[phkahler]

Has anyone ever tried to capture outside air and mix it in the rocket exhaust? The reason would that more mass expelled at lower velocity can produce the same thrust with less energy.

Momentum is MV while kinetic energy is 1/2 MV*2. what a rocket really needs is change in momentum, so high energy exhaust.

[panick21_]

Yes. See:

https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

https://en.wikipedia.org/wiki/Skylon_(spacecraft)

But there are lot of problems with this and it adds a huge amount of complexity.

[phkahler]

Maybe too small an effect, but what if they put 8 NACA scoops on the sides of Falcon 9 and just fed the air in between the center and outer engines. It would get accelerated out with all the regular exhaust, and I suppose would cause a small back pressure on the center engine in particular, but they could throttle it slightly if that were a problem.

[mhio]

How would feeding air outside of an engine increase performance?

Unless the gas is accelerated inside the engine bell, I don't see a force being applied to the rocket.

[addaon]

This approach is often called a thrust augmentor, e.g. https://doi.org/10.1023/B:FLUI.0000045678.92653.98. Basically you're using excess energy in a high-speed jet exhaust to accelerate ambient air, increasing the total momentum at a lower exhaust speed.

[trhway]

That was the approach on a USSR N1 Moon rocket. And also on a small air-defense missile Gnom.

[NickNameNick]

Are you thinking of Air augmented rockets? Which capture and entrain surrounding air into their exhaust.

https://en.wikipedia.org/wiki/Air-augmented_rocket

I have the impression that they see periodic use in various categories of mid-size military missiles.

[phkahler]

Missed the edit window. Does not need high energy exhaust, it needs high momentum exhaust.