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History of the Jet

 

What can we say about Sir Frank Whittle that hasn't already been said on countless websites and in thousands of books?

Well we know most of you will know quite a bit about the man without us patronising you and reiterating it all, but we feel that on a site such as this we are duty bound by our domain name to mention his impact on our lives.

Reading the numerous books on him, some listed on our books page and also links to websites we feel that he wasn't just a great mind and pioneer in aviation, but he was a true engineer in nature, not just on paper.

Why?  Simply because he had the guts and sheer determination to forge ahead when everyone, including the British Government, were against him. Although discouraged with his attempts to bring his ideas to life, he soldiered on in a true British stiff-upper-lip way.  He knew he could do it and he did.

Sir Frank Whittle

Some would call it stubbornness. Some would call it pigheadedness.  We call it sheer courage and perseverance, a quality that is fastly disappearing from the engineering world.  When you've got no money and the faithful friends end up becoming the faithless then there are two ways you can go - up or down. True engineers go up.

We hope when reading this that those of you who would love to have a go at this fantastic hobby will be like us and be damn stubborn and get on with building your engines in the biggest, bodgyest way!

 

So the history of the jet engine then:

 

OK, so most of you know that Hero of Alexandria first thought of the idea of using steam hissing out of a pipe in 120 BC and called it the "Aeolipile". He built it as a toy and unbeknown to him at the time for obvious chronological reasons he obeyed Newton's third law of motion "for every force acting on a body there is an equal and opposite reaction".

As far as history can tell it was never intended for mechanical analysis but merely for amusement. It worked by boiling water in a circular vessel and ejecting the resultant steam at high velocity through opposing nozzles which would cause the vessel, or sphere to rotate under this power.

Hero's Aeolipile

 

 

In the 11th century the Chinese took this principle of Newton's third law (still not alive yet) and applied it to the worlds first recorded rocket. Starting with fireworks (remember the little people invented gunpowder!) and gradually moved on to rockets to propel weaponry. They probably had some great parties with them too.

Ye olde rocket scientist

 

Not a lot happened between the Chinese pioneering their whizz-bang fireworks and the late 19th Century. Apart from things like windmills and watermills, both of which are examples of turbines, or converting the kinetic energy of wind and water into rotary motion or power to drive machinery.

 

 

Some bright spark called Thomas Savery hit upon the idea of developing a steam pressure cooking vessel idea from a chap preceding him by some years called Denis Papin. Savery did this in 1698 by taking Papin's original concept and modifying it thus allowing him to harness the power of the high pressure steam to drive water out of coal mines.

Thomas Savery

Thomas Saverys' Steam Engine

 

Then in around 1712 Thomas Savery partnered up with Thomas Newcomen and developed this weird contraption on the left.  This Newcomen called the "Atmospheric Steam Engine".  It took the high pressure steam of Savery's engine and used it to drive a piston up and then used water to cool the cylinder/steam chamber, thus creating a vacuum on the inside of the cylinder and the differential air pressure from atmospheric caused the piston to operate on its downward stroke.  The only problem was the cylinder needed to be heated by the steam and cooled by the water which was very inefficient.

Thomas Newcomen

Thomas Newcomens' Atmospheric Engine

 

 

Along comes a Scottish chap in 1769 by the name of James Watt with a much improved version of the Newcomen engine. It allowed the steam to be condensed outside of the cylinder so the cylinder could stay at a higher operating temperature therefore vastly increasing the efficiency. The unit of power, the Watt, takes it name from James Watt. One Horsepower (HP) is equal to 745.7 Watts. One Watt is equal to One Volt times One Amp.  Once these new fangled  Watt "steam engines" started to become popular it was the beginning of what we know today as "The Industrial Revolution"

James Watt

 

 

Steam now seemed to be the way forward. In the early 1800's Richard Trevithick, a Cornish mining engineer soon found ways to improve on the Watt engine and made the pioneering step of plonking it on a set of wheels and said "ooh arr" a lot in that Cornish way and ate pasties.  He developed the High Pressure engine, raising the previous design pressures from a tiny 8 psi to a whopping 250 psi.  People said he was mad. They really did.  His creation was the Puffing Devil, seen below.

Then along comes Robert Stevenson who furthered the work of Trevithick and produced the worlds first really useful engine by inventing a way of superheating the steam via boiler tubes inside the firebox which increased the surface area allowing greater energy to be released to create a more efficient engine and harness more power from the same amount of water/coal.

        

Richard Trevithick            Trevithick's Puffing Devil                Robert Stevenson            Stevenson's Rocket

 

Still with us? Still wondering how this ties in with jet engines?

Well in 1884 some industrialist called Charles A. Parsons, an Irishman, decided it would be a good idea if he blew steam inside a casing onto a big fan, known as a turbine. This fan looked like a big waterwheel.  This type of turbine is known as an "IMPULSE" turbine.  It spins as the steam hits its blades, and transfers it power through a shaft to drive machinery. Parsons patented the idea and earned himself a Knighthood in the process after coupling his invention to a dynamo and generating 7.5 kW of electricity.

Across the pond an American chap by the name of George Westinghouse saw this and scaled it up somewhat to generate bigger powers. Gustav de Laval looked upon this work and decided it would be great if he could accelerate the steam to full speed before it hit the turbine. A decent idea as the turbine can then run with any pressure steam, but it ran less efficiently so generally the world stayed with Parson's Impulse Steam Turbine.

High pressure steam that is many times the pressure of nominal atmospheric pressure (1 bar or thereabouts) wants to escape back to atmosphere. It sees a way out via the turbine and in doing so part of its energy is released by turning the turbine.  The turbine is connected to a shaft of which rotates with the turbine. There may be many turbines on one shaft to extract as much energy from the flow as possible. Steam turbines were used in all sorts of places, from generating electrical power to providing the power to drive the propellers on board a ship.

This is exactly how a jet engine, or gas turbine works and what Frank Whittle and Dr. Hans von Ohain saw in the 1930's.

 

Modern Times: Frank Whittle, Hawker Siddeley and Rolls-Royce -

Frank Whittle was born in Earlsdon, Coventry on the 1st of June 1907.

The son of mechanic, his father owning a small workshop in Coventry, Whittle grew up around engines and machine shops and the smell of machine oil.  After a strict upbringing for a timid lad such as Whittle he tried unsuccessfully several times to join the RAF being turned down on account of his height.

Eventually in 1923 after leaving Leamington College he joined the RAF , at the age of 16, as an aircraft apprentice at RAF Cranwell in Lincolnshire.

This was to be his making, proceeding quickly to Officer Cadet and Pilot. Flying was in his blood and his intense interest in the subject led him his thesis on "Future Developments in Aircraft Design" which first described how he intended to propel an aircraft with minimal drag and vast increases in speed and height as he realised the airscrew (propeller) was to be superseded by his own development of the gas turbine jet engine.

Initially, as reported in many a fine tome, Whittle had great difficulty in impressing the Air Ministry (British Government) in particular A.A. Griffith of the Air Research Committee.  Again, he tried hard for several years until in 1936 he formed Power Jets Ltd. with a former RAF colleague R.D. Williams of whom the latter had remembered Whittle's ideas and struggles of years ago, and with J.C.B. Tinling they set about the business of putting together Whittle's idea in the newly acquired workshop space at British Thomson-Houston (BTH) in Rugby who were in the business of making steam turbines.  It was here on the 12th April 1937 that Whittles' first engine, the WU (Whittle Unit) was run up.

Whittle's WU Prototype turbojet

Eventually Whittle and his team outgrew both their works and their welcome. There was a strained relationship with BTH and Power Jets Ltd. moved to the BTH Ladywood Works in Lutterworth, Leicestershire.

While all this was going on a young and brilliant mathematician by the name of Stanley Hooker had just started with Rolls-Royce in Derby under the leadership of Lord Hives.

Within Rolls-Royce everyone was known by their initials, in Hookers case this was to be SGH. Hookers career was to be a long and eventful one within R-R. Leaving to go to Bristol Siddeley in 1948 after a disagreement with Hives but later to return to the company, retire in 1967 and return again in 1971 after the collapse and bankruptcy of Rolls-Royce in 1971 to oversee the RB211 turbofan engine. To this day the company is still known as Rolls-Royce (1971) Ltd.

SGH was crucial in the encouragement of Whittle's idea, and indeed it has been stated that when Hooker reported back to Lord Hives on the invention in Lutterworth and how simple the idea was Hives was to return "we'll soon design the bloody simplicity out of it"

The Gloster E28/39

In May 1941 the first flight approved test engine was ready to provide powered flight for the first time in British skies.  The aircraft was designated the Gloster E28/39 and was powered by the experimental Whittle W1X.  This engine gave 750 lb (340 kgs) of thrust.  The W1X went on to be shipped to the USA to General Electric and was the basis for the Bell XP-59.

To Whittle and his team this was amazing, but Air Ministry officials weren't so impressed as they looked at the figures.  The design specification was for an engine of 850 lb thrust. At the time R-R were producing powerplants (the mighty supercharged 12 cylinder Merlin) at over 1,200 HP.

They didn't understand that thrust in lb and power in HP were not the same thing.  When people saw the E28/39 and its subsequent derivative the Gloster Meteor they came up with all sorts of ideas as to how it worked. Some believed it was sucking itself along!

Whittle had now proved his point and the concept was beginning to make big noise in the Air Ministry.

It is fair to say that across the pond in Germany a chap by the name of Hans von Ohain had coincidentally at the same time begun to think like Whittle and had actually beaten the British to the first powered flight in the Heinkel He178 in June 1937 powered by their He S 3B turbojet

The Ernst Heinkel He178 Turbojet of Germany in 1937

By the mid 1940's jet engines had literally taken off in a big way.  Several engineering firms in Great Britain were now excited with this new powerplant and aircraft firms such as Bristol Aeroplane Company and Hawker Siddelley were now very keen to power their own aircraft with this new efficient beast,

As everyone was now building gas turbine jet engines, some more successfully than others, this initiated a spate of takeovers eventually leading to Bristol Siddelley which gave us two of the most formidable engines of their time: Olympus starting life at 10,000 lb thrust and going on to reach 38,000 lb, powering the Vulcan B1 Bomber with the Olympus 301s and the famous Olympus 593s powering of course Concorde ; and the Pegasus - something that SGH pioneered by adding a stage or two onto an existing Orpheus engine and produced a staggering 21,500 lb for use in the Harrier VTOL aircraft, known colloquially as the Harrier Jump Jet.,

Our own Viper 522 started life on the drawing board and in the factory at Bristol until R-R took them over in 1966.

 

So then: How does it all work?

Ahh, we were building up to this bit...

Right, jet engines are termed Reaction Engines. Imagine this :-

1) Get yourself an ordinary dustbin (trash can if you're the other side of the Atlantic). Not a wheelie bin, don't get all posh on us. Just an ordinary old fashioned dustbin.

2) Turn it on its side (that was easy wasn't it?)

3) Stick a big fan at either end of the bin and connect them with a common shaft. The front fan we call the compressor.

4) Start one of the fans spinning so that it starts drawing air and makes the other fan spin too.

5) Add a continuous mist of fuel into the middle and ignite it.

6) The heat generated by the exploding fuel expands the compressed air that's being drawn in. The air wants to get back to normal pressure - just like it wants to in a balloon - and rushes out as quickly as it can over the fan at the rear end - this we call the turbine.

7) As the hot air hits this fan it makes it turn, like a windmill. This turns the shaft which is connected to the front compressor which draws in more air which so long as fuel is continually burnt in the middle will keep the whole process going,  The exiting air still has enough energy and speed after the turbine has extracted what it needs to drive the shaft and compressor that it provides this thing called THRUST.

8) Do NOT try this with a dustbin.

 

So this thingy is obeying Newton's (remember him from above?) third law of motion "for every force acting on a body there is an equal and opposite reaction". The exiting gasses create a reaction on the front of the engine driving the mass forward - just like a balloon when you let the air out.

In a balloon the air is trying to escape from all directions when it is tied. Each face of the balloon has equal pressure on it. When the neck is let go then the pressure is all at the front of the balloon due to the escaping trapped high pressure air. The air escaping from the neck of the balloon provides thrust in the forward direction.

This is exactly how a jet engine works - the compressed air inside the engine has expanded due to heat (same as a car engine) and wants to get out to return to normal atmospheric pressure, passing the turbines on the way.

So, how can we sum all this up?

Simple.

Suck, Squeeze, Bang, Blow....

The Jet Engine principle

So, it's this principle you have to remember when you're building your own jet engine, whether it be out of a turbocharger or a small microjet for a model or a flippin' big one.

The one important and extremely weird thing about jet engines - with all the mechanics, physics and mathematics are all very well - the technology that make it all work is complex and amazing BUT without a simple tube on the back it is rendered USELESS!

This tube forms the JET PIPE and is where the thrust is developed and focused through and out via a convergent nozzle. Take that off and let the turbine go straight to air and apart from a big noise nothing happens. A piece of hollow tube - that's it!

 

Technology and Types:

First of all, compressors. These fall into two main categories - Centrifugal and Axial.

 

Centrifugal Compressors

Usually only have one or two stages per engine and offer high compression ratios per stage but at the expense of weight and losses due to the air flow having to be turned through 180 . Whittles first engines were all of this ilk and for a while in the early days this was all they built.  Each stage needs a compressor wheel, diffuser and casing.

These are the sort of compressors that are in turbochargers and superchargers.

 

 

 

 

Axial Compressors

These are the ones we are most familiar with when looking at modern jet engines. They are multistage compressors, each stage having a rotor that turns with the shaft and a set of stator vanes that are static in between each rotor that turn the air and direct it efficiently at the next stage.

Low pressure changes between stages are achieved so lots of stages are needed, typically between 6 and 10. Sometimes split between a low pressure and high pressure sections with each section rotating at a different speed and having different sized rotors being driven from separate shafts (one inside the other) - referred to as aTwin Spool. engine.

INTERESTING POINT : The Pegasus engine used on the Harrier VTOL aircraft utilises a twin spool engine, with a difference.  This being contra-rotating shafts (spinning in opposite directions) to counteract the gyroscopic effect of a spinning body to keep the aircraft stable when in flight performing VTOL manoeuvres.  In helicopters the rear rotor (the little one) provides the same stability by counteracting the direction of the main rotor. If the tail rotor was not there then the helicopter would screw itself into the ground!

In some helicopter engines they use a mixture of axial flow and centrifugal flow compressors, typically a 3 stage axial feeding a single stage centrifugal, mainly for size advantages.

 

TurboJet

The first of the family to be produced historically and the simplest concept as described above. Compress the air, ignite fuel and use the expanding hot gasses to drive the turbine and the rest for thrust.

 

 

 

TurboFan

These fall into two categories :

Low Bypass - Like the R-R Spey and the Eurojet EJ200 (as used in the Eurofighter) and the Olympus engines. The front fans (1-3) provide more air than is needed and some of it is taken by the core engine, the rest divereted around the engine and mixed with the hot gas in the jet pipe. The most common engine now found in military aircraft and usually employ reheat (afterburner)

 

High Bypass - Like the R-R Trent, RB711, GE 90, etc of the passenger and private jet range of aircaft (747, HS125, bizjets like the Lear Jet.

These use one large fan at the front that provides about 90% of the total thrust. A large fan providing large air movements at relatively low rotational speeds. The R-R Trent range and RB211 range actually use a three spool system.  The rest of the air is used by the core engine as a gas generator for the big fan which acts like a big enclosed low drag propeller.

 

 

TurboShaft

These are used mainly in helicopters. Thrust is not developed in the jet pipe as fast moving escaping air. In the jet pipe a separate second stage of turbines, not connected to the main shaft, ie FREE RUNNING, more commonly known as a Free Power Turbine. This is connected via a gearbox to the rotors.  The exhaust of these engines is low velocity hot air, all the energy having been extracted by the free power turbines.

 

 

TurboProp

An early examples of this is the R-R Dart used on the Fokker Friendship, the first passenger carrying plane to use jet engines. This is where the main shaft along with driving the compressor stage also is directly coupled to a gearbox at the front of the engine to drive a propeller. A mixture of old and new!

 

 

 

 

Reheat / Afterburner.

As jet engines by comparison to their piston equivalents are more efficient and continuously burning there is enough oxygen left in the exhaust jet stream to facilitate further burning.

Remember how the compressed air when heated had so much energy that it wanted to explode and get back out to normal pressure? Well in the jet pipe we can add more fuel and ignite it just after the turbines causing that familiar flame shooting out the back increasing the thrust by up to 50% extra.