5 ms·

ICBMs already do this, and we've been precisely deorbiting satellites for decades. Landing safely is the really tricky part. During the Corona spy satellite da

by grinich 13y ago

ICBMs already do this, and we've been precisely deorbiting satellites for decades. Landing safely is the really tricky part. During the Corona spy satellite days, they used to catch the parachuting pods with a plane+hook. http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_capsule_recovery.jpg http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_c...

InclinedPlane 13y ago
ICBM warheads have about a 90% chance of landing within a circle with a radius of about half a kilometer, at least according to declassified info, the actual accuracy is likely higher. Even so, that hardly compares to a precision landing on a small landing pad.
- mikeash 13y ago
  I think your info on ICBM accuracy may be a bit out of date. Some searching around shows modern American missiles with a CEP of 100 meters or better. I'm not quite sure how to translate a CEP (50% chance of landing in the circle) to a 90% figure, but I wouldn't think it would be a factor of 5. (I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still the case. I don't know what the story is for other ICBM-equipped powers. The Soviets compensated for their lesser accuracy by fielding larger bombs.) Also keep in mind that the major factor limiting ICBM accuracy these days is the need to be completely self contained, since they're intended to operate in the middle of World War III, and no external navigation sources can be counted on. Slap a JDAM guidance system on the warhead and your ICBM can deliver it to within whatever square meter you're most interested in. ICBMs armed with conventional bombs have been considered, since accuracy can be enough for that to work now. As far as I know, the only reasons these haven't been deployed are cost (using up an entire ICBM to deliver a chemical bomb seems wasteful) and, probably more importantly, the fact that there's no way to tell whether a given ICBM is carrying a nuclear warhead or not, thus the strong possibility that using such a weapon could trigger a nuclear war by accident. See: http://en.wikipedia.org/wiki/Prompt_Global_Strike http://en.wikipedia.org/wiki/Prompt_Global_Strike
  - InclinedPlane 13y ago
    The Minuteman III (the backbone of the US ICBM fleet) has a listed CEP of 200 m. The Trident D5 has a CEP of as low as 90 m, but it's an SLBM not strictly an ICBM, and it varies depending on range. The CEP figure represents the radius of a circle where there is a 50% chance of the bomb landing within that circle. The CEP figure is basically 2/3 of a standard deviation.
    - mikeash 13y ago
      I suppose I should have said "some". Anyway, when looking for the state of the art, we should look at the best value available, rather than the average or typical. The Peacekeeper missile lists a CEP of 120m, although it's now retired. As for Trident, with a 11,000km range, I think we can put it in the same category when looking at reentry accuracy. In any case, accuracy is gated by navigational accuracy, not so much guidance, as long as your initial aim isn't too bad. ICBMs have a 100m CEP because they're using inertial navigation all the way. Switch to GPS and you'll have no trouble doing better. In the context of SpaceX, getting the rocket to within a few hundred meters of the landing pad is perfectly fine initially, because it can then maneuver to the pad more precisely.

by grinich 13y ago

InclinedPlane 13y ago

ICBM warheads have about a 90% chance of landing within a circle with a radius of about half a kilometer, at least according to declassified info, the actual accuracy is likely higher. Even so, that hardly compares to a precision landing on a small landing pad.

mikeash 13y ago
I think your info on ICBM accuracy may be a bit out of date. Some searching around shows modern American missiles with a CEP of 100 meters or better. I'm not quite sure how to translate a CEP (50% chance of landing in the circle) to a 90% figure, but I wouldn't think it would be a factor of 5. (I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still the case. I don't know what the story is for other ICBM-equipped powers. The Soviets compensated for their lesser accuracy by fielding larger bombs.) Also keep in mind that the major factor limiting ICBM accuracy these days is the need to be completely self contained, since they're intended to operate in the middle of World War III, and no external navigation sources can be counted on. Slap a JDAM guidance system on the warhead and your ICBM can deliver it to within whatever square meter you're most interested in. ICBMs armed with conventional bombs have been considered, since accuracy can be enough for that to work now. As far as I know, the only reasons these haven't been deployed are cost (using up an entire ICBM to deliver a chemical bomb seems wasteful) and, probably more importantly, the fact that there's no way to tell whether a given ICBM is carrying a nuclear warhead or not, thus the strong possibility that using such a weapon could trigger a nuclear war by accident. See: http://en.wikipedia.org/wiki/Prompt_Global_Strike http://en.wikipedia.org/wiki/Prompt_Global_Strike
- InclinedPlane 13y ago
  The Minuteman III (the backbone of the US ICBM fleet) has a listed CEP of 200 m. The Trident D5 has a CEP of as low as 90 m, but it's an SLBM not strictly an ICBM, and it varies depending on range. The CEP figure represents the radius of a circle where there is a 50% chance of the bomb landing within that circle. The CEP figure is basically 2/3 of a standard deviation.
  - mikeash 13y ago
    I suppose I should have said "some". Anyway, when looking for the state of the art, we should look at the best value available, rather than the average or typical. The Peacekeeper missile lists a CEP of 120m, although it's now retired. As for Trident, with a 11,000km range, I think we can put it in the same category when looking at reentry accuracy. In any case, accuracy is gated by navigational accuracy, not so much guidance, as long as your initial aim isn't too bad. ICBMs have a 100m CEP because they're using inertial navigation all the way. Switch to GPS and you'll have no trouble doing better. In the context of SpaceX, getting the rocket to within a few hundred meters of the landing pad is perfectly fine initially, because it can then maneuver to the pad more precisely.