Science / Medicine : Spaceplanes : U.S. and Other Nations Looking Beyond Shuttle

The world’s space-faring powers are already looking beyond the space shuttle and are preparing to build their replacements: spaceplanes.

These aircraft, powered by advanced engines and having no practical limit on speed or altitude, will take off from airport runways rather than from space centers such as Cape Canaveral. After carrying their cargo into orbit, they will return from space to land again at the airport, ready to perform another mission.

These spaceplanes are being designed to overcome the shortcomings of shuttles and other rockets.

“Our shuttle takes months to prepare for a flight, with thousands of technicians in attendance,” said Gary Hudson, president of Pacific American Launch Systems in Redwood City, Calif.

The United States and Europe also have unmanned rockets for launching satellites, which are somewhat simpler to prepare for launch, but these rockets fly only once, then fall into the ocean rather than return for reuse.

“They thus are about as costly as the shuttle itself,” Hudson said.

Spaceplanes, by contrast, are to take advantage of decades of experience in building high-performance aircraft and flying them routinely. The Air Force’s SR-71 spy plane, for instance, is the world’s fastest and highest-flying, reaching 2,200 m.p.h. and 85,000 feet.

“We fly it every day under those conditions,” said Capt. Steve Grzebiniak, a pilot at Beale Air Force Base in Northern California.

The ground crew, which numbers only a dozen or so, can prepare an SR-71 for a new flight in as little as six hours. The SR-71 can operate in this fashion because it is simple and rugged, in contrast to the delicate and complex space shuttle.

The reason planes can be built more ruggedly than rockets is that rockets, for all the drama of their fire and thrust, lack the performance to allow designers to build in ruggedness. This is because a rocket carries all its oxygen aboard, as a liquefied gas stored in a tank. This liquid oxygen is quite heavy and compels the rocket to expend much of its power simply in carrying it along.

There is plenty of oxygen in the outside air, and rockets fly through this air during the early stages of their flight. But there has been no way to get the air into the rocket motor.

By contrast, turbojet engines, such as those of the SR-71, rely entirely on oxygen in the air through which they fly and thus are said to be air-breathing. By avoiding the need to carry liquid oxygen, they achieve particularly high performance.

But such engines overheat at high speed, limiting their top speeds in the atmosphere. By contrast, orbital velocity is 17,500 m.p.h.--eight times faster than the fastest flight in the atmosphere.

In preparing to build spaceplanes, researchers are going beyond the SR-71 and designing novel engines to gain the best possible advantages from air breathing. Though there are still a number of obstacles to overcome, designers are confident that they are on the verge of a new generation of aircraft.

The farthest-advanced spaceplane effort, the X-30, is America’s National Aerospace Plane project, which is being funded at $300 million for 1989. The question is whether the Air Force and NASA will follow up on these designs and actually commit to building the world’s first experimental spaceplane. According to the project’s program manager, Robert Barthelemy of Wright-Patterson Air Force Base, this decision will be made during the summer of 1990.

The X-30 will pioneer a new type of engine called the scramjet, currently under development at the firms of Rocketdyne, Pratt & Whitney and Marquardt Corp.

Fred Billig of the Johns Hopkins University’s applied physics laboratory, a leading scramjet specialist, describes the engine as a carefully shaped duct for the airflow that is fitted with fuel injectors. It is likely to come equipped with a small rocket engine, mounted within this duct, which gives thrust to help the plane take off.

Then at 2,000 m.p.h., the scramjet can kick in. It rams full tilt into the air up ahead, compressing it and forcing it to flow through the duct. Hydrogen fuel, injected into this airflow, burns and adds heat; the hot air then expands through a rocket-like nozzle and produces thrust.

“The scramjet thus is to give the high performance of a jet engine but over a rocket-like range of speeds,” Billig said.

Its designers talk hopefully of operating scramjets all the way to orbit. However, aerodynamic heating produces formidable problems in designing such engines.

“The materials we’ve developed for use inside turbojets aren’t good enough,” Barthelemy warned.

While the hot parts of turbojets withstand temperatures of 2,800 degrees Fahrenheit, the X-30’s hottest areas will top 5,000 degrees.

“The X-30 will be a risky vehicle,” he declared. “There won’t be any zero-risk technology.”

The emphasis is on using the X-30 to push the frontiers of flight, much as Chuck Yeager in the X-1 pushed those frontiers by breaking the sound barrier in 1947. If all goes well, the X-30 will be built and will fly to orbit in September, 1996, according to Barthelemy’s current plans.

The Japanese are pursuing a similar approach.

“Their advanced-materials program is comparable in scope to ours; they are pursuing scramjets as well,” said Robert Williams, former director of the National Aerospace Plane. “They have laid down a program of spaceplane research with objectives nearly the same as our own.”

In particular, Mitsubishi Heavy Industries is developing an air-breathing rocket. Rocket motors operate at high pressure, and it is hard to take in outside air and pump it to the even higher pressures necessary to feed it into the engine.

The Mitsubishi approach applies a solution developed at Marquardt Co. in Van Nuys. They use supercold liquid hydrogen to chill the incoming air and turn it to liquid. The liquefied air is readily pumped to any desired pressure.

Shigeo Kobayashi of Tokyo’s Metropolitan Institute of Technology, who reviewed this work at a recent conference, asserted that current plans call for developing a spaceplane beginning in 1997, which will use such engines. The spaceplane then would enter regular service in 2006.

In Germany, the firm of Messerschmitt-Boelkow-Blohm is pursuing what may be the most formidable spaceplane concept of all because it is attempting to use existing or near-term technology. It is called Saenger, after a 1940s-vintage prophet of such craft and is being designed with $170 million in government support.

A decision is due in 1992 on whether to actually build the first version. Rather than seek the most advanced technology, as in Japan and the United States, this concept takes a much more conservative approach.

“Our Saenger is based on technologies that are at least partially available or are likely to be in the near future,” said Heribert Kuczera, a senior manager.

Saenger is to be a two-stage craft, taking off and landing from airports in Europe. Its first stage, the size of a wide-body airliner, is to use a combination of turbojets and ramjets--low-speed versions of the scramjet--for propulsion. These are to allow Saenger to cruise at 2,900 m.p.h. and 80,000 feet, climbing to 4,500 m.p.h. and 100,000 feet to launch its second stage, after which the first stage turns around and retraces its flight path to land at the airport.

The second stage is to be a rocket-powered craft about the size of our space shuttle and would similarly come down from orbit and land after its mission.

The second stage is to represent an enlarged version of a European mini-shuttle, Hermes, which France and other countries are currently preparing to build. Hermes is to fly to orbit in 1998, said Christian Dujarric of the European Space Agency. The Germans thus hope to offer Saenger as the next major project to receive Europe-wide support, after Hermes.

Saenger receives high marks from Williams.

“It’s readily apparent that it is relatively low-risk,” he said. “The first stage accelerates to speed ranges that can be handled by state-of-the-art propulsion and materials.

“The second stage avoids the need for advanced technology by ascending very steeply, as a rocket. All the difficulties of the X-30 are avoided by this approach. Politically, this two-stage design makes sense because it can be divided up between the different countries.”

What is more, the Germans hope to convert the Saenger first stage for use as a commercial airliner.

It would carry 230 passengers at 2,900 m.p.h. over a range of 7,000 miles, Kuczera said. It thus might fill the emerging need for high-speed aircraft able to span the Pacific. Such commercial flights today take up to 15 hours.

A supersonic transport, flying at 2,000 m.p.h., would cut this to four hours. Executives at Boeing and McDonnell Douglas, the major U.S. plane builders, anticipate a market for up to 600 such aircraft early in the next century. Saenger, if it proceeds, could offer strong competition.

The hope of the spaceplane designers is to turn the space near the Earth into an extension of the atmosphere. Flight to orbit is to represent an extension of aviation, rather than a highly publicized set of events that receive extensive television coverage.