Booster unit and Rocket engine

Booster unit noun - The first stage of a multistage rocket.

Rocket engine and booster unit are semantically related. In some cases you can use "Rocket engine" instead a noun phrase "Booster unit".

Rocket engine noun - A jet engine containing its own propellant and driven by reaction propulsion.

Booster unit and rocket engine are semantically related. Sometimes you can replace phrase "Booster unit" with "Rocket engine", this phrases are similar.