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BEAR,SABLE, and building a better balloon.

  1. Sep 25, 2009 #1
    Since the SABLE project broke on slashdot and youtube, I've been kind of keeping an eye on the tech, and science these average Joe's put into it. (I'm one of these average Joe's).

    For those not familiar what these projects, here's an overview. The SABLE and BEAR projects basically strap a camera, GPS receiver, and HAM radio transmitter to a Styrofoam cooler. They then strap this cooler to a latex weather balloon, fill it with helium or hydrogen, and let it fly. They use the GPS coordinates, transmitted back to earth via the HAM radio signal to track down the cooler and recover the goodies in the form of photos or video. They've punched out to 117,000 feet and have some pretty amazing photos to show for it, considering their $350 or so investment on hardware.


    Here comes the science question preamble. SABLE and BEAR basically fill a latex balloon with hydrogen or helium and let it fly until it pops. Presumably it pops because the air pressure outside the balloon drops to almost nothing, which causes the gas inside the balloon to want to expand with such force that it breaks the 'surface strength' of the latex.

    Here comes the science question. Can a balloon be made smarter? I mean, if you know the 'surface strength' of the latex, and you can measure the tension exerted by the expanding gasses in the balloon across the surface of the balloon, is it possible to vent gas to keep the balloon from bursting? I'm not talking about the specifics of such a device, but if there were, say a magical vent that opened when the balloon was at the verge of bursting, and closed when the balloon was structurally sound, or just kept a steady stream gas venting as the balloon rose to lower and lower external pressures, would the balloon keep rising?

    I understand that eventually the pressure outside the balloon will equalize with the pressure inside the balloon once so much mass is vented. However, will this 'magic' venting device raise the balloon to higher heights?
     
  2. jcsd
  3. Sep 25, 2009 #2
    My first feelings are that if you vent the lower density gas (helium/hydrogen) that you would decrease the balloons buoyancy (by decreasing its voume of less dense gas) and it would rise less.
     
  4. Sep 25, 2009 #3

    Cleonis

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    Gold Member

    The technological (and very expensive) solution is to not use a latex balloon, but another material (which, as it happens, is not an elastic material) and fill the balloon only partially. (Like, 1/20th of its capacity)
    Since the balloon is filled only partially on departure the size of the balloon must be scaled up, hence the cost.
    At its maximum altitude the balloon will finally assume an all round shape.

    The lift of a balloon comes from its buoyancy, and the higher the altitude the harder it is to achieve neutral buoyancy. At higher altitudes the density of the atmosphere is much less, and for neutral buoyancy you need to displace a volume of air that matches the total weight of balloon plus payload. Venting helium at any stage will reduce the maximum altitude that can be reached.

    If I hazard a guess the only way to go (to gain more altitude), is to start with only partially filled balloons.

    And, the simpler the better, I'd say. I think that in this case any valve mechanism or so will add weight and be just one more component at risk of malfunctioning.

    Cleonis
     
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