Jet-propulsion can be defined as the force which is generated in the opposite direction
to that of a discharge of fluid under pressure escaping through an opening
The force that makes a lawn sprinkler similar to the one shown here rotate when water flows
through it is one example of jet propulsion that is readily apparent in everyday life
And the thrust that sends rockets like this one into space is another which is perhaps
not such an everyday occurrence
Whatever the form that the device utilizing jet propulsion takes it is essentially a reaction
engine which operates on the principle of the Third Law of Motion, as stated by the
English physicist Sir Isaac Newton in 1687, which is that every action has an equal and
opposite reaction.
The first recorded use of a rear engine was my Hero of Alexandria in 250 BC. Hero's engine,
a representation of which is shown here, consisted of a sphere into which steam was introduced
under pressure. The steam was fed through apertures in the centre of the bearings upon
which the sphere was allowed to rotate. Allowing the steam to escape through nozzles in two
bent tubes mounted opposite one another on the surface of the sphere created thrust which
caused the sphere to rotate around its axis.
It's alleged that hero invented his engine while he was investigating different methods
of opening the doors of a temple in Alexandria
In recent times, one of the first attempts at creating jet-powered aircraft resulted
in the development of a hybrid design. An Italian Secondo Campini designed a system whereby
an external power source, in this case a conventional piston engine, powered a compressor. The output
of the compressor was mixed with fuel the mixture of air and fuel ignited and jet thrust
resulted. Campini collaborated with the Italian aircraft manufacturers Caproni and in August
of 1940 the Campini Caproni C.C.2 flew.
Although the Italians were unaware of it a year earlier than this the Germans had flown
their version of a jet aircraft – the Hankel 178. This aircraft was powered by an engine
designed by a young German scientist called Hans von Ohain
Despite the relatively sparkling performance for that era, it could travel at speeds in
excess of 400 miles per hour, the German air force initially paid it scant regard and it
was never produced in any quantity.
On the 8th of April 1941 the first official flight of the Gloster E.28/39 aircraft took
place at Brockworth in Gloucestershire. Sir Frank Whittle had submitted patents for his
jet engine in 1930 but it was not until 1939 that sufficient financial and technical backing
was found to enable him to manufacture the flyable version of his engine. The firm of
Rover had reluctantly initially being contracted to produce
the Whittle engine, but, Rolls-Royce who could see its potential eventually took over development
of the engine one of which is shown here.
Rolls-Royce reworked the Whittle design to produce the Derwent engine an example of which
is depicted here. This engine was capable in its mark for version of producing two thousand
four hundred and fifty pounds of thrust. Later versions of the engine capable of three thousand
six hundred pounds of thrust were used to power the Gloster Meteor F8, like the one
shown here flying at an open day at Royal Air Force Kemble in the year 2000.
During a visit to the United States in early 1944 the leader of the Rolls-Royce design
team found that General Electric were developing engines capable of producing up to 4,000 pounds
of thrust. As a response to this after his return he initiated a project which culminated
in the Nene engine which at the time was the most powerful engine in the world with 5,000
pounds of thrust.
Possibly one of the most important but least publicized uses of this engine was its incorporation
into the Russian MIG-15 aircraft, which was so effective in the Korean Conflict. One critical
difference between the German engines used in the Heinkel and the later Messerschmitt
262 and those developed from Sir Frank whittles original
engine was the type of compressor employed. While the Whittle engine used a centrifugal
compressor, similar to the example shown here. The German engines like the BMW 003 model
used in the Heinkel 162 utilize an axial flow compressor, similar in
design to the cutaway model shown.
Axial flow compressor x' have several advantages over the centrifugal compressor. For instance
whereas the centrifugal compressor compression ratio is limited to approximately twelve-to-one
when the maximum of two stages are used in series. By adding more stages to an axial
flow compressor compression ratios as great as forty-to-one can be obtained. The term
compression ratio refers to the ratio of the pressure at the outlet of a compressor to
that at its Inlet
A second advantage of the axial flow compressor, almost as important as the first, is that
the mass flow which can be obtained through an axial flow compressor is potentially much
greater than the mass flow which can be achieved through a centrifugal compressor.
As a consequence of these factors the development of the early centrifugal compressor engines
was subjugated in favour of the advancement of the axial flow compressor engines which
continues today.
The principle of the gas turbine engine is basically
the same as that of the piston engine propeller combination – they both propel a mass of
air backwards.
Mass times acceleration equals force. In a gas turbine engine the mass (m) mentioned
in the equation is the air delivered by the compressor. The acceleration in the equation
is the difference in the outlet velocity of the air (Vo) to that of its inlet velocity
(V1) due to the addition of heat energy. Force equals mass times V naught minus V1 which
equals thrust.
With the piston engine propeller combination the propeller drives a relatively large mass
of air backwards fairly slowly while the gas turbine throws a small mass of air backwards
relatively quickly. Newton's Third Law states “for every force acting on a body there
is an equal and opposite reaction”. In the two cases quoted earlier the piston engine
propeller combination and the gas turbine engine. The force created by the mass of air
being thrown backwards and it's velocity generates a reaction in the opposite direction – driving
the aircraft forwards.
It must be remembered that the jet reaction does not result from the pressure of the jet
on the atmosphere In all instances the resultant reaction or thrust exerted on the engine is
proportional to the mass, or weight of the air, expelled by the engine and the velocity
change imparted to it.
The working cycle of the gas turbine engine is called the Brayton Cycle, after George
Brayton an American mechanical engineer who invented the continuous ignition engine – which
was the basis of the gas turbine engine. The Brayton Cycle and the working cycle of the
four-stroke piston engine, the Otto cycle, are very similar – as can be seen in this
diagram.
The induction, compression, power and exhaust strokes of the Otto cycle are each matched
by induction, compression, combustion and exhaust stages in the Brayton cycle. One major
difference however exists, in that in the gas turbine engine combustion theoretically
occurs at a constant pressure, whereas, in the piston engine it occurs, once again theoretically,
at a constant volume. Power is developed in the turbine of the engine.
Other differences between the piston engine and the gas turbine engine concern the continuous
manner in which these processes occur in the gas turbine engine as opposed to the intermittent
procedure occurring in the piston engine. In the piston engine only one of the strokes
is utilized in producing power. The other three are effectively absorbing power. While
in the gas turbine engine the three idle strokes have been eliminated thus allowing more time
for the burning of fuel. This is just one of the reasons why the gas turbine engine
has a greater power weight ratio than the piston engine.
The pressure-volume diagram shown here otherwise known as the Brayton cycle represents the
working cycle of the gas turbine engine in its simplest form. Air, at atmospheric pressure,
enters the engine at point A and is compressed along
the line A-B. Fuel is added in the combustion chambers which is signified by point B and
the mixture is burnt, in theory at a constant pressure. In fact the reduction in pressure
between points B and C indicates that pressure losses do actually occur in the combustion
chamber. The drop in pressure is created by the need to produce the swirl and turbulence
necessary for efficient combustion and this causes a pressure drop throughout the combustion
chamber lengths of between 3 to 6 percent. Notwithstanding this drop a considerable increase
in the volume of the air is generated within the combustion chamber. Between point C and
D the gas generated through combustion expands in the turbine, where mechanical power is
extracted from the energy in the gas stream. And the jet pipe where the remainder of the
gas stream energy provides a propulsive jet as it discharged. In theory the gas stream
pressure attains a value equal to atmospheric pressure before being ejected.