More Power to You

Bolted motionless on a test stand, the little monster is not impressive. It has no coolly symmetrical propeller, no phalanx of cylinderheads, none of the hard geometrical grace of the conventional aircraft engine. Yet the unprepossessing turbojet engine has thrown the air designers into ecstatic confusion: nobody yet knows how fast the jet will enable man to fly, but the old speed ceilings are off. In their less guarded moments, sober designers talk of speeds so high that aircraft will glow like meteors.

The coming of the turbojet does not mean that the engine in use since the first days of the Wright brothers (pistons and propellers) is done for. It will be a long time, if ever, before that old stand-by disappears; it still has the edge over jets for many purposes, including long-range bombers. But where the power range of the old engine stops, the power of the jet begins. An air revolution is beginning too.

Next Moment, Whoom! To watch* a jet engine spring into life is to feel that power. Dimly visible inside is the turbine, like a small windmill with close-set vanes. When the starting motor whines, the turbine spins. A tainted breeze blows through the exhaust vent in the tail, followed by a thin grey fog of atomized kerosene. Deep in the engine a single sparkplug buzzes. A spot of fire dances in a circle behind the turbine. Next moment, with a hollow whoom, a great yellow flame leaps out. It cuts back to a faint blue cone, a cone that roars like a giant blowtorch. The roar increases to thunder as the turbine gathers speed. Then it diminishes slightly, masked by a strange, high snarl that is felt rather than heard. This is "ultrasonic" sound (a frequency too high for the ear to hear). It tickles the deep brain, punches the heart, makes the viscera tremble. Few men like to stay in a test room when a jet is up to speed.

The engine now has the fierce beauty of power. Its massive rotor, the principal moving part, is spinning some 13,000 times per minute (though with only the faintest vibration). The fire raging in its heart would heat 1,000 five-room houses in zero weather (though much of the engine's exterior is cool). From the air intake in its snout, invisible hooks reach out; their suction will clasp a man who comes too close and break his body. The blast roaring out the tail will knock a man down at 150 ft. The reaction of the speeding jet of gas pushes against the test stand with a two-ton thrust. If the engine were pointing upward and left unshackled, it would take off like a rocket, each pound of its weight overbalanced by more than two pounds of thrust.*

"Get Busy." When an airplane designer looks at this engine, his gloating eyes light up. He knows that it develops some 5,000 horsepower--even more when moving through the air at high speed. But it weighs less than half as much as an equivalent piston engine, and is much more simple to build.

The early jet planes were hurried improvisations (Lockheed wrapped the famed P-80 around a jet engine in 141 frantic days) which did not begin to utilize the new engine's capabilities. Even later airframe designs have not kept up with the fast-growing muscles of the engine. Britain's first turbojet flew successfully in 1941. Designed by Britain's Air Commodore Frank Whittle,* it developed only 850 Ibs. of propulsive thrust. Now engines with 5,000 Ibs. of thrust are available, and soon there will be bruisers with 8,000-10.000 Ibs. No one thinks that even these will be the last word.

Before the jet engine came into the picture (like a young wife smashing the habits of a sot-in-his-ways bachelor), airframe designers were screaming for more power. Now they have it, they do not know quite what to do with it. The power-plant men are doing the screaming now. The great engine builders (Pratt & Whitney, Allison, General Electric, Westinghouse) are working on more powerful engines. "Get busy," they warn the airframe men, "and design some airframes that can keep up with us."

Gargantuan Thirst. Only one present type of airplane, the fast, short-range fighter, is well adapted to the jet engine, whose great failing is its gargantuan thirst for fuel. Consumption varies with speed, altitude and other factors, but a fair figure for the big jets flying at low altitudes is 1,000 gallons of kerosene an hour. This means one gallon every 3.6 seconds. Fighters and interceptors justify this drain, but for bombers and commercial airliners, jet engines still use too much fuel.

This is mostly because the "propulsive efficiency" of the jets at ordinary speeds is low compared with conventional airplanes. Propeller-driven planes are pushed forward by the reaction from the blast of air forced backward by the prop. When the plane is standing still on the ground with its propeller roaring, all the engine's effort is wasted on merely moving air. None goes into forward motion; the propulsive efficiency is zero. When the plane is in the air, the propulsive efficiency is high. Propellers are designed in such a way that when the plane is flying at full speed, the air blast from them washes backward at a mild 10 or 15 m.p.h. Comparatively little energy is wasted; most of it goes where it should go: into moving the airplane.

Jet planes are pushed by the reaction of the blast of hot gas shooting out the tail pipe. The present problem is that this blast is speeding much too fast--at about 1,300 m.p.h. Even when the plane is flying at 600 m.p.h., the blast is shooting backward in still air at 700 (1,300 minus 600) m.p.h. The engine is still wasting too much energy merely stirring up the air. To approach the normal propulsive efficiency of a propeller plane, a jet plane would have to fly faster than sound. But one arresting compensation--for the jets--is that some day they may fly even that fast.

Promising Hybrid. One promising way to make the turbojet engine more efficient at lower speeds is to turn it into a "turboprop." A second turbine behind the first extracts more of the power from the stream of hot gases, and uses it to spin a conventional propeller. This hybrid has many of the advantages (high power, low frontal area) of the true turbojet, and it has a higher propulsive efficiency at reasonable speeds, using less fuel per hour.

There are no satisfactory turboprops flying in the U.S. at present, though they are on the way. The British have gone in for them heavily. Several British models (from 1,100 to 2,300 h.p.) have been thoroughly flight-tested with encouraging results, and a 3,500 h.p. model is about to fly. The British Air Ministry proposes to use these engines in long-range airliners cruising at 300-400 m.p.h. It hopes that Britain's early lead in turboprops will help her to draw even with current U.S. transport types, then perhaps replace U.S. airliners in the international skies, as British steamships once replaced U.S. sailing clippers on the seas.

Some U.S. air engineers believe that turboprops will be used instead of piston engines on commercial planes that fly from 400 to 500 m.p.h. They dread the thought that if the U.S. does not develop turboprops more quickly it might have to buy them from the British. Other engineers say, "Why bother with turboprops? Why not jump right to turbojet airliners designed to fly fast enough to show fair economy?"

U.S. military aviation is interested in turboprops for use in future bombers. Meanwhile it is happy with turbojets. Last week the Air Force signed contracts for $367 million in engines, more than half of them jets. Unknown (but very large) sums are being spent on development. In that part of the speed spectrum where it is thoroughly at home (550 m.p.h. and up), the jet plane is a solid military reality.

New Beauty. For the layman, the most conspicuous thing about such successful operational jet types as the Republic F-84 Thunderjet and the Grumman F9F Panther is their extraordinary beauty. They have no propeller to break their flowing lines, no clumsy bulk to house a broad-hipped piston engine. The body is slim and dart-shaped; the wings are thin. There are no bumps or excrescences to roil the air. Everything has been tucked away inside the smooth skin.

When built into an airplane, the turbojet engine runs much more quietly than it does on a test stand. Much of the ultrasonic snarl is gone. Even the blowtorch roar is less. When the engine starts in full sunlight, no trace of flame can be seen, only the blast of transparent gas shooting out the tailpipe. It expands in an even cone, and its heat makes it shimmer like watered silk. On each edge, as it leaves the tailpipe, is a slightly darker band. This is turbulence where the hot gas is mixing with still air. When the plane takes off, the jet roars a little louder (not very loud). Easily, without apparent effort or thunder of flailing propeller, the plane slides into the air, trailing its silken wake.

To fly as a passenger in such a plane (the Lockheed TF-80C is the only two-place fighter-type jet in the U.S.) is an oddly soothing sensation. The cockpit is remarkably quiet for a military airplane. Little engine noise gets into it; most of the roar and snarl is blown back with the wake. The air ducts grumble below the floor; a ventilator hisses. When the plane is up to speed, the airstream rushing over the canopy makes a moderate, roar. There is hardly any vibration. Experienced pilots say that the plane handles "like a kiddie-car." When it makes a "low pass," flying close to the ground at 550 m.p.h., objects far ahead seem to vanish before the eye has time to take a second look.

But the designers' sights are aimed at higher things than the speeds of these sweet-flying planes. Newer models, such as North American's F-86A, are already nibbling gingerly at the speed of sound, and no one doubts that the turbojet engine can soon push properly designed airframes across the threshold.

Ghosts Know. The speed of sound! That magic and frightening quantity dominates the dreams of high-speed plane designers. It is no mere landmark, no mere handy figure for public relations officers. It is basic: the speed with which a "compression wave" (whether a faint whisper or the crushing shock wave of an atomic explosion) moves through air. The actual speed varies considerably with the air's temperature (the colder the slower). To eliminate this variability from their figuring, scientists have given the speed of sound a special name. In aerodynamics, the speed of sound in any air under consideration is called "Mach I,"* no matter what the actual speed may be.

When an object is moving through air at less than Mach I, compression waves speed out ahead of it. They "warn" the air molecules that a moving body is coming, so the molecules have time to rearrange themselves to flow evenly around it. But above Mach I, the moving body (like the raiding Mongols of the Middle Ages) out-speeds the news of its coming. The air molecules, taken by surprise, are pushed aside sharply.

This is why air designers approach the speed of sound with infinite trepidation. The most troublesome speed begins just below Mach I. When a wing is moving at, say, Mach .80, the air passing over it has to hurry to get around its bulge. If, in doing this, it reaches Mach I, violent things may happen. The smooth airflow breaks into turbulence as hard shock waves jump around on the wing (see cut). The drag increases enormously; the wing's lift drops. The buffeting from the irregular airflow may be strong enough to tear the wing apart. This sometimes happens when a fast subsonic airplane dives too rapidly. The results are hard on the pilot--"as is well known," the training manuals say, "to many ghosts."

The "transonic" (transition) speed is the worst. After the wing gets moving well above Mach I, the air behaves reasonably again, but in a novel manner. From the leading edge of the wing, two intense sound waves flare off like the bow waves of a boat. Two more flare off from the trailing edge. If the moving object has any irregularities or sharp curves, these are apt to trail their sound waves too (see cut).

Brute Plane. The only airplane, so far as is known, to fly faster than sound is the rocket-propelled X-I (TIME, Jan. 5). But the X-I is not a real operational airplane. It is very small and heavy, made largely of metal plates nearly half an inch thick. It carres no useful load except the pilot, some instruments and fuel for two minutes of flight at full power. It smashed through the transonic speed band by sheer brute force, not aerodynamic virtuosity.

To achieve an airplane that will have range and load-carrying ability above Mach i is an extremely difficult problem. Such a plane must be several planes in one. It must take off and land at a practical speed and fly at first below Mach i. It must pass through the dangerous transonic band without being thrown out of control or damaged by buffeting. Then it must deal with the new air behavior and enormous drag encountered above Mach i.

There is no known design that will do all these things and still be a useful airplane. Wings that are efficient below Mach i do not serve above it. The behavior of an airframe in the transonic region is still a frightening unknown. But designers are working hard and hopefully. They are sure that by the time they have the proper airframe, they will have engines with plenty of power for the job. Engine men predict confidently that turbojet engines will work efficiently at least as high as Mach 1.5 (1,145 m.p.h. at 59DEG F.).

Such "future business" of aviation is the special province of the National Advisory Committee for Aeronautics. In three great laboratories (Langley Field, Va., Cleveland, and Ames, Calif.) the earnest, enthusiastic scientists of the NACA are digging out deep-hidden facts about high-speed flight. They put experimental wingshapes in big & little wind tunnels, and test their behavior far above Mach i. They test engines and engine components in wind tunnels too, to see how they behave at great speed, low pressure, low temperature. They devise new, more powerful fuels and high-temperature alloys.

For some speeds, thinks the NACA, the most efficient airplane may be shaped like an arrowhead. For others, it may have short, broad "stub" wings. They do not stop at "moderate" speeds such as Mach 1.5, but think boldly about speeds two or three times as fast. Obstacles do not discourage them. At Mach 4, they calculate, air friction will heat the leading edge (perhaps the whole body) of an airplane to about 1,200DEG F. This is red hot, and above the softening point of ordinary structural metals. "But," say the NACA men, "wings can be cooled artificially if necessary."

The Hungry Speed Animal. Above Mach i, thinks the NACA, another and stranger type of jet engine begins to come into the picture. This is the "ram-jet," which used to be called the "flying stovepipe" before its proper design was found to be enormously difficult. The ramjet does look simple. It is a hollow cylinder open at both ends and subtly shaped inside. When it is moving rapidly, the air coming in the nose is compressed as if by the compressor of the turbojet. Fuel is burned near the point of highest compression. The energy added to the compressed air by combustion shoves a jet of hot, high-speed gas out the rear end with a noise like thunder. There is nothing inside a typical ramjet except fuel nozzles and a gridlike "flame-holder" to keep the flame from being blown out by the airstream.

Ram-jets develop some power at low subsonic speeds, but they are efficient only above Mach i. For speeds even higher, it is possible to design a 20-inch-diameter ramjet that will develop (theoretically) as much as 30,000 h.p. They use a corresponding amount of fuel. Northrop's turbine expert Tom Quayle calls the ramjet "the hungry speed animal."

The NACA, the U.S. Air Force and various private companies are enthusiastic about ram-jets. They think of them chiefly as power plants for guided missiles, those "uninhabited aircraft" with which warring continents might blast one another to rubble from different sides of the earth. Super-enthusiasts think they may have a peacetime future also. A speed-hungry traveler, ramjet propelled at Mach 3, may start from New York at noon and flying west would see the sun sink rapidly in the east. He'd be in Honolulu in time for breakfast the same day.

Why should people want to go that fast, or even at a conservative Mach 1.2? When asked this question, air designers look stunned or hurt. They recover quickly and answer with ringing confidence: "People want to go fast."

*Only when the engine is set up with a pipe to catch its gases is it safe to watch the fires kindle. *The power of jet engines is measured in pounds of thrust. The propulsive horsepower developed varies with the speed. At 375 m.p.h., one pound of thrust equals one "thrust horsepower." * Spruce, tweedy Whittle, 41, comes nearest to being the inventor of the turbojet. Recently the British Labor government, with a grand Old Regime gesture, handed him a tax-free thank you of -L-100,000. *Pronounced mack. Named after Austrian Physicist Ernst Mach, 1838-1916.

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