In article ,
h (Rand Simberg) wrote:

- How can I estimate the leak rate through the seal?

Depends on design. I see no reason it couldn't be designed to zero
leak.

That's a bold thought -- my understanding is that pressure seals are
assumed to always leak at some rate, and that it was just a matter of
how expensive you want to make it to get the leak rate lower. But
that's a stance I'll question more carefully now.

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

In terms of the basic mechanics and strength requirements of the seal,
I'd think so. The issue is that there may be problems on the vacuum
side from material sublimation or welding. Not to mention radiation
(and perhaps monatomic oxygen, if it's in a suitable
environment--e.g., LEO). Just off the top of my head.

Thanks, that's what I was thinking too. So if we wanted to, say, build
working models of such seals on Earth, we could do it, and tests thereof
would be as accurate as tests of any other machinery intended for space
on Earth -- i.e., you just have to watch out for the standard effects of
the space environment.

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face?

Rotating face would be my first choice, but again, just off the top of
my head. Think of a giant (hold breath) O-ring. Just don't do it in
January. You'd probably heat it, particularly on the space side,
though rotisserie effect might ease the issues..

Thanks again. I confess that the space-environment thing makes me a
little nervous when elastic materials (like an O-ring) are in such a
critical path, but it's still worth looking into. Failure of an O-ring
in this case wouldn't be catastrophic; you'd just start venting
atmosphere at a prodigious rate, but it would still take many years to
depressurize an entire space habitat. No serious damage would result.

Of course, *replacing* the O-ring, or otherwise doing maintenance on
this giant rotating seal, could be a real PITA. Certainly something to
plan for up front.

I don't suppose anyone knows any mechanical seal engineers (if there is
such a thing) sufficiently interested in space to kick around some of
the details with me?

Thanks,
- Joe

P.S. It's probably a test of maturity to see how long you can use the
phrase "giant mechanical seal" before a silly image pops into your head.
(I didn't fare very well on this test.)