Page 14 - The Jet Engine

Basic mechanics

15. The mechanical arrangement of the gas turbine
engine is simple, for it consists of only two main
rotating parts, a compressor (Part 3) and a turbine
(Part 5), and one or a number of combustion
chambers (Part 4). The mechanical arrangement of
various gas turbine engines is shown in fig. 1 -9. This
simplicity, however, does not apply to all aspects of
the engine, for as described in subsequent Parts the
thermo and aerodynamic problems are somewhat
complex. They result from the high operating tem-
peratures of the combustion chamber and turbine,
the effects of varying flows across the compressor

Fig. 1-7 A pulse jet engine.

the valves to close, and the expanding gases are
then ejected rearwards. A depression created by the
exhausting gases allows the valves to open and
repeat the cycle. Pulse jets have been designed for
helicopter rotor propulsion and some dispense with
inlet valves by careful design of the ducting to control
the changing pressures of the resonating cycle. The
pulse jet is unsuitable as an aircraft power plant
because it has a high fuel consumption and is unable
to equal the performance of the modern gas turbine
engine.
13. Although a rocket engine (fig. 1-8) is a jet
engine, it has one major difference in that it does not
use atmospheric air as the propulsive fluid stream.
Instead, it produces its own propelling fluid by the
combustion of liquid or chemically decomposed fuel
with oxygen, which it carries, thus enabling it to
operate outside the earth's atmosphere. It is,
therefore, only suitable for operation over short
periods.

14. The application of the gas turbine to jet
propulsion has avoided the inherent weakness of the
rocket and the athodyd, for by the introduction of a
turbine-driven compressor a means of producing
thrust at low speeds is provided. The turbo-jet engine
operates on the 'working cycle' as described in Part
2. It draws air from the atmosphere and after
compressing and heating it, a process that occurs in
all heat engines, the energy and momentum given to
the air forces It out of the propelling nozzle at a
velocity of up to 2,000 feet per second or about 1,400
miles per hour. On its way through the engine, the air
gives up some of its energy and momentum to drive
the turbine that powers the compressor. Fig. 1-8 A rocket engine.

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