Bypass in the Middle

Recent pictures of new Russian bombers caused a flurry of speculation among engine experts. The Tupolev Bison, for instance, is about as big as the U.S. B-52. which has eight J57 jet engines, each rated at 10,000 Ibs. of static thrust (but capable of substantially more). But the Bison has only four engines, which led the experts to conclude that they must have at least 15,000 Ibs. of thrust.

Close study of the Bison's air intakes, which are 6 ft. in diameter, has convinced some experts that the new Russian engines are merely large, clumsy turbojets built on old principles and probably rather inefficient. Others draw a less comforting conclusion: that the large intakes point to bypass engines, a much discussed type that may prove ideal for long-range, high-speed bombers.

The Too-Fast Stream. One of the disadvantages of the conventional turbojet engine is its lack of propulsive efficiency. The stream of hot gas from its tailpipe speeds backward much faster than the airplane is flying forward. Much of its energy is wasted in creating air turbulence. The efficiency would be greater if the speed of the gas stream were only moderately greater than the speed of the airplane. But conventional jet engines cannot slow their gas stream without losing efficiency in other ways.

One cure for this situation is the turboprop, which is coming into use in transports, notably the British Viscount. Most of the energy that it develops spins a geared propeller that moves a large volume of air comparatively slowly and yields almost ideal propulsive efficiency. But propellers have many failings at high speed, and few enginemen think they will serve above. 450 m.p.h.

The bypass engine is an intermediate type that stands midway between the turbojet and the turboprop. It has two compressors, each driven by its own turbine through its own shaft (see diagram). Some modern turbojets have this arrangement too, but all the air that is compressed passes through the combustion chambers to form the high-speed jet. In the bypass engine, part of the air from the forward compressor flows around the combustion chambers (incidentally cooling trie engine's skin) and mixes with the speeding gas in the tailpipe. It cools the stream and slows it, but adds greatly to its mass. The net result is a large, comparatively slow stream that does not waste a large amount of energy by outspeeding its airplane.

Range & Quiet. Bypass engines are hard to design, and few of them have been built. Probably the leading model is the British Rolls-Royce Conway, which has been ordered for the Vickers 1000 airliner. Critics say that it does not bypass enough air to yield full efficiency, but Rolls-Royce claims that it will give top range and safety to airplanes flying above the practical speeds of turboprops.

Another advantage may prove more important than efficiency. The bypass engine is comparatively quiet, and this is a vital virtue for airlines that fear to fly screaming jets from airports besieged by embattled neighbors. The bypass principle can even silence the afterburner, whose bone-shaking thunder would otherwise keep it from being used to get heavy transports off the ground.

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