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How long would it take for an interstellar spaceship to reach Alpha Centauri?

  1. May 17, 2012 #1
    I'm writing a science fiction story and have a basic understanding of theoretical propulsion systems (ramjets, ion drives, solar sails, etc.). But I'm also trying to make my story plausible. So, that means no faster-than-light travel, infinite energy sources, artificial gravity generators, etc.

    In the story I want to consider the travel time and acceleration experienced by the occupants of a ship traveling from Earth to the Alpha Centauri star system (about 4 light years away) in the mid-22nd century.

    My early assumption was that the ship would travel at a constant .2c, thus a travel time of 20 years. But after doing a bit of reading, that doesn't make any sense; the ship must, of course, accelerate. I am now assuming that the ship will constantly accelerate to the mid-point of the journey, turn around, and negatively accelerate until it reaches its destination. The question, then, is the value of acceleration.

    For normal operations, a large section of the ship rotates to provide a 1G (9.81 m/s^2) centripetal force for the crew. But during interstellar transit, most of the occupants are in suspended animation/cryosleep/hibernation (perhaps not as realistic). Via the rotating section or engine acceleration, does the ship need to maintain an acceleration of 1G for the crew to remain healthy?

    I suppose it depends on the ship's design; that is, whether the engines are located at the "rear" and the decks are located parallel to direction of travel, or the engines are at the "bottom", and the decks are perpendicular to direction of travel.

    According to an online calculator I found (I would post the link but cannot since I am a new user), an acceleration of 1G and distance of 4.3LY would only take about 6 Earth years. That is dramatically shorter than what I had figured for the story. Halving the acceleration to 0.5G only takes 7.2 years. So is there a reason for the ship's acceleration to be so high, and in the future is that conceivable for something like a nuclear fusion-powered ion rocket?

    At 0.1G, travel time is 13.6 years, which might work into the scale of the story a little better. According to that calculator, acceleration has to be taken down to .01G before travel time takes around 40 years. But at such low acceleration, would the crew become prone to effects of microgravity, or could the rotation section of the ship take care of that?

    Core question: What is a feasible constant acceleration rate, in terms of g, for an interstellar spacecraft?

    I want to make it feasible for hard sci-fi fans. I wish I had some more knowledge of physics to bring to the table, but I appreciate any discussion or help. Good day. :)
  2. jcsd
  3. May 18, 2012 #2

    Filip Larsen

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    Gold Member

    Welcome to PF!

    If you haven't been there already you can perhaps find a bit of inspiration in some of the serious design studies made so far, like project Orion [1], project Daedalus [2] and especially project Longshot [3] and [4], and perhaps combined with more speculative concepts like fuel scooping [5] or beam riding [6]. You can find an overview of the different possible technologies at [7].

    As you probably are aware, going to answer your question is difficult and relies heavily on imagining available technology and social structure at the time of your story, while still adhering to some of the "hard limits" of physics, like conservation of energy, material limits in temperature and energy density, and so on. If it were me writing a story like that, I would probably try to eliminate the "wild" solutions first and then pick one of the solutions that goes nicely what you story is trying to tell. For instance, if you want to portrait the life on a generation ship then you obviously have to select a solution that utilizes generation ships.

    [1] http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion [Broken])
    [2] http://en.wikipedia.org/wiki/Project_Daedalus
    [3] http://en.wikipedia.org/wiki/Project_Longshot
    [4] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007533_1989007533.pdf
    [5] http://en.wikipedia.org/wiki/Bussard_ramjet
    [6] http://en.wikipedia.org/wiki/Beam-powered_propulsion
    [7] http://en.wikipedia.org/wiki/Interstellar_travel
    Last edited by a moderator: May 6, 2017
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