Robert Gilchrist, age 10, of Salt Lake City, Utah, for his question:

WHEN WAS THE JET ENGINE INVENTED?

Jet engines in aircraft, and rocket engines also, work on Newton's old law of action and reaction. The law of

physics,states that any push in one direction causes an equal push in the opposite direction.

A Chinese scientist in about 1500 was one of the first to try rocket power. Named Wan Hu, the adventurer strapped himself into a chair equipped with kites for wings to which were attached 47 huge cylinders filled with gunpowder. He anticipated pressure and thrust. There was a very large explosion in which everything vanished, including the pilot.

For years the principle of jet propulsion was known, but it wasn't until 1940 that the first successful jet propelled aircraft flight was made. It happened in Italy. A Caproni Campini CC2, powered with a jet engine built by Isotta Fraschini, flew for 10 minutes. A year later the CC2 flew from Milan to Rome at about 130 miles per hour.

A Gloster monoplane powered by a jet engine developed by Air Commander Frank Whittle in England made a most successful flight in 1941 and in the same year Bell Aircraft of Buffalo, N.Y., was given an order for an American version powered with two of the jet powered Whittle engines. In October, 1942, the Bell P 59A Airacomet made a successful maiden flight with Robert Stanley, former gliding and soaring champion, as test pilot..

Then came World War II. The Germans were successful with a jet propelled program which included the V 2 unmanned rockets.

Jet engines receive power from the air. The engine compresses the air and also heats it with both actions producing power. After air is taken in and compressed, it is ejected into a series of chambers where it is superheated by the introduction of burning fuel.

The air within the jet engine expands and then rushes from the heating chamber on through the blades of the turbine wheel on the rear of the compressor shaft and out the rear vent of the jet engine in a powerful thrust of many thousands of pounds. This staggering thrust of air produces an opposite thrust on the engine and on the plane to which it is attached.

It is this opposite thrust rather than the blast of air behind the plane that moves the engine and the plane forward in jet propelled flight. Actually, air outside the jet aircraft hinders rather than helps in the operation. Air behind the jet retards its escape from the engine, and air ahead of the plane retards its progress through the air.