booster unit vs rocket engine

booster unit and rocket engine both are nouns.

Word	Noun	Adjective	Verb	Adverb
booster unit	Yes	No	No	No
rocket engine	Yes	No	No	No

As nouns, rocket engine is a hypernym of booster unit; that is, rocket engine is a word with a broader meaning than booster unit:

booster unit: the first stage of a multistage rocket
rocket engine: a jet engine containing its own propellant and driven by reaction propulsion

Other hypernyms of booster unit include rocket.

booster unit (noun)	rocket engine (noun)
the first stage of a multistage rocket	a jet engine containing its own propellant and driven by reaction propulsion