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Old 31st Dec 2010, 12:42
  #41 (permalink)  
Join Date: Nov 2007
Location: United Kingdom
Age: 66
Posts: 710
Having completed a spot of research into various websites I discovered the follwing statement on a NASA related subject; which was the Shuttle Training Vehicle.

Viewed from the engine's point of view... As the incoming air is compressed in the inlet (and compressor), it is also decelerated to quite low speed within the engine. The engine very nearly brings the air to a halt, creating a great deal of drag on the engine. After adding some heat, the engine then expands and accelerates the air through the exhaust nozzle (and turbine), creating thrust.

Using the terminology loosely, the net useful thrust of the engine is nozzle thrust minus inlet drag. Nozzle thrust and inlet drag are both typically several times the net thrust; an engine with 20klb of net thrust may well be generating 100klb of nozzle thrust and 80klb of inlet drag. (This is one reason why the net-thrust/weight ratios of jet engines are so puny compared to rocket engines, which are all nozzle and no inlet.)

So killing the nozzle thrust while retaining the inlet drag instantly gives you a "lot" of braking force, and it's not really necessary to divert the exhaust forward.

If the thrust reverse is from inlet drag, why does the pilot advance the thrusters after engaging the reversers? Is inlet drag related to throttle position? Correct -- it's more or less proportional to the amount of air the engines are swallowing, which is dictated by the throttle setting.
I'm somewhat surprised that none of my textbooks make any reference to "Inlet Drag" as defined by thrust reversal systems.

I might have to revise my thinking!

TheChitterneFlyer is offline