2.3.2 Buoyancy Compensation System (BCS)

The buoyancy of Ranger NBV is a major factor that effects the fidelity of the space flight simulation. For high fidelity simulation, the vehicle should be as vertically and rotationally neutral as possible. To make the vehicle vertically neutral, the magnitude of the buoyant force must be equal to the magnitude of gravitational force. To make the vehicle rotationally neutral, the center of buoyancy must be spatially co-located with the center of mass.
The method most frequently used to date in the SSL for balancing a vehicle's buoyancy consists of an arrangement of weights and foam attached to its frame. These are designed to make the vehicle close to neutral both vertically and rotationally. To fine tune this, small (1-5 lb.) weights are attached to the outside surface of the vehicle. The divers can attach and remove them as needed to balance the vehicle. One shortcoming of this system is that the buoyancy can only be adjusted by discrete amounts at discrete locations on the vehicle. Also, this method is fairly time consuming, often using 10 to 15 minutes of test time. The accuracy of the balance for a specific pool test is directly related to the skill of the diver balancing the vehicle.
The approach for Ranger NBV has been to allow the vertical and rotational buoyancy to be adjusted independently. Additionally, both the rotational and vertical buoyancy are continuously variable. To simplify adjustment of rotational buoyancy, each of the axes may be adjusted independently. Each of these adjustments may be made by the vehicle itself, without diver intervention, and may be sensed and automatically adjusted to a great degree of accuracy.
The Buoyancy Compensation System consists of three systems, each with a specific function.
The Coarse Buoyancy Compensator (CBC) is a static system of lead and foam attached to the vehicle frame within the propulsion module. This system counteracts the large buoyancy offsets naturally inherent to Ranger NBV and makes it close to close to neutral. The following two systems are then used to fine tune the buoyancy.
The Vertical Buoyancy Compensator (VBC) adjusts the magnitude of the buoyancy of Ranger NBV. It is located between the four scuba bottles in the propulsion module because these are likely to experience the greatest change in buoyancy during the course of a pool test. Adjustment of the VBC also slightly changes the location of the vehicle's center of buoyancy.
The Rotational Buoyancy Compensator (RBC) system adjusts the position of the center of mass of Ranger NBV. The system consists of five individual RBCs; two along the x-axis, two along the y-axis, and one along the z-axis.