You are apparently referring to the United States Space Force. Its official motto is "Semper supra," which is claimed, by the USSF itself, to mean "Always above," and that is indeed a sensible translation.
Supra is not an adjective, it is an adverb meaning "on top, above, beyond." It also functions as a preposition taking the accusative, but that is not really applicable here.
Superus, on the other hand, is an adjective meaning "upper," although supera doubles as a variant of supra. The idea of a wordplay with the neutral plural supera, -orum (= upper regions, heights) standing in for space is indeed compelling. As Joonas commented, it has another interpretation fitting for a military unit: Since supera is the imperative of superare, it also means "Always be victorious" (as a command). But evidently the U.S. military did not think of, or decided not to go with that idea.
Even if it may sound a little strange in English, it is not unheard of to use supra absolutely in Latin. Example: supra, non infra est deus - "God is above, not below."
Other service branches have similar mottoes but tend to use adjectives: Semper paratus (U.S. Coast Guard: always prepared), Semper fortis (U.S. Navy: always brave), Semper fidelis (U.S. Marines: always loyal). The modified noun, although not explicitly mentioned, may in each case be assumed to be the service branch itself.