Why is thrust increased when exit pressure of a jet engine is increased?


I’m reading though the Rolls-Royce jet engine book and it states:

“With Vjet fixed at Mach 1, the new term for pressure thrust allows thrust to be increased by raising exit pressure”

My question is: If velocity can no longer be increased (this is in a choked scenario), then how can an increase in exit pressure increase the thrust? I see how this works in the equation, but it just seems rather counter-intuitive. I thought an increase in pressure meant a decrease in velocity? It also then says that by running the engine hotter, the speed of sound can be increased. So Mach 1 is no longer really 1234.8 km/h?

Another question I have, is that if Vjet (exhaust) is fixed at Mach 1, how is the exit pressure meant to increase? Hasn’t the critical pressure ratio already been reached?


In: 2

The way I’ve always managed to make it make sense is like this:

Imagine you have a vast reaction going in a contained space that manages to fully contain the reaction. The exit is small, so the actual velocity is limited. If you increase the size of the exit, as long as the reaction is still under some amount of contained pressure, the velocity should increase. However, if the exit is increased to the point to where the total output of the reaction is allowed to escape the exit, then the velocity should decrease.

But, it’s been a while since I’ve read into that stuff in any detail, so I could be completely fucking wrong.

Thrust comes from mass flow.

The flow is choked so the *velocity* is fixed, but if you increase the exit pressure you increase the density so the mass flux at equal velocity goes up, hence thrust goes up.

Increase pressure = decrease in velocity is Bernoulli’s equation…that only describes a constant energy situation. If you’re raising exit pressure you’re increasing energy input, straight Bernoulli doesn’t apply.

And yes, if you crank the temperature Mach 1 is no longer 1234.8 km/h or whatever it was, it gets higher. Mach 1 scales with the square root of temperature. If you hike the temperature Mach 1 is a higher speed. 1235 kph is Mach 1 at standard temperature (68F). Mach 1 in a jet exhaust is *far* faster.

Critical pressure ratio is the minimum to achieve Mach 1 at the exit; there’s nothing preventing you from increasing the pressure upstream of the choke. You’re above (usually well above) the critical pressure ratio.

For every action there’s an equal and opposite reaction. Thrust is a product of mass not pressure. If you’re pushing tons of air backwards very fast you get pushed forwards. You can push a miniscule amount of air backwards at the same speed and pressure but you’re not going anywhere.