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

Mikolaj Habryn dichro at rcpt.to
Mon Dec 21 22:46:53 UTC 2009

On Mon, Dec 21, 2009 at 2:13 PM, Christie Dudley <longobord at gmail.com> wrote:
> 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?

Bearings that will provide less friction at -100 degrees than the
force required to spin a 150' tether? That could be an interesting
research project on its own.

(actually, I just checked FAR101 - maximum tether length is 50' unless
you have pennants visible from a mile away)

But, thinking about it, you could just do a horizontal axis and not
have to worry about it - it would be even more effective at pulling
the balloon around.

And while I'm thinking about it, I wonder if we could use an oversize
kite to generate extra lift as well?

> 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.

Yeah - some of these balloons use a coating on the latex that makes
them less permeable to helium, but it's definitely an issue. I was
thinking of setting up a static test with one of the big balloons in
some corner of Noisebridge - inflate it up to 9' while hooked up to a
data-logging luggage scale (ooh, look - another arduino project!) and
see how much lift it retains over a week.

> Christie

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