1. jonathan hernandez13
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    jonathan hernandez13 Contributing Member Contributor

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    Need specific help with hard SF computations

    Discussion in 'Plot Development' started by jonathan hernandez13, Sep 23, 2011.

    I love SF and especially the hard stuff, unfortunately I know very little about rocket science and physics and am having a hard time wrapping my head around some of the math.

    I don't need any hard numbers, (although those may be nice), but I need some kind of accurate and realistic means of determining some values for a story.

    Premise(s):

    1)A starship is traveling from Earth to the Sirius star system

    2)The ship is traveling at one gravity (acceleration)

    3)In order to travel at a sustained velocity of one G it has a fusion based star drive accelerating to the halfway mark, and then deccelerating for the other half of the trip (so it can stop)

    4)Because of time dilation, trip time will be 4.6 years, while observers will experience 10.3 years


    Problem: In order for my rocket ship to burn fuel (I'm thinking deuterium fusion) for 4.6 years straight, I'm trying to figure out roughly how much of the stuff its fuel tanks would need to carry. That way I can get some sort of idea of how big my ship would be at least, and I can describe it.

    I'm thinking it would have to be immense, even for a small crew. I don't know what to base that assumption on however, other than I know it's probably true.

    I have been checking some online resources, but much of it is just going over my head. I believe I have correctly discovered that one gravity equals 9.806 Newtons of force per kilogram of mass.

    But, I have no idea how much mass my ship will have/needs to have, so I'm stuck in a chicken and egg type thing.

    Let's say, with all the life support, electronics, mechanical systems, etc, there is one astronaut in a ship of about 20 metric tons (that is really light).


    If anyone can shine I light I would be mighty relieved, I'm stuck:redface:
     
  2. mugen shiyo
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    mugen shiyo Contributing Member

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    I think you would need to know the mass of your ship first.

    Then you would need to know the efficiency of your fuel. How much power or force can be harnessed from a certain amount of your fuel.

    Then you would need to know the efficiency of your engine.

    Overall, a pseudoscience will have a psuedo-answer. Make up numbers by relating them to actual space ships. If a space ship is so big, determine how much bigger/smaller your own would by and multiply or divide the mass/weight by that factor

    Same for the fuel. How much more potent is your choice of fuel to that used by real rockets. If the fuel of real rockets moves a real spaceship so much distance, than calculate the same for your fake ship and fuel using ratio's. I don't think anyone will be paying TOO much attention to the numbers that you come up with. Only if they were just glaringly wrong, but there is no way to prove or disprove the pseudomechanics of a pseudoscience. Have fun.
     
  3. jonathan hernandez13
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    jonathan hernandez13 Contributing Member Contributor

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    I don't think pseudoscience is the word you're looking for, theoretical maybe. Everything I mentioned in the problem exists or is demonstrably possible, right down to the fusion reactor.

    There are actual formulas and equations that are used to derive knowns from unknowns, I just don't know what they are and get confused with some of the technical terms. I gave a hypothetical mass of 20 metric tons.

    Chances are not many people know this kind of stuff, unless maybe they work for NASA or something.:rolleyes:


    I read somewhere about ship mass/fuel mass ratios. Some of the ones I saw were 20 to 1 and higher. For a 20 ton ship it would need the fuel mass equivalent to the cubic volume of about 400 freight trains. That sounds ridiculous.:eek:
     
  4. psychotick
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    psychotick Contributing Member Contributor

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    Hi,

    You need some more data before you can begin to calculate this. You need to know how much force your fusion drive can generate from how much deuterium. With that and the mass of the ship including fuel you can calculate a burn rate to achieve one G acceleration. (Guess the mass of the ship initially - its actual weight doesn't really matter.) With a burn rate you can then calculate how much fuel you need over the length of the journey - by the way it would only be burning for just over two years max, but I'll explain that shortly.) When you have the amount of fuel you can then compare its mass against the mass of the ship and see if you have space left over for a crew, or whether as I suspect it quite possibly will be, the fuel mass is greater then the mass of the ship.

    As for the time of travel, relativity is going to kick in something wicked on your flight. Sirius from memory is eight or so light years from Earth. Relativity says that you cannot exceed the speed of light. And accelerating at one G you will achieve the speed of light in just over a year. Assuming you can do that, once you had achieved light speed, you would stop burning fuel, there would be no point, and then just coast until you hit the deceleration point. My calculation would be roughly ten years each way.

    Hope that helps.

    Cheers, Greg.
     

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