Bob,<div><br></div><div>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.</div>
<div><br></div><div>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). </div>
<div><br></div><div>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.</div>
<div><br></div><div>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.</div>
<div><br></div><div>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. </div>
<div><br></div><div>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....</div>
<div><br></div><div>73,</div><div>Robert KC8UCH</div><div><br></div><div>*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).</div>
<div><br></div><div><br></div><div><br><div class="gmail_quote">On Sat, Mar 24, 2012 at 8:22 PM, Bob Bruninga <span dir="ltr"><<a href="mailto:bruninga@usna.edu">bruninga@usna.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
> The descent rate right now is consistent with what I<br>
> have seen from zero pressure balloons<br>
> The big question is, how is their ballast-release<br>
> controlled?<br>
<br>
Robert,<br>
<br>
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).<br>
<br>
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.<br>
<br>
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.<br>
<br>
Bob, WB4aPR<br>
<br>
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