The new Orient express – hypersonic transport

The new Orient express – hypersonic transport – HST

Jerry Grey

THE NEW ORIENT EXPRESS

Many people in the U.S. breathed a sigh of relief in 1971when the supersonic transport SST, that symbol of runaway technology was dealt its death below by a skeptical Congress. A few years later one of the American plane’s foreign ??, the Soviet Iupolev Iu-144 came to a less publicized but even more inglorious end, it was gounded because of poor performance. And while the Anglo-French Concorde remains a favorite well-heeled jetsetters, its more a succes d’estime than a commercial triumph.

Yet despite the SST’s bittersweet record, the dream of highspeedflight is very much alive today. Encouraged by new technology, the aeronautics industry has begun to think about a bread of superfast aircraft that should fly lines around its 1960s-vintage ancestor at speeds that could make obsolete the very idea of a SST which travels at about twice the speed of sound. This hypothetical aircraft is the hypersonic transport or HST, and it would roar across the skies at Mach 5 to Mach 8, or from five to eight times the speed of sound. What’s more say it’s advocates, it might even fly at a profit.

In Such a plane travel timeacross the Pacific–say, from Los Angeles to Tokyo–could be sliced from about ten hours to as little as dizzing speeds, they’re only a crawl compared to those of a more futuristic aircraft under more than casual consideration by NASA, the U.S. Air Force, and the British aviation industry: a transatmospheric transport–the term airplane really won’t do–that would take off from runways, accelerate in the atmosphere to near-orbital speeds (about Mach 25, or about 17,000 m.p.h.), and then boost itself literally out of this world.

Such high-flying ideas, nowbeing seriously discussed by very level-headed engineers, show how far and fast we’ve come since the British engineer Frank Whittle invented the jet engine in 1941, finally turning visions of breaking the sound barrier into more than flights of fancy. Aviators instantly became obsessed with that goal, in spite of warnings from experts like Sir Ben Lockspeiser, the British Ministry of Supply’s director general of scientific air research, who declared somberly in 1946, “We have not the heart to ask pilots to fly the high-speed models, so we shall make them radio-controlled.’ These fears were justified later that year: young Geoffrey de Havilland, son of the famous British plane maker, was killed when his DH-108 Swallow disintegrated in mid-air because of then unknown aerodynamic forces that radically change the behavior of control surfaces (rudder, elevators, and ailerons) as aircraft approach the speed of sound.*

* Which is about 760 m.p.h. at sealevel and somewhat less at higher altitudes, where the sound waves are slowed by the lower temperatures. Ernst Mach, the nineteenth-century Austrian physicist for whom the speeds are named, was a pioneer in the study of shock waves.

Theorists had long predictedthat these forces would appear at near-sonic speeds, and both wind-tunnel and flight tests at high subsonic speeds had shown a marked rise in aerodynamic drag as air velocity around the plane neared sonic speed. The physical problems were clear. The air piled up in front of the plane, increasing the force needed to plow through it. Worse, as the plane flew faster, the pressure waves it created couldn’t move out ahead of the aircraft–a region that the Hungarian-born aerodynamics pioneer Theodore von Karman, a founder of Caltech’s Jet Propulsion Laboratory, called the “zone of silence’ or the “zone of forbidden signals.’

But theory could take aircraftdesigners only so far in the mid-1940s, and experiments had their limitations as well. Wind tunnels capable of moving air at transonic and supersonic speeds weren’t equipped to make the precise measurements needed to study the subtle but important effects on aircraft wings and control surfaces at those velocities. When, on October 14, 1947, Air Force test pilot Chuck Yeager finally crashed through the sound barrier, flying his rocket-powered X-1 to Mach 1.07, he stripped away some of the mystery from von Karman’s zone of silence. It had been known, for example, that there’d be sudden jumps in air pressure caused by the pile-up of air in front of a supersonic plane. But it was only from the X-1’s supersonic flights that we learned these shock waves could spread for miles behind the aircraft, rattling windows and shaking wooden buildings on the ground. The waves–one from the front of the plane, another from the rear–became known as sonic booms, a graphic, if not very accurate, description of them (because they’re really supersonic).

As we entered the jet age,with 200-passenger 707s and DC-8s regularly crisscrossing the oceans, plane makers concentrated increasingly on aircraft that could carry more people and get them to their destinations faster, thereby making planes more profitable (Airlines measure an aircraft’s “productivity’ in seat-miles per week or month: the number of passengers it can carry times the distance it can fly in a given period.) But the development of the 747 jumbo jet showed we had reached a practical limit on size; there weren’t many markets that could absorb more than 500 people on a single flight. The byword among aircraft designers became faster and more efficient, not bigger.

Subsonic jets like the 747 flyin the range of Mach 0.8 to 0.85. With more sophisticated wing designs and computer-operated controls to reduce drag, they could perhaps be pushed to 0.9. But the drag at this speed becomes so great that it makes no sense to burn the gallons of extra fuel needed to go just a little faster. And at higher speeds this so-called transonic drag is replaced by “wave drag,’ the force needed to propel those wide-ranging supersonic shock waves through the atmosphere. Although wave drag is low at first, it increases with speed, demanding more and more fuel to overcome it. And at high supersonic speeds–Mach 3 and more–there’s an even more serious problem: heat. As the plane rams through the air, the air in front of it compresses and heats up, rapidly raising the temperature of the aircraft’s skin. At Mach 2 the Concorde’s nose temperature is 260|F., even though the air temperature outside is a numbing -60|F. If the aircraft were to exceed Mach 3–the Concorde flies as fast as Mach 2.2–its aluminum skin and struts would begin to weaken and might fail.

This heating also limits thetop speed of conventional turbojet engines to about Mach 3.5. Already warmed by the plane’s forward motion, the incoming air would enter the combustion chamber at such high temperatures that the additional heating caused by the burning of the fuel would weaken the turbine’s blades and burn other critical engine parts.

It’s these problems that limitedthe speed of the first commercial supersonic jets. The Soviets’ Tu-144 edged out the Concorde by reaching Mach 2.3; Boeing’s proposed 2707 SST, if it had been built, would have flown at Mach 2.7. But there are other caps on SST performance as well. For take-off, climbing, and transonic acceleration, the turbojet engines use fuel-gulping afterburners, which ignite the extra fuel downstream of the turbine to avoid overheating the turbine blades. Besides being inefficient, the afterburners are quite noisy, and for a while New York closed its airports to the Concorde–though tests eventually showed it wasn’t much louder than older, subsonic jets like the Boeing 707 on take-off and landing. Still, the Concorde’s sonic booms remained a headache, literally. To protect American eardrums, to say nothing of windows and dishes, the Federal Aviation Administration (FAA) forbade commercial SSTs to fly at supersonic speeds over any part of the U.S.

But fears that the SST wouldcause major environmental problems–there was also concern (much exaggerated, many experts say) about what the plane’s exhaust might do to the earth’s protective ozone layer–weren’t what killed the American SST or hurt the Concorde commercially. Their undoing was the high cost of operations. After the 1973 “oil war’ the price of fuel rose almost tenfold. This combined with other difficulties–inefficient performance during take-off, restrictions on supersonic flight over the U.S., and the limitations of range and passenger payload imposed by its 1960s technology–to imperil the Concorde’s commercial viability. Its market eventually shrank to a few North Atlantic routes (London-New York, Paris-New York, and London-Washington-Miami). Even runs between London and Bahrain, in the oil-rich Middle East, couldn’t be sustained economically.

The Soviet Tu-144, a far lessefficient twin of the Concorde, was even more unsuccessful. Although the Soviets hoped eventually to fly it on the showcase North Atlantic route, it suffered many technical and operational problems, including the crash of a prototype at the 1973 Paris Air Show that killed the entire crew and eight people on the ground. The Soviets finally–and without fanfare –took the plane out of service in 1978.

In light of the SST’s lacklustreperformance, why have the prospects for superfast transport planes now improved? First, since the design of the Concorde was frozen in place two decades ago, there have been innumerable advances in aeronautical technology. Second, there’s the growing importance of long-distance passenger and air cargo routes between such far-flung places as New York and Tokyo, London and Sydney, and San Francisco and Peking–to name a few. These expanding markets might support the kind of heavy capital investment in new high-performance aircraft that was unthinkable even a decade ago. As Deputy Assistant Secretary of Commerce Crawford Brubaker told Congress earlier this year, “The Pacific Rim is the fastest growing region. In 1984 exports of $56 billion to [it] accounted for 26 per cent of all U.S. exports. Seven Pacific Rim countries accounted for over half the U.S. trade deficit of $123.3 billion.’

An obvious first step in exploitingthis burgeoning traffic across the Pacific would be to upgrade the Concorde/2707 SST concept. The technical base already exists–thanks to NASA’s supersonic cruise technology program, which has been quietly proceeding since the demise of the American SST in 1971. The space agency’s scientists and engineers have pursued such cost-saving goals as doubling the thrust-to-weight ratios of turbojet engines (from 4:1 to 8:1) and increasing overall engine efficiency (from the 40 per cent of the Concorde’s Olympus engine to nearly 55 per cent) while reducing by half the number of engine parts. They’ve also been experimenting with new superstrong lightweight materials–titanium and graphite composites, for example–as well as using superplastic “sandwich’ structures in many parts of the aircraft, which could achieve reductions of 30 per cent in the weight of the planes. Improvements in design could increase lift/drag (L/D) ratios in supersonic planes–a measure of an airplane’s efficiency and a direct indicator of how far it can fly–from the Concorde’s 7:1 to more than 11:1. In the near future, these could reach from 17:1 to 20:1. (The Boeing 767 has the best L/D of today’s subsonic airliners, about 18:1.) Add to these developments a whole gamut of weight-reducing refinements that are already found on aircraft: a new generation of lighter seats, carbon brakes, radial tires, high-pressure hydraulics, and electronic flight controls. The space agency’s planners already have a pretty clear notion of where such innovations could take them.

In a recent report, RichardPeterson, director of NASA’s Langley Research Center, said, “The preliminary design studies suggest an exciting two-engine supersonic 250-passenger configuration.’ This Mach 2.7 vehicle would provide an unrefueled range of 6,300 miles and permit a Los Angeles-Tokyo run in a little more than three hours. By including a less developed but promising technology, laminar flow control (which reduces drag by smoothing air flow over the plane’s surfaces), says Peterson, range could be boosted to 9,300 miles (New York to Tokyo nonstop in only four hours, versus about 14 today).

To meet some of the originalobjections of environmentalists to the SST, NASA has created new noise suppression techniques. These can reduce engine noise 20 decibels below the Concorde’s, and ?? 10 decibels above the current background noise level at American airports.

But even NASA’s aeronauticswizards couldn’t nullify a fundamental physical law of supersonic flight. The sonic boom wouldn’t go away. Aerodynamic streamlining might help a little, as would making the plane lighter, which would reduce the lift the wings would have to provide and thereby weaken the shock waves they create. The only effective way to reduce the boom’s strength enough to make land overflight acceptable would be to fly where the air is less dense and the shock waves have much farther to travel before they reach the ground.

While theU.S. restrictions on overland flights aren’t serious drawback on the transatlantic run, they would severely limit the operational flexibility needed for economical performance of future SSTs, because some of the best routes would pass over inhabited areas at one or more points–the New York-Tokyo run, for example. Unfortunately, conventional turbojets can’t operate at high enough altitudes to stop rattling windows and startling those on the ground. To fly higher, researchers must look to a whole family of engines that could propel planes well beyond Mach 3.5, the generally accepted upper limit for conventional engine and airframe technology.

In a standard turbojet engine,a rotating compressor raises the pressure of the incoming air. This air is mixed with fuel in the combustion chamber; as the gases burn, they rapidly heat up, expand, and rush out of the rear of the engine at a much higher speed than that of the incoming air. Although some of their energy is tapped by a turbine to drive the compressor, the gases still exit with enough speed to provide the thrust that pushes the plane forward.

When the incoming airreaches high subsonic speeds, the so-called ram pressure of the air entering the engine generates enough thrust to propel the plane without any additional mechanical compression. Such engines, called ramjets, have been known for many years; they’ve been used to propel subsonic drones and several supersonic missiles, such as the U.S. Bomarc and Rigel, back in the 1950s.

However, a pure ramjetwon’t produce enough thrust to drive a plane until it reaches flight speeds that create sufficient ram pressure. So it can’t be used by itself as a plane’s engine. Some other propulsion system has to boost the plane to ramjet speed. The Bomarc and Rigel were assisted by rockets, which were quickly jettisoned after they provided the required boost.

But the Orient Express–the nickname the industry uses for the hypersonic, high-altitude transport that would serve the Pacific market– couldn’t use expendable rockets; they’d be too expensive. Nor could it use an auxiliary turbojet; such a two-engine system would create excessive drag. Instead, the Orient Express would be powered by an ingenious hybrid called a turboramjet. When it starts operating, it acts something like a turbojet, carrying the plane through subsonic speeds. But as it accelerates into the low supersonic range, this aeronautical chameleon begins to take on the characteristics of a ramjet. The ramjet’s best operating regime is at speeds of more than Mach 2, when the compressor serves merely as a supercharger that slightly improves the efficiency of the ramjet engine. To accommodate the changing air flow needed to fly efficiently over a wide range of speeds, computer-operated controls would adjust the dimensions of the engine’s inlet duct and exhaust nozzle.

But the turboramjet’s realplus is that its compressor is powered by a turbine that’s driven in turn by a rocket-like “gas generator,’ which is wholly independent of the incoming air stream. This device burns either a so-called monopropellant fuel (one that creates heat by its own decomposition), like hydrazine or ethylene oxide, or a “bipropellant’ fuel-oxidizer combination, of the sort used by rocket engines. In the turboramjet, the gas generator is designed to produce a fuel-rich combustion gas at a constant temperature tolerable to the turbine (about 1,500|F.) no matter how fast the airplane flies. The gas, still relatively hot after it passes through the turbine, mixes with and burns in the main engine’s air flow, just as it would in a pure ramjet.

This version of the turboramjetoffers excellent flight performance up to perhaps Mach 5 or even Mach 6, but then it too runs into a temperature wall. When the rapidly moving incoming air is abruptly braked in the engine’s combustion chamber at these speeds, it gets so hot (perhaps 3,000|F.) that even the nonmoving parts of the engine will overheat and fail.

The remedy would seemsimple: don’t let the air slow down as it flows through the engine; then it won’t get too hot. But that turns out to be quite complicated. One fuel that can burn fast enough to be used in a supersonic airstream is hydrogen, but learning how to burn hydrogen in this high-speed air flow without blowing out the flame in the combustion chamber took a great deal of theorizing and experimenting. The result was a supersonic combustion ramjet (scramjet for short), which uses a weak shock wave created by a very sharp-edged or sharp-pointed inlet to slow the incoming air, and then some computerized ingenuity: a variable-geometry aerodynamically shaped duct, whose dimensions are adjusted automatically by computer to meet the engine’s air-flow needs at different flight speeds. Although the air is still traveling at supersonic speeds, the flame doesn’t blow out. The air remains hot enough for rapid and efficient combustion, but not so hot as to burn out the engine’s combustion chamber. Finally, the air accelerates out of the engine exhaust exactly as in a conventional ramjet, but at much higher speeds.

If this scheme works, thescramjet, or a variant of it, what I call the scramrocket (see diagrams, page 78), should be able to boost a plane all the way up to earth-orbiting Mach 25. However, the high-speed portion of the flight must take place in the rarefied upper atmosphere to keep aerodynamic heating from incinerating the plane’s skin. Of course, we can’t be sure the plane will work until it’s flight-tested, because the wind tunnels needed to test scramjet engines go to only about Mach 8 before temperatures become unmanageable.

Such flight testing is in thecards, according to Charles Buffalano, who until recently was deputy director of the U.S. Defense Advanced Research Projects Agency (DARPA). “Several hundred million dollars over two to three years will be required to perform “proof of concept’ tests,’ he says. “After that we would be ready to proceed to a flight demonstration phase.’

The new high-performancetransatmospheric transports under study by both NASA and the Air Force would be intended mainly for military uses, such as inspecting and destroying enemy satellites, rapidly deploying an observation or control platform in space, and perhaps carrying into orbit the very large space-based battle stations that would be needed for a Star Wars ballistic missile defense system (DISCOVER, Sept.). James Martin, of NASA’s Langley Research Center, predicts that an orbit-on-demand vehicle will be practical before the year 200–although NASA’s version won’t necessarily take off like a plane. It may be rocket-boosted and rise vertically from a launch pad, like the shuttle. On the other hand, both DARPA and the Air Force, anticipating a need to lift into space and service the various components of Stars Wars, are much more interested in a true “aerospaceplane’ powered by scramjets that would fly almost all the way to orbit. At an altitude of 40 miles or so, where there’s not enough air for either propulsion or aerodynamic controls, small rockets like the shuttle’s reaction-control thrusters would take over and maneuver the aerospaceplane into orbit.

Even if scramjets turn out tobe impractical, another actor waits in the wings: the cryojet. Essentially, it’s a turboramrocket, with a built-in rocket for the final boost into space, but with one important difference that enables it to reach extremely high flight speeds. Its hydrogen fuel, carried as a supercold (or cryogenic) liquid rather than as a gas, would be used to cool the incoming air before combustion. Taking this idea even further is the liquid-air-cycle engine (LACE), which cools some of the intake air enough to liquefy it and use it as the oxidizer in the fuel-rich combustion chamber of the turboramrocket.

Proponents of ground-to-orbithypersonic transports hope that such craft can be developed in a decade or so. At a recent congressional hearing on high-speed flight, presidential Science Adviser George Keyworth said, “The ability to use scramjet technologies at extremely high altitudes to achieve Mach numbers in excess of 10 is now, we believe, technically feasible. It wasn’t as recently as two or three years ago. I think the [supersonic transport] is an anachronism. It will be bypassed . . . We have simply skipped over what would be, if you wish, the commercially feasible SST.’

The manager of DARPA’shypersonic technology program, Robert Williams, was more cautious. “While a thousand tests of scramjets give me a lot of good feelings,’ he said, “it isn’t until we actually build that thing and test it over a couple of years that we’ll be able to say it’s mature enough to go ahead.’ Still, he described himself as excited by the prospects, adding “We don’t see any show-stoppers there’– nothing to halt progress.

In the future, the primaryresearch support for hypersonics will clearly be military, although NASA has done most of the research so far. So of what conceivable benefit can all this activity be to civilian aviation? History has a clear answer: much of the airline industry’s present success is directly traceable to military R & D. The big turbofans that power the jumbo jets were developed for military cargo planes; the first U.S. commercial jet–the 707–inherited key technologies from the B-47 and B-52 bombers. Looking even further back, the development of the first commercially practical airplanes early in this century was financed primarily by the Army and the Navy. Hypersonic space-launcher technology should have similar civilian spin-offs. Says Keyworth, “It’s possible to do what the shuttle does and at the same time provide commercial transportation for people wishing to fly to the Pacific Basin . . . It’s possible to do that with one research and development program and one basic technology.’

Although foreign competitionhas eroded American dominance in such industries as steel, electronics, cameras, and automobiles, aerospace continues to be a bright spot for the U.S. It has exported an average of $16 billion in aerospace products annually for the past five years. But Western Europe’s Ariane space launcher is taking commercial launch business away from NASA’s shuttle, and foreign air-frame and engine manufacturers are cutting into traditional American markets, even selling their products to our domestic airlines. Says Roger Schaufele, leader of McDonnell Douglas’s Orient Express research effort, “Foreign competition is a very serious issue for the U.S. commercial transport manufacturing business. Our market share has continued to shrink in recent years, and we can look forward to more and more serious competition in the future.’

The British are already predictingthey’ll capture a large chunk of the commercial space launch market by the year 2000 with a new ground-to-orbit concept called HOTEL (horizontal take-off and landing). At the Paris Air Show in June, British Aerospace exhibited a very elaborate model of this aircraft. HOTEL would use rockets to boost it to near-orbital speed, but, unlike the U.S. Air Force’s proposed transatmosphere vehicle, would rely on air-breathing liquid-air-cycle engines at lower altitudes. British Aerospace, which wants to join in a partnership with other Western European nations to develop the plane, says such a high-flying transport could carry 60 passengers from London to Sydney in only 67 minutes, wheels off to wheels on.

The likeliest propulsion systemfor this speedster’s American rival, the Air Force’s transatmospheric vehicle, is the scramjet, but with a self-boosting feature that DARPA officials aren’t willing to divulge. My guess is that the scramjet will borrow the turboramjet’s gas generator for its fuel injector, but will run it at high enough power at low flying speeds to drag in enough air so that the ramjet and gas generator together produce the thrust required to boost the plane up to a speed at which the ramjet works alone. Engine geometry changes can then handle the transition from ordinary ramjet to scramjet operation. Even in the secrecy-shrouded aerospace world, there’s nothing entirely new under the sun: this seemingly esoteric concept, called a ramrocket, was popular among propulsion theorists as far back as the 1950s. They even demonstrated the idea in ground tests.

Another option, less elegantbut possibly more practical, would be a two-stage vehicle: a turboramjet-powered plane would carry a scramjet-powered orbiter to Mach 5 or 6; then the first stage plane would cut loose and return to its base while the orbiter, under pure scramjet power, would accelerate to near-orbital speed. A civilian spin-off of the first stage might be the Orient Express.

Once the flight phase isreached, it’s not clear who’ll carry the ball. The Air Force is a good bet to continue with a military transatmospheric space launcher, but when the civilian transport effort diverges from the military, as it must once the R & D phase is complete, who picks up the tab?

Congressman Dan Glickmanof Kansas, a long-time supporter of new flight developments, says, “We need the hypersonic transport, but not with a tax increase. Spending billions of federal dollars would be very difficult without a Star Wars-like presidential commitment.’ Investments by individual corporations on this scale–perhaps $6 billion to $10 billion–are unheard of. As Martin Marietta executive vice president Norman Augustine says, “You’d be betting your company on success.’ The Department of Commerce’s Brubaker adds, “The aerospace companies would like to have at least some public opinion on their side before they put significant efforts or monies into a new high-speed aircraft concept.’ He thinks money might come from multinational industries, airlines, and institutional and private investors.

The experience of the Britishand French in building the Concorde should certainly help guide the builders of the HST. The partnership was troubled throughout. There were endless internal squabbles, horrendous cost overruns, and an extraordinary 5,500 hours of test flying before the Concorde was certified as airworthy (compared with 1,500 for the Boeing 747). Still, the Concorde, whose final price tag was more than $3 billion, burned out to be a magnificent aircraft, in an engineering sense, and a stunning example of international cooperation in high technology.

Although the British andFrench were sure the Concorde would be a commercial as well as a technological triumph, they can be forgiven for their overoptimism. In 1970 the FAA forecast that 500 to 800 SSTs would be in operation by 1990–an error of about 5,000 per cent! As it happens, even the Concorde’s bleak financial picture has recently brightened. In his 1982 book, Concorde, the British aviation writer Kenneth Owen notes, “The two airlines [Air France and British Airways], having been allowed to write off the capital cost of the aircraft, are now making operating profit with their Concordes. People are buying the time-saving and the service that Concordes are providing.’

On the Concorde’s inauguralpassenger flight in 1976, a passenger wondered what all the fuss was about. He commented to Concorde designer Sir George Edwards, “Mach 2 feels no different.’ According to Owen, Sir George replied, with characteristic British understatement that only hinted at the dozens of brilliant engineering breakthroughs hidden behind this exceptionally smooth voyage, “Yes. That was the difficult bit.’ Therein perhaps lies the Concorde’s legacy to the future builders of the Orient Express.

Photo: ANGLING FOR SPEED

Traveling at speeds greaterthan Mach 5, the HST would slice the flying time from Los Angeles to Tokyo from ten hours to two.

Photo: EXPANDING THE ENVELOPE YET AGAIN

As an aircraft approaches Mach 1:

The air begins to pile up in front of it, and the pressure (orsound) waves created by its motion can no longer move ahead of it, thereby sharply increasing drag. But if the plane is properly streamlined and powerful enough, it will eventually move faster than the pressure waves and break the sound barrier.

As an aircraft files faster than Mach 1:

It leaves behind two conical shock waves–one from its nose,the other from its tail. These cause the “sonic booms’ heard on the ground after the plane passes overhead. In the hypersonic range (above Mach 5), the angle of the cone decreases, weakening the sonic boom by the time it reaches the ground.

Photo: The Concorde: more a succes d’estime than a commercial triumph

Photo: The Soviet Union’s SST, the Tu-144, came to an inglorious end.

Photo: TURBOJET

As the air enters, it’s compressed,mixed with fuel, and ignited. The burning gases expand, powering the turbine, which drives the compressor. Escaping at high speed, the gases push the plane on. Most airliners have turbojets.

Photo: RAMJET

In this type of jet engine, theincoming air is traveling so fast it doesn’t require a compressor: “ram pressure’ alone can drive the aircraft. But ramjets don’t develop enough thrust until the plane reaches supersonic speeds.

Photo: TURBORAMJET

At lower flight speeds, thishybrid engine compresses the incoming air as a turbojet would. But above Mach 2 it relies on ram pressure. To prevent overheating, it uses a temperature-controlling gas generator to drive its turbine.

Photo: SCRAMROCKET

In this variation of theramjet, the incoming air, traveling at supersonic speed, is controlled by intake ducts whose shaped are altered by computer for different conditions. The rocket provides thrust at lower velocities.

Photo: VARIATIONS ON AHIGH-SPEED DREAM

Although the American SSTdied in 1971, U.S. and foreign engineers are keeping alive the dream of passenger planes that travel at many times the speed of sound. A sampling of their designs:

COPYRIGHT 1986 Discover

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