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The Physics of Propellant

  1. Dec 5, 2014 #1
    Let's say a ship is beyond Earth's meaningful gravity pull, and has solar panels. Assume that it can collect solar power and use it in a system which blasts propellant away from the ship to accelerate through space. The ship's goal is to leave the solar system and reach a new star, but to continue accelerating for half the distance, and decellerating the remainder, versus just drifting at a constant velocity. To me, this gives us two options:

    1. The ship must leave our solar system with enough propellant to continue accelerating. This seems problematic because for every kg of propellant it takes with, the mass grows, negating the benefits of bringing the propellant with.

    2. The ship must collect propellant along the way. This seems problematic because the ship is unlikely to encounter an appropriate amount of propellant along its linear path, or to solve the logistical problems associated with capturing them at high speed.

    This makes me think it's unlikely we could ever build a ship capable of propelling to another star.

    I hope my thinking is flawed?
  2. jcsd
  3. Dec 5, 2014 #2

    Doug Huffman

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    See Bussard Ramjet. Even at Earth, solar irradiation is only 1350 Watts meter^-2
  4. Dec 5, 2014 #3


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    It's extremely unlikely that any interstellar ship will continually accelerate halfway and then decelerate. The amount of fuel required is staggeringly huge. Gathering fuel on the way probably won't work either, as the interstellar medium is not very dense and the faster you're traveling, the less effective this method is. You'll slow down when you gather the fuel since the fuel has to accelerate up to your speed. The faster you go, the more energy this takes.
  5. Dec 5, 2014 #4


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    We can. The Voyager and Pioneer probes are doing exactly this and New Horizons will follow them. They are just all way too slow for practical purposes and won't stay active long enough.

    There are concepts that would allow significantly higher speeds - up to ~10% the speed of light, that would allow to reach the nearest stars in decades. Most of them use fission or fusion (some even consider antimatter) as their high energy density allows high exhaust velocities. Some use sail-like structures that would rely on a beam sent from the solar system. See Project Longshot and the various links in the navigation box at the bottom, or Category:Interstellar travel.
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