In a high speed fluid flow, like that in a rocket engine, an inlet in the direction of the flow will tend to bring in other fluid, even if the other fluid is at low pressure. This can be used in a rocket engine to aspirate air into the rocket nozzle, where it can then react with unreacted fuel in the exhaust, adding a bit of thrust. This can also be used in a rocket engine to aspirate boiling hot water into the rocket nozzle, which can add a great deal of thrust and permit the engine to operate at sea level pressure with a high expansion ratio.

Another version is where the rocket chamber can itself have a rocket engine in it exerting thrust around the edge, to spin the fluid in the chamber, which can then be used to aspirate yet more rocket fuel from a low pressure tank. In effect, the rocket chamber can be used as its own turbopump. The rocket fuels would be let in around the edge, where they would be accelerated by the gas flow from previously reacted fuels. Then they would run into a sub rocket engine at the part of the rocket chamber with the greatest radius, which can be looked at as putting the sub rocket engine at the bottom of a valley where rocket fuels flow down into. This would be heavier and more complicated than a conventional rocket chamber; but, less complicated than a turbopump / rocket chamber combination.
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