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Kooky Idea: Electron Balloon Rocket

  1. Jul 18, 2012 #1
    One of the biggest problems with launching things into space is that fuel weighs a lot. With conventional rocketry, mass of fuel per unit of stored energy is high enough that it requires huge rockets to lift even a couple of tons into orbit.

    Electrons are very light, and can be used to store up potential energy, so they outwardly seem like a decent candidate for propulsion fuel. In fact, if you could somehow cram 2 nanograms (thats about 2*10^19 electrons) into a ball of radius 1 metre, you would have stored up more than enough potential energy to lift a one-ton spacecraft fully out of Earth's gravitational well.

    So my kooky idea is this: given that you have this ball of electrons, you remove a little section of whatever it is confining them so that electrons start whizzing out from that opening, generating thrust. I call it an electron balloon rocket because it looks the same as when you blow up a balloon and then release it without tying the end off: the air comes whooshing out the end and the balloon flies around like a rocket.

    My question is whether it is possible to confine so much charge (and more importantly, contain such a huge electric field) in one place. Would something like a Penning trap work? Could it be done with electrical insulators?
     
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  3. Jul 18, 2012 #2

    ZapperZ

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    This IS a kooky idea. How did you come up with such numbers? And how do propose to convert the PE into propulsion?

    Zz.
     
  4. Jul 18, 2012 #3
    Here's a sketch of my crude calculations:

    As we all know the potential energy stored in the electric field of a system of two electrons is [tex]U = \frac{k_c e^2}{r}[/tex] where kc is the constant in Coulomb's law, e is the charge of an electron and r is the distance between the electrons.

    With another electron present, the situation can be viewed as a body with charge equal to 2e interacting with a body of charge e, so the potential energy is given by [tex]U = \frac{2 k_c e^2}{r}[/tex] Carrying on this way gives a series for the amount of potential energy stored by bringing N electrons into proximity in this way: [tex]U = \frac{k_c e^2}{r} \sum^{N-1}_{n=1} n[/tex] which evaluates to [tex]\frac{1}{2} \frac{k_c e^2}{r} N(N-1)[/tex] Plugging in all the relevant numbers, like r=1m, and setting equal to the amount of energy needed for a 1 ton spacecraft to escape earth's gravitational well, [tex]1000 \frac{G M_e}{R_e}[/tex] gives me the numbers I supplied above.

    On the question of how thrust will be generated, the idea is that the electrons that are sent whizzing out of the opening in the confinement will impart momentum on the confining system itself according to newton's third law.
     
  5. Jul 18, 2012 #4

    Drakkith

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    That's a tremendous amount of negative charges you have there. We can't let them get anywhere near anything else, like the walls of a fuel tank, air, etc or they will immediately ionize anything they touch. This would require the use of magnetic fields to store the electrons. However this in itself poses huge problems too.
     
  6. Jul 18, 2012 #5

    jbriggs444

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    I have a couple of concerns.

    If you have assembled all these electrons into one place then you will have a rather large negative charge. You had to get those electrons from somewhere else. That somewhere else will have a rather large them from somewhere. That somewhere will have a rather large positive charge.

    What can we say about the force between a negative charge and a positive charge?

    I seem to recall a calculation that if one were to take the electrons from one cubic inch of a Saturn V rocket and place them on the launch pad that the resulting electrostatic attraction would be sufficient to prevent the rocket from taking off.

    Be that as it may -- let us suppose that you have taken the positive charges left from filling your object with electrons and transported them far enough away that the electrostatic attraction is no longer a problem.

    You still have an electric field. Your object is still attracting positive charges and repelling negative charges. Would you not expect this to polarize the ground beneath your craft, thus setting up an electrostatic attraction anyway?
     
  7. Jul 18, 2012 #6
    The amount energy it would take to get all these electrons together in such a tiny arrangement... Along with all the other things mentioned... the cost would just set you back to using current payload fuel....

    This is one of the great challenges for us. We have sources of potential energy in so many situations that exist naturally. But the ease of turning this potential energy into useful work is where technology melds with basic research to give us really useful things. But clearly this is not easy. Fossil fuels are still extraordinarily easy for use in so many situations. (Some due to pre-existing technology and infrastructure; and some just because it works really well in many situations.)

    It is nice that people are always thinking of this kind of stuff though. I want me a lightning bolt to store energy in a battery that holds useful energy for very long periods of time and can dole it out in small rations to my electric car and other appliances. Im still waiting. I want some sort of belt that goes around my torso while I am asleep that will use the rhythmic supply of kinetic energy due to my breathing to charge my toaster for the morning. (May take a week of good sleep depending on efficiency for my toast.)
     
  8. Jul 19, 2012 #7
    To get to orbit, rockets will push a spaceship for several hundred km. The rocket engine will put out a force of several times the weight of the space ship.
    Your ball of electrons wich has the same energy will lose most of its energy, just by expanding a few metres. Since energy = force * distance, this means that the force to contain the electrons will be about 100000 times bigger than the force of the rocket motor. We can't contain anything like that.
     
  9. Jul 19, 2012 #8

    sophiecentaur

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    It seems to me that what is being described here is just half of the well known Ion Drive motor, in which atoms of propellant are ionised and both ions and electrons are ejected at very high speed from the back (easy to achieve with two accelerators). This maintains the ship's electrical neutrality and the ejected ions and electrons will eventually re-combine, so they don't 'chase' the ship and slow it down again.
    This system has all the advantages of extremely high speeds for the ejecta, and, hence, the low mass of propellant needed as mentioned in the OP. The only downside is the small forces available and the long time that the motors need to run compared with the quick burst that conventional rockets use. Ion drive is really only suitable for interplanetary propulsion and not for launching.
     
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