Rods From God

kinetic_loki

It’s been over a year since the last science post, so I had probably better either a) make another science post or b) rename the site. What I’m about to talk about is more science fiction than proper science, but I was wondering about it and did the research and it’s close enough to the real thing to pass for it if you squint a bit, and most importantly it ends the science drought.

The hokum military fantasy plotline of Call of Duty: Ghosts is kicked off when the baddies hijack a US space station to drop a number of very heavy objects onto the continental United States, devastating the country and providing the developers with an excuse for one of the more mediocre first person shooters I’ve played in recent years. As with most concepts explored in Call of Duty, while the way its portrayed in the game is complete nonsense the idea of launching weapons into space that can bombard targets below is a legitimate one that’s been around for a very long time –since before we actually got into space in the first place,  in fact — and Ghosts even references a specific one: its Loki satellite has an obvious link to Project Thor, a proposal originating from the 1950s but which was being mentioned in news reports as recently as four years ago under its sexier nickname: Rods from God.

As the particular variant of kinetic bombardment that’s been drawn up by the US, Rods from God is by far the most well-known version of the kinetic bombardment idea, even worming its way into popular consciousness via “entertainment” products like CoD: Ghosts and the execrably awful GI Joe: Retaliation. The idea is that you would have a pair of satellites in orbit: a command and control satellite that handles targeting and guidance and a launcher satellite that houses the actual projectiles, which in this case are long, thin rods composed of a very dense material (tungsten is the one usually cited due to its frankly ludicrous density of 19.3 g cm-3 – for comparison, lead clocks in at a measly 11.3 g cm-3) and whose only attachment would be a set of small fins required to steer the rod through the atmosphere to its target. The high material density is required to maximise the projectile’s mass, and hence its kinetic energy once it strikes its target; kinetic energy is a function of mass times velocity squared, and the velocity of something dropped from orbit is in theory going to be pretty damn fast.

kinetic_london

Now, the popular depictions of kinetic bombardment mentioned above portray these rods as being absurdly destructive, striking with the force of a nuclear weapon and wiping entire cities off of the map. The reason I’m writing this post is because I got curious: tungsten may be dense, and orbital velocities may be pretty fast, but how much kinetic energy would one of these rods actually have once it reached the Earth’s surface? The rods can vary in size, but the larger versions are mentioned as being 6m long with a 30cm diameter, giving them a mass of roughly 8,000 kg. Assuming we want our rod to hit with the force of a small nuclear weapon (15 kilotons of TNT, say, or 63 terajoules), we’d have to ensure that it hits the Earth’s surface travelling at a velocity of

kinetic_eq1

For reference the fastest man-made object in existence is the New Horizons probe, currently in the penultimate year of its journey to Pluto and cruising at a cool 16.26 kilometres per second relative to the Sun — and it managed that only with the aid of a gravitational slingshot provided by Jupiter. Clearly, then, the idea of one of these rods striking with the force of a nuclear weapon is somewhat overstating their destructive capacity, especially since you’d lose a lot of the kinetic energy on the way down thanks to the Earth’s atmosphere. The question now is: how much kinetic energy would one of these rods realistically have once it reached the surface? There’s a bunch of complicated ways we can figure this out, but I’m going to cheat and simply assume that the rod is going to be slowed down enough by its passage through the atmosphere that it will be moving at terminal velocity when it hits the ground. The equation for working out terminal velocity is

kinetic_eq2

Where m is the mass of the object, g is its acceleration due to gravity (9.81 m s-1), ρ is the density of the medium through which the object is falling (in this case air with a density of 1.225 kg  m-3), A is the projected area of the object (the rods are very long and thin and present only a small surface area along their direction of travel), and Cd is the drag coefficient of the falling object (I’m using the drag coefficient of a cone, since my assumption is that the tip of the rod will be cone-shaped). Plug all of these numbers into the equation and the terminal velocity of the rod comes out at

kinetic_eq3

This gives the rod a kinetic energy of around 14.5 gigajoules, which sounds like a lot but which, when converted to something more analogous to explosive force, is equivalent to only 3.46 tons of TNT. That’s plenty of power, more than enough to wreck a small neighbourhood, but a single kinetic impactor isn’t going to be destroying a city any time soon.

More than that, though, there’s the question of practicality to consider. Rods from God do have a couple of advantages over regular missiles – dropping them from space would give any prospective target little to no warning before they struck, and the sheer density of the rod would give it an awful lot of penetrating power ideal for destroying hardened targets – but they have to be weighed up against the massive disadvantages inherent in the concept, first and foremost of which is that in order to drop an 8 ton rod onto somebody’s head you first have to lift it up into orbit. With typical launch costs currently standing at around $5,000 per kilo at the cheap end, putting just one of these rods into orbit would cost a whopping $40,000.000. Putting a whole array of them (plus launch system) up there would be ruinously expensive – and before you say “But Hentzau, a single stealth bomber costs a billion dollars,” you have to bear in mind that that price tag includes the R&D and engineering work required to develop the bomber in the first place. There would be similar costs involved in developing a kinetic bombardment array that would massively inflate the cost, and all for something that has less destructive capacity than a WW2-era Lancaster bomber,  and whose job can easily be performed by much cheaper conventional missiles and/or bombs. Little wonder, then, that the Rods from God concept has never made it out of the pages of science fiction.

Tagged , , , ,

16 thoughts on “Rods From God

  1. Gap Gen says:

    Interesting! I’d never actually looked at the energetics, and you’re right that this seems a bit underwhelming. Another thing would be that aiming might be hard if it’s just a dumb piece of metal falling into the atmosphere (even with aerodynamic fins, etc). Plus you’d have to fire it at a certain velocity so that it didn’t just follow you around in orbit and Gravity anything in its path. Certainly if you can capture asteroids firing one at the surface isn’t beyond the realms of possibility, but yeah, seems like a lot of effort when you could fire missiles at the target instead.

    • Hentzau says:

      Thinking about it the chances of a dumb piece of metal hitting anywhere near on target are next to nil, unless your target was several miles in diameter. Since this is a precision weapon you’d have to strap some sort of rocket motor to it to correct the trajectory, and then a targeting package to figure out what those corrections should be, and then all you’ve achieved is a very expensive missile.

  2. Strudel says:

    Yay for science! A welcome return.

  3. Reminds me of old joke: Chinese have developed pig iron bomb. Its area of effect is equal its area.

    I’ve always thought that the point of having space weapon is that you can drop it quick and without flying over all those borders with AA guns. Why would you travel to space to make a glorified catapult?

    • Hentzau says:

      It was a much better idea before the development of ICBMs and cruise missiles, that’s for sure. As it is modern precision guided munitions can be launched from outside a country’s airspace, evade AA defences and strike the target with a similar degree of force, so while it’s an idea that captures the imagination it’s also one that essentially obsolete.

  4. Zenicetus says:

    The “crowbars” the aliens used in Niven/Pournelle’s “Footfall” novel had a more realistic impact effect, more like a cruise missile explosion. They were aimed at tactical targets like Army tanks. It was a nice demonstration of the military advantage of the ultimate “high ground,” which was one of the big themes of the book.

    The really nasty stuff would be redirected asteroids including smaller ones, not just Dinosaur Killers, since they could have much higher relative velocity and kinetic energy. Of course you’d need to go out there and give ‘em a push to alter their orbits, but if you have that capability it would make a neat weapon.

    Speaking of which, it always amazes me how often people talk about the potential for mining asteroids and sending cargo payloads to Earth, without mentioning that any ability to fool around with asteroid trajectories is also automatically a weapon system. There is no way we’ll ever mine asteroids without some kind of ironclad safeguards and cooperation between participating nations. The idea that a private company would be allowed to do this is laughable.

    • Hentzau says:

      Less “neat”, more “terrifying”. You ever see that bit in Babylon 5 where Londo is staring out the window of his warship as it fires asteroids at the Narn homeworld? There’s a reason he looks utterly horrified; it’s pretty much the ultimate weapon of mass destruction simply because asteroids are so much more massive than anything we can put into space.

      That being said, asteroid mining *will* happen at some point (Earth resources ain’t going to last forever) but I don’t think anyone’s going to be moving full-sized asteroids around any time soon, both for reasons of practicality and because the very idea would make a lot of people extremely nervous.

      • Gap Gen says:

        There’s the idea of using an asteroid in Earth orbit as a counterweight to a space elevator. But yeah, I imagine people might get nervous. That said, assuming capitalism doesn’t implode and wealth & power disparity continues, I assume private companies will absolutely get away with moving asteroids about.

        As for the precision weapon against tanks, I doubt tanks will be a major weapon in the near future; missiles and drones will probably tear them apart, they’re useless for counter-insurgency and difficult to use safely in urban environments. Plus yeah, aimed kinetic weapons would probably end up with something like a depleted uranium missile, not a huge rod buffeting about in the troposphere.

  5. Why does there need to be a separate satellite for targeting and guidance?

    Regardless, this does pose some other questions: how long would it take from the start of the drop to the impact? Would it be long enough that the target area could be evacuated?

    Would there be any potential for some sort of defence system to intercept and destroy, or possibly nudge a falling rod somewhere else?

    • Hentzau says:

      They don’t technically have to be separate, but the advantage is that one targeting satellite could control many launcher satellites.

      Length of flight depends on how high up the satellite is. I think for something like this it’d have to be in LEO simply because lifting the rods higher than that is exponentially more expensive, which means 300-1000km altitude. It’s not quite as simple as dropping the rod to the surface, either, because you’d need to give it a kick of delta v that’d set it into some re-entry trajectory that isn’t straight down. A guesstimate would be somewhere around an hour from launch to impact an the short end.

      However, the difficulty in evacuating the target area stems from the fact that you have to see the rod coming first. This is pretty much impossible to do, since there’s no exhaust plume to aid in spotting a launch and radar won’t pick it up until it’s minutes away from impact. You could certainly knock a rod off course if you could hit it, but current difficulties with intercepting ICBMs show just how difficult that is to do when you have good tracking of the target.

      • Zenicetus says:

        There wouldn’t be an exhaust plume, but if it’s coming in fast enough it will heat up and glow, at least through the initial encounter with the atmosphere before aerodynamic drag slows it down. Might be enough for IR detection of an incoming strike, especially if the defense sensor is also in orbit and looking down, not up through the atmosphere..

        If it’s coming in REALLY fast it will leave an ionization trail. I’ve seen fast meteors leave a glowing trail across half the sky that lasted almost a full minute, at a dark sky observing site. Although I guess we’re talking less velocity than that.

        Anyway, by the time this kind of technology could be deployed, the ability to detect an incoming strike — either through better radar or heat sensing — might also be advanced enough to launch a defense of some kind. Ground-based laser maybe, for the more predictable military targets.

    • Gap Gen says:

      For things like GPS you need multiple satellites to triangulate your position – the more the better. If the enemy can detect the launcher they could also try to shoot it down. I wonder if the first space war will end up looking like Gravity and leaving us Kesslered until someone goes up and nudges all the debris out of the way.

  6. innokenti says:

    Gosh that’s a terrible name *shakes head*.

    Yay for science!

  7. well, the moon offers a big supply of rocks. slinging those into an earth orbit shouldnt be too hard. a railgun using a steel sled? And then you just have to nudge them into reentry. Targetting becomes the tricky part but if you get it aimed at a continent then a JDAM guidance attachment might be able to guide a cylindrical piece of moonrock to approximately the location of the separatists in eastern ukraine,.

  8. Anonymous says:

    I love to pick apart ideas like these.
    First: why two satellites? Two components are twice as likely to fail.

    Second: putting up atomic bombs would be way more cheaper.

    Third: Tungsten sounds good, but it costs arms and legs. There are other alternatives (like Osmium or Plutonium) but I’d wager that it would be better if the designers would load plain Basalt into the tubes. Otherwise you’ll end up with the same effect the US had in Afghanistan: blowing up a 10$ tent with a 40k$ missile. :-)

Leave a Reply