As the world watched the progress of the space shuttle Endeavour’s mission to the international space station and back last month, it was hard not to suffer from flashbacks to past space disasters.
The presence of Mission Specialist Barbara Morgan, for one, evoked memories of the 1986 space shuttle Challenger disaster, since Morgan had served as backup to Christa McAuliffe under the NASA Teacher in Space program for that doomed mission.
Even more pertinent, though, was the potential similarity to the space shuttle Columbia disaster when it was discovered that protective tiles on the Endeavour’s belly had been gouged by foam debris during liftoff. The Endeavour ultimately completed its mission and returned safely to Earth, but only after detailed analysis of the gouge and considerable anxiety on the part of watchers around the world.
NASA has taken many steps to reduce the potential risks associated with the space shuttle’s protective foam, but the evidence seems to suggest the problem isn’t going away anytime soon.
The External Tank
The space shuttle’s foam problem stems from its external tank, which is the orange, 15-story-tall fuel tank loaded with the 535,000 gallons of liquid hydrogen and oxygen needed to get the shuttle into orbit. Just before liftoff, these super-cold fuels are mixed and burned by the shuttle’s three main engines, which guzzle the mixture at a rate equivalent to emptying an average backyard swimming pool in 20 seconds.
All three of NASA’s shuttles — the Endeavour, the Discovery and the Atlantis — use the same sort of tank and are built on essentially the same design. In all three cases, the external tank is the only part of the shuttle that cannot be reused after a mission. About 8.5 minutes into a flight, by which time all the fuel has been used up, the tank is jettisoned and disintegrates over the ocean.
That, of course, makes it harder to figure out where problems arose.
Thermal Protection System
Foam is actually one of two parts making up the thermal protection system, or TPS, that is used to insulate the tank before and during liftoff. The other part, sitting below the foam, is a dense composite material known as “ablator” that’s made of silicone resins and cork and functions to dissipate heat.
In general, the purpose of both parts is to keep the shuttle’s liquid hydrogen fuel at minus 423 degrees Fahrenheit and the liquid oxygen tank near minus 297 degrees while preventing a buildup of ice on the outside of the tank.
“It’s sort of like a Coke can, where you use a styrofoam holder to keep it cold,” John Pike, veteran space policy analyst and director of GlobalSecurity.org, told TechNewsWorld.
A Harsh Environment
There is a demanding set of requirements. The low-density, polyurethane-type foam must be durable enough to endure a 180-day stay at the launch pad; withstand temperatures up to 115 degrees Fahrenheit and humidity as high as 100 percent; and resist sand, salt, fog, rain, solar radiation and even fungus. Then, during launch, the foam must tolerate temperatures as high as 2,200 degrees Fahrenheit generated by aerodynamic friction and radiant heating from the 3,000-degree solid rocket boosters and 6,000-degree main engine plumes.
When the external tank falls back to Earth after being jettisoned, the foam must be able to maintain its structural temperatures, allowing it to safely disintegrate over a remote ocean location.
More than 90 percent of the foam is sprayed onto the tank automatically, leaving 10 percent that needs to be applied manually on trickier areas. Though it is only one inch thick on most parts of the tank (increasing to as much as three inches thick on parts subject to the highest heating), it adds 4,823 pounds to the tank’s weight.
NASA actually uses four slightly different types of foam on the external tank. One covers roughly three-quarters of the tank, while the others are used for particular areas and manual application.
Environmental policy changes such as the implementation of the 1987 Montreal Protocol phasing out class I ozone depleting compounds and emerging Environmental Protection Agency regulations have necessitated changes to NASA’s foam over the years, so what’s used today isn’t the same as what was originally used.
The space shuttle has always lost foam to some degree, but “the original foam was a formaldehyde-based foam and very tough and resilient,” Paul Czysz, professor emeritus of aerospace engineering at St. Louis University, told TechNewsWorld. “Suddenly NASA went ‘green’ and the new foam would not adhere as well and crumbled easily.”
When pieces of foam break off, they have a very high drag and very low mass, causing them to slow down quickly, Czysz explained. So, when the briefcase-sized, roughly 2-pound piece of foam broke off the Columbia tank, for example, the shuttle was moving at about 275 meters per second; the impact energy was equivalent to dropping a 10-pound object 100 feet, he explained.
The result: Damage sustained to the Columbia’s left wing ultimately caused the shuttle to disintegrate upon reentry into Earth’s atmosphere.
“The problem is that the external tank is big and complex,” Pike noted. “It is not really intended to vibrate, so when it does during liftoff, it does so with considerable complexity.” So, too, does the foam it is coated with. “That foam doesn’t like vibration either, and it has a tendency to come loose or pop off in small pieces.
“Before Columbia, NASA thought that foam-shedding was a maintenance problem, not a safety one,” Pike added. “After Columbia, they came to understand that it was a bigger problem than they thought it was.”
Indeed, “They looked at this very soft piece of material on Earth, and couldn’t imagine it would put a hole in the shuttle,” agreed Harry Lambright, a space policy expert at the Maxwell School of Syracuse University.
“I think the foam problem is very interesting in that something that you think is not a particularly serious risk based on your experience here on Earth may turn out to be quite a risk up in space,” Lambright told TechNewsWorld. “At the same time, what we’ve got now are a lot of problems.”