During the course of a pool test, Ranger NBV changes buoyancy
slightly due to several factors. One of these is air loss. The
internal volumes of many parts of the vehicle are pressurized to a
few PSI above the ambient water pressure. This air is supplied from
four high pressure scuba tanks near the rear of the vehicle, and is
slowly used during the course as the vehicle ascends and descends
purging the pressurized air on ascent. Air is also lost through small
leaks in sliding seals. The loss of air from the SCUBA tanks causes
the vehicle to slowly become lighter. Buoyancy also changes as a
function of depth. As the water pressure increases, unpressurized
compartments (most notably the electronics module) are reduced in
volume. The general trend is for the vehicle to become negative and
front heavy as it goes deeper.
To compensate for these buoyancy changes, the Vertical Buoyancy Compensator has been included in the BCS. The VBC design consists of two concentric cylinders each closed on the outside ends that can slide with respect to one another along their axes thus changing the total volume that they displace. This is accomplished by single motor under computer control. As the VBC increases its buoyancy through its full range, its own center of buoyancy shifts forward along the x-axis of the vehicle. The effect of these changes on the buoyancy of the vehicle are shown in Table 2-10.
Table 2-10 VBC buoyancy characteristics
Figure 2-11 Side and end views of VBC showing location of motors, lead screws, and guide rods.
Figure 2-12 VBC location within the Propulsion Module