War in space

Once the weight is assumed to be negligible, I have no problem with
that calculation. It's just that I expected there to be a
significant amount of mass there, without doing an calculation.

What radius are you assuming? Are you assuming no shielding mass
will rotate? How are you assuming the interior structure will be
supported, or are you just assuming it has negligible weight?

I also had in the back of my mind, although it's not directly
relevant to the simple cylinder, that we wouldn't want to pressurize
the whole interior because we're likely to have limited use for low-
g pressurized volume where no one can live much of the time without
bone loss. If the colony is large enough that the low-g interior
volume is more than you want to pressurize, but not so large that
you have a whole sky-full of atmosphere overhead, then part of the
pressure becomes tensile stress on interior components connecting
the inner cylinder to the outer. Odd to imagine your buildings
hanging from the ceiling supported by air pressure, but I suppose a
colony could be designed that way.

Btw, I note that your formula has the structural material being
metal, which I don't assume it would.

--- In spacesettlers@yahoogroups.com, Ian Woollard
wrote:
>
> On 1/12/06, Dan Wylie-Sears wrote:
> > > Actually there's also longitudinal stress, 50% of the
> > circumferental
> > > stress, which is caused by air pressure on the end caps.
> >
> > How does it come out so simple?
>
> I don't know of any deep reason for it, but I know how to prove it.
>
> It's easy to work out how much metal you need to stop a pressure
> vessel exploding. You cut it with imaginary planes and work out how
> much force (=pressure*gasArea) there is trying to blow the two
halves
> apart and then you make sure you have enough metal to stop it doing
> that (tensile strength*metalArea/safetyFactor) in that plane.
>
> In this case, the amount of metal needed to stop a cylinder
exploding
> lengthwise is half that needed to stop it bursting
circumferentially.
>
> > It seems as though it has to be
> > something more complicated, because the circumferential stress
> > depends on the g's whereas the longitudinal stress depends only
on
> > the pressure.
>
> To a first order approximation the rotation can be neglected. The
> atmospheric pressure is 5-10 tonnes, but the weight of the floor is
> only about 10-20% of that.
>
> > > Well, you've reduced the load on the structure by more than the
> > size
> > > of the hole.
> >
> > I still don't get it. How do those quantities even have the same
> > dimensions?
>
> You're removed the hole, which means the strength has gone down,
but
> you've also removed the load where the hole used to be. Also the
air
> pressure around the hole is lower too, so you've reduced the load
> surrounding the hole as well.
>
> --
> -Ian Woollard
>
> Identity cards, cameras on every corner, long term detention
without
> trial, electronic tapping and reading of email/phone calls, speed
> cameras that increase the death rate, extradition for torture -
It's
> all double plus good, Big Brother Blair and Uncle Bush are my
friend!