Well-Behaved Engine

Every new airplane engine starts out in life as a fairly simple machine but gets more & more complicated as it grows up. Ram jets are no exception. The early models were hardly more than hollow cylinders--enchantingly simple, but also extremely inefficient. Their "fires" often blew out, and there was no adequate way to control their power output. So the engineers went to work to educate the ram jet and teach it how to act as a well-behaved engine should. Recently the Wright Aeronautical Corp. told how a grown-up supersonic ram jet works.

The model shown was designed to work best at Mach 3 (about 2,000 m.p.h.). The air "ramming" in at the open front is slowed down and compressed in the ring-shaped space between the outer shell and the pointed inner section. Some of the compressed air is diverted by a scoop and used to run a turbine and drive the fuel pump. The rest is mixed with fuel and fired by a small flame that burns in the shelter of the conical igniter. The hot gases roar out through a nozzle lined with heat-resistant ceramic. Their reactions propel the machine through space.

Push & Pull. To overcome the bad habits of earlier ram jets, Wright engineers have installed in the new model a complicated system of instruments that measure temperature, pressure, etc. As the ram jet changes speed and altitude the instruments feed in just the amount of fuel needed to keep the engine working at top efficiency. If a sudden change of conditions makes the main flame go out, it is reignited immediately by the small, sheltered flame.

So far, Wright ram jets have "flown" only in a monstrous test chamber at the Wright plant at Wood-Ridge, N.J. Compressors blow air into the ram jet's nose. Simultaneously, three steam "ejectors,"' fed with steam from the plant's main boilers, pull combustion gases out of the ram jet's exhaust. By regulating the compressors and ejectors, the engineers can feed the ram jet with air of almost any speed and density. It is no trick at all to make it act as if it were speeding 2,000 m.p.h. at an altitude of 15 miles.

For Better Missiles. The biggest ram jet tested so far is 20 inches in diameter, but bigger ones are on the way. Exact details are a military secret, but Wright engineers are already talking about a monster (probably no bigger than an automobile) that will develop 75,000 h.p.--about one-third as much as the Queen Mary.*

These Paul Bunyan engines are intended primarily for expendable guided missiles. Since a ram jet does not have to carry its own oxygen, as a rocket does, it is seven times more efficient. But since ram jets have no power at all when standing still, the missile will probably be tossed into the air by a great booster rocket. When it reaches a sufficient speed (something like 500 m.p.h.) the ram jet will take over. Wright's experts also foresee another use for the big new ram jets: when airplanes fly at 2,000 m.p.h., they believe a ram jet will power them.

* The thrust-horsepower developed by a ram jet increases fantastically with its speed. This is because the engine swallows more air at high speed and therefore can burn more fuel, and also because its jet has more propulsive efficiency. Roughly, the power of a ram jet increases with the square of the speed. If it develops 5,000 thrust-horsepower at 500 m.p.h., for instance, it will develop 20,000 thrust-horsepower at 1,000 m.p.h., and 80,000 at 2,000 m.p.h.

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