"Propellant" - Thank you! I knew it wasn't technically fuel, but it's still being expelled (as you mention, if you don't expel mass with a different velocity re

"Propellant" - Thank you! I knew it wasn't technically fuel, but it's still being expelled (as you mention, if you don't expel mass with a different velocity relative to the rocket, then you won't be able to accelerate a rocket)