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Thread: rocket fuel for interstellar travel

  1. #1
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    rocket fuel for interstellar travel

    I once read about a rocket for interstellar travel that would fly in a stream of it's own fuel. The fuel would be accelerated to the target and the ship would fly down that path, gobbling up fuel as it went. How much fuel would you have to accelerate to do such a thing, assuming you were using hydrogen? It seems like it would be a ridiculous amount of fuel, but I can't even imagine how much that would be.

    Would all the hydrogen on Earth be enough or are we talking about harvesting fuel from a gas giant?
    Solfe

  2. #2
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    It wouldn't be that much, because it would stop being useful as soon as your ship accelerated to a velocity relative to the fuel equal to its exhaust velocity. This would probably happen at a much lower velocity than if it'd been a conventional rocket with a decent mass ratio. (You only need a mass ratio of 7.4... for the craft to have a delta-v equal to double its exhaust velocity, so assuming that's how fast you could launch the fuel ahead, that's the mass ratio a conventional rocket would need to beat one ingesting an external propellant stream.)

  3. #3
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    I suspect what you're talking about is the preseeded trajectory for a Bussard ramjet. (Useful search terms.)
    It has the advantage that you don't need to burn fuel to accelerate fuel to be used later in the voyage - the idea is that the fuel packets are launched using a rail gun. The velocity profile of the fuel packet trail is tailored to match the velocity of the spacecraft, so that drag is minimized.

    Grant Hutchison
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  4. #4
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    The correct answer is NONE! After your craft has reached sol escape velocity it can turn off its engine and coast to the star! it will need fuel to get it to orbit ( and what ever) around the target star.

    Space craft are not like cars --- there is no friction in space --- it does not work the same way as on earth!

    Mark

  5. #5
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    rocket fuel for interstellar travel

    I think Solfe was describing a technique in which a constant supply of fuel was available so that the ship could reach relativistic speeds, or at least some significant percentage of the speed of light. That was the intent of the ramjet method which would ingest interstellar hydrogen.


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  6. #6
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    But you would need the same amount of fuel to reverse the process at your destination. You will need the time, the fuel, the astrogation ,
    and the conscious operation of that vehicle , unless you expect to trust your machine to do everything for you , at precisely the correct time
    and place. Unless you want to slip past your destination at some terrific velocity .

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    Interesting thought to send a factory bot to a Kuiper Belt mini-object made from various ices, then build engines on one end designed to use the mini-object's mass as fuel.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Doesn't that assume that the Kuiper Belt object (from which fuel manufacture is suggested ) is moving at the same vector and velocity as the primary ship in question? It would appear that you would have to decelerate to that Kuiper belt object (certainly a substantial cost in
    delta V , and than try to accelerate on vector , just to reach the velocity you had before . Where do you realize a significant gain or advantage? Maybe I missed something.

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    Quote Originally Posted by danscope View Post
    Doesn't that assume that the Kuiper Belt object (from which fuel manufacture is suggested ) is moving at the same vector and velocity as the primary ship in question? It would appear that you would have to decelerate to that Kuiper belt object (certainly a substantial cost in
    delta V , and than try to accelerate on vector , just to reach the velocity you had before . Where do you realize a significant gain or advantage? Maybe I missed something.
    The Kuiper Belt mini-object is the ship itself. You add the parts.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by danscope View Post
    But you would need the same amount of fuel to reverse the process at your destination. You will need the time, the fuel, the astrogation ,
    and the conscious operation of that vehicle , unless you expect to trust your machine to do everything for you , at precisely the correct time
    and place. Unless you want to slip past your destination at some terrific velocity .
    There are other proposed braking methods as well. A magsail "parachute", for instance.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  11. #11
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    It sounds like a rocket using a pre-seed flight path isn't riding in a stream of material, it's in a bolus* of fuel to be consumed. Does the image of a "stream" come from the idea that the system to create that mass of fuel probably would continue to feed it as the ship travels or perhaps is seeding the same area for another ship to make it's passage later?

    *I like that word. Its organic sounding while describing a "simple" physical principal of delivering stuff to the right place at the right time, which is a necessarily complex problem.
    Solfe

  12. #12
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    It's a stream, rather than a bolus. If braking at destination is required, the first packages to be launched are those that will be consumed at the destination, which are moving slowly and have a long way to go. Then faster packages for the earlier braking trajectory. Then the acceleration trajectory is seeded, with slower packages for early acceleration, and then faster packages which will overtake the slower ones, and provide fuel later in the acceleration phase. Then the ramjet launches. The whole is carefully tailored to place a fuel package in the right place at the right time with the right velocity, as the ramjet passes through. Any delay in the ramjet launch, and the stream will compact at the far end and disperse at the near end, eventually becoming unusable.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  13. #13
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    Quote Originally Posted by holmes4 View Post
    -- there is no friction in space --
    A rogue planet, wandering between stars would provide enough friction to vaporize any reasonable interstellar craft.
    We are starting to get some stats on how come such things are, one time star occultations; but you are not going to fend such a thing off with your magnetic deflection screens. It remains to be seen how big a problem interstellar debris will prove to be. Radar may not be good enough to pick such things up before collision is unavoidable.

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