam on the tank?
Because it stays on only when it's been perfectly applied. If NASA engineers leave any air pockets or bits of dirt in the foam, or miss a tiny spot, then the extreme conditions of liftoff can knock it right off. As the shuttle accelerates to more than 3,000 mph in two minutes, the foam needs to withstand violent vibrations, air friction, and sudden changes in temperature and pressure. Unfortunately, the workers applying the foam can't always see very well because they wear protective suits and masks. There's also no way to test for cracks before launch; the only inspection tool is the naked eye. According to the Columbia accident report, NASA officials found foam loss in more than 80 percent of the 79 missions for which they have pictures.
The foam itself consists of different kinds of lightweight polyurethane. Anywhere from half an inch to 6 inches of the stuff insulates the tank, which holds the shuttle's fuel at cryogenic temperatures. The liquid hydrogen inside has to be kept below minus 423 degrees Fahrenheit—no easy job considering that the tank sits out under the Florida sun for days before launch, and then gets as hot as 600 degrees on the outside during the ascent. In case the weather forecast calls for rain or just a very humid day, the foam also has to keep ice from forming on the surface of the super-cold tank—ice that could break off with the foam in flight and strike the shuttle.
Most of the time a machine applies the foam, but for the irregularly shaped parts of the tank, people do it. First, they spray a primer on the tank. Then they spray the foam in several layers. The foam goes on in liquid form and rises as the cells inside expand with gas. When the first layer is done foaming, there's a 30-second window to add the next one; too late and the second coat won't stick. The spray-on stuff doesn't need an adhesive, but some parts of the tank use premolded foam, which is glued on.
Since the Columbiadisaster, NASA has developed more consistent foam and now uses heaters in some places to prevent ice from forming. Still, air bubbles trapped within the foam or between the foam and the tank can explode during liftoff. This happens when gas in the pockets liquefies because of the cryogenic temperatures in the tank and then heats up and vaporizes during liftoff. At that point, it can blow off pieces of foam above it.
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Explainer thanks Brad Lerch of NASA and Steve Nutt of the University of Southern California.