Btw, if both reheat pipes have the same external diameter, why was the Avon pipe 28.8″ and the Spey 36.8″ id?
OK, I meant to get back to this quite a while ago, but didn’t.
Now that this thread has been “zombied”, I’ll answer this more directly.
Simply put, the Spey passes more air through than the Avon, and thus requires a larger diameter jet-pipe. This is because of the difference in the type of engine.
As a turbojet, the Avon relies on high temperatures to expand the air passing through the engine. This expansion, passing through a constricted jet-pipe, accelerates the air to a high speed, which provides the thrust.
As a turbofan, the Spey relies only partially on expansion thrust, and augments it by using a fan section larger than the engine core to push significant amounts of air around the combustion chamber. This cooler air, together with the core of high-temperature air, is then forced out the jetpipe. The cooler air is slower as it leaves the engine, thus a greater mass of air is required to produce a similar amount of thrust. This requires the larger diameter jet-pipe to prevent back-pressure which would lessen the effectiveness and efficiency of the engine.
This is shown by the air-mass flow figures of each engine (in pounds of air per second).
Avon 200/300 (10,000-12,600 lb.s.t. without reheat) – 170-175 lb/sec.
Spey (11,030-12,550 lb.s.t. without reheat) – 200-210 lb/sec.
As the reheat module has already been discussed, no more need be said about it.
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