Building a better shuttle: NASA turns to new designs and materials.

By now, the space shuttles can be considered the dinosaurs of the space age, as obsolete as a 386 computer. But they're still flying, and when and if NASA lifts the moratorium it imposed after the shuttle Columbia broke apart on Feb. 1, the shuttles may fly for the rest of the decade. When the space agency originally built a fleet of four shuttles, no one expected that the very same vehicles would still be on the launch pad more than 20 years later. Had NASA adhered to the once-a-month launch schedule that it once envisioned, the 100-flight proposed lifetime of each vehicle would have expired years ago.

But the $3 billion annual cost of the shuffle program and the labor-intensive efforts required to maintain the vehicles after each bruising space flight has led to a much slower schedule. On average, there are only five shuttle flights per year. Even Discovery, the most flown shuttle, has taken only 30 trips.

In one respect, that's fortunate because NASA has yet to choose a successor to the space shuttle. In the 1990s, the agency failed in two costly attempts to design and begin building next-generation, reusable spacecraft. On the other hand, the continued use of the shuttles is only a stopgap on the way toward truly 21st-century space vehicles.

VARIATION ON A THEME Although the Columbia tragedy may force NASA to speed up development of the next-generation spacecraft, the agency is currently addressing the future of space flight with a conservative, two-pronged approach--one evolutionary and one more revolutionary.

The evolutionary approach, announced last November, calls for the construction of a spacecraft that would serve initially as an emergency escape vehicle for the crew of the International Space Station. NASA hopes that the first of these rescue vehicles, dubbed the Orbital Space Plane, will attach to the space station by 2010. But if the craft proves durable, a modified version would begin ferrying space-station crew to and from Earth by 2012.

The space plane isn't intended to replace the space shuffle, notes former astronaut Vance D. Brand, now deputy director of Aerospace Projects at NASA's Dryden Flight Research Center in Edwards, Calif. The space plane would be roomier than the Russian Soyuz module that currently serves as the space station's lifeboat. But, unlike the shuttle, it wouldn't have the space to serve as a cargo container or host a science laboratory.

Moreover, the plane would be launched piggyback atop a disposable rocket. That's in contrast to the shuttle, where the rocket boosters are often recovered and refurbished, at great cost, for reuse.

BEYOND THE SHUTTLE In addition to pursuing the Orbital Space Plane, engineers are starting from scratch with more radical designs. It's likely that any shuffle successor will resemble a streamlined airplane, perhaps a fighter jet, predicts Daniel Rasky of NASA's Ames Research Center in Mountain View, Calif.

That's in stark contrast to the blunt features of the space shuttle, whose broad, flat underbelly--designed to distribute the heat of reentry over a large area--is anything but sleek.

And like jets, the next generation of space vehicles might take off from a runway rather than a launch pad. Getting these vehicles into space would probably require a combination of engines to accelerate the craft up to 25 times the speed of sound. After an initial ascent by a conventional rocket or turbo jet engine, the plane would zoom horizontally, scooping up oxygen from the air at high pressure and mixing it with a tank of liquid fuel.

Because the oxygen comes from the atmosphere, the craft doesn't have to carry huge amounts of it to propel the vehicle. In contrast, each space shuffle carries more than a half-million kilograms of liquid oxygen. Scientists are working on alternative fuels that would consist of solid wax (SN: 3/22/03, p. 187).

High speed, air-breathing engines are known as ramjets or scramjets because they ram oxygen into the front of the engines. Aerospace engineers first began thinking about such engines several decades ago in a project that became known as the National Aerospace Plane. But at the time, says Brand, the technologies required weren't mature enough to make the concept work. After sinking more than $1 billion into the project, NASA abandoned it in 1994.

Rasky says that it's inevitable that the ramjet concept will be incorporated into any shuttle successor, but the vehicle probably won't be ready until 2020 or so.

In 2001, NASA scrapped a $912 million venture aimed at developing a reusable, single-stage-to-orbit spaceliner. The streamlined, wedge-shaped craft known as X-33 proved problematic. It was too heavy, and its liquid-hydrogen fuel tank didn't work properly. The failure of the X-33 shuttle-replacement venture has prompted NASA to focus on multistage engines for the next generation of spaceliners.

With several multistage designs now under consideration, technical advances to make a lighter-weight, more durable space plane are "like putting tools in a toolbox," says John Rogacki of NASA headquarters in Washington, D.C. "We're not sure [right now] what the tools will be used for."

Among the tools are a variety of ceramic and metallic composites that could provide a tougher, more heat-resistant skin for a next-generation space shuttle. At NASA Ames, researchers are studying low-density ceramic composites known as toughened uni-piece fibrous insulation (TUFI), which are 20 to 100 times more resistant to impact damage than are the glass-coated ceramic tiles still standard on the shuttle and that have become a focus of the Columbia investigation.

The coating on the heat-resistant tiles now in use is so fragile that technicians servicing the shuttle must remove jewelry lest they scratch the surface, notes Rasky. The problem, according to NASA scientists, is that the coating gets little support from the porous structure of the underlying tile. As result, when the tile surface gets hit, whether by a micrometeoroid or shuttle debris, it cracks or chips.

Although TUFI tiles have the same silica-fiber interior as the standard tiles do, they have a more durable surface coating. The coating permeates the pores near the surface of the tile, creating a strong, crack-resistant outer surface. In contrast, when a standard shuttle tile is hit, a crack spreads from the point of impact as it does when window glass is struck.

TUFI tiles are already being used on the shuttle's base heat shield, which covers the main engines and some other areas that are prone to damage in space but aren't subject to extremely high heat during reentry.

These tiles aren't being used on the belly of the shuttle and other areas that are most stressed during reentry, because TUFI isn't as good an insulator as the original shuttle tiles are. But researchers are studying a new version of TUFI that consists of silica fibers interspersed with a layer of aerogel. Sometimes referred to as solid smoke, this super-lightweight gel fills the tile's air spaces. Because the filling has such small pores, it traps air and other gases and prevents them from transporting heat through the material.

If these aerogel-filled tries prove to be as good an insulator as the standard shuttle tiles, says Rasky, "then TUFI could easily be [applied] all over the shuttle--to the belly and other regions--as damaged tiles are replaced"

NASA scientists are also studying thermal blankets, which would provide further insulation during reentry. Rolled out like carpet, the blankets feature a main insulating layer of a ceramic-fiber batting sandwiched between layers of ceramic fabric. Other layers would include a screenlike metal fabric woven from wire and an outer layer of metal foil, most likely made of a nickel alloy. These layers would be stitched together by ceramic thread. Researchers are also testing smart structures, materials embedded with computerized sensors that would alert the crew and ground controllers to sudden problems and suggest a fix. Smart structures could be affixed to parts of the existing shuttle fleet.

HOT STUFF There's an even more radical approach that some designers are considering. So-called hot structures would replace thermal tiles, heat blankets, and other thermal-protection devices external to the craft's skin. Instead of relying on the continuous shunting of heat away to prevent structural materials from melting, engineers are developing metallic alloys or ceramics that don't melt--or even lose strength--at any temperature they might encounter during space flight. Among the materials now under study are titanium- or nickel-based alloys and silicon carbide ceramic reinforced with carbon fibers.

Internal insulation, akin to the fiberglass insulation inside houses, would be applied wherever a hot structure might otherwise come into contact with the crew.

Such a change in strategy would have important several advantages. Not only are tiles difficult to repair, but the absence of any material covering up the skin of a space vehicle would make it easier for engineers to spot faults, such as underlying cracks or metal fatigue.

Rogacki notes that NASA spacecraft have had little margin for error as they've carried crews and cargo into space. For example, there's no procedure for repairing tiles while a shuttle is in orbit. Any new design for human space flight must include a greater tolerance for problems that may be encountered during launch, when reaching orbit, or during reentry, he says.

Of course, the direction that NASA will pursue most actively in the development of a shuttle alternative will depend, in part, on critical factors not yet evident, including the conclusions reached by the committee investigating the Columbia tragedy and how soon the shuttle fleet will be back in business.