nuclear rocket vs rocket engine

nuclear rocket and rocket engine both are nouns.

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

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

nuclear rocket: a rocket engine in which a nuclear reactor is used to heat a propellant
rocket engine: a jet engine containing its own propellant and driven by reaction propulsion

Other hypernyms of nuclear rocket include rocket.

nuclear rocket (noun)	rocket engine (noun)
a rocket engine in which a nuclear reactor is used to heat a propellant	a jet engine containing its own propellant and driven by reaction propulsion