Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A rather large interplanetary ship

  1. Aug 8, 2014 #1
    Right, this is for the same universe as the "Two Planets" thread. To reiterate the backstory, the star system was colonized by an interstellar human civilization at some point in the future, but that civilization fell, the colony worlds were set back to the bronze age, civilizations rose, and now I've got to make one of them cross interplanetary distances with roughly modern technology.

    So now I'm designing a ship. That may seem simple, as we've got stuff on the drawing board for craft that would take us to Mars and suchlike, but there's a couple of added wrinkles. Firstly, the worlds are far farther apart than that--this is a double-star system, and world A orbits a different component than world B. The other is that a lot of people are being sent. The stars won't be this close together again for thirty-odd years (because of the way they orbit their barycenter), so if the nation that's doing this wants to pull a march on its competitors, it's got to get everything there now.

    There are ~200 people being sent. Not enough for a viable colony, but they're assuming they can interbreed with the natives (rather than them having been genetically engineered for their environment, which is possible as far as they're aware).

    I've decided to cram them all tightly into a spherical module; after calculating the weight of the lead shielding (there's other forms of shielding, but that'll be the most massive), it was apparent that that was the only viable option. (I figured the dimensions out, but I don't have it on hand.) The ship is a spinning bola; considering the length of its journey, and that the destination planet has gravity that's higher than the homeworld, gravity's a necessity, and this is the easiest way to do it. Once the ship has been accelerated to speed, it'll spin 90 degrees to the angle of thrust, extend the hab section at the end of a tether, and start spinning. (I gave myself quite a headache trying to balance the two sections before realizing, literally just today, that not only was it not necessary, I'm actually better off not doing so--if the "rear" is many times more massive than the "front" the barycenter will be closer to the rear, and therefore the tether doesn't need to be as long as it would otherwise be. Derp.) There's also a whole bunch of equipment that they'll need, but I haven't gotten around to massing that yet.

    Not real happy with the fact that I'm forced to use nuclear salt water rockets, given that there's controversy around that concept, but nothing else has the ability to make the journey (about 30 or so AU) in the time I need without having a mass ratio of ridiculous, and even so I find my self having to stage the sons of bitches.

    So, does all this check out, or are these the ramblings of a madman? Got any way I can weasel out of using the NSWRs (no disrespect to Dr. Zubrin)? Any other thoughts or concerns?
    Last edited by a moderator: Aug 8, 2014
  2. jcsd
  3. Aug 14, 2014 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    You're going to need to provide numbers. How much does your ship mass unfueled for a start? Then you'll need to get a figure for exhaust velocity and decide on a cruise speed. Using the rocket equation you can find out how much fuel you need and then work out how fast the ship can accelerate.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook