Saturday, June 11, 2005

The Rods from God is an article about space-based weaponry.

The system would likely be comprised of tandem satellites, one serving as a communications platform, the other carrying an indeterminate number of tungsten rods, each up to 20 feet in length and 1 foot in diameter.

It's an interesting approach. Very heavy objects dropped from a very high altitude carry a lot of potential energy, which is then converted to kinetic energy, which is then used to destroy things.

I wondered though, where does that energy come from? We have to lift those rods into space. How much would it cost?

- What is the volume of each rod? The volume of a cylinder is
`pi * radius`

=^{2}* length`pi * 0.5ft * 0.5ft * 20ft`

=.`15.7 cubic feet`

- What is the density of tungsten? Google tells me 0.7 pounds / cubic inch.
- What is the weight of each rod? First convert
`15.7 cubic feet * (12 in/ft)`

. Then multiply^{3}= 27143 cubic inches`27143 cubic inches * 0.7 pounds/cubic inch`

and we get.`19000 pounds`

- What is the cost of tungsten? Google pointed me to a Reuters story suggesting prices are quoted in hundreds of dollars per ton. That means the tungsten in this rod should be fairly cheap (under $5000).
- What is the cost of lifting something into orbit? This is harder to find; Google pointed at many different costs. The ad I got on that search led me to
`spacetethers.com`

, which gives`$1000-$2000 per pound`

. Given that it's the military doing this, I have to take the upper estimate, and say that the cost is`$2000/pound * 19000 pounds`

, giving.`$38,000,000`

I suppose $38 million could be considered a “reasonable cost” to the military.

Labels: future

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I first heard about this concept in a Warren Ellis comic, of all places. It sounded like one of those feasible-and-scary ideas that work great in fast-paced pop-thrillers...

Finding out that it is being seriously advocated is another matter...and the name just somehow makes it all the worse. It was hard enough taking the US armed forces seriously with the whole 'Star Wars' nonsense...this, well, at this stage I find it about as primitive and ludicruous as if we'd discovered the Iraqi's had a similar 'Fists of Allah' project underway.

That's a lot of power to be put in the hands of people who seem to be ignorantly proud of the category errors they're making.

At the same time, I can't help but appreciate the elegance of the idea...

-alex

IEEE Spectrum covered this sort of thing as their March '05 Cover Story...

It's an old SciFi notion (e.g. Footfall, by Niven & Pournelle, 1985). I wonder how many of these we'd have to make before it would be cheaper to get the mass from outside our gravity well.

Simplicity is beauty. No complex laser weapons or particle accelerators. What can we wait for the future? Asteroids instead of tungsten bars?

Terminal velocity limits how dangerous these rods would be. I suspect that the rods would reach terminal velocity after only a few hundred meters of free fall so lifting them into outer space would be a complete waste. If you would like me to calculate for you the terminal velocity, I can do it, it's related to the coefficient of drag, you would obviously want the rods to have an aerodynamic nose cone and rear-cone as well. The terminal velocity is solved for when the drag force is equal to the weight of the object. Vterm = sqrt(2*m*g/rho/A/Cd) where rho is density, m is mass, g is gravity, A is frontal area, Cd is coeff of drag.

I had expected terminal velocity to be rather high, given the mass of the rods. Do you have the coefficient of drag for a cone shape? It seems like it could get complicated.