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SF Writer's Speculative question on Space Combat

  1. Nov 7, 2011 #1
    I am a sciencefiction writer who specializes in military science fiction. I have a question dealing with the affects of high speed targeting. Take two ships travelling on a recropical course. Each is seeking to target the other with weapons that can accelerate to close to C. Make one going at 0.65C, the other moving at 0.15C.

    Assuming both have targeting systems and weapons that are equally capable. Would the the ship going at 0.75 C will be harder to target?

  2. jcsd
  3. Nov 7, 2011 #2
    Any space faring race engaging in this sort of combat will have targeting computers capable of compensating for any relative movement of the ships. The calculations are pretty straight forward. What will make a difference is the closing speed of my projectile and your ship. Take the following 4 senerios...

    1. Your speed is .75c, I fire at you from in front.
    2. Your speed is .25c, I fire at you from in front.
    3. Your speed is .75c, I fire at you from behind.
    4. Your speed is .25c, I fire at you from behind.

    In 1 you have very little time to react with evasive maneuvers, in 2 you have more time to react so if you are easier to hit in 1. In 3 you have a more time to react then in 4. So weather you would be harder to hit in a faster ship depends on weather I am in font of you or behind you.

    If my weapon is a laser then the closing speed will always be c and you will have zero time for evasive maneuvers because you can never see it coming before it hits you.
  4. Nov 7, 2011 #3
    Due to the principle of relativity the situation is completely symmetric. The velocities of the ships in the rest frame of their opponent differ by the sign only.
  5. Nov 8, 2011 #4


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    You must always specify a velocity relative to something. That is to say, "it is travelling at 0.65c" is meaningless because if I am moving relative to you, its speed will be different from my perspective. We get away with it in every day life because everyone automatically assumes that when you say "I was doing 30mph" you actually mean "...relative to the ground". But note that pilots have to make a distinction between their speed relative to the air (airspeed - related to whether or not you are about to fall out of the sky) and relative to the ground (groundspeed - related to how long it will take you to reach your destination), so it's not that simple even in fairly ordinary circumstances.

    Your question as phrased implies that there is an onlooker - probably the narrator - who is watching the battle from a separate vantage point. One of the ships is doing 0.15c relative to him; the other is doing -0.65c relative to him. But the important perspectives from a shooting point of view are those of the ships themselves and, as DrStupid pointed out, they both see the same thing in opposite directions. Velocities don't add straightforwardly at relativistic speeds, so it turns out that each ship sees the other coming at it at about 0.72c. The situation is symmetric.

    The major problem for combatants in the situation you describe is related to the fact that the ships are travelling at sizeable fractions of the speed of light, and are (I presume) gathering information about each other using light or radar. That means that the information they have is significantly out of date, and they will have very little time to react to updates. Also, your targets are very small relative to the distance between them and the distances they travel within the reaction time of even a computer, and small inaccuracies mean a miss.
  6. Nov 8, 2011 #5


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    Also; niether one is going ".75c"
    ...unless we say that both are moving at .8c(which would not be strictly accurate either, but close).
  7. Nov 9, 2011 #6
    All combat in science fiction seems to be WWII 1945 style, since this is more exciting than what one would expect real combat to be like.

    I think that in real space any combat would be highly computerized and over before any human would be aware it had begun. Everything is relative so there is no advantage to any sort of maneuver. There is no place to hide either. Each of these two ships would be moving exactly the same with respect to one another. If you want these sort of things to matter then you need to introduce a third ship or body like a planet or star.
  8. Nov 10, 2011 #7
    For an interesting space battle, you may wish to consult Larry Niven's "Protector." It contains a space battle between two sets of "totally rational actors."
  9. Jun 15, 2013 #8
    If you have any questions about space combat then maybe this site can help you out:military-sf.com
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