[aprssig] Superpressure was: Re: [aprsisce] Transatlantic balloon attempt (ballast?)

[R. Rochte]

Bob,

Constant pressure balloons (aka superpressure balloons) have no need for
ballast, assuming that you have enough initial superpressure to offset the
radiation cooling that you are sure to experience at night time.  That is,
the gas in your balloon will almost certainly be colder than ambient at
night, so there is the potential for loss of volume.

Whatever excess lift is present on the ground (assuming gas temperature has
stabilized, etc) will be converted into superpressure at float.
 Specifically, the percentage of free lift vs total system mass (including
lift gas mass) will be exactly equal to the superpressure in the balloon vs
ambient pressure at float.  So if you launched with 5% free lift and the
ambient pressure at float is 100mb, then the superpressure will be 5mb
(i.e., a 5mb overpressure).

This initial, by-design level of superpressure is what allows the balloon
to keep flying at night.  It also - in the case of true long-duration
balloons - provides a reservoir of gas to offset losses due to pinhole
leaks and diffusion losses.  The 5% initial superpressure that I used as an
example (and I have often used in my own sp balloons) is actually very low
by professional standards.  At the same time, I wasn't trying to offset gas
losses over a year-long flight.

Now your balloon envelope must, obviously, be able to withstand the initial
superpressure created by the free lift.  Once you get past that hurdle,
then you have to consider the additional superpressure caused by solar
heating during the day.  In a lot of the Air Force and NASA reports on the
subject, the maximum supertemperature (and thus superpressure, if we're
using an absolute temperature scale) that small sp balloons experience in
the troposphere is around 20%.  That's assuming a polyester (i.e., Mylar)
envelope and tropical conditions, if I remember correctly.  So now, for
example, you have the initial 5% superpressure due to free lift and then
another 20% increase in pressure during the day from solar heating.  Using
the 100mb example again, we're up to 25mb of overpressure that your balloon
has to withstand.

I would be curious to see whether your party balloon experiment works or
not... Other than a 1989 (I think - have to check) Winzen study on the
subject, I don't think that nylon film (what "mylar" balloons are really
made from) has been used in superpressure envelopes.  I don't recall the
results of that study offhand, but I do remember that I was going to try it
myself (so the results must have been encouraging).  The "paddle balloon"
shape - if that's what you're using - is certainly *not* the best in terms
of minimizing stress, and the typical aluminized coating will likely cause
significant solar heating*.  Creating a successful superpressure "Pleiades"
cluster would also be complicated by the fact that the balloons will likely
have different individual free lift percentages at launch, and so differing
superpressures at altitude.  If I had to guess, I would say that one will
pop and the payload will slowly fall, dragging the remaining balloons with
it.

As for HF beacons, the little ones that Pierre KA2QPG made for me were
heard for hundreds of miles and they only put out 50mw, I think.  I tried
listening for the current balloon here in Michigan with an Icom 746Pro and
couldn't hear a thing.  Maybe just band conditions....

73,
Robert KC8UCH

*Contrary to "common sense", metalized films make really good solar
Montgolfieres due to their very low emissivity (typically much lower than
their absorptivity at solar wavelengths).



On Sat, Mar 24, 2012 at 8:22 PM, Bob Bruninga <bruninga at usna.edu> wrote:

> >   The descent rate right now is consistent with what I
> >   have seen from zero pressure balloons
> >   The big question is, how is their ballast-release
> >   controlled?
>
> Robert,
>
> Can you elaborate?  Is it possible to do a constant pressure balloon
> mission without ballast release?  We are going to fly a 50g payload any
> time now on 6 three foot mylar party balloons at about 22,000 feet (the max
> possible without even an ounce of payload is only 35,000 feet due to the
> weight of the balloons).
>
> These balloons seem to have a very low loss rate, and they also can stand
> some overpressure..  Can we survive without active ballast release?  We are
> going to carry some water and let it evaporate, but do not want to be
> totally dependent on such an untried system.
>
> Also I am concerned that I never heard their HF beacons even though I was
> less than 300 miles away and well inside the 400 mile range.  If anyone did
> hear the HF beacon, I'd like to hear more about it.  That will be all we
> have on ours.
>
> Bob, WB4aPR
>
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