Jason Davis, age 13, of Longview, Wash., for his question:

CAN YOU EXPLAIN JET PROPULSION?

Jet propulsion is the thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high velocity liquid or gaseous stream.

A simple example of jet propulsion can be found in the motion of an inflated balloon when the air is suddenly discharged. When the stem is released, the internal pressure is less at the open end than at the opposite end, causing the balloon to dart forward. The balloon is propelled not by the pressure of the escaping air pushing against the outside atmosphere but by the difference between high and low pressures inside the balloon.

An actual jet engine does not operate quite as simply as a balloon, although the basic principle is the same.

More important than pressure imbalance is the acceleration to high velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the gas to flow backward, forming the jet.

High thrust engines require both large rates of mass flow and high jet exit velocities, which can only be achieved by increasing internal engine pressures and by increasing the volume of the gas by means of combustion.

Jet propulsion devices are used primarily in high speed, high altitude aircraft, in spacecraft and in missiles. The source of power is a high energy fuel that is burned at intense pressures to produce the large gas volume needed for high jet exit velocities.

The oxidizer required for the combustion may be the oxygen in the air that is drawn into the engine and compressed, or the oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding atmosphere.

Engines that depend on the atmosphere for oxygen include turbojets, turbofans, turbo props and ramjets.  Nonatmospheric engines are usually called rocket engines.

All atmospheric engines depend on the flow of a large mass of air that is first compressed, then used to oxidize fuel, and finally expanded to low pressure though a nozzle in order to achieve a high jet exit velocity.

The most widely used atmospheric engines are turbojets. After air has been drawn into the engine through an inlet, the air pressure is increased by a compressor before it enters the combustion chamber. The power required to drive the compressor is provided by a turbine that is placed between the combustion chamber and the nozzle.

Practically all airborne jet engines use an axial flow compressor, in which the air flows generally in the direction of the shaft axis through alternate rows of stationary and rotating blades, called stators and rotors. The blades are arranged so that the air enters each row at a high velocity. As it flows through the blade passage the air is decelerated to a lower velocity, thereby increasing the pressure.

Modern axial flow compressors can increase the pressure 24 times in 15 stages, with each set of stators and rotors making up a stage.