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Aerospace Futuristic propulsion of spacecraft

  1. May 9, 2012 #1
    I read about ion thrusters. If they had enough energy, is it possible, that the descendants of the current engines fires the ions with relativistic speeds, and a small yacht could go from LEO to Moon and back, with small amount of propellant, in hours?
     
  2. jcsd
  3. May 9, 2012 #2
    They already fire them with relativistic speeds. So I think the answer to your question is no.
     
  4. May 9, 2012 #3
  5. May 9, 2012 #4

    mfb

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    LEO to Moon and back is not limited by fuel even for current ion drives. The limit is the maximal thrust (limited by the design, and to a lesser extent the power), which is quite low for current engines.

    Relativistic speeds would require large accelerators structures (>10m) and probably a small fission reactor for the necessary power.
     
  6. May 9, 2012 #5
    Bigger than 10m? That fits to my image of small yacht or ship.

    Although i dont know, what should be the minimum size of the fission reactor.
     
  7. May 9, 2012 #6

    Ryan_m_b

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    To get to the Moon and back to Earth in just under a day would require a constant acceleration of 0.1G (with two periods of acceleration/deceleration). That's quite a high level of thrust, this is beyond any current ion drives. You may have come accross the proposal for VASIMR which is in the early stages of development and is intended to be operational at higher thrust but even that has (IIRC) a proposed specific impulse of ~3000 seconds. You'd have to make your ship 30 parts fuel for every 1 part ship for this venture and that's ignoring the mass of the fuel itself, the strain this would put on the engine and the fact that you'd need a hefty power source to run the whole thing.
     
  8. May 9, 2012 #7

    mfb

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    Well, high-tech cavities reach ~30MeV/m for singly charged particles. If you accelerate protons (which is a bad idea for current ion drives, but probably unimportant for relativistic speeds), this would need ~10m for some significant fraction of c. This does not account for the ion source, the issue that the cavities need pulses and not constant beams, and other problems. In addition, the fission reactor needs space, mass, a cooling cycle and large radiators.

    Let's see: Assume a ship with a mass of 10 tons (probably too low, but whatever), about 10.8km/s (~10h to the moon) and ~1 hour acceleration/deceleration at the moon. There, I neglect its gravity and details of orbits - which is a good approximation at the moon (but not at earth). This requires an acceleration of 3m/s and therefore a thrust of 30kN.

    With E=300MeV per proton, the momentum is ~800MeV/c, which requires 7*10^22 protons per second and the nice power of ~3300 GW, which is a bit more than the total output of all power plants on earth ;).

    However, with non-relativistic speeds, the thrust to power ratio is better. There, 2P=Fv. With 1GW, v~100km/s and the journey would require some tons (~2 per direction) of reaction mass. With 100MW and 1/10 of the acceleration, you get the same numbers (apart from the time to reach the moon) and the power becomes more realistic. However, packing all this stuff in 10 tons of total ship mass is still unrealistic.
     
  9. May 10, 2012 #8
    I see.
    So a 100 ton ship and a voyage of a week would be more realistic?
     
  10. May 10, 2012 #9

    mfb

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    Longer journey times need a smaller acceleration and therefore a smaller power supply. Alternatively, they can use the same power and less reaction mass.

    "Realistic"... well, up to now, no ion drive could generate a thrust of this order of magnitude. So it is science fiction anyway, but at least it has some science in it.
     
  11. May 10, 2012 #10

    Ryan_m_b

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    I've just found an article that fits well with this thread, it outlines a slow (6 month) but relatively cheap Orbital Transfer Vehicle that continually can shuttle payloads to and from the Moon:

    Projected Lunar Cargo Capabilities of High-Power VASIMR Propulsion
    Tim W. Glover, Franklin R. Chang Díaz et al
    Presented at the 30th International Electric Propulsion Conference, Florence, Italy
    September 17-20, 2007
    http://www.adastrarocket.com/Tim_IEPC07.pdf
     
  12. May 13, 2012 #11
    Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?
     
  13. May 13, 2012 #12

    mfb

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    With antimatter, the stored energy density is enough for all needs in the solar system. The issue is just to store and extract this energy in an efficient way. Oh, and you have to produce the antimatter first, of course.
     
  14. May 13, 2012 #13

    Astronuc

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    Antimatter production is the biggest impediment.

    Even with a source of antimatter, one still has to have a chamber in which to allow the anihilation reaction, which then has to transfer that energy to propellant, which is usually hydrogen. The thruster chamber (usually assumed to be a magnetic confinement system) pressure is usually the limiting factor.

    The downside is the loss of about half the energy to neutral pions and (e+e-) to gammas.

    The current problem with VASIMR is the low thrust and relatively low specific energy.
     
  15. May 14, 2012 #14

    Ryan_m_b

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    An antimatter battery :eek: as if we didn't need more dangerous technology.
     
  16. May 15, 2012 #15
    Well, if we want to rescue a damaged ship for example, it is worth the risk, to get there fast... get near to it fast, but outside the radius of a possible anti matter explosion or whatever.

    Basically the best futuristic way would be convert stabil matter into energy, but as far as i know, currently there is no way to do that.
     
  17. May 15, 2012 #16

    Ryan_m_b

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    The point is you have to keep antimatter caged up and hope that the storage mechanism never fails. The energy we are potentially talking about here is horrendous, if you manage to bring antimatter production down to reasonable prices then you've proposed a system whereby biosphere destroying devices are available for an unreasonable but possible price. Think of it this way: any vehicle fitted with a few grams of antimatter will release a Hiroshima scale explosion when damaged. A few kilograms and you've got the release of >Tsa Bomba scale explosion when damaged.

    The potential harm of what you are proposing more than outweighs it's uses IMO.
     
  18. May 29, 2012 #17

    FTL

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    FTL drive :P
     
  19. May 30, 2012 #18

    Ryan_m_b

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    There is no good reason to think this is possible.
     
  20. May 30, 2012 #19
    The minimal power (dE/dT) for a voyage is when acceleration - forward or reverse - occupies 2/3 of the total trip time. To the Moon and back, 768,000 km, in six hours ('a few') requires 14.8 m/s2 acceleration, which is perhaps a touch uncomfortable for the four hours under thrust.

    Total delta-vee - the velocity change - is 213.3 km/s, which is doable by advanced ion drives, but impossible to achieve in just four hours under thrust. Ion drives have strict thrust limits due to arcing from excessive voltage. Plasma drives, particularly VASIMR, have no such voltage limits, but they do have heat-loading issues. Current designs take weeks to reach the Moon, albeit using much less propellant than chemical rockets.
     
  21. May 30, 2012 #20

    mfb

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    Why would you need a higher (peak?) power when you accelerate 1/2 of the trip and decelerate the other 1/2? In addition, you can reach the moon with 100W and an ion drive. It just needs ages.
    I think the 2/3 are the result of some other optimization process.
     
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