[Physicist]

Go check your physics for "On Earth there is always air or water available to push against."

I'm afraid I don't understand what you're driving at, either. Could you spell it out for us?

[cinFLA]

I'm afraid I don't understand what you're driving at, either. Could you spell it out for us?

A common misconception by laymen is that the force is derived from "pushing against" the water/air as in pushing against a wall. In reality, the force is derived from the acceleration of the water/air. The force would be the same in a vacuum for the same amount of water/air accelerated even though there would be no air/water to push against.

[tpaine]

"I'm afraid I don't understand what you're driving at, either. Could you spell it out for us?"

'Cin' can't explain, - because he's playing a semantic game, [about the word 'push'] with the point at issue, -- in an effort to show how clever he is, vs the supposedly 'inept' author.
The issue:

"The propellor or the jet engine of an aircraft push air backwards to propel the aircraft forward. A ship or boat propellor does the same thing with water. On Earth there is always air or water available to push against. But a rocket in space has nothing to push against, and so it needs to carry propellant to eject in place of air or water."
_________________________________

Perfectly simple point made above.
-- If you are on the ground you must push against it to move forward.
-- If you are in the water you must push against it to move forward.
--- If you are in the air you must push against it to move forward.
--- If you are in space you can't push against it to move foward, you must eject something away from you.

I submit that 'cinfla' can use his large surplus of internal hot air for this purpose.