interstage

Noun

 * 1)  The structure, generally containing mechanical components and instruments but no engines or fuel tanks, connecting the top of a stage of a multistage rocket to the bottom of the stage above it.
 * 2)  The components of a multistage thermonuclear weapon which channel the radiation produced by the primary's fission explosion so as to trigger the implosion of the secondary and ignite a fusion explosion in the secondary's fusion fuel.

Adjective

 * 1) Between stages.