Animated Engines, Jet Propulsion
|I’ve grudgingly included this section by popular
request. Rocket and turbojet engines are fabulous technological
achievements–But they’re so simple the animations are boring!…At least I think so. You be the judge!
The rocket engine is the simplest of this family, so I’ll
start with it.
In order to work in outer space, rocket engines must carry
their own supply of oxygen as well as fuel. The mixture is injected into
the combustion chamber where it burns continuously. The
high-pressure gas escapes through the nozzle, causing thrust in the opposite
direction.To illustrate the principle yourself, inflate a toy balloon and release
it (without tying it off!). …rocket propulsion at its simplest.
The turbojet employs the same principle as the rocket. It burns oxygen from
the atmosphere instead of carrying a supply along.
similarities: Fuel continuously burns inside a combustion chamber just
like the rocket. The expanding gasses escape out the nozzle
generating thrust in the opposite direction.
Now the differences: On its way out the nozzle, some of the gas
pressure is used to drive a turbine. A turbine is a series
of rotors or fans connected to a single shaft. Between
each pair of rotors is a stator — something like a stationary fan. The stators
realign the gas flow to most effectively direct it toward the blades of the next rotor.At the front of the engine, the turbine shaft drives a compressor. The compressor works a lot like the
turbine only in reverse. Its purpose is to draw air into the engine and pressurize it.Turbojet engines are most efficient at high altitudes, where the thin
air renders propellers almost useless.
The turboprop is similar to the turbojet, except that most of the nozzle gas
pressure drives the turbine shaft — by the time the gas gets past the
turbine, there’s very little pressure left to create thrust.Instead, the shaft is geared to a propeller which creates the majority of the thrust.
‚Jet‘ helicopters work the same way, except that their engines are connected
to the main
rotor shaft instead of a propeller.
Turboprops are more fuel efficient than turbojets at low altitudes,
where the thicker air gives a propeller a lot more ‚traction.’ This
makes them popular on planes used for short flights, where the time
spent at low altitudes represents a greater percentage of the overall
The turbofan is something like a compromise between a pure
turbojet and a turboprop. It works like the turbojet, except that
the turbine shaft also drives an external fan, usually located at the
front of the engine.The fan has more blades than a propeller and spins much faster.
It also features a shroud around its perimeter, which helps to capture and
focus the air flowing through it. These features enable the fan to generate some thrust at
high altitudes, where a propeller would be ineffective.
Much of the thrust still comes from the exhaust jet, but the addition
of the fan makes the engine more fuel efficient than a pure
turbojet. Most modern jetliners now feature turbofan engines.
As you can see all of these engines are conceptually very simple,
and have very few moving parts, making them extremely reliable. They also
have an excellent power-to-weight ratio, which is partly
why they’re so popular in aircraft.Like most of my illustrations, these
are extremely simplified. Turbine engines often employ more than one
shaft and have other more complex features that I really don’t understand
and, frankly, don’t care to investigate further.
For some terrific illustrations and a lot more information on these
engines, see the NASA web site:
…Now, don’t you think the other engine pages are a lot more fun?
2001, Matt Keveney. All rights reserved.