Slate's Bizbox
explainer: Answers to your questions about the news.

Get Your Foam OnHow hard is it to keep insulation from falling off the space shuttle?

By Michelle Tsai
Posted Monday, Aug. 13, 2007, at 6:52 PM ET
The shuttle Endeavour. Click image to expand.

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 Columbia disaster, 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.

Got a question about today's news? Ask the Explainer.

Explainer thanks Brad Lerch of NASA and Steve Nutt of the University of Southern California.

Print This ArticlePRINTDiscuss this in The FrayDISCUSSEmail to a FriendE-MAIL
Share on FacebookPost to MySpace!Share with MixxDigg ThisShare with RedditShare with del.icio.usShare with FurlShare with Ma.gnolia.comShare with SphereShare with Stumble Upon
Michelle Tsai is a writer living in Jersey City, N.J.
Photograph of space shuttle by AFP/Getty Images.
Join the Fray: our reader discussion forum
What did you think of this article?
POST A MESSAGE | READ MESSAGES

Remarks from the Fray:

I don't understand why they launch the shuttle from Florida, probably the most humid place on the continent, where the amount of water vapor in the air on a hot day guarantees that the sudden mega-chill of fueling the rockets will create massive amounts of condensate ice. Why not launch from the Mojave Desert, Edwards AFB, where it's as dry as the proverbial popcorn fart?

--JimBob

(To reply, click here.)

For the first flight (or two) of the shuttle, the tank was painted white over the foam. Paint was later dropped as it added significant weight to the vehicle, diminishing the payload capacity.

More than 20 years down the line, perhaps it makes sense to revisit the paint issue.

Surely a thin layer of some kind of rubberized or epoxy paint would add weight, but--applied after foaming was complete--it would provide aerodynamic smoothing to the foam, as well as making the exterior of the ET slicker, making ice less likely to stick.

In hindsight we can see, as some did in foresight, that the shuttle was not what it was sold to us as--reliable, inexpensive transport to LEO. However, for the moment, it's all we have. I say paint it, and if NASA would like to get sponsors to pay to have their names on the ET, all the better...

--Andy M-S

(To reply, click here.)

It seems that there are more ways to insulate the liquid hydrogen than foam on the outside. Have the overly educated college bred professionals at NASA even considered any other way? Why couldn't the inside be lined with some form of insulation prior to completing the tank assembly or make it clamped together for repeated inspection and relining? It could be a double hulled tank that would provide the insulation as well as be filled with an insulating medium in between hulls, and any arguments saying that it would become too heavy or less capacitory are moot as there are materials light enough and gas pressures could be increased with double lined tanks. Where has common sense gone? It would seem that colleges remove any semblance of such a needed talent. In any case, I have read that the Space Shuttle Program is so outdated given the state of the art technologies available as alternate means of obtaining atmospheric escape that it is fairly criminal to go on with the current space program. The Space Program has to be revamped and updated, period.

--Watcher49

(To reply, click here.)

(8/14)

Washington Post