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Science fiction writer needs help on sci-fi weapon

  1. Sep 16, 2011 #1
    i'm not a physics major so i have to ask a question and i may not know what i'm talking about but here goes. if i had a giant circular accelerator accelerating a metal slug from zero velocity to high velocity with say 10^12 watts per electromagnet in a circle of 10 electromagnets is it possible to accelerate the slug to a kinetic energy of greater than 10^12 joules? how does this kind of thing work?
    Last edited: Sep 16, 2011
  2. jcsd
  3. Sep 16, 2011 #2
    Maybe this is not a good idea. To keep an object with a mass m and a velocity v on a circular path with a radius r you need a centripetal force of

    [itex]F_{circular} = \frac{{m \cdot v^2 }}{r}[/itex]

    To accelerate the same object to the same velocity on a linear path with the length of 2·r (the diameter of the ring) you need the force

    [itex]F_{linear} = \frac{{m \cdot v^2 }}{{4 \cdot r}}[/itex]

    That means with the same Force the linear accelerator gives the double velocity. Furthermore it is easer to target with a linear coilgun.

    The circular accelerator would be the better choice if the maximum radial acceleration is at least four times greater than the tangential acceleration.

    That depends on the construction.
  4. Sep 16, 2011 #3
    thanks DrStupid
    this is very helpful.
  5. Jan 24, 2012 #4
    O.K. i get the math on the cannon but have anther question about it. a hypothetical scenerio where the iron slug is charged up to a kinetic energy of 10^22. this is 100,000 times the energy of an H-bomb. suppose i used the weapon on a planet? some how i do not think it will actually deliver that power...would it? what would be the effect?
  6. Jan 25, 2012 #5
    Well, for one, I'm pretty sure it would disintegrate at those velocities.

    In the hypothetical scenario that it doesn't, upon impact its momentum would be transfered to the body it hits, so I would expect a pretty big hole :biggrin:

    I doubt, however, that it would reach the destructive power of an H-bomb, since it's a different mechanism of releasing that energy to its surroundings.
  7. Jan 25, 2012 #6
    Thank you meldraft. i was thinking it would explode upon hitting the atmosphere. i guess since it hasn't been done i do not know except i like your answer about the big hole. Thanks! i was thinking about something i learned in physics class in high school. (M1XV1)+(M2XV2)=(M1+M2)X(V1+V2). i'm not even sure that's right, it's been a while, but i was thinking that a lot of the energy would be absorbed this way rather than massive destruction.
    Last edited: Jan 25, 2012
  8. Jan 25, 2012 #7
  9. Jan 25, 2012 #8


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    Assuming a "large" iron slug, much much larger than a normal bullet, I could see it surviving reentry and impacting the surface. The energy released would be equal to whatever it's kinetic energy is. It woudn't be quite like a nuke, but if it was large enough it could do significant damage.
  10. Jan 27, 2012 #9
    If the slug is compact, e.g. of order of 1 m in size. Then it is a relativistic object with energy of each proton compared with the value that BHC can provide. If such object is shot into air it will be bombarded by relativistic flux of atoms with intensity of order of 10^30 1/cm^2*sec. So I imagine the slug will be disintegrated within few nsec. Since mass of air column with length of order of 1 km is compared with mass of the slug, I believe remains of the slug must lose most of the kinetic energy within this distance. So I think the explosion will be similar to explosion of a huge H-bomb. I think also, since electrons are prone to scattering on heavy particles considerably more than heavy particles, there will be a pronounced directional flux of positively charged particles for some short time (I estimate the equivalent current may be more than 10^18 Amp), that should produce powerful EMP, though I can’t compare it with EMP of H-bomb explosion.
  11. Jan 27, 2012 #10


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    ...including the shock wave that would precede it, which would be larger than the projectile alone.
  12. Jan 27, 2012 #11
    My knowledge of particle physics is not that extensive. My intuition, however, says that a transfer of momentum can't possibly have the same destructive effect as an H-bomb, simply because the mechanisms which distribute the energy through the environment are different. Mechanical waves should generally be dissipated in a much shorter distance than EM waves (at least in this context), which really shrinks the potential area of effect.

    As far as I know, there is a limit to how large the shock wave can be, for a slug of a given size (along the same lines as saint-venant's principle). Practically, in order to have a slug with that kind of (kinetic) energy and not have it disintegrate in nanoseconds, you would have to increase its mass. Considering that you got a power of 10^22, you would need a pretty ridiculously high amount of mass to manage to hit something.

    I am really curious as to whether the hypothesis in my first paragraph is justified, so if any particle physicist is reading this, I would be delighted to read an expert opinion!
  13. Jan 27, 2012 #12
    Thank you guys for all the input. i was thinking the mass of the slug would be around 600 metric tons putting a 2,200m/s recoil on a 8,000,000 M.T. ship for a 10^22 joule kinetic energy...i think. i hope this is fun for you guys because it sure is to me, especially with all of tour help.
  14. Jan 28, 2012 #13
    I’m not a particle physicist also and I try to use general consideration. I imagine very high energy is released within a limited volume. It leads to very high temperature and intense flux of various particles. I believe only neutrino can escape far away and carry their energy. All of the rest particles as well as EM radiation are high-energy and must ionize surrounding air losing their energy. Since intensity of EM radiation is proportional to 4th power of the temperature I believe considerable amount of total energy is released just due to it. Air is not transparent for EM with wavelength shorter than 200 nm. So the fireball will explode until the temperature decrease below approximately 10 thousand degrees. I estimate its diameter at that moment will be of order of 100 km. The picture should be similar to H-bomb fireball evolution except size and shape (since height of atmosphere is limited).
  15. Jan 28, 2012 #14
    I think if the slug mass is 600 tons, while the launcher weights only 8,000,000 tons the recoil must be significantly higher than 2.2 km/sec (more than 10 km/sec to my estimation).
  16. Jan 28, 2012 #15
    Sounds like someone is building a Death Star :biggrin:
  17. Jan 28, 2012 #16
    the equation i'm using for recoil is V=E/mc which was given to me on another thread, in the physics forums.
  18. Jan 28, 2012 #17
    maybe the equation works differently because it was designed to describe the recoil from firing an energy weapon and this is a kinetic energy weapon can anyone elaborate as to the difference?
  19. Jan 29, 2012 #18
    I don’t know that is energy weapon, but I suppose you mean some device emitting electro-magnetic radiation, such as a laser. In that case the recoil is equal E/Mc indeed. Concerning kinetic energy weapon, shooting a projectile, the recoil velocity is V=p/M (if V<<c), where p- impulse of the projectile, M – mass of the gun. If velocity of the projectile is compared with speed of light, we must use relativistic formula for its impulse. Let’s estimate the velocity. Using classical formula for kinetic energy E=m*v^2/2 leads to v approx. equal 1.8 * 10^8 m/sec. It is relativistic velocity, but not “very” relativistic. So, as a rough estimation, we can use classical expression for impulse p=m*v. So the recoil is m/M*v, or approximately 12 km/sec.
  20. Jan 29, 2012 #19
    Thanx AlexLAV for the breakdown it was very helpful. this is really kool.
    meldraft it's not a death star but maybe it could destroy the death star without having to run fighters down a trench where they are vulnerable.:rofl:
  21. Jan 29, 2012 #20
  22. Jan 29, 2012 #21
    The defence against this is probably to put up some random junk between the planet and the ship. In all probability a grill of tungsten bars (or anything harder than the iron projectile) will fragment the projectile in the same way that some tanks have holes "drilled" into their armor to cause penetrating projectiles to be subjected to offcentre forces and therefore shear or shatter (it's called [STRIKE]spaced armor[/STRIKE]). [edit: I forget what it's called. But it exists]. [edit: I checked. It's "perforated armor". These are holes about 1/2 the expected diameter of the projectile, and causes the projectile to break up when it strikes]

    You might or might not still have heat transfer from the fragments burning up in re-entry. I think that's a different problem.

    I am fairly sure even a second hand "space motorbike" can tow such a device and can probably maneuver it more easily than an 8 million ton battlewagon. Likely you're going to need your own X-wing fighters after all. :-)

    I find it hard to envisage a situation where such railguns are used but I have no first hand knowledge of interplanetary warfare(!) ... Or if I do I am keeping it secret until after we have captured all the Earthlings.

    However, it seems only useful against planets with atmospheres and probably isn't the most effective way to force a surrender or kill all the life forms.

    If they live underground, will it work?

    If the planet is airless, will it work better than a much simpler and less complex bomb?

    If the target is a ship, how do you detect, identify, target and aim? Most likely you are outside visual range. Can your radar distinguish a decoy e.g. an inflatable balloon with metallic coating, from a ship? A ship is pretty small in the vastness of space. How do you aim?

    If the target is a ship, what happens if it's not solidly built, and the projectile enters one window, passes through the (empty) cargo space and then out the other window, doing nothing much? Happens plenty of times when you shoot an armor piercing anti-tank round at a jeep. The target does not have enough mass to convert the energy of the projectile.

    Perhaps the option for an explosive projectile with a hardened nose cap, or even one that can shotgun for use against biodomes on an airless world?

    BTW I see a lot of sci-fi that falls behind, or is not aware of, current military technology. For the benefit of readers with some knowledge of military systems it would be nice to cover why simple obvious defences like the aforementioned tungsten grids cannot be used.
    Last edited: Jan 29, 2012
  23. Jan 30, 2012 #22
    The object causing the Tunguska event was likely Cometary in nature, and 10 meters across. It delivered roughly 20Ktonnes of explosive power. Your object is 600 times more massive than an iceball 10 meters in diameter, suggesting that this should be similar to an H-bomb is 500 times more powerful.

    Part of what makes cometary objects so explosive is that the material can build up pressure and explode, rather than disintegrating from the outside in.

    If the object were to be made of distinct layers, with higher thermal expansion rates the deeper into the object we go, the pressure would build as the object gained heat entering the atmosphere before exploding.

    For example, the outer layer could be silicon thermal shielding, followed by carbon-steel, concrete, brass, lead, and finally a large core of gasoline that would ignite from the high temperature and release further energy (assuming an oxygen rich atmosphere).

    The mass could even be composed of a handful of kernels tethered, so that the tether burns on entering the atmosphere, allowing the objects to separate on their way towards the surface and impacting a higher surface area.
  24. Jan 30, 2012 #23


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    You sure about this? Heating would be only on one side for one thing. Also, if the object is large and massive enough, the object would pass through the atmosphere without having enough time to heat up on the inside and trailing side.
  25. Jan 30, 2012 #24
    It is sci-fy, so, sure :)

    It is likely the objects will be rotating, so the heating could be more uniform. Looking at the speed, assumed above to be 2x10^8m/s, (neglecting the fact that this is 2/3 the speed of light) with an atmosphere of thickness 5km, this gives us 25 microseconds for thermal dispersion. With the relatively small values of thermal expansion coefficient, this would require extreme temperatures, on the order of millions of degrees, to disperse enough heat to cause a pressure build up before impact.

    One way to mitigate against this is to aim the weapon tangentially to the surface. This would significantly increase the distance and time traveled in the atmosphere, increasing the impact of the heating, diffusion and expansion pressure, reducing the required temperature to a few dozen thousand degrees. This would also maximize the separation of multiple objects once the tethers burned away.
  26. Jan 31, 2012 #25
    It´s sci-fi.
    10^22J is, if my math is right, the energy equivalent of ~1100kg of mass. A technology capable of generating* or storing this much energy in a weapons-use-compatible timeframe will have better means of destruction than shooting oversized cannonballs.
    And there are other constraints: How long will it take to accelerate the slug without destroying it by its own acceleration? What about reaction forces on the firing structure?
    Talking of sci-fi: IIRC the lensmen used "neutralisation of inertia" for a comparable project ...

    *) How? controlled annihilation of matter? anti-matter? tapping the next convenient sun?
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