Monday, August 22, 2011

The Great Step Forward

Going over some old thoughts the other day.  Pondering, as it were, the way jet engines have changed over the years.
No change in the basic theory, of course, but a lot of refinements and improvements in materials.
Of course, the manufacturers have put in a lot of effort to fine tune engines and improve gas flows and aerodynamics—especially in the turbine but there has been one thing that, to my mind, was the real turning point.  That moment in time when jet engines ‘grew up’.

Before I get into that, let’s just think about the ‘old days’ of  gas turbines.  Aero-gas turbines as opposed to hulking great industrial LP gas turbines with, or without, closed loop systems as made by the likes of Solar and ASEA Brown Boveri.

[NB: The Solar Saturn engine, first designed in 1950 for the US Navy and produced in 1960, went on to become the world's most widely used industrial gas turbine with some 4800 units in 80 countries. It remains in production today in two up-rated and enhanced configurations.]

Our good old ‘clunky’ engines were highly reliable and ruggedly built, for the most part.  They were pretty heavy for their thrust output but the idea was to contain all that heat and energy.
Of course, many of the old engines were centrifugal flow.  The wonderful sounding Derwent, Ghost, Goblin were all centrifugal flow as were the first turbopropellers—chief among them being the redoubtable Dart, still in use today.  First produced in the late 1940s, it powered the first Vickers Viscount maiden flight in 1948, and was still in production until the last F27s and H.S 748s were produced in 1987.
These centrifugal flow engines were very robust and had very simple systems; the fuel system on the Dart is a model of simplicity.

“Whittle… stressed the great simplicity of his engine. Hives [Director of Rolls Royce] commented, ‘We’ll soon design the bloody simplicity out of it.’ ” [From Genesis of the Jet]
How successfully they have achieved that!

Is an interesting web page—especially the early history.]

Of course there were axial flow engines, too.  The Avon, Sapphire, Viper were all axial flow as were many of the early Pratt & Whitney engines.
Some engines were compounded.  The reverse flow Proteus turbopropeller engine had axial flow and a centrifugal last stage to ‘turn’ the air around the corner into the combustion section.  This is still something that is widely used, not least by the wonderful PT6 engine.
All these engines had alloy front ends with aluminised mild steel rear compressor sections and outer combustion casings.  The materials used were fairly standard in those days, the hot parts were various forms of nimonic—‘Hastalloy’, ‘Waspalloy’ (Registered Trade names of ‘Special Metals Group), which is a nickel alloy in various guises.
Materials have moved on.  We still use nimonics in conjunction with crystals for turbine blades but there is increasing use of lighter and stronger metals like titanium.  There is also more use being made of composites here and there on colder parts.

[NB: The ‘colder’ parts are not all ‘cold’.  Remember that the temperature of the gas coming off the compressor of a twin spool engine like a ‘Spey’ is around 550°C which is the same temperature as the glowing end of a cigarette when you suck on the other end.]

So where is the ‘breakthrough?  Is it the multi-spool engine like the JT9D or Spey that led to the Triple Spool RB211—a machine of great beauty and elegance?
Is it the development of high by-pass engines for the military airlift aeroplanes?
No, no.  None of these.
Lets go back to the Viper.
No, really!
The Viper was developed from the Adder engine that was developed, in turn, from the Mamba that came from a Metrovick project.  The twin Mamba was successfully installed in the Gannet and the Adder engine was the prototype Saab Draken engine.
‘Power by the Hour’ leasing was started in those days as the Viper had maintenance issues resulting from it being developed as a limited life engine (10 hours) for the Jindivik target drone.  Operators would pay a fixed hourly rate to Bristol Siddeley for the continual maintenance of the engines.
There was one major step forward.
But the main one was that these engines had vaporising burners.  Not a big issue.  Vaporising, hockey stick, burners were very efficient but hard to start so atomising burners (4) were used as well to start the engine and get the vaporising burners going.  The atomising burners would remain burning while the engine was running.
How is that a major leap forward?
Because they were in an annular combustion chamber.
Why is that so significant?
This engine, like the constant volume engine and the external combustion engine is a heat cycle machine.  It relies for its effectiveness and operation on the addition of heat energy.
Irrespective of the temperature of the gas coming off the compressor, fuel is burnt in the combustion chamber in order to add (heat) energy to the working fluid (air).
Separate combustion cylinders and can-annular (cannular) systems are limited to the number of burners they can hold and control—in terms of flame shape; this includes multiple burner systems on early Pratt & Whitney engines.
But annular chambers can have as many burners as you can fit in them.  This means you can add as much heat as the engine will take without a disproportionate increase in temperature.

Now, from that, we leap forward.  Old engines ran at 10:1 compression ratios.  It was thought that the maximum possible would be around 20:1 before all sorts of problems occurred.
Now we are running at over 40:1 compression ratio on a routine basis.  We have high by-pass engines where 80% of the airflow is going down the by-pass duct and doing most of the work and that means that the fan is being driven by 20% of the airflow in a small core engine.
Huge amounts of power are generated in the core turbine to drive the big fans which means that the pressure ratios must be higher, the temperatures are higher, the stresses are higher.
All this is possible because the amount of heat that can be added is exponentially increased by the use of annular combustion chambers

And that, dear friends, is all thanks to Bristol Siddeley and the humble, disposable, Viper engine.

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