Buoyancy

Variable Buoyancy System (VBS)

Type (locked): an oil + external-bladder buoyancy engine, pressure-tolerant. A leadscrew-driven piston pushes hydraulic oil from a dry 1-atm reservoir out to an external bladder in a free-flooding section. Bladder inflates -> more displaced volume -> rise; retract -> sink / land. The oil is a sealed loop, so seawater never enters the hull and no sliding seal touches seawater. A self-locking ACME screw holds buoyancy at zero power.

Sizing (finalize after the wet trim test)

• Static trim (ballast weights) handles the bulk + the fresh-vs-salt offset; the VBS only covers the dynamic range - depth-hold, gentle landing, small-sample trim.
• Target swing ~= 0.1-0.2 L. Small bore + long stroke + slow keeps the force sane: a 20 mm bore at 300 m sees ~100 kgf, driven through the screw's mechanical advantage.

Fail-safe

A stuck or leaked VBS is not fatal - the independent drop-weight + potted timer surface the vehicle regardless. Trim slightly positive at rest so loss of power floats her up. A piston-seal failure leaks oil into the isolated module, not seawater into the hull.

Build sequence

Design now, build after the first thruster-only wet tests - so bore, stroke, and force are sized from *measured* trim, not guessed. Bench-cycle dry -> shallow-tank buoyancy swing -> pressure-test bladder/penetrator/cylinder to >1.5x depth -> tune the depth-hold loop on the tether.