2012 is the 70th anniversary year of the first test runs of the Power Jets W2/500 centrifugal reverse-flow turbojet engine. Based on the W2 design, an unsuccessful attempt to specify a turbojet suitable for series production by Rover at Barnoldswick, the W2 developed into the W2B series that Rover attempted and Rolls-Royce succeeded in launching full-scale manufacture for the Gloster Meteor twin-engined fighter, and the W2/500 and &00 series that Power Jets developed to pre-production standards at Lutterworth.
The technology advances, achieved by Rolls-Royce on the one hand and Power Jets on the other, were combined in a highly successful collaboration which lead to the Welland engined Meteor being available for action against the V-1 ‘Doodle Bug’ missile (see #102 above)
In view of the approaching anniversary of the first run of the W2/500 on 13 September I thought it useful to reiterate some of the achievements of the test programme and how they relate to the W2B programme.
The design and construction of the W2 was authorised by the MAP in 1940 and Power Jets immediately started to scheme out the main features of this new engine. The drawings were produced and a set handed over to Rover who also constructed it, with some changes in mechanical design, under direct contract to the MAP. Roxbee Cox wrote:
“In this engine, the design efficiencies of the compressor and turbine were not achieved and, as a result, it did not equal expectations and was subject to surge.”
Power Jets performed a series of modifications, including complete changes in blower casing and diffuser design, brought their W2 engine, built by BTH, up to a relatively satisfactory condition, known as the W2 Mk IV, which differd on slightly from their latest design, the W2B.
The most important aspect of Power Jets development of this engine is the step-by step increase in throughput by successively lengthening the turbine blades.
The W2B engines had a turbine blade 2.455 in. long adopted for the first designs of the W2/500. Soon this was increased to 2.73 in. long. This was the engine that ran on September 13, 1942 and at a maximum rpm of 16,750 rpm the engine delivered a thrust of 1,755 lb at a jet temperature of 879 deg K and an sfc of 1.13.
The next step was to increase the blade height at the trailing edge to 3.03 in. Once again performance improved, but the next step of raising both leading and trailing edge to 3.03 in. did not result in any further performance improvement. In this condition the engine delivered 1,850 lbt at 17,750 rpm at 893 deg K jet temperature and for a sfc of 1.12.
The front end of the engine now became a limiting factor and so Whittle undertook a complete redesign of the diffuser in order to remove any restrictions to better performance that may be there. Instead of a classic diffuser topology – long, smoothly varying channels connecting the flow from the impeller to the inlet to the combustion chamber the channels picking up the impeller flow were swung out into the plane of the impeller disc and then the flow was directed into fore and aft channels that lead directly into the combustion chambers. This design, known as Type 16, delivered a worthwhile improvement. The engine test results for the two designs on what was then called the W2/700 were:
Rpm Old Type 16
Diffuser Diffuser
RPM 16,750 16,750
Thrust lb 1,850 2,040
Jet temp deg K 893 870
Sfc 1.12 1.17
Delivery pressure lb/in**2 43.5 47
The rotating machinery on these engines suffered a series of impeller failures so the then current design of impeller was replaced by a design modelled on that being used on the I-16 engine in the States. This had turned out to be a safer design on test over there. It is this modification that may also have led to some commentators thinking the Type 16 diffuser was a diffuser from the I-16, especially as GE promptly adopted the design.
The impeller although better from a mechanical integrity standpoint was slightly less aerodynamically effective and the thrust dropped by 150 lb, a small price to pay for eliminating the failures!
Development was able to proceed apace and soon the W2/700 was delivering a thrust of 2,130 lb at a jet temp of 920 deg K and a sfc of 1.077, all at 16,750 rpm.
All this improvement was adopted by RR and GE for their particular versions.
The W2/700 with original and Type 16 blower casing and diffuser are shown in photographs below... more detail of the Type 16 will be shown later in Derwent V/Nene discussions.
….tbc: Photos to illustrate changes will follow.
