Urban myths
Apologies to ChristiaanJ if I appear to be pedalling urban myths. Not intended. I'm just trying to get to the physics involved. Thanks for the discussion.
I appreciate that substantially increased pressures and temperatures exist in supersonic inlets, and that due to the receding slope of Concordes' deeper ramps (and the SR71's inlet structures downstream of the inlet cones) these surfaces are subject to very substantial forward pressure forces, and yes, these must be absorbed by the wing structure.
However, the air has still been slowed by the inlet, and none has spilled. Air enters the intake at M=2.0 (~570 m/s at 200K). To have the compressor ingest air at M<1, with no temperature change, would mean that the momentum of the airflow had more than halved. But the air has been shock heated, by a factor of up to ~3 (?). This would lessen the change in momentum, as the sound speed in m/s at the compressor face has increased by ~root(3), and so the condition that M<1 can be met at a higher flow speed. Nevertheless, there is still a slowdown, which requires a net forward force on the air (from the inlet shocks?) and thus a net rearward reaction force on the whole inlet.
Where is the discrepancy? Is it a key fact I'm missing or a wrong assumption I'm making, or is there just a bigger drag force on the front ramp and forward 1/3 of the intake (that supports the shocks) than the thrust force on the rear ramp.
If the section of wing incorporating the inlet was broken clear from a Concorde cruising at M2, all the way back to the compressor face, would the released inlet really accelerate forward due to this inlet thrust, no longer burdened by the airframe and engine behind? Think of the engine and bypass ducts still connected by extendable hoses if that removes some ambiguity.
If the inlet does head forwards, then where does it get the kinetic energy from, and is it not a perpetual motion machine?