Science: Fancy Jets

The simple jet engine is a thing of the past. The latest designs are highly efficient and powerful, but they pay for their superiority in complication.

Until recently, jet engines had only one essential moving part: the rotor. The forward end of its shaft spins an air compressor, which usually looks like a series of small windmills on the rear end of the shaft. High-pressure air from the last windmill goes to the combustion chambers where the fuel is burned. Hot gases formed there spin a turbine. The turbine turns the compressor, and the gases that pass through it shoot out the tailpipe in a high-speed jet whose reaction pushes the airplane forward.

This simple design has a bad limitation. All the rows of blades in the single compressor must revolve at the same speed. This works all right up to a compression ratio of about eight to one. Above this ratio single compressors are found to be inefficient, even though they may have as many as 16 rows of blades. Since the efficiency of the engine depends largely on the pressure of its incoming air, the single compressor sets a limit to the engine’s performance.

Two-Spooler. The “two-spool” engine, now in fashion both in the U.S. and Britain, solves this problem by means of two compressors, each driven by its own turbine through concentric shafts. The first compressor, which spins comparatively slowly, compresses the air part way. Then the second compressor, spinning faster, takes over and crams the air into the combustion chambers at much higher pressure (up to twelve to one) than could be reached by a single compressor alone. The result is to increase the power of the engine and to reduce its fuel consumption per Ib. of thrust by as much as 25%.

Practically every engine builder on both sides of the Atlantic has a two-spool model under development. Out in front is Pratt & Whitney Aircraft Co. of Hartford, Conn., whose two-spool J57 is already in limited production. Several British manufacturers are not far behind.

Another limitation of the simple jet engine is under attack too: its lack of “propulsive efficiency” at subsonic speeds. When a jet plane is standing still on the ground, the gases shooting out of its tailpipe at 1,300 m.p.h. spend all their energy in pushing air backward; the propulsive efficiency is zero. When the plane takes off, and flies faster & faster, less & less of the energy is wasted on the air. More of it goes into pushing the plane forward; i.e., the propulsive efficiency increases.

Less Fuel. Supersonic fighters fly fast enough to get good propulsive efficiency, but bombers and transports (up to 600 m.p.h.) do not. Such craft will fly more efficiently with an engine whose gases shoot out of the tailpipe more slowly. Rolls-Royce Ltd. of Britain is testing such an engine, which it calls the Conway, after a river in Wales. Rolls will not give details of its construction, except that it uses the “bypass” principle.

One type of by-pass engine (see diagram) is a two-spooler whose forward compressor passes some of its air around the combustion chamber. This air remains cool, and it does not flow very fast. When it is turned into the tailpipe behind the second turbine, it cools the gases and slows them down. It also adds to their mass. The final result is a heavy, cool, slow jet of gases instead of a light, hot, fast one. Less energy is wasted as heat, and the airplane can get good propulsive efficiency without flying above the speed of sound. Rolls claims that on bombers and transports the Conway will use less fuel than any other jet engine.