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Thread: Spaceships Powered by Fusion!! How to you convert the energy into propulsion?!

  1. #1 Spaceships Powered by Fusion!! How to you convert the energy into propulsion?! 
    Forum Junior Double Helix's Avatar
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    A number of people have postulated the use of fusion reactors to power spaceships. Their apparent advantage is the ability to create a large amount of energy from a little bit of mass (fuel). However, this energy is created inside of the reactor, in a plasma at super high temperature and pressure.

    So, how do you convert this enormous amount of energy into propulsion? It would seem that one would need to convert that energy into an energized propellant that could then be used to start the spacecraft moving, and presumably reach very high speeds if the enormous efficiency of fusion can be utilized for this purpose. Of course it could also be used for all other operations, one should think, eliminating the need for solar panels. fuel cells, RTGs, etc.

    But how do you get propulsion from a super high temperature plasma that is contained in an immense magnetic field? It seems like a tall order, but perhaps there are some clever people out there who can help put this spaceship together, and get it moving.

    Or not!


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    The first thing that needs to be done is to invent a fusion reactor. The second thing would be to then invent a fusion reactor that is small enough to fit on a vehicle. These are way more difficult than using the plasma as a propellent.


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    Forum Junior Double Helix's Avatar
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    Well, yes of course one has to have a functional fusion reactor. Its "size" for a spaceship will depend on the output vs. mass of the reactor and spaceship. This is all "science speculation", as opposed to science fiction.

    The question assumes that you have both the reactor and the right "size".

    So as posed, there is only one issue to address:

    How do you convert the energy into propulsion?


    https://www.nytimes.com/2020/09/29/c...n-reactor.html


    https://news.mit.edu/2020/physics-fusion-studies-0929
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by Double Helix View Post
    Well, yes of course one has to have a functional fusion reactor. Its "size" for a spaceship will depend on the output vs. mass of the reactor and spaceship. This is all "science speculation", as opposed to science fiction.

    The question assumes that you have both the reactor and the right "size".

    So as posed, there is only one issue to address:

    How do you convert the energy into propulsion?


    https://www.nytimes.com/2020/09/29/c...n-reactor.html


    https://news.mit.edu/2020/physics-fusion-studies-0929
    The trick would be to create the conditions of a self-maintaining Fusion reaction without having to completely contain the plasma. Plasma would be allowed to "escape through a nozzle". This would be a lot harder to do then a standard Fusion reactor, So I would expect that. even if possible, it would not be accomplished until we had a ton of practical experience with Fusion reactors. The other approach would be to use the Fusion reactor output to power something like a VASIMR rocket, or to just use the heat from the reactor to directly heat your reaction mass to produce thrust (Nuclear Thermal Propulsion).
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Forum Junior Double Helix's Avatar
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    Quote Originally Posted by Janus View Post
    The trick would be to create the conditions of a self-maintaining Fusion reaction without having to completely contain the plasma.
    Yes, it seems that venting the fusion plasma would provide propulsion, but the mass of the propellant would be quite small (although this likely does not matter as any other approach is less efficient?). And one may not want to lose all that heat directly, or so it seems. But if it is shot out at relativistic speeds, that might work. You only need to get to cruising speed in the vacuum of space, and for orbital insertions, etc. later in the "flight". Surely this would be the ultimate form of "ion drive"!

    And just as surely, this is where the engineering gets rather difficult.

    If you use the energy for a nuclear thermal rocket, it seems like you can gain a lot by using more "fuel" mass, and maintaining the plasma at optimal temperature and yield. And as noted, venting the high temperature plasma as a drive might be more difficult to engineer than the reactor itself.

    At such high temperatures, one might imagine a "thrust chamber" connected somehow to the plasma, which you could dump just about anything into for the plasma to heat up and drive the spaceship. The "anything for fuel" design would even allow cannibalizing parts of the spaceship no longer needed during the flight. After all, you are dumping it into a "plasma chamber" at millions K. Anything would serve as a fuel, assuming the plasma thrust chamber is even possible.

    Let me know when you have figured it out!
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