In military parlance, a rocket differs from a missile primarily by lacking an active guidance system; early missiles were known as "guided rockets" or "guided missiles". Some rockets were developed as unguided systems and later upgraded to guided versions, like the GMLRS, and these generally retain the term "rocket" instead of becoming missiles.[1] Rockets or missiles that travel underwater, like the VA-111 Shkval, are known as torpedoes no matter what their propulsion system is.