# [Space] space is hard, let's go shopping!

Christie Dudley longobord at gmail.com
Mon Dec 21 14:13:08 PST 2009

```On Mon, Dec 21, 2009 at 1:10 PM, Mikolaj Habryn <dichro at rcpt.to> wrote:

> On Mon, Dec 21, 2009 at 12:42 PM, Christie Dudley <longobord at gmail.com>
> wrote:
>
> Definitely - vertical axis windmill? Though I guess that might just
> spin up the tether...
>

Uh.  Well, you just solved the problem with your description of it... a
"windmill" wouldn't be fixed to the tether, but rather rotate around it,
no?  Bearings anyone?

It'd be interesting to have a rigid tether to give the center part something
better to rotate against.  Would this cause our balloon to spin?  I would
think we could get it down to less than we'd experience with a tether that
would twist.  I'd also see it as a sort of a gyroscopic stabilizer as an
interesting side effect.

> I don't know, but I have a feeling that that when the volume is
> smaller, the pressure required to inflate is larger, and there's some
> point at which the bursting pressure is less than inflation pressure -
> um, I'm having trouble explaining this. You know how when you blow up
> a balloon by hand (or rather, by mouth), you have to blow *really*
> hard to get it going at the start? And then the bigger it gets, the
> less pressure you need to apply to keep inflating? As the latex
> stretches it gets thinner, so the amount of pressure differential it
> can handle decreases - so you can't just measure pressure to know how
> far from bursting you are; you also have to know your inflated volume.
>

Oh, I suppose I was misleading there. I was thinking pressure *difference*.
While the instantaneous rate of change as a function of time/pressure/size
is significant in figuring out how fast we need to vent, it's not important
in initial calculations, I wouldn't think.

The tension of the elastic on the balloon increases as the pressure
difference (between the inside and the outside) increases, making the
balloon larger as the gas inside expands to try to equalize the pressure.
As the elasticity resists expansion, the pressure difference increases and
the integrity of the material is challenged.

So I see it as being a two step process to figure out.  First, figuring out
what tension causes the latex to break. Second, what pressure differential
will generate this tension on the skin of the balloon.  These might even be
known values if there are spec sheets on the balloons ordered.  Or we could
run up an experiment to empirically determine this.  Does this sound right
to people who maybe remember their physics/materials science better than me?

Another interesting thought.  Someone pointed out that latex is porous.  I'd
put money on that porosity increasing as the balloon stretches, actually
creating a bit of a relief valve.  It probably (obviously) wouldn't be
enough to keep it from bursting though.

Christie
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