The Big Bang Theory: North Korea Hwasong-12 Missile Test and Nuclear Warhead Dynamics

Like I said, don’t forget the RV when it comes to North Korea’s Hwasong-12 missile test.

David Wright has a great post up on at the Union of Concerned Scientists blog on the RV angle. He finds, pretty much as I had done, that

This shows that North Korea could get significant data from the recent test—assuming the RV was carrying appropriate sensors and sent that information back during flight, and/or that North Korea was able to recover the RV from the sea. But it also shows that this test does not give all the data you would like to have to understand how effective the heatshield might be before putting a nuclear warhead inside the RV and launching it on a long-range missile.

That’s because the lofted trajectory of the Hwasong-12 test leads to high peak temperatures, good from an ICBM perspective, but the total heating time is less than that for a 10,000km ICBM fired on an MET trajectory and so the total heating is less, which is bad from an ICBM perspective. In this case, that is the Hwasong-12, the total heat absorbed by the 10,000km MET ICBM is twice as large.

So, in short, North Korea would, or perhaps better still could, have gained important information regarding RV dynamics for an ICBM but by no means would this data be sufficient for a reliable ICBM RV. North Korea needs to develop an RV able to withstand the high temperatures of re-entry, among other things, and this remains an issue for Pyongyang.

One of the interesting things about North Korea and the question of RV dynamics is accuracy. As Wright, correctly, points out high atmospheric drag leads to lower missile accuracy. The Hwasong-12 is a modified KN-08 with the third stage removed, and the two remaining stages shortened. The KN-08 is widely seen as the basis of North Korea’s liquid fuelled ICBM programme, and the Hwasong-12 test, partly, likely was a testbed in that regard.

Now this is where the RV gets interesting. The KN-08 (never tested), a mockup thereof, was first paraded in 2012 and the RV was a triconic warhead. Modern RVs, for example used by the US and Russia, are conic, but for North Korea a triconic warhead is easier for dealing with reentry vehicle dynamics for relatively heavy nuclear weapons delivered to long ranges. But a triconic warhead comes at the cost of increased atmospheric drag.

Now in 2015 North Korea paraded a KN-08 but this time not with a triconic warhead, but rather with a blunt warhead. The blunt warhead travels through the atmosphere at lower velocity than the triconic warhead, but it enables North Korea to deliver lighter nuclear weapons.

Now the warhead on the Hwasong-12 tested on May 14 was not that of the KN-08 paraded in 2015. It is not as blunt a warhead. It pretty much looks like the original KN-08 RV. The North Korean’s, in part, are probably concerned about missile defence. Blunter warheads are easier to intercept because of their lower velocity. The thing about the original RV for the KN-08 is that the tip of the RV is subject to higher peak temperatures than for a blunter RV, so the matter of peak temperatures is of significance here.

Anyway, the thing I wanted to focus on was accuracy. The accuracy of a missile is largely a function of errors in velocity from that planned at the moment when the RV separates from the missile. The greater the error of velocity the greater the inaccuracy. So far as we know the guidance and control system of the KN-08 is based on the, vintage, Soviet R-27 guidance and control system. The total flight time of the KN-08 would be greater than the R-27, which was a submarine launched missile, so whatever errors the R-27 guidance system exhibited would be greater for the KN-08 given the greater time for errors to accumulate.

Blunter RVs are also more inaccurate. The RV on the Hwasong-12 is an improvement upon the RV displayed on the 2015 paraded KN-08.

A North Korean ICBM would have an accuracy, measured in terms of CEP, in the kilometres. That means that a North Korean ICBM targeted at the centre of a city has a 50% probability of missing the designated ground zero by kilometres. Estimates of how many kilometres vary but a reasonable estimate for a lower bound is 4km. What this means is that North Korea wants a nuclear weapon with an explosive yield to compensate for these errors.

To date North Korea has achieved a nominal yield of 20 kilotonnes for a first generation nuclear device. We know that North Korea has been digging tunnels at its nuclear test site at Pungyye-ri to contain explosions with an upper bound of 282 kliotonnes. North Korea wants to develop boosted fission weapons not just to increase the efficiency of fission to make its warheads more compact, but to compensate for the inaccuracy of its missiles especially for an ICBM. The US tested a boosted fission weapon in 1951 and it had a yield of 45 kilotonnes of TNT. A 45 kilotonne nuclear weapon would have an airbust radius of approximately 2.5 to 3km.

Those tunnels at Pungyye-ri, I submit, were excavated for a nuclear weapon to be delivered to intercontinental ranges with sufficient punch to act as a credible countervalue nuclear deterrent given the technical limitations of its missile programme.

The first image below depicts the KN-08 mockup paraded in 2012

The second depicts the KN-08 paraded in 2015.

The third shows the RV for the Hwasong-12 tested on May 14 2017.