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Theoritical weapon question.

  1. Apr 12, 2012 #1
    Ok, so let's say I have a weapon that can fire a 2 centimeter diameter rod that is 25 centimeter long at a speed of 1/3 the speed of light, or 100,000,000 meter seconds with a weight of 5kg.

    I estimate the energy at 2.5 e16 joules, or approximately 6 megatons.

    Now, my question is complicated, I figure a large part of that energy will be heat as it starts to touch the atmosphere at the barrel's mouth as it exit the weapon.

    How do I go about calculating how much energy would probably be wasted burning up in the atmosphere on the surface of the earth if I shoot the weapon to the horizon, so say going through roughly 100km of dense atmosphere at normal sea level.

    Next how far do you think such a rod would travel down to the center of the earth?

    Next loud would this potentially be? from this site

    http://www.makeitlouder.com/Decibel Level Chart.txt

    I'd estimate between 275 decibel or so.

    finally, the air molecule it hits as it exit the weapon, would the rod cause a shower of mainly x-ray? Or is it possible that it would create mainly gamma ray as it travels and interacts with matter?

    thank you very much for your answers.
     
  2. jcsd
  3. Apr 12, 2012 #2

    sophiecentaur

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    It is going so fast that it will only be travelling through the air for a very short time. Hence, I wouldn't expect it to transfer much of its energy. I imagine that it will not have time to interact with a big cross section of air so, perhaps you'd get a good idea of the energy put into the atmosphere would be to consider the momentum transfer to the mass of air molecules (the long cylinder) it would encounter on the way through. On second thoughts, that would represent quite a few kg of stuff! What speed to the fastest meteorites hit the atmosphere, I wonder? They tend to vaporise unless they are pretty massive.
    Certainly, an object travelling at that speed will be generating extremely high frequency EM.
    It's interesting that you ask whether X rays or Gamma rays would be generated. If you look at the loose definitions of the regions of the EM spectrum at that end, there is a massive overlap between the two. This is largely, I think, because the name Gamma, implies Nuclear origin and X ray means other sources. A Photon with so many squigahertz frequency could be either. Not worth worrying about.
     
  4. Apr 12, 2012 #3

    K^2

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    The thing won't make it through the atmosphere. Not as a rod. To punch through as a rod, it needs to take that 2cm diameter cross section column of 100km of air and bring it to its own speed, because compared to c/3 transverse velocity of air is negligible, and air won't be able to get out of the way. So if fired straight up, you will need to take 3kg of air and match speeds with your 5kg rod. The rod would have to decelerate from 1/3 of the speed of light to a little over 1/5 almost instantly due to incoming pressure. Nothing can withstand that sort of stress or that sort of energy release. The rod will obviously exit atmosphere, but as a burst of plasma that's a mix of ions from the rod and these from atmosphere.
     
  5. Apr 12, 2012 #4

    OldEngr63

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    Have you considered how you will power this weapon? That's a lot of umph to get it going!
     
  6. Apr 12, 2012 #5

    phinds

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    +1 on that.

    I don't think that kind of acceleration (0 to 1/3 c in next to no time) is even remotely close to obtainable with any currently known technology (for an object of that size).
     
  7. Apr 12, 2012 #6

    davenn

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    ~ 40 km/sec is typical from what I have read

    Dave
     
  8. Apr 13, 2012 #7
    Would it necessarily have to accelerate a volume of air equal to its cross section times traveling distance up to its own speed? If it gets slightly narrower in the front or has an aerodynamic shape, it would give the air molecules transverse acceleration also. Also wouldn't the increased pressure in the front of the rod cause acceleration in the transverse direction?
     
  9. Apr 13, 2012 #8

    mfb

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    In the system of the rod, the air (in good approximation: 100% nitrogen) would move with 1/3 c, which gives you an energy of ~50MeV per proton and neutron or ~700MeV per nitrogen nucleus (and ~25keV per electron). This is small enough to stop the particles, which means that nearly the whole energy is transferred to the rod.
    The atmosphere has a total area density of approximately 1kg/cm^2, which means that the rod will receive an energy of 1/2 rho v^2=5*10^15J/cm^2. This is enough to make a nice, dense plasma, and if you shoot this rod towards the earth it will probably make a small, sharp hole and give a significant explosion somewhere inside, when the plasma is slow enough to expand significantly.

    There is no aerodynamic shape. The particles hit the rod with relativistic speed and impact it. The transverse acceleration is very small, compared to the speed of the rod.
     
  10. Apr 13, 2012 #9

    sophiecentaur

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    Could you explain what area density means in this case? Won't that figure depend upon the path taken? The scenario was to fire the projectile horizontally at one stage.
     
  11. Apr 13, 2012 #10

    sophiecentaur

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    So a totally different ball park (not surprisingly).
     
  12. Apr 13, 2012 #11

    sophiecentaur

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    Recipe for tiger stew:
    First catch your tiger.
     
  13. Apr 13, 2012 #12
    Well, lets be honest, there is no way with any known technology to achieve this kind of power in about any type of engine or motor, or whatever, so I use magic science.

    The way I envision it, the technology is based on multiple dimension above our common 4, 3d and time, so the whole thing is using what I call dimensional mirrors, because the human mind can not comprehend or visualize the higher dimension, all we see is a mirror with very weird light refraction.

    Those mirrors are powered by matter/antimatter reactors, and can make complicated shapes around very exotic object. So, all the formula I can basically make from scratch, and it's basically tapping into "free energy" yet as usual, I'll impose some restriction an the such.

    The most realistic yet at the same time unrealistic part of it, is what the actual weapon is made of. I'll get back to that later.

    But basically, you create a stable loop with the dimensional mirror, imagine portable worm hole, that you can assemble any way you want. so you put an exit 100,000km away from a black hole, drop the rod into a very well define "tube" of mirrors, and then "flash" a window right before the rod hit's the event horizon, sending it back to 100,000km away. So as the rod falls it builds up speed, and when the right speed is attained, you open a window at the muzzle of the weapon where the projectile is then aimed at the target.

    I first thought about using a neutron star, however even though I know they can have gravity strong enough to accelerate to close to a third of the speed of light, my main concern was the gigantic electromagnetic field they possess that would probably wreck havoc on any matter.
     
  14. Apr 14, 2012 #13

    mfb

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    I was under the impression that the rod is fired from space towards the surface, as Khursed asked how far this rod could go into the earth. The area density I used is the one you get at sea level and orthogonal to the sea.


    @Khursed: That looks like a strange Science Fiction fantasy. In addition, if you can create such a loop (and transversing your mirror does not slow down the object), you can use the earth as well. With ~10m/s^2, the required time for the acceleration might be a bit longer, but who cares?
     
  15. Apr 14, 2012 #14

    sophiecentaur

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    I gave up on this once I read bits of post no 12.
    Real Sci Fi is more interesting and has better plot lines.
     
  16. Apr 14, 2012 #15

    OldEngr63

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    uh-huh.
     
  17. Apr 14, 2012 #16
    Well, you mean real science, because if it's science fiction, by definition, they are using basically extrapolation at best, and complete ******** at the worst.

    I realize it's impossible to get some result without cheating, I'm simply trying to bring novel ideas as to the "how" and try and be realistic with the consequence of what happens.

    I love science, yet one of my favorite movie ever, is "The Core" probably one of the worst science fiction movie ever for realism. Lets be honest, I didn't enjoy the movie because the science was awesome, and on the spot, I enjoyed it because I simply let go of all the nonsense, and let the story entertain me.

    My setting has faster then light travel, strike one, since at the moment we have no realistic way of making any ship go faster then light. My setting has exotic material, completely beyond any technology we know of, strike two. My setting has advanced being with completely out of this world powers, strike three.

    My goal, is to have the details as credible as possible in as many aspect as I can.

    I believe some people will enjoy that aspect, and many won't even care, and some will hate it. That is life, you can't please anyone.

    Yet, I'd like to describe whats going on with some realism, as opposed to simply fudge everything, and go for the rule of cool.

    The main problem i have, is that a lot of the stuff I'm thinking about doesn't have any type of real life equivalent, so ok, I posit a new weapon, that does something probably impossible, yet is it bad of me to want to explain the impact as realistically as possible, as opposed to simply say big boom, end of story?

    I want people to read the stuff, and go "wow, I wonder if that could ever happen" instead of "what utter ********, that wouldn't and couldn't happen at all, this is ridiculous".
     
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