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Need Help About Nuclear Fusion

  1. Jul 5, 2014 #1
    Hey guys,

    So, I'm writing a screenplay about space travel. I've been thinking of using nuclear fusion as the method of space travel. I've read it's a viable option. But, I don't know much about nuclear fusion.
    I know cold fusion is, at this point, a near-impossible option. But is it true that cold fusion means the reaction takes place in room-temperature environments?
    How would nuclear fusion affect space travel? How far could we go in the Milky Way?
    Exactly how long would it take to get somewhere else in the galaxy and how would that affect time dilation?
    Sorry for the vagueness. But, I'm looking for education in this matter.
    Thanks everyone!
     
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  3. Jul 5, 2014 #2

    HallsofIvy

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    Yes, that's what "cold" fusion means. But in space temperature isn't really relevant is it?

    It would reduce the amount of fuel required since, with fusion, you get a lot of "bang" (force) out of a small amount of matter. To answer questions like "how far we could go" and "how long" depends upon what acceleration you want. Of course, as you increase speed, you will need more force to get the same acceleration so you will need to either increase the amount of fuel used or accept less acceleration. Eventually you should be able to get to a decent fraction of c. Yes, time dilation, relative to the earth, which depends upon velocity relative to earth, will make the trip shorter than a simple "distance divided by velocity" (from the point of view of the ship's crew, the distance traveled is shortened) but, of course, even if the time required is only a few years to the ship's crew, it may be hundreds of years to people on earth.
     
  4. Jul 5, 2014 #3

    Drakkith

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    Cold fusion is the production of a sustained fusion reaction at significantly lower temperatures than is normally required. Since temperature is a measure of average particle velocity, this means that cold fusion collides ions together at very low speeds, much lower than what they really need to fuse. This is why it doesn't work. The ions have to have enough kinetic energy to overcome the repulsive barrier between them and fuse. Hence, cold fusion does not exist.

    It is MUCH more efficient than chemical rockets, but the overwhelming distances in space mean that even with nuclear fusion you still need huge quantities of fuel and long burn times for your engines. There is no set distance we could go. It all depends on the level of technology and how you build the ship.

    For that you would need to learn a little bit about Special Relativity.
     
  5. Jul 6, 2014 #4

    Ibix

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    Basically, all space travel at this point works by taking some chunk of fuel, heating it up really hot and spitting it out of the back of the craft. As the chunk goes out backwards, your ship must go forwards as per Sir Isaac ("For every action, there is an equal and opposite reaction" - hence the term reaction drive). Fusion reactions allow you to make the exhaust really, really hot, so you go faster for the same chunk of fuel expended.

    Generally, fusion reactions proceed by heating hydrogen to extremely high temperatures and letting its nuclei (which repel each other) slam into one another really, really hard so that they collide and stick together. The result is a helium atom (or similar) that weighs slightly less than the hydrogen atoms that made it up. The difference in mass gives us energy (E=mc2 - thanks, Einstein) that can be used to accelerate a rocket (by making the fuel chunk really, really hot).

    Cold fusion is the idea that one can get repelling hydrogen nuclei to stick together without slamming them together really, really hard. It's way beyond "fringe science" and into "so cranky you could start a Model T Ford" territory, post Pons and Fleischman. More respectable (less disreputable?) research continues into "Low Energy Nuclear Reaction" (LENR) simply because it would be awesome if we could do it, rather than because we've any immediate hope it would work. Muon catalysis is a possibility for this - there are only three impossible things we'd have to do before breakfast to get this working. Also, Arthur C. Clarke used it in his novel 2061, if memory serves, so it has a respectable SciFi pedigree if nothing else.

    How far you can go and how time dilation factors into things are hairier questions. The basics ae: (1) once you start moving through space you don't stop, and (2) time dilation depends on how fast you are moving.

    If what you are writing is a Anne McCaffrey Pern style "we struck out into the great night and landed on this planet where adventures happened" style thing, it's relatively simple (pardon the pun) to figure out. Your question becomes: how high a speed can you reach while still retaining fuel to slow down again, and how long can your life support hold out while you're floating along at that top speed? Or, more simply, how much fuel can you afford to drag along?

    If you are going for a David Weber Honor Harrington "adventure among the stars", it's a bit more complex since there's a lot of accelerating and decelerating.

    We need more information, unless you want to follow Drakkith's suggestion and learn Special Relativity for yourself.
     
  6. Jul 6, 2014 #5
    I appreciate the help everyone!

    The essential premise of the script is a private corporation that has found the Goldilocks planet, and sends out a spaceship to find and land on it. The script is more soft sci-fi. But, I wanted to understand the hard sci-fi aspect of it more before I begin it.

    So then the problem, for my purposes anyway, is modeling a ship capable of carrying enough fuel for travelling further to the farthest reaches of the galaxy. Or possibly there could be space stations throughout the galaxy that allow for "fueling up", like space gas stations?
    And more importantly, the bigger problem is trying to figure out a (fictional) method of reducing time dilation so the time lapse between the crew on the ship and people back on Earth is more equal. (Making the assumption that they are traveling and can travel at 99% of c) Any ideas or feedback?

    Thanks guys. Really.
     
  7. Jul 6, 2014 #6

    Drakkith

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    Remember that once you start moving in space, you don't stop until you hit something or you make yourself stop. So you don't need X fuel to travel Y distance, you need X fuel to get up to Y speed and slow down again once you arrive.

    Also, you could significantly reduce the size and mass of the ship if you use some sort of external launching device instead of carrying all that fuel on board to accelerate you. An example I made up just now: some sort of very long magnetic launcher could accelerate your ship to a sizable fraction of c during launch, reducing the initial mass and size of the ship by around 80% since you don't have to accelerate massive quantities of fuel right from the start.

    There are no realistic or plausible options. You'd have to invent some sort of purely sci-fi device to do this. Honestly, if you aren't traveling FTL, you don't want to do this, as it extends the length of time that the ship experiences significantly, meaning you will need to carry more supplies, which increases the mass of the ship, and your 5-year journey becomes a multi-generational voyage.
     
  8. Jul 6, 2014 #7

    Ryan_m_b

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    If you want to get really correct with the fuel requirements look up the rocket equation (and the relativistic rocket equation for higher velocities). Unfortunately even with a hypothetical fusion drive getting to a higher percentage of light speed and slowing down again requires impossible amounts of fuel (depending on how optimistic/pessimistic you are, though some estimates are still thousands of times the mass of the ship).

    As others have said there's simply no way of making the time passed on earth to be short unless you involve FTL nonsense or place the planet extremely close, which seems even more unrealistic.

    My advice would be to not explain it too much at all. Just write the ship as having a FutureDrive that runs in FutureScience and can get the crew up to a very high fraction of the speed of light. The ramifications of this are considerable (if relativistic travel is cheap and easy then so are planet destroying weapons) but they can often be quietly ignored and good fiction still produced.
     
  9. Jul 6, 2014 #8

    Ibix

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    Fueling up is a pain. The stations have to be... stationary... with respect to the stars, and slowing down to transfer fuel is very expensive. Imagine if petrol were free but you had to pay to go on or off a major road - you'd do your damndest to be sure to only refuel at start and end of a journey, no? Especially since you'd pay to go in and pay to go out of the fuel station. That's more or less what you're looking at.

    Regarding avoiding the effects of time dilation, there really isn't a way that I can think of. I'd be inclined to either live with it (have a read of the Orson Scott Card's Speaker for the Dead, Xenocide and Children of the Mind) or take the Mike Okuda defence. Star Trek transporters can't really work because of Heisenberg's uncertainty principle. Fortunately they've got Heisenberg Compensators built in. When asked how these worked, Okuda replied "very well, thank you".
     
  10. Jul 6, 2014 #9
    Fair point, Ryan_m_b.

    What about a jump drive that is not instantaneous? And thus it creates a reduced time dilation? But it's perceived as instantaneous from the crew on-board.
    (Time dilation is a key subplot of the script.)
     
  11. Jul 6, 2014 #10
    What about using handwavium and just jump through hyperspace? That actually there is a shorter route than a straight line...

    (Because civilization that is able to send ships fast enough to travel to stars during human lifetime must have effectively unlimited amounts of easy to harness energy. And sending frozen settlers that would arrive in next millennium is a bit boring)

    If you want to have fun, make clear that the crew is also not sure how it exactly works, just that they have to have check in professional garage every 100 light years or their insurance premium would go up. ;)
     
  12. Jul 6, 2014 #11

    Ryan_m_b

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    So it takes X time for the ship to disappear and reappear at its destination but for the crew no time has passed? Sounds similar to the light drive used in Ken Macleods work. You can write interesting fiction with that, it has no basis in real life and still violates causality but that doesn't stop good science fiction. Especially if you dont go into details, ensure consistent enforcement of in-universe rules and work through the ramifications logically.
     
  13. Jul 6, 2014 #12

    Drakkith

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    I like the FTL jump drives used by the ships in the newer Battlestar Galactica series, they are depicted similarly to the FTL drives used in the Battletech universe. Another possibility is "jump points" used in the Freespace games. You could easily modify these to involve time dilation. (Though it won't be realistic. But who cares?)
     
  14. Jul 7, 2014 #13

    Ryan_m_b

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    They are quite cool and have the standard limitation of needing to charge before jump which can make for good scenes. My favourite form of jump drive is from the lost space series of novels. In that there are specific points in each star system that can be used for jumping. The range of the jump drive though isn't very long which limits where you can actually go. Practically this means that to get from System A to System X you have to go through B, C, D...etc rather than in a straight line. Which leads to some interesting tactical considerations.
     
  15. Jul 7, 2014 #14

    Bandersnatch

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    That sound almost exactly like the "collapsar" wormholes in Haldeman's "Forever War". It's got time dilation properly applied too. With ships getting technologically outdated by the time they get to the target, the soldiers returning to Earth that seems alien, and the army having to plan for years ahead(further dehumanising soldiers in the process).
     
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