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I have a fictitious country, let’s call it Democratica, which is a democratic western nation, threatened by a neighbouring communist state (called Sovietania.) Sovietania has a number of Soviet and Russian thermonuclear ICBMs, namely the RS-24, which are aimed at Democratica.

The Democratican government want to invest in a missile defence system that will protect cities and towns from this threat and have decided to either invest in an existing defence system (such as the PAAMS system) or to develop their own. Would a missile defence system such as PAAMS reliably (at least 90% of the time) be able to shoot down a ICBM, such as the RS-24?

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    $\begingroup$ I'd wager that when the 10% of missiles you don't stop are able to wipe you out, MAD would be a much better nuke deterrent. Get yourself some nukes too. $\endgroup$ – Renan Oct 24 '18 at 20:57
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    $\begingroup$ (a) You're asking for something fictitious, so it's impossible to give you any guarantee whatsoever of a 90% success rate (if anybody had a solution that could do that, they wouldn't post it as an answer, they'd be running to the patent office). (b) The purpose of this site is to help you develop consistent rules for your world. What rules of your world are you asking about? Because it's easy to write into a story, "a breakthrough in missle defence with a 90% success rate" and then move on with your story. At this time, this doesn't appear to be about worldbuilding. $\endgroup$ – JBH Oct 24 '18 at 21:13
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    $\begingroup$ I suggest you look into existing and past ballistic missile defense systems: sentenal program in the '60s and '70s was made of the terminal SPRINT missile (some amazing YouTube vids of this thing) and the midcourse SPARTAN interceptor. Current interceptors are AEGIS, GBI, and THAAD. $\endgroup$ – MParm Oct 24 '18 at 21:48
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    $\begingroup$ The new guys are all kinetic kill (ram the target), but the older interceptors were less accurate and were armed with neutron bombs (nukes) so they only had to get close. Canada wasn't a huge fan of those since intercepting Russian ICBMs flying over the arctic involved detonating nukes over Canada. But for 90%... $\endgroup$ – MParm Oct 24 '18 at 21:57
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    $\begingroup$ Effective decoys are pretty effective, small, and easy to add to the missile. $\endgroup$ – RonJohn Oct 24 '18 at 22:20
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TL;DR: No, modern ICBMs move too fast for on the spot defenses

To understand why defending against ICBMs is so difficult, we have to understand how their launch, flight, and attack patterns work. Modern ICBMs have 4 phases:

  1. Boost Phase: In this phase, the ICBM is accelerating from its silo into orbit. It'll spend a few minutes in its silo, and then start accelerating into space. It is during this early launch phase that the missile is most vulnerable since it is relatively slow. At this point, its destruction can be accomplished by fighter, drone, cruise missiles etc. However, destroying a missile in boost means taking the war to the enemy since it almost certainly will be launching from enemy territory. This makes stopping a missile in boost difficult unless you have complete air superiority.
  2. Post-Boost Phase: In this phase, the ICBM has entered orbit, but is still acquiring its target. If it can be struck now, its complete destruction is essentially guaranteed. However it's going many times the speed of sound, and it's in space, so intercepting it here is by no means easy, but still possible.
  3. Midcourse Phase: The missile has acquired its target and is coasting until it is in position to begin its decent. Now, a modern ICBM doesn't have just one warhead or just one decoy. It has what's known as a weapons bus where there are numerous warheads, decoys, and other countermeasures, like radar reflectors and flare launchers. The weapons bus takes care of targeting, then releases its payload. Thus, during this phase, a modern ICBM will release its warheads, decoys, and countermeasures to form what is known as an attack cloud. This cloud can be more than a mile across. Any interception in this phase would have to be done via satellite laser weapons, since getting a projectile based defense in orbit poses a humongous technical hurdle.
  4. Terminal Phase: Warheads are coming down. At over Mach 10. And there are a lot of them. You know where each one is, you can predict where each one is going to hit. You just don't have anything fast enough to hit them. At this phase, it's pretty much over.
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https://en.m.wikipedia.org/wiki/Anti-ballistic_missile

A MIRV warhead as your soviet missile carries is nigh unstoppable, getting a 10% successrate is in all likelyhood high, and I'm talking about individual warheads rather than the entire ICMB. https://www.google.nl/amp/s/www.popularmechanics.com/military/weapons/news/amp28653/us-missile-defense-accuracy/ This article mentions the 56% successrate (for individual warheads). While this one (https://www.google.nl/url?sa=t&source=web&rct=j&url=https://aip.scitation.org/doi/pdf/10.1063/1.5009222&ved=2ahUKEwjJx8SG_5_eAhUNy6QKHXoWDEcQFjACegQICRAB&usg=AOvVaw2algaSzUSD8cCJA8vJFJCD) mentions that almost all interception tests are highly scripted, and that simple ABM-countermeasures that have been known for decades still have no solution.

The big problem is that an ICBM is incredibly fast, the best chance to shoot it down is close to it's launch, after that the missile will release it's warheads high in the atmosphere. These warheads are ballistic and dont have any propulsion, so they are extremely hard to detect. By adding fake warheads along the mix it becomes even harder to intercept the actual ones and makes stopping the missiles even more expensive as you fire multiple multi-million missiles to intercept individual warheads.

I only see two solutions: extreme saturation, as in for each warhead the enemy has you have at least 10+ ABM's ready (for a 6 warhead ICBM that's 60 missiles, more when decoys are deployed). That combined with secret missile systems mere kilometers from soviet launch sites to alert for launches and intercept a portion in the early stages. Otherwise you can forget it.

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As has been pointed out, interception is arguably best done (from a technical standpoint) during or just after boost phase, when the launch vehicle is a pretty unambiguous target. If the warhead bus has time to deploy countermeasures the job gets much tougher. During terminal phase atmospheric drag strips away the decoys and such, so target identification becomes a whole lot easier, but then you're faced with incoming at Mach 10 and only a short time until impact.

Previous answers have claimed that the available time to intercept is too short to do anything, so Democratica is hosed.

That, as it happens, is not entirely true. I present the early-70s answer to the problem, the Sprint missileenter image description here

It uses explosive silo hatch bolts, a gas-piston ejection mechanism to get the bird in the air quickly, and accelerates at 100 g's to Mach 10 in 5 seconds.

Drawbacks? Oh yeah. The big one was the warhead, which was a radiation-enhanced fission jobber (read: neutron bomb). This is not entirely as bad as it might seem, since the high altitude of detonation means that there are no ground fires started, and nuclear winter is not much of an issue. Since the warhead is fairly small, "only" a few kilotons, fallout is not as bad as you might think.

Modern technology might or might not address the problem, since the last couple of decades has seen the development (and deployment) of kinetic-kill interceptors, which do the job by direct physical contact - no explosives, nuclear or otherwise, required. Of course, making a window to look for the target in a vehicle going that fast that low is not what you call a non-trivial exercise.

But it does serve as a starting point.

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