The original note, as written at the time
COMPACT MODULAR CABIN. A set of sketches describing a spaceship design that takes advantage of compact modular units for purposes of counter-depressurisation redundancy.
Ongoing development
A spaceship design built from compact modular units, for redundancy against depressurisation.
The redundancy is the point and it is a different kind of safety from what spaceflight usually does. A spacecraft today is a set of pressure vessels inside a structure, and a breach in the outer shell is an emergency handled by sealing a whole compartment. In a ship made of small pressurised modules, a breach costs you one module.
The modularity also changes repair. A damaged unit can be isolated, emptied and replaced at a docking port rather than requiring a spacewalk with a patch. That is the difference between a vessel designed to survive an accident and one designed to be recovered from it.
The cost is mass and volume. Every module needs its own pressure boundary, its own seal, its own fittings, and the seals are the failure mode — a modular ship has dozens of joints where a monolithic one has none. So the design trades a rare catastrophic failure for a common small one, which is normally the right trade in safety engineering and is also a trade that has to be argued explicitly.
The other cost is that a modular ship is hard to move. Its shape is a stack of units, which is a poor aerodynamic form for launch and a poor pressure form for a long mission.
But the idea generalises beyond space, and that is why it is worth keeping. Any vehicle carrying people into a hostile environment is safer as a set of cells than as one shell. Submarines learned this; spacecraft have not.
Lose a cell, not the ship.