Can It Be Built? : Space Plane: Flight Test for Science

It will be able to take off from an ordinary runway, accelerate to 25 times the speed of sound, climb into orbit and then, mission accomplished, land like any other airplane.

To President Reagan, it would be a new “Orient Express” that would carry passengers from Washington to Tokyo in less than two hours.

To NASA, it would replace the rocket-launched space shuttle.

To the Pentagon, it would reduce reliance on highly vulnerable launch pads and lift space weaponry into orbit.

When the President called for increased research funds for such a triple-threat aircraft in his State of the Union address Feb. 4--only a week after the Challenger explosion--visions of the futuristic vehicle understandably struck a responsive chord.

New Sense of Urgency

Plans for such a “trans-atmospheric vehicle” have been under consideration all during the 1980s. But they have taken on new significance and a new sense of urgency after the destruction of the space shuttle Challenger on Jan. 28.

Yet, knowledgeable engineers say the undertaking is an incredibly complex project that may have great difficulty simply getting off the ground--much less into space.

The problem, they say, is that such an aircraft would have to go through three distinct phases of flight. Therefore, such a plane would probably require three distinct propulsion systems.

Invoking an automotive analogy, engineers argue that developing such an aircraft is like trying to combine the four-wheel drive system of an off-road vehicle, the sustained power of an Indy 500 racer, and the high acceleration of a nitro-powered dragster in one chassis that is no larger than the family sedan--while still leaving room for the passengers.

Competition for Funds

But despite such potential difficulties, U.S. aerospace companies are vigorously competing for their share of the research funds, even though commercial airlines remain somewhat skeptical.

Those funds total $60 million for this fiscal year, and NASA and the Department of Defense have requested $212 million for next year. Space and defense officials estimate that the project will require $500 million over the next three years and as much as $3 billion before a prototype of the craft could be test-flown sometime in the 1990s.

The United States is not alone in undertaking to develop a new generation of aircraft to replace jet airliners and space shuttles.

The day after Reagan called for increased research funds in his speech, the British government authorized $4.1 million for a “proof of concept” study for a similar craft called the Hotol--for horizontal takeoff and landing.

In the United States, government officials appear to be lumping two distinct vehicles into one project under the rubric “National Aerospace Plane.”

The first is a more or less conventional passenger plane, frequently called the hypersonic transport, or HST. It would be a successor to the Concorde supersonic transport that now ferries passengers from Europe to the East Coast in just 3 1/2 hours. The HST would attain speeds two to five times that of sound--Mach 2 to Mach 5, or 1,500 to 3,750 m.p.h.

Such an aircraft “could be built with existing technology,” said Roger Schaufele, vice president for engineering at McDonnell Douglas Corp.

The second vehicle is a true spacecraft that could accelerate to escape velocity--Mach 25, or about 17,600 m.p.h.--to enter orbit. Such a craft would require considerable amounts of new technology, most engineers agree.

Range of Speeds

The key to both types of planes, however, is the development of appropriate propulsion systems that can power a plane over a range of speeds, from takeoff at about 100 m.p.h. to entering orbit at Mach 25.

Those new systems will probably be extensions of the turbojet engine that powers most modern planes. In the turbojet, propeller-like blades inside the engine suck in and compress air. Kerosene is added to the compressed air and ignited; the rapid expansion of the combustion gases forces them out the rear of the engine, providing thrust and turning a turbine that drives the compressor.

The turbojet is efficient at the 550-m.p.h. speeds of conventional airliners. But at much higher speeds, temperatures within the engine approach the melting point of most present-day materials.

So a more efficient device above Mach 2 is the ramjet, which is little more than a hollow tube with a restriction in it. “Air flowing into the front of the ramjet compresses itself at the restriction and is slowed below the speed of sound,” said Robert A. Jones of NASA’s Langley Research Center in Hampton, Va.

After the compression, combustion is the same as in a conventional turbojet.

High-Speed Problems

“But as you go to higher speeds,” Jones said in a telephone interview, “the temperature of the air increases because of the slowing. When you get near the temperature of the flame--at about Mach 6, or about 4,500 m.p.h.--the air will no longer support combustion” because at that temperature molecules break apart and combustion cannot be sustained.

Beyond Mach 6, it is necessary to use a supersonic-combustion ramjet, or scramjet. In the scramjet, the restriction is “very carefully designed so that it produces compression without significantly slowing the airflow,” Jones said. “Theoretically, there is no limit to the speed that can be obtained with a scramjet.”

But building a scramjet is not easy. “You have to mix the air with the fuel and have it burn in microseconds,” Jones said. “That’s barely enough time for combustion.”

One way to reduce combustion time is to use as a fuel either methane or hydrogen, both of which burn much more rapidly than kerosene. And because both gases must be stored as low-temperature liquids, they also can be used as coolants before combustion to protect both the engine and the airframe.

Conventional Source

Neither ramjets nor scramjets, however, function at subsonic speeds. Hence, the aircraft must also carry a conventional turbojet to allow it to reach supersonic speeds.

Such engines also do not operate at high altitudes, where there is too little oxygen to sustain combustion. The national aerospace plane will therefore also carry a rocket motor and oxygen as well as fuel.

One of the promising potential engines for the national aerospace plane is the air turbo ramjet developed by Aerojet TechSystems Co. of Sacramento. This engine illustrates how engineers are attempting to combine characteristics of the turbojet, the ramjet, and the rocket in one hybrid propulsion system.

The air turbo ramjet was first designed in the 1950s, said Ronald A. Samborsky of Aerojet. But it was shelved because no airframe, or body, of that era could absorb the engine’s thrust without being torn apart. “New materials and designs,” he added in a recent interview, “make use of the engine feasible.”

Conversion to Gas

The key to the engine’s design is the fact that the turbine that powers its compressor is turned by the engine’s fuel before the fuel is burned. Liquid hydrogen, for example, must be converted into a gas before it is burned. The fuel’s volume expands sharply during that conversion and the expanding gas can turn the turbine.

Because the turbine is no longer in the exhaust gas, it does not overheat at high speeds.

The lack of oxygen at higher altitudes, however, would require modification of the engine.

Aerojet is working with General Electric Co. to build a hybrid air turbo ramjet that would incorporate a rocket motor. That rocket would boost the plane into orbit and provide braking for reentering the atmosphere.

The Langley Research Center is also developing a hybrid scramjet. Most of the details are classified, but the overall configuration of the Langley engine is probably similar to that of the proposed Aerojet engine.

Wind Tunnel Tests

The NASA scientists have tested a model of the engine in a wind tunnel at speeds as high as Mach 7 and Jones is confident that the engine will surpass Mach 12.

Another engine that may be similar in concept was conceived by Alan Bond, a nuclear fusion researcher for Great Britain’s Atomic Energy Authority. Bond designed the engine in his spare time and took it to British engine maker Rolls-Royce Ltd.

Both Rolls-Royce and the British government were apparently favorably impressed by the design: the company began work to develop the engine and the government stamped it “top secret.”

About the only thing that is publicly known about Bond’s engine is that it will burn liquid hydrogen, using oxygen from the air at low speeds and altitudes, and that it will switch over to its own liquid oxygen supply at higher altitudes.

Lockheed, McDonnell Douglas, Boeing, and British Aerospace are all working to develop airframes to be powered by these new engines. Not surprisingly, all of the airframes look much alike.

Integrated Design

The chief feature that the designs all share is the integration of the engine into the fuselage. Engines hung from the wings, as is the case with most commercial airliners today, simply provide too much drag as the aircraft passes through the atmosphere.

Most of the designs, furthermore, are delta-shaped, somewhat like a cross between the space shuttle and the Concorde, because that configuration provides the most lift with the least resistance to passage through the air.

McDonnell Douglas envisions a hydrogen-fueled, 305-passenger, Mach-5.5 hypersonic transport that would be about the size of a DC-9 airliner. Such a craft could make a 6,500-nautical-mile trip in 2 hours and 16 minutes, Schaufele said, “allowing the crew to return to their home base during the same working day.”

For the national aerospace plane, Lockheed envisions a craft 205 feet long and 60 feet high, with a wing span of 95 feet. The plane would be slightly smaller than Lockheed’s C-5 Galaxy, the world’s largest transport, the company says, “but almost twice as heavy since its high temperature metallic structure must stand up to the rigors of multiple flights to the threshold of space.”

Heavy and Fast

The plane would have a launch weight of 1.5 million pounds and could carry payloads of as much as 20,000 pounds. It could jump from New York to Los Angeles in less than 30 minutes, reaching an altitude of 300,000 feet in a suborbital trajectory.

Perhaps the most ambitious space plane is the Hotol, contemplated by British Aerospace and incorporating the Rolls-Royce engine. The unmanned vehicle is designed to carry a cargo of 7 tons into a 120-mile-high orbit.

Because of the need to carry oxygen, Hotol will be five times as heavy at takeoff as at landing; the same ratio for a conventional aircraft, in contrast, is two.

Because of that weight ratio, Hotol will be launched from a specially designed sled that will never leave the ground. The sled will be powered, which could eliminate the need for a turbojet. The craft would land on conventional landing gear.

Question of Cost

Development of Hotol is expected to cost at least $3 billion and perhaps as much as $7 billion, so Britain hopes to gain support from other European countries. Executives of British Aerospace predict that Hotol could launch satellites into orbit by the end of the century for less than half the cost of launch by the shuttle. Eventually, it might be able to transport 70 passengers on a suborbital flight.

The potential military value of the National Aerospace Plane might justify its eventual cost, but the transport of passengers is more of a problem. Just for starters, a hydrogen-powered HST would require elaborate new fuel-storage and loading facilities at airports at a potential cost of hundreds of millions of dollars.

“When people suggest we can afford that, I just shake my head in disbelief,” said Roger Fleming of the Air Transport Assn. in a recent interview.

Fares would also be very high. British Aerospace estimates that a one-way ticket from London to Sydney, Australia, via Hotol might cost $5,600, several times the current cost. The market for such flights would obviously be limited.

THE AEROSPACE PLANE

The national aerospace plane would require three types of propulsion systems--a conventional turbojet for takeoff, a ramjet for high-speed flight and a rocket motor with its own oxygen supply for space. New hybrid motors combine all three technologies. A plane powered by such an engine would not be dependent on complex and potentially explosive rocket boosters that may have destroyed the shuttle Challenger.

TURBINE ENGINE At takeoff, air sucked into the engine through an opening in the fuselage is compressed by motor, fuel is injected and ignited.

RAMJET ENGINE The ramjet scoops air, compressing it, and ignites the oxygen after fuel is injected. The engine is capable of high speed.

ROCKET ENGINE Once in space, where there is no oxygen, a rocket engine is fired, using liquefied oxygen fuel carried inside the plane.