A deep-submergence vehicle (DSV) is a deep-diving crewed submersible that is self-propelled.[citation needed] Several navies operate vehicles that can be accurately described as DSVs. DSVs are commonly divided into two types: research DSVs, which are used for exploration and surveying, and DSRVs (deep-submergence rescue vehicles), which are intended to be used for rescuing the crew of a sunken navy submarine, clandestine (espionage) missions (primarily installing wiretaps on undersea communications cables), or both.[citation needed] DSRVs are equipped with docking chambers to allow personnel ingress and egress via a manhole.
Strictly speaking, bathyscaphes are not submarines because they have minimal mobility and are built like a balloon, using a habitable spherical pressure vessel hung under a liquid hydrocarbon filled float drum.[citation needed] In a DSV/DSRV, the passenger compartment and the ballast tank functionality is incorporated into a single structure to afford more habitable space (up to 24 people in the case of a DSRV).
Most DSV/DSRV vehicles are powered by traditional electric battery propulsion and have very limited endurance, while a few (like NR-1 or AS-12/31) are/were nuclear-powered, and could sustain much longer missions. Plans have been made to equip DSVs with LOX Stirling engines, but none have been realized so far due to cost and maintenance considerations. All DSVs to date (2023) are dependent on a surface support ship or a mother submarine that can piggyback or tow them (in case of the NR-1) to the scene of operations. Some DSRV vessels are air transportable in very large military cargo planes to speed up deployment in case of emergency rescue missions.