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jet, rocket, gas turbine, turbo/ram jet or turbo-rocket,
differ only in the way in which the ’thrust provider’, or
engine, supplies and converts the energy into power
for flight.
11. The ram jet engine (fig. 1-6) is an athodyd, or
’aero-thermodynamic-duct to give it its full name. It
has no major rotating parts and consists of a duct
with a divergent entry and a convergent or
convergent-divergent exit. When forward motion is
imparted to it from an external source, air is forced
into the air intake where it loses velocity or kinetic
energy and increases its pressure energy as it
passes through the diverging duct. The total energy
is then increased by the combustion of fuel, and the
expanding gases accelerate to atmosphere through
the outlet duct. A ram jet is often the power plant for
missiles and .target vehicles; but is unsuitable as an
aircraft power plant "because it requires forward
motion imparting to it before any thrust is produced.
12. The pulse jet engine (fig. 1-7) uses the principle
of intermittent combustion and unlike the ram jet it
can be run at a static condition. The engine is formed
by an aerodynamic duct similar to the ram jet but,
due to the higher pressures involved, it is of more
robust construction. The duct inlet has a series of
inlet ’valves’ that are spring-loaded into the open
position. Air drawn through the open valves passes
into the combustion chamber and is heated by the
burning of fuel injected into the chamber. The
resulting expansion causes a rise in pressure, forcing
Basic mechanics
3
Fig. 1-4 Hero’s engine - probably the earliest
form of jet reaction.
Fig. 1-5 A garden sprinkler rotated by the
reaction of the water jets.
Fig. 1-6 A ram 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.
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 temperatures
of the combustion chamber and turbine,
the effects of varying flows across the compressor
Basic mechanics
4
Fig. 1-7 A pulse jet engine.
Fig. 1-8 A rocket engine.
Basic mechanics
5
Fig. 1-9-1 Mechanical arrangement of gas turbine engines.
Basic mechanics
6
Fig. 1-9-2 Mechanical arrangement of gas turbine engines.
and turbine blades, and the design of the exhaust
system through which the gases are ejected to form
the propulsive jet.
16. At aircraft speeds below approximately 450
miles per hour, the pure jet engine is less efficient
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