I picked this up from a very old thread by a gentleman of the name Henri Spencer:
In most thrust-reverser designs, little or none, despite the name. The
key purpose of the thrust reversers is not actually to reverse thrust, but
to kill thrust by sending the exhaust off in a useless direction -- more
or less sideways in most modern designs. The braking effect comes from
the fact that the engine is swallowing very large amounts of air, creating
a considerable drag on the engine, without producing any compensating
thrust. Almost all of the braking comes from intake drag, not from the
forward component of the exhaust.
>> ...The braking effect comes from
>> the fact that the engine is swallowing very large amounts of air, creating
>> a considerable drag on the engine, without producing any compensating
>> thrust. Almost all of the braking comes from intake drag, not from the
>> forward component of the exhaust.
>
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.