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H/He lighter-than-air craft

  1. Apr 26, 2015 #1

    DaveC426913

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    I've been playing with an idea for a lighter-than-air craft, and I'm trying figure out how large an envelope it might need to be to hold its own weight plus one person aloft. Have there been any advances in gas mix that can reduce the volume?

    I've been reading a little about mixes, such as H/He or He and hot air, but not a lot of facts about it.
    Has there been any study done on the H/He mix required to make a relatively safe non-explosive mix?
     
  2. jcsd
  3. Apr 26, 2015 #2
    Well Helium is inert and therefore totally safe, Hydrogen on the other hand can be disatesterous explosive, but has approximately 4 times the amount of potential lift.
    Mixing hot air with hydrogen sounds like a non runner to me.
    Hot air plus helium might be a nice low risk idea though.
     
  4. Apr 26, 2015 #3

    DaveC426913

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    Yeah, He is expensive, and not as bouyant as H. I was reading some Arxiv about lower flash point and upper flash point or some such terminology - how much He you'd need to mix with H to make it nonexplosive.
     
  5. Apr 27, 2015 #4

    CalcNerd

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    Actually He is has nearly the lift of H2. He is monatomic and H2 is diatomic so, at worst it has half the lifting ability. But I seem to recall the US Navy found it is much better than this (see reference below, indicates He has 90% of the lift of H2). Prior to WW II the US had lots of He on hand, but withheld it from the Germans to cripple their Airships ability to be weaponized hence the Hindenburg incident.

    While He is relatively expensive, the word is relatively. The small amount you are looking for, the difference in cost from hydrogen probably is not significant.

    http://en.wikipedia.org/wiki/Lifting_gas
     
    Last edited: Apr 27, 2015
  6. Apr 27, 2015 #5
    A lot of areas have limits on the amount of H2 gas you can have in one area at given time. IIRC its a fairly small amount here where I live. But this was years ago, I dont know if or how the laws have changed.
     
  7. Apr 28, 2015 #6
    The lifting capacity of He and H2 are very similar, and not close to a factor of 4. The following example explaines why.
    Lets say we have a balloon with exactly one mole of gas inside which will have volume of around 24l. If we fill the balloon with H2, the mass of the gas inside the it will be 2g and if we fill it with He the gas will weight around 4g. It will displace one mole of air, which has a weight of around 29g. The boyancy of the H2 balloon will be:

    29g-2g = 27g

    and the He balloon will have a buoyancy of

    29g-4g = 25g.

    The difference between the lifting capacity between the He and the H2 balloon is:

    (4g-2g)/25g = 8%

    ...which is far from the factor of two stated in the quote.

    If we could "invent" a gas with a weight of 0 the increase in the lifting capacity would be the weight of the helium which would increase the lift with a factor of: 4/25 = 16%, and the volume of the balloon could be reduced by the same amount. 16% is so little, that I do not think it is worth the effort. To do better than that, you need a stuff with a negative mass, and if someone can come up with that, I think they are sure of getting a Nobel price.

    H2 is easy to produce and therefore cheap, but has the option of going "Hindenburg", whereas He is difficult to produce as it is inert. There may be a point in dissolving H2 to below a combustable consentration with He to reduce the cost of the gas, but lift wise, there is little point in doing it. Likewise reducing the already low density of He by heating it, makes only a little increase in lift, and is likely not to be worth the energy cost.
     
  8. Apr 28, 2015 #7

    russ_watters

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    The buoyancy calculation is pretty straightforward, Dave -- have you tried it?

    For mixing, look into the "lower explosive limit" of hydrogen. That would be a good starting point for determining the highest fraction of hydrogen allowable. It isn't perfect because the oxygen is external and only mixes in the event of damage, but it should provide a safety factor (however, I'm not going to look it up for you, but if I remember correctly, it is pretty low).
     
  9. Apr 29, 2015 #8

    DaveC426913

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    Yeah it's less about the buoyancy and more about the mixes I'm asking. Wondered if the flammability problem had been "solved" recently.
     
  10. Apr 29, 2015 #9
    well originally you asked:
    so It was natural to think that that was what you were what you were looking for. As my previous post suggests, that is probably not possible. If you want to reduce the volume significantly, I think the way to go is to reduce the weight of the balloon, not the gas inside it.
     
  11. Apr 29, 2015 #10

    russ_watters

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    I'm not sure if you looked up the LEL for hydrogen, Dave, but i checked and as I remembered, it is very low. So I would say that no mixture involving hydrogen will be feasible/safe at any concentration.
     
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