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Need help figuring out a possible fusion engine

  1. Jul 8, 2015 #1

    DHF

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    Hello,

    I am in the process of writing space based sci fi novella, I made a few posts last year and earlier this year regarding different aspects of the ship and crew. If anyone is interested, I wanted to concentrate on the fusion engine of the ship itself and hopefully come up with a feasible model.

    The novella takes place several hundred years in the future and details Earth's first trip to Alpha Centauri. The ship will take 40 years to get there and will be traveling at a peek velocity of 0.12c . The ship will be coasting for most of the journy because there was no way I could work out a constant acceleration trip without making the ship the size of texas just to hold the fuel :)

    The question is regarding how the engine might work, Its a Fusion engine but the problem I have is that most types of fusion reaction require hydrogen isotopes that are only stable for a decade or so. This isnt a problem for shorter journeys but for a trip lasting 40 years I couldn't come up with an engine that would still have viable fuel to decelerate with.

    That had me thinking about a proton to proton fusion reaction. I know we are no where near that level of design today so this will all be pure speculation. Assuming the civilization in question could develope a proton to proton engine, what might it look like? I know stars get away with it but is this something that is even feasible for us to reproduce and what are some of the ways we might go about it?

    If the idea of a proton to proton reaction is completely off the table then what are some of the other methods my ship might utilize to facilitate a 40 year journey?

    Thanks for all your help.
     
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  3. Jul 8, 2015 #2

    mfb

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    Proton-proton fusion is extremely challenging and pointless. You can use deuterium-tritium fusion to accelerate and deuterium-deuterium fusion to decelerate. Deuterium is stable, only tritium is radioactive (but you use it within the first years, that is fine).

    Deuterium-helium3, Boron11-proton and other fuels are possible, but DT and DD are more convenient. Avoiding neutrons in the reaction could become interesting in the future.
     
  4. Jul 8, 2015 #3

    DHF

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    ok that is good to know. Is there a reason for using the two different types of fuel? why not just rely solely on Deuterium all the way?
     
  5. Jul 8, 2015 #4

    mfb

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    DT fusion releases nearly twice the amount of energy of DD fusion (per mass). It also gives a higher fusion power for the same conditions (a factor of 50 more!), which helps to accelerate the ship while it is still heavy due to all the fuel.

    You could also load your ship with deuterium and lithium and breed the tritium on the way. That is the same concept fusion power plants on earth would use. That is probably a better solution for both acceleration and deceleration.
     
  6. Jul 8, 2015 #5

    DHF

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    that makes sense. using the more volatile fuel first allows them to pack much less fuel mass.

    Thank you very much.
     
  7. Jul 8, 2015 #6

    DHF

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    I like the idea of breeding the Triterium. Triterium has a lifespan of about 12 years yes? so the ship would keep the lithium in storage then a few years before they reach the star system it would begin the breeding reaction.
     
  8. Jul 8, 2015 #7

    mfb

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    Breeding and fusion would be directly connected and happen at the same time - you need neutrons for breeding, and the DT fusion reaction is the most reasonable source for neutrons.

    The breeding reaction is not as efficient as fusion, so starting with some supply of tritium for the acceleration phase could still be interesting, but for deceleration a closed tritium cycle (breed -> fuse -> use released neutron to breed new tritium) is important.
     
  9. Jul 8, 2015 #8

    DHF

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    In that case a breeding system might not work. The ship will be accelerating for about a year then it will coast for almost 40 years before it decelerates. If the triterium is bred during the acceleration, wont it have decayed by the time the ship arrives at its deceleration point?
     
  10. Jul 8, 2015 #9
    Could you use a proton / antiproton? You'd require a lot less fuel and get much more power. If you calculated the density of interstellar hydrogen compared to how much energy you need, you might be able to not require matter at all. If you brought antimatter with you and sucked up hydrogen from the heliosphere / interstellar space.
     
  11. Jul 8, 2015 #10

    DHF

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    I had considered an antimatter drive but ultimately discarded it because the trip will take several decades and I feel that it would be too difficult for the ship to safely contain antimatter for that long. Even with your design it would not be safe because the stored antimatter would react with anything it touches so if the containment field were ever to fluctuate at any point during the decades long cost, it would come in contact with and react with the liner of the fuel storage container.

    There is also a matter of cost. Economics always have to be considered and I just do not feel that any organization would have the resources to fabricate several thousand tons worth of antimatter. Fusion ultimately won out because we are activly striving to master it today and I feel in a few hundred years it will be a viable fuel. It would also be safe to store and relatively cheap compared to antimatter.
     
  12. Jul 8, 2015 #11

    mfb

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    You don't need to store tritium for longer than a few days (in reprocessing materials).

    You start with deuterium, a lot of lithium and a small amount of tritium for a few days of initial fusion reactions. You fuse this tritium with deuterium, creating new tritium in the process. You can use the produced helium for ion drives, that is probably the most efficient way to use the fuel.
    Once you reached cruise speed, you reduce the fusion power to levels necessary to support your ship. That will always keep some amount of tritium on the ship.
    To decelerate, you probably want much more power than during cruise time. This can be achieved with a different lithium composition - lower power output for a while, but more tritium so you increase the possible power output quickly. Alternatively, have some small neutron source around to activate as necessary. Either way, it is sufficient to breed enough tritium for a few days of operation, because using this tritium allows to breed more again.
     
  13. Jul 8, 2015 #12

    DHF

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    Ok gotcha. So keep the reactor on at a trickle of power, just enough to keep producing more triterium.

    You also mentioned D-h3 reaction. what would be the downsides of that?
     
  14. Jul 8, 2015 #13

    mfb

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    It needs much higher temperatures and the reaction rate is much lower. The second issue could be a very fundamental problem, as you get more energy losses from Bremsstrahlung compared to heating from fusion, so the plasma could cool down too fast for stable fusion.
    Also, He-3 is quite rare.
     
  15. Jul 8, 2015 #14

    DHF

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    fair enough. D-T with Tritium breeding it is. Thanks for the help.
     
  16. Jul 13, 2015 #15

    DHF

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    MFB, I would like to follow up on your previous post regarding the engines. Going with a D-T reaction you mentioned using the H4 for ion engines. So in my design of the ship. the main power would come from a D-T fusion reactor then the H4 produced from that reaction would be channeled to a bank of Ion engines to provide thrust to move the ship.

    Do I have that right?
     
    Last edited: Jul 13, 2015
  17. Jul 13, 2015 #16

    mfb

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    Something like that. There are certainly more futuristic options, maybe some of them give a bit more thrust, but that is certainly something that should work centuries into the future.
     
  18. Jul 13, 2015 #17

    DHF

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    Ok good to know. I doubt I will go into detail about the mechanics of the engine on page because the story is about the crew, however I like to have things sorted out ahead of time. This way if the crew make a passing remark about the engines, I sound like I half know what I about :)

    Thanks for the help.
     
  19. Jul 15, 2015 #18

    DHF

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    Quick question on shielding: to save mass, can I position the deuterium tanks between the engine and the sensitive parts of the ship to act as shielding?
     
  20. Jul 15, 2015 #19

    mfb

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    The shielding is needed between plasma and (cold) superconducting coils anyway, you also need it to extract heat and to breed tritium. Shielding is not an issue.
     
  21. Jul 15, 2015 #20

    DHF

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    excellent. thanks.
     
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