Hatch Hinging

In the enclosed drawing, the hatches are just round circles, but in reality they will probably be a bit D-shaped -- a circular hatch with one edge straight to accommodate a hinge -- or square with rounded corners.

There are alternatives to hinges. On the space station the hatches move on a yoke mechanism which guides the hatch so that it slides along the wall of pressure shell instead of swinging into the airlock. That reduces the swept volume of the airlock hatch, gaining more useful interior space at the expense of additional weight. On Apollo, the Lunar Module had a D-shaped hinged hatch. Between the Command Module and the Lunar Module, the hatches were closed with simple removable plugs. The Command Module's main hatch, on the side where the crew entered on the launch pad, was a plug.

Out of all these alternatives, my guess is that the best choice is an adaptation of the space station design. A square hatch with rounded corners has an advantage over a circular design because it can move through its own diagonal. Circular hatches can't do that. That feature means we have more operational flexibility. A hatch over an opening which sometimes sees pressure on one side and sometimes on the other can be moved to the other side depending on the conditions. Theoretically, that makes for lighter hatches and door frames. It also means we could move a hatch from one location to another as we plug additional modules onto the moon base.