The turbojet engine is a highspeed, highaltitude powerplant.
The Army, at present, has no requirement for this type of engine.
Because it is simple and easy to operate and maintain, however, the
primarily one rotating unit, the compressor/turbine assembly. The
turbine extracts from the gas stream the energy necessary to rotate
the compressor. This furnishes the pressurized air to maintain the
engine cycle. Burning the fuelair mixture provides the stream of
hot expanding gas from which approximately 60 percent of the energy
is extracted to maintain the engine cycle. Of the total energy
development, approximately 40 percent is available to develop useful
If we had ten automobile engines that would equal the total
shaft horsepower of a turbine engine, it would take six of these
engines to turn the compressor, and the other four would supply the
power to propel the aircraft. The amount of energy required to
rotate the compressor may at first seem too large; however, it should
be remembered that the compressor is accelerating a heavy mass
(weight) of air towards the rear of the engine. In order to produce
the gas stream, it was necessary to deliver compressed air by a
mechanical means to a burner zone. The compressor, being the first
rotating unit, is referred to as the N1 system.
With a requirement for an engine that delivers rotational
shaft power, the next step is to harness the remaining gas stream
energy with another turbine (free turbine). By connecting the
turbine to a shaft, rotational power can be delivered to drive an
an air cushion vehicle (ACV), or whatever is needed. The power shaft
can extend from the front, back, or from an external gearbox. All of
these locations are in use on various types of Army engines at
The following sketch shows a turboshaft engine with the power
shaft extended out the front. The bottom sketch shows the same
engine with the power shaft extending out the back.
The basic portion of the turbine engine, the gas producer,
extracts approximately 60 percent of the gas stream energy
(temperature/pressure) to sustain the engine cycle. To develop
rotational shaft power, the remaining gas stream energy must drive
another turbine. In Army engines today, a power turbine that is free
and independent of the gas producer system accomplishes this task.
The power turbine and shaft (N2 system) are not mechanically connected
to the gas producer (N1 system). It is a free turbine. The