Roberta Flanders, age 13, of Portsmouth, Ohio, for her question:
HOW DOES A JET ENGINE PRODUCE POWER?
A jet engine builds up inside pressure which becomes power by two methods: burning a fuel that gives off hot gases and squeezing or compressing air that is mixed with fuel.
Some jet engines use only the burning of fuel while others use, a combination of burning fuel and compressing air. Gasoline or some other chemical combination can be used as fuel.
After a jet engine has built up inside pressure, it exhausts burning gases from its tailpipe in a stream called the jet, or jet exhaust. The reaction inside the engine to this jet exhaust drives the engine forward.
Thrust, or the forward driving force, is measured in pounds or in newtons. The thrust stays about the same at all flying speeds, which makes a jet engine particularly useful in high speed aircraft.
At the same time, the jet engine's power or ability to do work in a given period of time increases at higher flying speeds. The jet engine produces thrust by moving a relatively small volume of gases at high speed.
A turbojet is a jet engine that is used in many commercial and military airplanes. It uses either a centrifugal flow compressor or an axial flow compressor to build up thrust.
A centrifugal flow compressor squeezes the air by bringing it into the center of a rapidly spinning wheel which throws the air toward the rim. There it enters a nearly circular expanding passage where its speed decreases and its pressure increases.
An axial flow compressor has several disks with a large number of small winglike blades around their rims, as in an electric fan. These disks are attached to a shaft that spins at about 12,000 revolutions a minute. Between each pair of disks is a row of blades that do not rotate. The air flows parallel to the shaft. Each row of blades compresses the air a little more than did the preceding row. The pressure of the air is raised to 12 times that at the inlet.
After a jet's air is compressed, it rushes into a set of combustion chambers. The air is mixed with fuel and burned. This creates greatly expanded hot gases which rush toward the tailpipe.
Before the gases escape from the engine, they turn a turbine wheel which spins the compressor. This uses energy from the exhaust gases. This turbine wheel must be made of materials which can stand temperatures as high as 1600 degrees Fahrenheit.
To obtain a large thrust from a jet engine, it is necessary to have a large weight of gas exhausted at high speed. The weight of gas in the jet exhaust comes from the air and fuel taken into the engine. A turbojet of moderate size uses about 540,000 pounds of air an hour.
A turbojet engine requires about 1,200 gallons of fuel an hour.