A sphere is strong because it is continuous. Every opening interrupts that, so the trick is to cut them where they hurt least, reinforce them, and seal them so the sea helps rather than fights you. Because SubHab only sinks occasionally and shallow, the pressure demand is small - this is boat and aquarium glazing, not deep-submarine engineering. Below: the rules, the numbers, and the fittings.
Cut a hole in a pressurised shell and the load that used to flow through that material has to divert around it, crowding at the edges. Two consequences drive the whole design: keep openings round (corners are far worse stress raisers - it is what cracked the square windows of the early de Havilland Comets), and reinforce them by replacing the removed section with a thickened boss. Drag the opening size and watch the numbers.
Area-replacement rule (used in pressure vessels and submarine hulls): the reinforcement should put back roughly the load-bearing section you removed, which for a hole of diameter d in a wall of thickness t is about d·t. Spread it as a boss around the rim and the ~2× stress crowding drops back toward the parent shell.
How big a push does a door actually take, and how thick must the acrylic be? Set the submersion depth and the door size. The two thickness figures bracket reality: a flat disc (worst case, bending-governed) and a curved cap that matches the hull and so carries pressure in compression - acrylic's strong direction, and dramatically thinner.
Acrylic taken at ~70 MPa tensile / ~100 MPa compressive ultimate with a PVHO-style safety factor of ~6 (human-occupancy viewports are deliberately thick-bodied). Flat disc uses the simply-supported plate relation σ = 3(3+ν)/8 · p·(a/t)²; the cap uses membrane compression σ = p·R/2t. First-order sizing, not a fabrication drawing.
Whichever way a door swings, close it so external water presses it onto its seat, not off it. For your outward-opening door that means the leaf laps over the outside of a raised sill: sinking jams it tighter with depth, and the pressure load runs straight into the sill and shell, not the hinges. Dogs only hold it snug at the surface until pressure takes over. It is pressure-assisted sealing, the same principle as the main hatch.
Acrylic creeps under sustained load, so a big thin pane deflects over time - keep it thick enough, or curve it. For a seal that must hold underwater, prefer one monolithic curved leaf over true French doors: a centre split is a mullion that becomes part of the pressure boundary, a harder seal. Make it one wide cap that reads as a picture-window door and hinges from one side.
A spherical-cap window is not exotic. Manned-submersible viewports are standardised under ASME PVHO-1 (Pressure Vessels for Human Occupancy), which defines spherical-sector and hemispherical acrylic windows and ties thickness to diameter, pressure and temperature.
Public-aquarium fabricators produce thermoformed and monolithic cast acrylic in almost any curve - single panels over 30 m wide and hundreds of millimetres thick. A door-sized spherical cap is trivial by comparison, either thermoformed over a mould or cell-cast to shape. You are ordering a catalogue capability, not inventing one.
Cast marine-grade threaded sockets (316 stainless, isolated from rebar) into a reinforced perimeter ring so a deck bolts on for surfaced living. Keep it free-flooding grating or removable so it never traps air and upsets trim when you dive. An awning above doubles as a rainwater catchment feeding the tanks - which already serve as ballast and thermal mass.
Consolidate the services into one bronze sea-chest plate in the keel: coolant loops for the deep-water heat exchanger, ballast flood/vent, and waste discharge - one reinforced boss, one isolated fitting, one place to inspect and protect. This is a deliberate trade against the "no holes below the waterline" ideal on the Thermal page, bought back with fail-closed valves and redundancy.
Surfaced, waste drains by gravity; submerged, external pressure is higher than inside, so overboard discharge needs a pump beating that head. Coolant runs as a closed loop - no raw seawater through the house, so no internal fouling.
Silicon bronze is the classic below-waterline choice: it resists seawater and, unlike 316 stainless, does not suffer crevice corrosion in the permanently wetted, low-oxygen spots a through-hull lives in. Do not mix metals underwater, isolate the plate from any rebar, and fit sacrificial zinc anodes - the same zincs every boat carries.
On a hab that sinks on purpose, each below-water penetration is a way for the sea to get in, so the rules are few, redundant, and fail-closed valves you shut before diving. Keep the penetrations small and round - easy to reinforce, unlike the big door - and cluster them in the one serviceable plate. Black-water discharge also carries regulatory limits (holding and treatment, MARPOL and local rules); mechanically simple, legally not.
Representative handbook figures used in the models above. Every row links its source.
| Material / parameter | Value used | Role in the model | Source |
|---|---|---|---|
| Cast acrylic (PMMA) density | 1180 kg/m³ | Leaf mass | MakeItFrom |
| Acrylic tensile / flexural ultimate | ~70 MPa | Flat-disc bending allowable | iPolymer datasheet |
| Acrylic compressive ultimate | ~100 MPa | Curved-cap allowable | Laminated Plastics datasheet |
| Acrylic Young's modulus | ~3.0 GPa | Stiffness / deflection | MakeItFrom |
| Acrylic Poisson's ratio ν | ~0.37 | Plate coefficient | MakeItFrom |
| Viewport safety factor | ~6 (PVHO-style) | Design allowable | ASME PVHO-1 |
| Large curved acrylic manufacture | thermoform / cast | Feasibility | Reynolds Polymer |
| Through-hull metal (below waterline) | silicon bronze | Corrosion / crevice | Eugene Fastener |
| Aluminium set in concrete | avoid / isolate | Alkaline corrosion, H₂, cracking | Arizona Chapter ACI |
| Seawater density | 1025 kg/m³ | Pressure = ρgh | Seawater (Wikipedia) |