Rudy Lowrey, age 11, of Springfield, Mass., for his question:
WHAT IS A TURBOJET ENGINE AND HOW DOES IT WORK?
A turbojet engine i s a jet propulsion engine i n which the air from the atmosphere is compressed for combustion by a turbo driven compressor. It is made up of a compressor, combustion chambers, a turbine and a tail pipe or exit nozzle.
Turbojet engines are used i n many military and commercial airplanes.
There are two kinds of compressors used in turbojet engines: a centrifugal flow compressor and an axial flow compressor.
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 i n an electric fan. These disks are attached to a shaft that spins at about 1 2, 000 revolutions a minute.
Between each pair of disks in an axial flow compressor 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.
Most modern turbojets use axial flow compressors which raise the pressure of the air up to 12 times that at the inlet.
After the air is compressed, it rushes into a set of combusi on chambers. This creates greatly expanded hot gases which rush toward the tail pipe. Before these gases escape from the engine, they turn a turbine wheel which spins the compressor. This uses energy from the exhaust gases.
The turbine wheel must be made of materials which can withstand temperatures as high as 1,600 degrees Fahrenheit from the hot gases.
To obtain a large thrust, it is necessary to have a large weight of gas exhaust at high speed.
Turbojet engines can produce a thrust ranging from about 2,500 to 30,000 pounds or more.
A turbojet takes i n large amounts of air and fuel. The exact amounts depend on the altitude and flight speed, as well as on the spinning speed of the turbine and compressor.
A turbojet of moderate size uses about 540,000 pounds of air an hour. It also needs about 1,200 gallons of fuel an hour. If a four engine turbojet airplane flies at 600 miles per hour, it travels only about one eighth of a mile per gallon of fuel i t burns.
A turbojet engine i s started by spinning the turbine with auxiliary power. A spark plug i s used to start the fuel burning. After that, the fuel mixture burns steadily as long as the engine keeps running.
Compared with the propeller and piston engine combination, a turbojet i s lighter, smaller, more powerful and faster. It has little vibration. It has the disadvantage of using more fuel at the lower speeds, however.