Joe Strout wrote:
Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them,
FWIW, there is an alternate method, assuming you are willing to let
your "constant" traffic travel in discrete chunks.

In the center of the junction between the two (on the axis of
rotation), you have a sealable "elevator". To go from the spin section
to the no-spin section, you enter the elevator, close the hatch. The
elevator is undocked, spun down and docked to the no-spin section.
Going the other way, you spin up. With careful design of the
(non-rotating) seals and hatches, the atmosphere loss at each
docking/undocking can be very small. The volume containing the elevator
shaft can be made such that while it isn't completely sealed, it leaks
at a low enough rate that if the elevator or one of the adjoining
segments depressurizes rapidly, it doesn't leak down too fast to do
something about it. You should be able to create a long lasting
rotating 'seal' that keeps the leak reasonably low without too much
trouble.

If you put hatches on the both the spin and no-spin ends of the
elevator, the range of travel just needs to be enough to unmate from
one side and mate to the other.

If you want to have multiple elevators (obviously not all located
exactly on the axis of rotation), the situation gets more complex. They
look a bit more like train cars, but it could be done.

Whether any of this is a win depends how hard large rotating seals in
vacuum really are.