Could it be that the supersonic intake produces thrust in an indirect way, rather like the convergent propelling nozzle?
The force on the propelling nozzle acts rearwards, but the overall effect of the nozzle is to increase the total thrust. But the extra thrust does not act directly on the propelling nozzle.
The Intake Momentum Drag thread cited by Mr Optimistic above includes the following text.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi. This extremely large pressure differential led to a forward thrust vector .”
A pressure of 18 psi inside the intake sounds impressive, particularly when the ambient pressure is only 0.4 psi. But is it really as good as its sounds?
Let’s start by assuming that the external (front facing) area of the intake is equal to the internal (rear facing) area. Any pressure on the external surface will produce a rearward acting force and any pressure on the internal area will produce a forward acting thrust force. The relative magnitudes of these forces will be determined by the relative magnitudes of the pressures.
The pressure pushing aft on the external surface is static pressure plus dynamic pressure.
But dynamic pressure acts only in the downstream direction, so the pressure pushing forward on the internal surface will only be the (considerably increased) static pressure.
If the total pressure has remained constant as the air flowed into the intake, then the total pressure inside will be equal to the total pressure outside. This means that the static pressure pushing forward on the inside surface can be equal to the static plus dynamic pushing aft on the outside, only if all of the dynamic pressure has been converted into static pressure. This would require the air to be brought to a full stop inside the intake. This clearly does not occur. So the rearward force exerted on the external surfaces must be greater than the thrust force exerted on the internal surfaces. This does not explain where the extra thrust comes form.
In reality the situation will be somewhat worse because some of the total pressure is lost when air flows through shockwaves.
So by what other means might the supersonic intake be producing extra thrust?
Let’s look again at that quote.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi.”
That represents a pressure ratio of 45 to 1, which is greater than that achieved by many jet engine compressors. What the intake has done is to recover kinetic energy from the incoming airflow and convert it into static pressure energy.
This will have (at least) the following effects.
A. Increase the air density and mass flow rate of air passing through the engine.
B. Increase the pressure of the gas at all of the subsequent stages of the engine. If the compressor pressure ratio is 20 to 1 for example, then the pressure at the compressor outlet would be 360 psi with the intake but only 8 psi without it.
Both of these factors will increase the thrust produced by the engine. But the extra thrust will not actually act upon the intake.
This explanation might also explain why the extra thrust is lost if the engine is shut down. Without the benefit of the secondary compression process carried out by the engine, and the additional energy provided by the combustion of fuel, the kinetic energy that has been recovered from the incoming air is simply wasted windmilling the engine.