Hybrid Vigor

Turbojet engines are fine for fast fighter planes and fast, short-range bombers. But in long-range bombers and commercial airplanes there is much to be said for the dependability and fuel economy of the old-fashioned piston-and-propeller engine.

This week the Air Force released a rather sketchy description of a sort of hybrid of the old & new: Pratt & Whitney's Wasp Major-VDT. Basically, it is the 28-cylinder, 3,500-h.p. Wasp Major used in the Air Force's B50 bombers. Combined with a "variable discharge" turbosupercharger manufactured by General Electric Co., it turns into a new engine with greatly increased power and flexibility.

The hot, high-pressure exhaust gases from the cylinders spin a two-stage turbine that drives a compressor. The air from the compressor passes through a cooler, which gets rid of the heat of compression, makes the air contract and become denser, able to burn more fuel. The dense supercharged air goes into the piston engine, burns with the fuel and passes on to the turbine. As it shoots out through a tailpipe, it exerts several hundred pounds of jet thrust.

The discharge opening can be varied in size to get the best division of exhaust energy between supercharging and jet thrust. The result is an engine with 4,000 (instead of 3,500) h.p., without proportionate increase in weight. The Air Force intends to use the hybrid in its newest heavy bomber, the B-54.

The other big manufacturer of piston engines, Curtiss-Wright Corp., is making a hybrid (for the Navy) on a different plan. Its Turbo-Cyclone 18 is a regular, 18-cylinder piston engine whose exhaust drives three turbines geared directly to the crankshaft. The energy recovered gives the engine more horsepower with over 15% more fuel economy.

Neither of these combinations is novel to engine designers. Both have been discussed for many years as promising possibilities. Technical improvements discovered during the development of the turbojet engine made them practical. The turbojet's threat to piston engines made them necessary.

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