booster rocket vs booster unit

booster rocket and booster unit both are nouns.

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

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

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

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