Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Supergun compared to nuclear bombs

  1. Sep 4, 2007 #1
    Imagine a theoretical scenario where an artillery 3.5m across and 150m long is built underground (its possible, according to a science magazine). A 20 ton tungsten shell is fired into orbit and strikes a target thousands of miles away. Since the escape velocity is 30000km/h, assuming that the shell does not lose speed during reentry, the speed would be 8333m/s.
    Here is the scary part. According to the equation ½mv^2, the KE would be 10 terajoules. That is almost as powerful as the hiroshima bomb. But if a hostile country (say Iran) were to fire shells like that as if it were artillery, I don't see a difference between this and the cold war.
  2. jcsd
  3. Sep 4, 2007 #2
    well what you say is true but the fact remains that the shell will lose speed in the re-entry and also gain speed. so it may or may not be the same.
    (visit my blog!)
  4. Sep 4, 2007 #3


    User Avatar
    Gold Member

    Welcome to PF, Longrange.
    With a minor modification, what you describe is a basic 'kinetic energy weapon' as was proposed by a famous SF author sometime in the middle of the last century and has since been seriously investigated by the military. The difference is that the tungsten rods (about telephone-pole size) would be fired downward from a satellite, with a guidance package.
  5. Sep 5, 2007 #4


    User Avatar
    Homework Helper

    Well, the cold war was cold.

    Firing bullets makes it a hot war, no?
  6. Sep 5, 2007 #5


    User Avatar

    Staff: Mentor

    If the impact energy is roughly equal to a nuclear bomb and the trajectory is ballistic, what kind of energy must the charge that fires the shell have...?
  7. Sep 5, 2007 #6
    Something moving that quickly through the atmosphere will most probably be massively decellerated by air friction. Saying that however on the basis that the artillery shell meets its target with said kinetic energy, it would be devestating yes, but you must remember that it is going to be falling vaguely vertically, and not exploding. Its force will be heavily centred on the pressure of the end of it hitting the ground and will probably make a rather impressive crater as it compresses mud buildings and the occasional unlucky person.

    However just because something has the equivalent kinetic energy of a nuke, doesnt instantly mean it is as effective as one, an artillery shell such as that wouldnt be stopped by any amount of armor probably, but its blast is primarily vertical and towards earth.

    By contrast a nuclear bombs burst is outwards and horizontal, this comes as a much more devestating explosion as the outward force will destroy buildings with a sidewards force (which they are much weaker against as they are designed to be resistant to the vertical forces of gravity ^_^) and the explosion is not majorly decellerated trying to pierce or compress mud and ground foundations, most of the destruction is focused on buildings, people etc etc.

    Also nuclear bombs have an ionising radiation component which will primarily irradiate people and cause them to get early signs of cancer, go crispy and die quickly and all those other frankly disgusting things that come with using nuclear weapons, not only that, nuclear bombs have an EMP portion that will disable electronic equipment making it even more devestating again.

    The artillery weapon may have some impressive kinetic energy, but what would you prefer, dropping a bus vertically on a few unlucky people or using the equivalent kinetic energy to ride the bus horizontally over land and run over a few even unlukier people?
  8. Sep 5, 2007 #7
    There will certainly be an explosion - consider large meteors and comets striking the earth, for example Tunguska and the Chicxulub crater marking the K-T boundary. There may be a scale factor - I'm not sure if the kinetic energy of a projectile can by directly compared with explosive yield in kT - but certainly large, heavy objects striking the earth can generate enormous explosions.

    No, it's a roughly spherical explosion. It can't really be anything else since the whole bomb vaporizes - there's nothing left to contain any part of the explosion and direct it in specific directions.
  9. Sep 5, 2007 #8
    It is primarily sperical, but the force can easily change perpendiculary and be deflected horizontally because the explosion is caused by the rapid superheating of gases around it, the gases can deflect.

    In contrast the tungston artillery shells explosion is caused by the kinetic movement of whatever it hits, and thus it will be much harder for the particulate matter to deflect horizontally i believe is the point I was attempting to make much simpler.

    As for large meteors hitting the earth, they do cause large craters, but this is because they are extremely heavy kinetic balls of mass, the devestation they cause is still barely comparable to a nuclear bomb with equivalent energy components.

    Also the meteors are sperically shaped and therefore whatever they hit will he compressed horizontally outwards as the groundlevel is pushed around towards its maximum radius (i.e. the equator of said meteor) and therefore they will have a larger blastwave then a cylindrically shaped artillery shell that was mentioned here
    Last edited: Sep 5, 2007
  10. Sep 5, 2007 #9


    User Avatar
    Gold Member

    Man, there's a concept—a nuclear shaped charge! That'd take the guesswork out of tunnel construction. :biggrin:
  11. Sep 5, 2007 #10
    I was just personally under the impression that although it is spherical, the explosion is caused by the superheating of gases around it, which causes fast expansion of the gases, the mushroom cloud is caused by convection currents pulling up the gases then decellerating them as they cool in the atmosphere?

    By the way i've never properly read into nuclear bombs before, I've just done light chemistry work on nuclear fission/fusion so I'm guessing thats how the bomb works. But I've always been the theoretical thinker ^_^
  12. Sep 5, 2007 #11


    User Avatar
    Gold Member

    A vast amount of the damage caused by a nuke is a result of atmospheric air blasting back in to fill the vacuum from the initial explosion. When it all hits in the middle, it gets deflected upward and back outward. That's where the mushroom shape arises.
  13. Sep 5, 2007 #12
    Ah so the explosion doesnt heat up the air as such, it just bursts outwards causing a vaccum, then the atmospheric air pulls into the void ^_^ knowing is half the battle hehe.
  14. Sep 5, 2007 #13


    User Avatar
    Gold Member

    Well, there's heat involved alright. Up to 45% of the yield can be in the form of thermal radiation, to the tune of close to 100,000,000 degrees. It's mostly electromagnetic, though, rather than conduction or convection. I wouldn't stand next to one without my sun-block.
  15. Sep 5, 2007 #14
    Atmospheric overheating

    I just realized something. The shell can initially start cone shaped, but during reentry, the shell will become overheated and stretch into an oval-spherical shape. That would definitely produce horizontal blast force (no wonder Saddam was pursuing this so much). Also, this completely different from the "Rods from God" project.
  16. Sep 5, 2007 #15


    User Avatar
    Homework Helper

    I sincerely hope that these comments are facetious.
  17. Sep 5, 2007 #16
    Lots of things about nukes are deadly - direct electromagnetic radiation, shock waves, thermal heat waves, convection currents, radioactive debris, etc. - but which is most deadly depends a lot on the altitude at which the bomb explodes and how far away you are. Some interesting stuff here - http://en.wikipedia.org/wiki/Effects_of_nuclear_explosions. There's an interesting old book on my shelf, http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330161139839 [Broken], that describes the explosions in more detail. A high-altitude air burst will produce a spherical explosion - any deviations from spherical symmetry are due to the interaction between the explosion and the ground, and the "optimum" altitude depends on what you're trying to accomplish with the bomb.

    So, a projectile impact is basically equivilent to an explosion of a given yield detonated at a particular depth in the ground.
    Last edited by a moderator: May 3, 2017
  18. Sep 5, 2007 #17


    User Avatar
    Science Advisor

    The projectiles would probably not be deformed by re-entry, as they would almost certainly be heat-protected for the purpose of surviving that stage of their flight. Also, it was discovered during test-detonations that a small nuke detonated at a lower altitude could do much more damage to surface targets, because the spherically-expanding gasses get deflected when they encounter the ground, spreading out in a sort-of "wedge" shape. The leading edge of this wedge acts very much like a razor shaving a leg; a long, hot, silky-smooth leg that...

    Oh, anyway, the effect of this mega-mortor would probably be more like that of an "Earthquake Bomb," shifting the ground underneath a city causing foundatuions to crumble, thereby precipitaing the collapse of structures.
  19. Sep 9, 2007 #18
    The railgun is also a kinetic weapon but instead of supplying the vast amounts of kinetic energy with mass it does it with velocity. Fired at many kilometers a second (according to wikipedia) it can hit fast and hard. The benefits of firing large chunks of metal as apposed to nukes include : More metal "missles" can be stored, they don't take as long and as much resources to make, they are safe from exploding if the railgun takes a hit and they don't leave behind radioactive material that can plague the earth for years.
    Last edited: Sep 9, 2007
  20. Sep 10, 2007 #19
    Most of its energy will be converted to viscous heat and more money will be needed to do this kind of thing than nuclear bomb.
  21. Sep 10, 2007 #20
    I don't know. How much would it cost and how long would it take to inrich some weapons grade radioactive material? I'm pretty sure it's pretty costly for time and money.

    A railgun only requires a metal projectile, a strudy set of rails and a huge amount of electricity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook