I believe that is the "Kzinti Lesson" from Larry Niven: "A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive." http

I believe that is the "Kzinti Lesson" from Larry Niven: "A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive." https://tvtropes.org/pmwiki/pmwiki.php/Main/WeaponizedExhaust https://tvtropes.org/pmwiki/pmwiki.php/Main/WeaponizedExhaus...