1940's German Jet Technology
During the mid 1930’s both Britain and Germany were engaged in
the pioneering development of jet propulsion. Companies such as Bramo,
BMW, Heinkel, and Junkers being lead by the likes of Hans-Joachim Pabst
von Ohain in Germany, in contrast to Frank Whittle, de Havilland, Rolls-Royce
and Metropolitan Vickers in Britain were all major players in this advanced
field. Building a reliable and powerful turbo jet engine was the goal,
however this was no easy task and all the company’s involved were
working on the limits of the materials and technology of the period.
For these countries to build a turbo jet engine two avenues are available
in the basic design; the first being the ‘Centrifugal flow’ turbo
jet engine and the second and more challenging engine concept is commonly
referred to as an ‘Axial flow’ turbojet engine.
At first both countries explored the two concepts with various engines
being built and tested to destruction. But at some point during the late
1930’s, Britain and Germany struck very different paths. Here in
England designers pursued the centrifugal compressor concept because
of better reliability and build quality issues, where as in Germany;
engineers strove to pioneer the more complex axial flow turbo jet design
with greater future development potential. So in the early days of the
war it was this development potential that probably pushed the Luftwaffe
and the German Air Ministry to axe most centrifugal engine developments
in favour of the more complex axial flow designs being currently developed.
And hence the very different look to the aircraft that would be powered
by these engines in the late stages of the war.
So what’s the difference?
Well, a ‘Centrifugal flow’ turbo jet engine [see figure A] works by sucking air though a frontal intake and throwing the air out at 90° to the fan blades and axial under the centrifugal forces created by the spinning fan, once through this first fan known as a ‘compressor’ the air has reached over 2-3 atmospheres of pressure. This air, now under pressure is directed into the combustion chamber by stator guide vanes where fuel is sprayed and the now oxygen rich mixture is ignited. The resulting hot and expanding gases are forced to escape to the rear of the engine, but on there way out they pass through the turbine which is basically another fan, but with twice as many more blades as the compressor fan. As the hot gas’s rush past this turbine they cause it to turn, but this turbine is coupled by a shaft referred to as the ‘axial’ to the compressor fan at the front of the engine, causing it to turn faster and forcing more air to be drawn in, and so this cycle continues, faster and faster till the revolutions per minute of the compressor and turbine fans can reach up to 100,000 rpm in some cases. The advantages of this engine design lie in being a simpler engine to construct and in its day, very reliable in operation with more than adequate thrust output for the task in hand.
The big disadvantage with this type of engine is that you can only have a maximum of two compressors and thus a limit to the maximum thrust you could ever achieve in future design improvements; they also tended to be heavy [power to weight ratio] with a large cross-section forcing plane designers to create a fat body to contain such engines.
However the ‘Axial flow’ engine [see figure B] differs by way of using multiple compressor fans on one axial or shaft to drive the air in a rearward direction into an ever decreasing space, rather than being thrown out side ways as in a centrifugal compressor idea. The advantages of this engine is that you can have many more compressors, sometimes as many as eleven fans [known as compressor stages] at the front of the engine to give huge increases in the volumes/pressure of air drawn in, maybe up to 30 atmospheres of pressure, thus giving the engine greater thrust and offers massive potential for future engine development.
Another benefit is that these engines have a small frontal cross-section allowing for more lean shaped airframes. The disadvantage is the difficulties of designing and constructing a reliable engine, to help you do this, you need for it’s time some exotic materials which the Germans simply did not have during the later part of the war. Because of these shortages German turbo jet engines had a very short operational life of around 10-25 hours before a major rebuild was required. This shortage also contributed to engine throttling problems and forced pilots to make slow approaches on landing giving experienced allied pilots an easy kill.
‘Axial flow’ engine
In Britain, the RAF’s own jet fighter project the ‘Gloster Meteor’ made its first test flight in April 1942, powdered by a de Havilland built centrifugal engine later to be named the ‘Goblin 35’. Rolls-Royce took Frank Whittles pre war designed centrifugal flow turbo jet engine and put it into full production as the RB23 Wellend and built some 167 units from September 1943. These engines were then fitted to the Gloster Meteor with deliveries to the RAF beginning in April 1944. Parallel to this was an improved design on the Wellend engine called the RB37 Derwent with first designs being drawn in 1942. These engines were fitted to the Gloster Meteor by late 1945 setting speed records of over 616mph.
Frank Whittles, Rolls-Royce built RB23 ‘Wellend’ centrifugal flow turbo jet engine, on display at Rolls-Royce heritage centre,
Sinfin site in Derby, England.
A rear view showing the business end of Frank Whittles ‘Wellend’ RB23 jet engine.
Arranged around the outside can be seen the flame cans, containing the combustion chambers.