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The cone, on both the SR-71 and the D-21 is adjustable to maintian the shock wave in exactly the correct position.
As the behaviour of the shock wave is dependent on the air temp, the humidity, the exact angle of attakc of the cone etc etc it is constantly shifting and the cone needs to respond, albeit by fractions of an inch.
The SR-71 powerplant functions as a partial ramjet at M3+.
The Maraquardt ramjet in the drone is a pure ramjet, needing subsonic air within the combustion chamber to sustain combustion.
The SCramjet, Supersonic Combustion ramjet does what is says on the tin....the combustion cycle takes places in airflows greater than M1.
The intake cones are designed to capture a shock wave from the tip to the inside of the intlet tunnel. It is then reflected down onto the cone afterbody within the inlet and a series of 'standing shock waves' serve to slow the air down whilst compressing it.
Whilst this creates sufficient compression in the ramjet to sustain combustion, in the J-58 engine the turbojet core is still needed to ensure the reheat section, used continously in supersonic flight remains ignited and can use the highly compressed air which has been diverted around the core engine and dumped upstream of the a/b nozzles.
If for some reason the shockwave in the J-58 loses its focus, and it 'escapes' the lip of the inlet the resultant loss of thrust and increase in drag is sufficient to slam the pilot's head against the canopy.
The inlet is said to have suffered an unstart an the cone needs to be cycled inwards before moving outwards again to recapture the shock wave.
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