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

Kinetic bombardment effects

  1. Mar 15, 2016 #1
    So, i have some questions, they might sound simplistic or fanboyish even, i apologize beforehand for that! I cannot reveal anything more at this point, sorry :D

    I would need some realistic assesments of a simple rod, launched from low orbit, designed to take out a city or more....

    Would need in general some pointers (or links to numbers), best material, terminal velocity, the mass required at that velocity to fulfill the objective.
    I need it to be better than USAF's vision from 2003 report, which envisioned 6 meter long tungsten rods, 0.3m diameter and Mach 10 terminal velocity, hiting at 0.12kt equivalent.
    My math skills suck too much to figure out the mass of this rod even though i know all the relevant info for calculating it is given.

    Furthermore, if this is launched at a coastal city, where water depth is shallow, say 100 meters or less, can it penetrate all the way down to the sea bed, and make an eartquake and a tsunami because of the eartquake?
    here is where the real problem begins, i can guesstimate with reasonable accuracy when enlarging the idea, leaning on the aforementioned USAF report (link http://www.rand.org/content/dam/rand/pubs/monograph_reports/2011/RAND_MR1209.pdf) , however i am completely clueless as to the energy that would be expended in the tsunami and subsequent eartquake, ie, approximate size of waves, severity of quake on Richter scale, area affected and so forth....

    Thanks for any answers.
    Last edited: Mar 15, 2016
  2. jcsd
  3. Mar 15, 2016 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Volume of a cylinder is πr2h. A 6 by 0.3m rod has a volume of 1.7m3. A quick google pegs tungsten's density as 19.25 tonnes per cubic meter. So a rod would mass 32.725 tonnes.

    Mach 10 is 3402.9m/s. Kinetic energy is calculated as 1/2mv2 (mass in kg, velocity in metres per second). Plugging our numbers in we get 1.9e11 joules.

    A tonne of TNT is 4.184e9 joules. So the rod is equivalent to 45 tonnes of TNT. That's a lot, but it's not enough to destroy a city or cause a tsunami (the 2004 Indian Ocean tsunami released the equivalent of 26 million tonnes of TNT).

    If you want a beefier space based kinetic weapon: increasing the mass will increase the energy linearly (I.e. 10x as massive rod, 10x energy) butt increasing velocity raises the energy as a square (I.e. 10x faster, 100x energy).
  4. Mar 15, 2016 #3
    This is exactly what i wanted to continue on my own, many thanks. just one question, you came to conclusion of 45 tonnes of TNT equivalent, while i am almost certain i remember the number mentioned to be 0.12kt, ie 12 tonnes equivalent from that report.

    So, basically, i need a bigger mass and faster velocity to get even near the threshold where earthquakes and tsunamis are triggered? Can this be like a cumulative effect, a few of these rods, sufficiently upscaled ofcourse hitting in quick succession?
  5. Mar 15, 2016 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    No problem. 0.12kt is actually 120 tonnes. I'm not sure what the discrepancy may be, if it's lower it may be because some of the rod ablates on re-entry but I'm not 100%.

    It would be commutative in the sense that multiple rods would grant more energy but that would scale linearly. In the example above you'd need to drop 500,000 to get the Indian Ocean equivalent (though that may be overkill). A good place to start would be to figure out how much energy you want to unleash, from there it would be easier to calculate what size and speed a rod would need to be.
  6. Mar 15, 2016 #5
    Oops, sry, ya 120 tonnes, im layman.... :D

    Okay, thanks, will think this through a bit, see how i can integrate it. but generally, if i increase the mass and speed there would not need to be 500.000 right?
  7. Mar 15, 2016 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    My advice would be to plug the kinetic energy equation into a spread sheet and play about with it. As I said above when you increase the mass the energy increases linearly. So if you made them 10x bigger you'd only need 50,000. However energy increases with the square of the velocity. So if you make it 10x faster then the energy is 10x10=100. So you would only need 5,000 if you sped them up by a factor of 10.
  8. Feb 13, 2018 #7
    wouldn't the possibility of earthquakse or tsunamis be dependent on the LOCATION of your target? Hit a known fault line might trigger an earthquake. Hit the edge of a cliff and drop 400tons of rock into the ocean might trigger tsunami. Hit a fault line that drops the ocean floor a few feet, maybe another tsunami scenario.... or you could just target a large dam and blow the side out of it.

  9. Feb 14, 2018 #8


    User Avatar
    Gold Member

    Devastation from TNT.
    Going back to 1917, in Halifax, when the ship Mont Blanc's cargo of TNT exploded after the ship caught due to a collision with the ship IMO in the Halifax harbor, the harbour at the time for North America for military operations during the First World War.
    The resulting explosion was the equivalent of 2.9 ktons of TNT, destroyed a half square mile of the city, and created a harbor tsunami from the water displacement. The Mont Blanc was obliterated, and components such as anchor and guns launched into the air, landed several miles away.
    This was the largest pre-nuclear explosion.
  10. Feb 14, 2018 #9
    Thank you for pointing out that bit of history. Wikipedia is addictive, took me almost an hour of reading before I replied but it was interesting reading. You are quite right that there are other ways than the ones I mentioned. But the point I was making was the target is important.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted