Understanding Jet Propulsion: History, Principles, and Engine Evolution

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

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 center 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 campaign II 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 campaign II collaborated with the Italian aircraft

manufacturers caproni and in August of 1940 the campaign II caproni CC - 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 one seven eight this aircraft

was powered by an engine designed by a young German scientist called Anne's 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 its can't regard and it was never produced in any quantity

on the 8th of April 1941 the first official flight of the Droste 28:39 aircraft took place at Brock Werth 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 fireball 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 f/8 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 Neen 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 mr. Schmitt 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 0:03

model used in the huncle 160 to 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 favor 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 V o 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 gentry

action 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

concerned 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 the line a bee

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

injected