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Space dirigibles

  1. Feb 27, 2010 #1
    Here is an idea for a space freighter. Build a dirigible covered with solar cells including an ion engine. Once the dirigible reaches the outer atmosphere the ion engine takes over slowly accelerating the ship to orbital speeds.

    An alternative idea would include a dirigible with a rail gun that would use solar power and capacitors to lunch projectiles into low earth orbits.

    What do you think?
  2. jcsd
  3. Feb 27, 2010 #2

    Jonathan Scott

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    Gold Member

    I think you should try some rough (order of magnitude) calculations before suggesting such ideas.

    For a dirigible to obtain lift, the atmospheric density has to be high enough that the mass of air the dirigible displaces is a bit more than the mass of the dirigible (where the difference depends on what gas the dirigible uses for lift). That means that when it gets moving, each time it travels its own length, it has to push out of the way its own mass in atmosphere. To accelerate to orbital speeds would need a LOT of thrust to overcome that resistance, and a lot of strength and heat resistance in the material of the dirigible!

    You should similarly try calculations on the second idea. How long and how heavy is a rail gun likely to be which can achieve orbital speeds even for a small payload? How high could a dirigible lift such a weight? How would the recoil be handled? Would it be easier and cheaper to build a ramp up to the same height? Note also that if a rail gun fires a projectile from within the Earth's atmosphere, then even if it achieves orbital speed it will need additional thrust mid-flight to circularize the orbit, otherwise it will re-enter before completing an orbit.
  4. Feb 28, 2010 #3
    How about a dirigible shaped like a wing? As the dirigible starts moving it would start climbing hopefully above the thick atmosphere. As the atmosphere gets thinner the friction will lessen allowing the ion engines to gradually nudged it to orbital speeds. I've heard of helium baloons that have exceeded 100,000 feet, how much atmosphere resistance can there be up there?

    As for the rail gun idea: I'm not sure how high a helium or hydrogen dirigible could climb but I heard maybe 100,000 feet. The dirigible can be made as big as required to lift the rail gun. The projectile can have additional fuel for orbital speed.
  5. Feb 28, 2010 #4
    The problem is that ion engines are only useful once you get outside of the earth's gravity well. Ion engines provide only a few pounds of force but can do that for months at time. Useful once you are in interplanetary space but pretty much useless once you are on the ground.

    I don't think that the energetics will work, but I haven't done the calculations. This is a pretty good intro physics problem since all you need to figure out if this has any chance of working is a few formula and some basic arithmetic.

    I think you'll be in good shape if you study some intro physics so that you can tell us if this idea will work or
    not :-) :-) :-)
  6. Feb 28, 2010 #5
    I like to think of the analogy of trying to put an automobile on top of a mountain by a rocket vs driving it up to the top. The fuel required is vastly different. It may take a gallon of gas to drive it up vs tons of rocket fuel.

    If the Earth is at the bottom of a gravitational well that's shaped like an inverted cone then a gradual spiral flight up the gravitational cone might work just like the car going up a mountain via a spiral road.
  7. Feb 28, 2010 #6

    Jonathan Scott

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    Regardless of shape or friction, my first point still holds. Each time it moves its own length, it has to push its own mass of atmosphere out of the way. Even if it was a very long thin needle shape, that would still apply, although the distance it had to move it sideways would be less, and you'd move further for the same amount of mass displaced. As you'll be wanting to exceed the speed of sound, you won't even get any thrust back from the air closing behind the vehicle.

    The lift you get with a dirigible is roughly the mass of air displaced by the volume of the dirigible minus the weight of the dirigible, so to get lift you need a significant fraction of atmospheric pressure (probably a few percent, even though large helium balloons can reach lower pressures), but to operate an ion engine efficiently and get low enough friction to allow acceleration you need near-vacuum, much smaller than millionths of normal atmospheric pressure.

    It is true that a balloon or dirigible can get above the bulk of the atmosphere, and hence could for example be used to launch small conventional rockets. However, the difference between air thick enough to support a balloon and thin enough to reduce friction at hypersonic speeds is many orders of magnitude.
  8. Feb 28, 2010 #7
    The reason that you have to do the math is that the math tells you what analogies work and which ones don't. The reason you need tons of rocket fuel is that rockets have to carry there own oxygen whereas cars do not. Also most of the rocket fuel is used to lift rocket fuel.

    No it won't.

    Also it would be a very, very good thing if you took an intro physics course since once you have that under your belt, you can quickly calculate what will work and what won't, and if something doesn't work, you can figure out why, and work around it.
  9. Feb 28, 2010 #8
  10. Feb 28, 2010 #9
    JP Aerospace claim to be working on a design that does exactly what you describe. The response from knowledgeable people in aerospace has been, usually, incredulity...

    http://www.jpaerospace.com/atohandout.pdf" [Broken]

    ...friction, drag, lack of power & lack of lift have been quoted as problems. I suspect JP is working on a dynamic soaring approach, perhaps using the extreme upper atmosphere's tidal bulges or something very tricky. Not sure if they can do it, but they're not giving away any juicy details.
    Last edited by a moderator: May 4, 2017
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