Abstract: A hydraulic cylinder assembly for use in a variable external pressure envnment is provided. A hydraulic cylinder exposed to the variable external pressure environment has two chambers separated from one another by a piston. Hydraulic fluid fills each of the two chambers. In an "at rest" mode, the hydraulic fluid in the two chambers is pressurized to the variable external pressure. In an "at work" mode, pressure of the hydraulic fluid is selectively varied from the variable external pressure in one of the two chambers such that the piston moves. The entire assembly can be installed between the pressure and outer hulls of a submarine and utilize seawater to equalize the pressure in a balanced power stroke circuit.

Abstract: An apparatus for ejecting a tethered underwater device, such as a communiions instrument package, from a naval vessel such a submarine is taught. The system incorporates a relatively long hydraulically operated launch tube thereby permitting slow acceleration of the device to achieve the desired muzzle velocity resulting in a reduced acoustic pressure wave upon launch.