The best time to take out the North Korean ICBMs are before launch during the 3-4 minutes of the boost phase when they are first launched.
Taking out the ICBMs when they are about to hit their target is a far harder problem. The ICBMs can deploy decoys and they can alter course.
Hitting the North Korean ICBM during boost phase is still very hard. We will need faster missiles or we have quickly flood North Korean airspace with drones and fighters during the early stage of a conflict.
It appears that US Mach 15 interceptor missiles stationed in the waters just off the coast of North Korea could hit a North Korean ICBM in boost phase. But this requires no time or at most ten seconds to decide if the detected missile is a threat or worth shooting down.
This would also require the US to provide the training for the navy crews to shoot SM-3 missiles against ICBMs. Currently the US ships have to board special civilian specialists for each SM-3 anti-ICBM test.
There are 90 to 120 seconds for an intercept after detection of a missile launch
ICBMs would need to be engaged on a very compressed timeline. The upper half of the figure shows the stages of the intercept process for both solid- and liquid-propellant ICBMs launched from Iran. The right hand end of the solid bars indicates when the ICBM stops burning. The last chance for intercepting the ICBM is five seconds before that, as indicated.
In the lower half of the figure, the lengths of the solid bars show how long the interceptors have to reach the ICBM, from interceptor launch to intercept. The left hand end shows the earliest possible launch of the interceptor after the ICBM launch taking into account the time needed for tracking the ICBM, determining a firing solution, and deciding whether to intercept.
These figures show where an interceptor would need to be based to successfully reach a solid-propellant ICBM launched from North Korea. Missile trajectories for different targets in the United States are shown. The left hand figures show basing areas if the interceptors are capable of achieving speeds of 6.5 km/s. The right hand figures correspond to a hypothetical interceptor capable of 10 km/s, which is beyond current capabilities.
The solid inner circles indicate the required basing area if 30 seconds of decision time is allowed. The outer dashed circles show the basing area if interceptors are fired as soon as a firing solution is obtained, with no decision time. The crosses show the position of the last chance to intercept, in Chinese airspace in all cases.
6.5 km per second is Mach 19
10 km per second is Mach 29.
It does not appear in unclassified information that US has 6.5 km per second let alone 10 km per second interceptor missiles. The newest SM-3 Block II missiles are 4.5 km/second Mach 15 missiles. The THAAD missiles that are based in South Korea do not have the speed or range to cover North Korea to hit ICBMs as they launch.
North Korean Missiles launched toward the USA would fly over China.
In several tests, the U.S. military have demonstrated the feasibility of destroying long and short range ballistic missiles. Combat effectiveness of newer systems against 1950s tactical ballistic missiles seems very high, as the MIM-104 Patriot (PAC-1 and PAC-2) had a 100% success rate in Operation Iraqi Freedom.
The U.S. Navy Aegis combat system uses RIM-161 Standard Missile 3, which hit a target going faster than ICBM warheads.
Hypothetical Mach 29 interceptor missiles could be placed on US Navy ships on either side of North Korea and in South Korea. They should have 300-500+ mile range.
High speed interceptors with mid-range placed onto stealth drones should also be developed.
Operational ranges of SM3 missiles
700 km (378 miles) Block IA/B
2,500 km (1,350 miles) Block IIA
Speeds of SM3 missiles
3 km/s (Mach 10.2) Block IA/B
4.5 km/s (Mach 15.25) Block II
These systems, as opposed to U.S. GMD system, are not capable of a mid-course intercept of an ICBM.
Stealth Drones with shorter range and slower missiles would need to be in North Korea airspace
The Avenger and other MQ-X class drones could be used to constantly fly near North Korea territory or even over North Korea to fire missiles at any ICBM that in the first 3-4 minutes when they are most vulnerable.
The US has 160 MQ-9 drones which can carries a variety of weapons including the GBU-12 Paveway II laser-guided bomb, the AGM-114 Hellfire II air-to-ground missiles, the AIM-9 Sidewinder, and the GBU-38 JDAM (Joint Direct Attack Munition). Tests are underway to allow for the addition of the AIM-92 Stinger air-to-air missile. The missiles have short range.
However, fewer than a dozen Avenger drones have been purchased since the product first flew in 2009. There is talk of a 90 Avenger purchase by India over the next few years.
Unlike the previous MQ-1 Predator and MQ-9 Reaper (Predator B) drones, the Avenger is powered by a turbofan engine, and its design includes stealth features such as internal weapons storage, and an S-shaped exhaust for reduced infrared and radar signatures. The Avenger will support the same weapons as the MQ-9, and carry the Lynx synthetic aperture radar and a version of the F-35 Lightning II’s electro-optical targeting system (EOTS), called the Advanced Low-observable Embedded Reconnaissance Targeting (ALERT) system. The Avenger will use the same ground support infrastructure as the MQ-1 and MQ-9, including the ground control station and existing communications networks.
The Avenger besides being bigger has some stealth capability.
The US has research for larger, faster and more stealthier drones and incrementally improved models are technologically possible for deployment over the next few years.
The US with current weapons (drones and fighters) and slower interceptors would need to take over North Korean airspace and place a couple hundred drones and fighters spread over the country to spot and shoot any ICBM as it is set up to launch or as it launches.
With current US weapons before fighting starts the ideal location to put the interceptor missiles would be in China. This of course is a tough political problem.
Basing on satellites 400 miles up
Nextbigfuture has previously written that basing missiles and kinetic weapons on satellites 400 miles over North Korea could also intercept weapons during the boost phase. The weapons could be designed for a speed of Mach 29 and those would cover the 400 miles in about 70 seconds.
Mach 10 straight kinectic interceptors would take 200 seconds to reach a land target. However, the ICBMs would flying upwards.