Can Vmg exceed the V of a jet exhaust?

[awblain]

Re: 47: Intake and turbine

I completely agree with the picture in 46. The uncompensated pressure on the left-hand wall of the balloon and the change in momentum of the escaping gas are equivalent ways to get the leftwards thrust force.

To try to draw a link to the ice-cream cone, if you were to snip off the balloon's blowing-end, it would be blown off to the right, because the air is trying to drag it to the right, and being slowed down in the process.

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Now, the main point I wanted to make is about the general discussion about thrust from supersonic intakes and the thrust on turbine blades mentioned in 47.

The turbine extracts energy from the exhaust, slowing it down. This reduces the rearward momentum of the exhaust, requiring a forward force on the air, and Sir Isaac demands a rearward force on the turbine in reaction. The exhaust stream does not push the turbine into the engine, it tries to pull it off. You could also consider this a pressure drop from upstream of the turbine to downstream.

I think if you have `front of the combuster' instead of `the back of turbine blades' in the first paragraph of post 47 then the statement is true. When you're swimming you're accelerating water backwards with your arms: that's an analogy to your arm being a fan blade rather than a turbine blade.

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For supersonic intakes, I don't think that the idea that they produce thrust is correct. In the intake the airflow is slowed to allow subsonic combustion. This is the same as the case for the turbine: the rearward momentum of the airflow is reduced, pushing the intake backwards. This typically happens, along with direction changes, at carefully-controlled shocks. Only by then burning fuel in the flow is it reaccelerated to ensure a net gain in rearward momentum through the whole engine, and thus a forward thrust. I can't see how any arrangement of components, ramps and shocks can extract thrust from slowing an airstream.

I suspect that if the J58's rotating core in an SR71 is producing drag at M3, then it must be providing mechanical power to a compressor to get enough airflow to the ramjet-like afterburner that actually provides the thrust.