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Questions for a Near-Future story

  1. Oct 5, 2013 #1

    Drakkith

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    Okay, I've been thinking of writing a story and I have a few questions.

    First, the idea is that a cargo ship on the way back to Earth from the outer solar system is sabotaged or has an accident, leading to a loss of communication with the rest of the solar system and, more importantly, their fusion reactor has been damaged and they cannot accelerate or decelerate. And to make matters worse, I was considering having them be on a collision course with Earth unless they can get their ship working again. (If feasible)

    Ship specifications:
    Some type of ion engine to provide propulsion.
    Fusion Reactor for power.
    Spinning ring where the crew live/work. No cryogenics or anything like that.
    Designed to haul large, bulky cargo. At least 1000 tons worth. Cannot enter the atmosphere of a planet.


    Now, specific questions.

    1. What kind of minimum mass would a ship like this have?

    2. What kind of velocity could they realistically reach given ship mass, engine, and fuel constraints? Would 0.01c be feasible?

    3. Without primary power, what kind of problems would they have to deal with? I've thought of things like shortage of fresh water, ship temperature regulation, oxygen generation, and difficulty repairing the ship. What else?

    That's it for now. If anyone has any suggestions please feel free to comment.
     
  2. jcsd
  3. Oct 5, 2013 #2

    Vanadium 50

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    .01c means you need to convert 0.01% of the ship's mass to energy - half when speeding up and half when slowing down. This is not a rocket - this is running on the ground, so it is hugely optimistic - this works out to something like 8 billion BTUs/ton.
     
  4. Oct 6, 2013 #3

    Ibix

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    Friction will spin down the living ring and spin up the rest.

    It's awful dark out there in the outer system.

    Earth is a small target to hit by accident. Maybe a line about the saboteurs picking the time for the attack carefully?

    I don't know how practical Bussard ramscoops are, but it might help with your fuel requirements.
     
    Last edited: Oct 6, 2013
  5. Oct 22, 2013 #4

    Ryan_m_b

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    Not sure if this scenario counts as near future but I'll do my best to help.

    A fusion reactor is going to weigh a lot. The ones on the ground that we've been experimenting with might weigh a few tonnes but one in space is going to have to dissipate a hell of a lot of heat meaning you'll need to carry a lot of radiators. No idea what kind of number would be feasible for a mass I'm afraid. But given that the ISS masses 500 tonnes perhaps as a ballpark figure go for ten times higher. At least to give something to work with.

    This is VERY back of the envelope, not to mention hand waving away the technobabble but using the rocket equation:

    Given a mass of 6000 tonnes (5000 craft plus 1000 cargo), a target speed of 0.01c and a specific impulse of 100,000 (near magic in other words but you did say fusion drive) you'd need ~128,000 tonnes of fuel. But that's only to speed up, to slow down you'd need a total of ~2,700,000 tonnes of fuel.

    Even if you did have that much fuel your thrust would probably be so low that you wouldn't get to 0.01c before halfway point.

    Heat is the main one. That ship is going to get dealthly cold, not right away but probably in an inconvenient amount of time. Regarding things you haven't thought of it all depends on what emergency power does. If you don't have any then darkness, a lack of computers and no space toilets spring to mind.

    It seems likely that in a setting where intrasystem is so routine that manned cargo vessels from the outer system are economical that tracking said vessels will be common. It probably wouldn't take too long for a system of satellites to detect that a ship had shut down and was drifting, slowly cooling. A rescue craft would probably be sent out ASAP.
     
  6. Oct 22, 2013 #5

    Drakkith

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    Awesome. Thanks guys.
     
  7. Oct 22, 2013 #6

    Borek

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    20 times more? And not just twice?
     
  8. Oct 22, 2013 #7

    Bandersnatch

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    Since you need 128 000 tonnes to decelerate a 6000 tonne ship, during the acceleration phase your ship will need to carry all that additional dead weigth.
     
  9. Oct 22, 2013 #8

    Borek

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    Obvious. Somehow I got confused.
     
  10. Oct 23, 2013 #9

    Ryan_m_b

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    The rocket equation is easily forgettable. It's so intuitive to think "it takes X fuel to go Y speed therefore 2X to go 2Y" forgetting the mass of the extra fuel.

    Incidentally if anyone wants to experiment with the equation but like me your maths is rusty here's a handy online calculator: http://www.quantumg.net/rocketeq.html
     
  11. Oct 23, 2013 #10

    Borek

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    It was a little bit more convoluted - I was first confused by misunderstanding what you wrote, and that put me on the wrong approach to the problem.
     
  12. Oct 25, 2013 #11
    Given how long you've been around here, you probably already know about this site, but just in case you don't, I think this will be helpful: Project RHO
     
  13. Feb 11, 2014 #12
    I'm thinking that in a future where spaceships are becoming ubiquitous there would be massive defenses in place around Earth to stop someone from using a spaceship as a massive bomb like that. Expect nuclear missiles and gigawatts of coherent UV-light to be delivered to you if your vector transects Earth and your speed is deemed too high.
     
  14. Feb 11, 2014 #13

    Ryan_m_b

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    Enter stage left a nation state with a grudge against their neighbour and a cyberwarfare division and suddenly those "defences" start to look a little bit ominous.
     
  15. Feb 12, 2014 #14
    The same line of thinking could be applied to the current US nuclear arsenal, is someone going to hack that anytime soon?
     
  16. Feb 12, 2014 #15

    Ryan_m_b

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    The US arsenal is not computer controlled unlike a hypothetical network of weaponised satellites. Regardless another concern is the polices turmoil this defence network would create. It's one thing for a nation to have nuclear weapons (and nations developing those programs is a huge political issue) but it's quite another to think of surrounding the earth in giggawatt lasers. What nation controls these? Is it international? Do different nations have their own to force a balance of power?

    All interesting topics for fiction to explore.
     
  17. Feb 12, 2014 #16

    Borek

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    Build the lasers on the dark side of the Moon.
     
  18. May 10, 2014 #17
    Write the story, not get bogged down by trivial details! Unless your story ends with hundreds of facts
     
  19. May 10, 2014 #18

    Borek

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    Well... I hate it when the author doesn't treat me seriously. Obvious holes insulting my intelligence are what makes me put the book aside.
     
  20. May 10, 2014 #19

    If I were reading my chemistry textbook I would completely concede to your statement but if I'm watching Star Wars I dont expect scientific accuracy. Which is analogous to when watching batman you don't turn off the tv because he accelerates faster than the woman who falls out.
     
  21. May 10, 2014 #20

    Borek

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    I never said you have to switch YOUR TV off, but let me use my remote my way :tongue2:
     
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