booster unit vs booster rocket

booster unit and booster rocket both are nouns.

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

As nouns, booster unit and booster rocket are synonyms defined as:

Other synonyms of booster unit include booster, takeoff booster, takeoff rocket.

booster unit (noun)	booster rocket (noun)
the first stage of a multistage rocket	the first stage of a multistage rocket