Posted by: bean
« on: November 14, 2012, 04:52:49 PM »If the rod is designed to punch through the atmosphere and can survive the heat of re-entry it does not need to be 20 tons.Yes. If it's a rod. I was speaking of a projectile designed to make a crater and do surface damage. Which has to be about the same in length and width.
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And impacting at hypersonic speed it will make a nice hole in the ground.A rod will do little else.
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20 tons is absurdly high. There is no physical reason for such a number.It is somewhat high. I made a mistake, and the actual number is more like 2.1 tons for tungsten. It will be higher for other objects.
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We aren't talking about an arbitrary potatoe shaped object striking the atmosphere but a guided projectile. Most of the energy is lost in the atmosphere and the end velocity of the projectiles is low, the less energy you give to the atmosphere (a more direct approach, a better areodynamic shape) the higher the impact velocity the lower the mass needs to be. What it has to do is not disintigrate in the upper atmosphere like most meteors do.Please do not lecture me on atmospheric entry and hypervelocity impact physics. I've read far more about it that you have. I know this because what you keep saying is wrong. And I'm also willing to share the source of my insights. For a basic background, check out Space Weapons, Earth wars. http://www.rand.org/pubs/monograph_reports/MR1209.html
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The material of the missile would vaporize on impact at those speeds, the plasma shock front of that would expand through the ship and tear it appart even if only a fraction of the momentum (1-10%) is transfered. Thousand of KPS you don't gain by a warhead.That's pretty much what I've been saying, although I think you vastly overestimate the amount of momentum transferred. Stuff will break before it passes much on, and become part of the shrapnel cloud.
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It doesn't need to be perfectly in-elastic since the ship is going to crumple around that impact point anyway, even if the plasma ball that used to be a missile punches through it the ship will be rendered mission killed since every human onboard will be reduced to so much thin paste. Getting slamed into a wall at 100G isn't something you survive.
You've been doing the math as if the collision was perfectly inelastic (the objects stuck together).
But you still are ignoring my arguments as to why the momentum transfer won't kill the ship. So I'll lay them out simply.
First, the projectiles you keep using are ludicrously large compared to the target ship, particularly given their speed. An Iowa-class was 33,333 times the size of its projectile at light load.
Second, the ship's structure will not be able to take the forces involved in 100G acceleration. The structure might be rated for 10G, but that's only from the engine. From any other direction, it will fail under much lower loads. The area that gets hit will behave more or less as a liquid, rip free very close to the projectile and turn into shrapnel (which is what I meant when I referred to spalling.) The total momentum transferred to the rest of the ship will be minor. The same goes for any other bulkheads it hits along the way, and the projectile goes out the other side as a cloud of plasma.
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The point is putting a fusion bomb on a kinetic weapon moving at thousands of KPS is needless. At those speeds you don't get spalling, or anything...6000 kps is 6,000,000 m/s that goes through a 100 m of ship in tens of microseconds. Nothing can move out of the way of that, that is faster than the scale at which molecules move. It might even be energetic enough to exceed the columb barrier but that is more akin to a particle beam hitting a material objec then what is normally considered an impact of two physical objects.Not even close. Particle beams operate close to c, which is 300,000 kps. You're still a long way short of that.
And again, look at what happens here. Force is equal to the change in momentum (impulse) times the change in time. The time is very small (.000017 seconds). The impulse available is simply too small to have a serious effect on the motion of either the projectile or the ship. So no jelly. Burned and lacerated humans, but not jellied.