retrorocket vs rocket engine

retrorocket and rocket engine both are nouns.

Word	Noun	Adjective	Verb	Adverb
retrorocket	Yes	No	No	No
rocket engine	Yes	No	No	No

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

retrorocket: a small rocket engine on a larger rocket or spacecraft that is fired to slow or alter its course
rocket engine: a jet engine containing its own propellant and driven by reaction propulsion

Other hypernyms of retrorocket include rocket.

retrorocket (noun)	rocket engine (noun)
a small rocket engine on a larger rocket or spacecraft that is fired to slow or alter its course	a jet engine containing its own propellant and driven by reaction propulsion