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2. A French engineer, René Lorin, patented a jet
propulsion engine (fig. 1-1) in 1913, but this was an
athodyd (para. 11) and was at that period impossible
to manufacture or use, since suitable heat resisting
materials had not then been developed and, in the
second place, jet propulsion would have been
extremely inefficient at the low speeds of the aircraft
of those days. However, today the modern ram jet is
very similar to Lorin’s conception.
3. In 1930 Frank Whittle was granted his first patent
for using a gas turbine to produce a propulsive jet,
but it was eleven years before his engine completed
its first flight. The Whittle engine formed the basis of
the modern gas turbine engine, and from it was
developed the Rolls-Royce Welland, Derwent, Nene
and Dart engines. The Derwent and Nene turbo-jet
engines had world-wide military applications; the
Dart turbo-propeller engine became world famous as
the power plant for the Vickers Viscount aircraft.
Although other aircraft may be fitted; with later
engines termed twin-spool, triple-spool, by-pass,
ducted fan, unducted fan and propfan, these are
inevitable developments of Whittle’s early engine.
1
Fig. 1-1 Lorin’s jet engine.
4. The jet engine (fig. 1-2), although appearing so
different from the piston engine-propeller
combination, applies the same basic principles to
effect propulsion. As shown in fig. 1-3, both propel
their aircraft solely by thrusting a large weight of air
backwards.
5. Although today jet propulsion is popularly linked
with the gas turbine engine, there are other types of
jet propelled engines, such as the ram jet, the pulse
jet, the rocket, the turbo/ram jet, and the turborocket.
PRINCIPLES OF JET PROPULSION
6. Jet propulsion is a practical application of Sir
Isaac Newton’s third law of motion which states that,
’for every force acting on a body there is an opposite
and equal reaction’. For aircraft propulsion, the ’body’
is atmospheric air that is caused to accelerate as it
passes through the engine. The force required to
give this acceleration has an equal effect in the
opposite direction acting on the apparatus producing
the acceleration. A jet engine produces thrust in a
similar way to the engine/propeller combination. Both
propel the aircraft by thrusting a large weight of air
backwards (fig. 1-3), one in the form of a large air
slipstream at comparatively low speed and the other
in the form of a jet of gas at very high speed.
7. This same principle of reaction occurs in all forms
of movement and has been usefully applied in many
ways. The earliest known example of jet reaction is
that of Hero’s engine (fig. 1-4) produced as a toy in
120 B.C. This toy showed how the momentum of
steam issuing from a number of jets could impart an
equal and opposite reaction to the jets themselves,
thus causing the engine to revolve.
8. The familiar whirling garden sprinkler (fig. 1-5) is
a more practical example of this principle, for the
mechanism rotates by virtue of the reaction to the
water jets. The high pressure jets of modern firefighting
equipment are an example of ’jet reaction’,
for often, due to the reaction of the water jet, the hose
cannot be held or controlled by one fireman. Perhaps
the simplest illustration of this principle is afforded by
the carnival balloon which, when the air or gas is
released, rushes rapidly away in the direction
opposite to the jet.
9. Jet reaction is definitely an internal phenomenon
and does not, as is frequently assumed, result from
the pressure of the jet on the atmosphere. In fact, the
jet propulsion engine, whether rocket, athodyd, or
turbo-jet, is a piece of apparatus designed to
accelerate a stream of air or gas and to expel it at
high velocity. There are, of course, a number of ways
Basic mechanics
2
Fig. 1-2 A Whittle-type turbo-jet engine.
Fig. 1-3 Propeller and jet propulsion.
of doing this, as described in Part 2, but in all
instances the resultant reaction or thrust exerted on
the engine is proportional to the mass or weight of air
expelled by the engine and to the velocity change
imparted to it. In other words, the same thrust can be
provided either by giving a large mass of air a little
extra velocity or a small mass of air a large extra
velocity. In practice the former is preferred, since by
lowering the jet velocity relative to the atmosphere a
higher propulsive efficiency is obtained.
METHODS OF JET PROPULSION
10. The types of jet engine, whether ram jet, pulse
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