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The SR-71 has a sort of twin cycle engine....the intake produced the majority of thrust, in the order of 70% when crusing a M3+ with the combined exhaust/ reheat section producing a further 22%, leaving the J-58 engine producing only 8%.
However without that core engine producing the thrust the ramjet cycle could not be sustained. At lower speeds the bypass tubes running around the engines are not working correcty and their % contribution dimishes to leave the J-58 working as a jet engine.....
The D-21 recon drone was powered by a ramjet engine, a hollow tube that uses the compressive effects of M1.25+ airflow to compress the air and a simple fuel pump and igniter system to burn the fuel to produce thrust. However it still needs a shock cone, the cone you see in the inlet, to moderate the airflow to below M1 within the combustion section. The cone itself needs mechnaical adjustments etc and a control system to make it work.
The SCRAMJET takes this one step further and does away with the inlet shock cone. The whole idea is to burn the fuel in a supersonic airflow.....getting rid of all the moving parts of the engine entirely....
Imagine trying to get your lihgter to burn whilst holding it out of your P-car window a 150mph.....
The exhibit in Seattle is a M/D -12 combination.
The M-12 'mothership' is derived from the A-12 Balckbird, forerunner of the SR-17 and the D-21 Tagboard drone vehicle. This was designed as a free flying drone, destined for covert overflights of hostile territory (Gary Powers anyone?). Once its preprogrammed flight was done it was designed to jettison the camera module to be recovered inflight from a parachute and then self destruct.
Following an inflight collision between a M-12 and D-21 at launch resulting in the drowning of the Launch control officer the program was cancelled.
The D-21 was adapted to be carried aloft under a B-52 with a booster rocket pack, as will be the X-43. Two operation missions were launched, one drone vanished entirely and the other, altohugh succesful in ejecting the module it was lost during the recovery.
After that the program was abandonned.
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