4 ms·

A reasonable impact speed for a Rod would be more than mach 20. That's what we get out of ICBMs today. And that's including atmospheric drag. As far as dropp

• credit_guy 10y ago
  A Rod at Mach 20 only has four times the energy of a Rod at Mach 10. Not really a game changer (in my first point I used 7km/s which is Mach 20.6). The vis-viva equation doesn't change what I was saying in my third point. I didn't know about it, thanks for pointing to it, but implicitly I applied it. I found a Hohmann calculator online, and I got that the delta v for going from a 400km orbit to a 100km orbit is about 175m/s. I plugged that in the rocket equation assuming a decent solid propellant (APCP used in the retrorockets of the descent stage of NASA's Curiosity and Spirit, for example, an application similar to the retrorocket needed here) and I got a propellant fraction of 6.7%. Now, at 400km you still have atmospheric drag, and you need a delta v of about 25m/s per year to maintain your orbit. If you want to go to 600km, you pay a price in the deorbiting delta v. At any rate, let's say the optimal poster that brings you down from space is between 5 and 10% of the mass of the Rod. We can agree to tall this "tiny". But this will need at least a revolution around the Earth (at 7km/s that's more than 95 min). The fact that you have plenty of Rods all over the place doesn't help you. A perfectly place Rod will still take 95 min to get from LEO to surface. If you want to hit the atmosphere in minutes instead of 1.5 hours, you need to pay a price in delta v, but you don't have a problem with the control at hypersonic speeds - we are still in vacuum. Once you hit the atmosphere you need control to hit the target, not to get to the target faster. Now let's see what happens if you put a few dozen or hundred in space. What can you do with a single Rod? Sure, you can destroy a civilian building, and that could be a skyscraper and you kill a few thousand civilians. I don't think either the US or Russia would adopt such a strategy (North Korea maybe). But in terms of military targets, what good are these? Remember, they are tiny in comparison with nuclear weapons. You can't destroy nuclear bunkers, you can't acquire mobile targets. If someone could solve the mobile target acquisition problem, these would be really good at killing tanks and armored vehicles. This could be useful, but this is hardly the application the alarmists on the internet have in mind. The typical perceived threat is some type of decapitation strike (alluded by the comment I was answering to). This simply is far from realistic.
  • gozur88 10y ago
    >If you want to hit the atmosphere in minutes instead of 1.5 hours, you need to pay a price in delta v, but you don't have a problem with the control at hypersonic speeds - we are still in vacuum. Once you hit the atmosphere you need control to hit the target, not to get to the target faster. If you're in low orbit you'll drop into the atmosphere almost immediately when you burn retrograde. And you can hit your target faster by entering the atmosphere later than you otherwise would and steering down. The question is when you really have enough control to make that happen. >Now let's see what happens if you put a few dozen or hundred in space. What can you do with a single Rod? The earth turns. Within 24 hours all your rods will have been in a position to hit any target on the surface. >But in terms of military targets, what good are these? Remember, they are tiny in comparison with nuclear weapons. You can't destroy nuclear bunkers, you can't acquire mobile targets. You could most certainly destroy missile bunkers if your targeting was accurate enough even with a relatively small rod. But there's no reason to assume the rods are small - 50 tons is a nice round number. Assuming you can't get volume pricing SpaceX will loft 100 of them for about the price of a carrier. And I don't see why you think you couldn't hit a moving target (assuming there exists a moving target worth destroying that way). The MRBM missile the Chinese have developed to destroy carriers can hit a moving target, and that's substantially the same bit of engineering. >If someone could solve the mobile target acquisition problem, these would be really good at killing tanks and armored vehicles. As I pointed out, this is already a solved problem. That said, it would be pointless to use something that expensive to destroy a tank. There are all sorts of ways to kill tanks and armored vehicles for a tiny, tiny, tiny (and that's not really enough "tinys") fraction of the cost. You're already a sitting duck in a tank. No, the role for the Rods would be strategic - bunkers, ports, airfields.