North Korea’s Hwasong-15 Reentry Vehicle and the Window of Vulnerability

Soyuz, Falcon-9, Ariane, Long March and so on are rockets whose purpose is to deliver a payload into space. Military missiles are no different, indeed this feature is more pronounced with military rockets, and the Hwasong-15 ICBM is a good case in point. The purpose of the Hwasong-15, the subject of much commentary, is best gleaned from an analysis of the reentry vehicle that sits atop for that is where the intended payload is also housed.

That is why a recently published examination by David Wright, of the Union of Concerned Scientists, is so valuable.

The key facet of the article, and the commentary engendered by it, focuses on North Korea’s capabilities especially with respect to RV survivability. That is appropriate and highly significant, especially when a lot of discussion regarding the possibility of a “preventive” war is based on a curious mixture of underplaying North Korea’s technical capabilities and overplaying US interdiction and missile defence capabilities.

I suggest here that an examination of the RV also provides insight for an analysis of the intentions animating the development of the Hwasong-15. It should be stressed, however, that an analysis of intentions, for obvious reasons, is more fraught than an analysis of capabilities. With that caveat in mind, let us proceed.

There are two physical characteristics to the Hwasong-15 RV that first attracted Wright’s attention; the relatively large diameter and the blunt body of the RV

A blunter RV can clearly accommodate a larger diameter warhead, and the warhead can sit farther forward toward the nose of the RV. This moves the center of mass forward and makes the RV more stable during reentry

That’s important because there has been some discussion regarding the stability of North Korea’s long range RVs, the Hwasong-15 included, that is that they would tumble through reentry. Notice that the larger diameter contributes to stability, a point to which we return.

The RV of the Hwasong-15 is related to a favourite little hobby horse of mine and that is the manner in which Pyongyang is reinventing the wheel. The understanding of reentry vehicle dynamics, for both the early US (and Soviet) space and missile programmes, was supported by a newly constructed body of physical theory known as blunt body theory. We are referring here to a theoretical science not just an empirical art. In the US, in the early 1950s, this was developed at the Ames Aeronautical Lab. It was discovered that a blunt body descending into the atmosphere with supersonic velocities generates a V shaped shockwave which deflects the frictional heat occasioned by reentry. A blunt body RV has a low ballistic coefficient, β, which is a measure of atmospheric drag that the RV experiences on reentry. An RV with low β experiences most of its atmospheric drag in the upper thinner part of the atmosphere so generates less peak temperatures than sharp nosed coned RVs but they decelerate more in the lower parts of the atmosphere so heat up for longer.

There’s more to successful reentry than the blunt body of the RV, but this was a critical advance in the early part of the missile age and one which wasn’t prefigured in the V2 era unlike other aspects of missile technology.

Blunt body theory was developed in 1952, and was published in 1958. In the 1950s with the heat shield and ablative technology at hand blunt body reentry was the way to go. One of the reasons for the publishing of blunt body theory was that the US was moving on to more technically demanding sharper nose coned RVs, which are also more accurate than blunt body RVs. See image below for a comparison of the Atlas B ICBM and the Minuteman 1 ICBM.

This shift was supported by intensive research on ablation, and it kind of made blunt body theory passe. For those interested in looking at nuclear weapons in the context of the history of science, and science and technology studies(as I very much am) it is an interesting exercise to explore whether the technical development of RVs beyond blunt body theory brought about counterforce nuclear strategy, supported by the advent of deterrence theory, or whether counterforce strategy was brought about by advances in physical theory and engineering in support of sharp nose coned RVs and so deterrence theory and counterforce came later.

There’s quite a debate regarding technological determinism in the literature, but I don’t think this aspect has been studied closely.

In January 1958, the same year blunt body theory was released publicly so enabling its public dissemination, the Soviets demonstrated the capability to deliver an ICBM RV intact to the surface of the Earth. It has been supposed that the USSR adopted sharp nosed coned missiles from the start, however this view is based on the shorter range R-5M missile which did have a sharper nose cone compared to the Thor IRBM and the Atlas ICBM. However, the Thor RV was over engineered precisely so that it could support the development of the Atlas ICBM. The heat loading of ICBM RVs is greater than MRBMs and IRBMs. The RV of the R-7 (Semyorka) ICBM tested in January 1958 (the 7th launch of the missile/rocket) was blunt and spherical. Interestingly the RV missed its target by 80 kilometres, and another test in March 1958 had a down range error of 7.5 kilometres and a cross range error of 1.1 kilometres bad by today’s standards but a whopping improvement on the first experiment. Ironically, in 1957 the R-7 could launch a satellite into space, Sputnik, but it couldn’t safely bring down a nuclear armed RV which is what it was designed for and which many at the time feared was a demonstrated capability by October 1957.

Still, the gap between October 1957 and January 1958 isn’t all that great something those who suppose a successful demonstration of a Hwasong-15 RV test launched on a depressed trajectory is years away might want to think about.

At any rate, the point here is that blunt body theory is known and provides North Korea with a predictive physical theory of reentry vehicle dynamics derivable from first principles. That is to say, North Korea is reinventing the wheel through a known predictive theoretical science, its RV programme is phenomenological, and this consideration lends weight to the thesis that Pyongyang has advanced its missile programme indigenously, and its testing programme, even though based on lofted trajectories, gives it useful data to feed into that predictive physical theory.

The matter of accuracy is important as it can provide insight into intention. It has been supposed by some that the relatively large diameter and throw weight of the Hwasong-15 means North Korea can MRV or MIRV the missile. Theoretically, after more technical advance including but not limited to moving beyond blunt body theory, it could but is that the intention? Perhaps it is better to gauge this by considering what North Korea’s capabilities currently are rather than what they may be down the line.

Consider the Hwasong-12 nose cone as compared to the Hwasong-15 nose cone. It is sharper, and based on what we know it appears that the Hwasong-12 is aimed at a specific military target, rather than a large city, namely the United States Air Force base on Guam.

In a previous post I had argued that the Hwasong-15 provides a “window of vulnerability” whereby a small and poor country is now able to bring all the major cities of the continental United States within range of a heavy thermonuclear warhead. The yield of the September 03 hydrogen bomb test is often cited in the media as being 120 to 150 kilotonnes, however there is good evidence to suppose it was higher than this something exceeding 250 kilotonnes which is a quarter of a megatonne. North Korea has had a history of underdoing its nuclear tests, for example its fifth test, most likely boosted fission, had a yield of approximately 35 kilotonnes (widely cited as 20KT which is nominal yield of first generation plutonium fission bomb) however all the preceding appeared well below the nominal plutonium first generation yield.

North Korea may well have done the same with the September 03 test. Less is known in the open public domain regarding thermonuclear weapons than fission only weapons so one must be careful in commentary, however it is known that a good portion of the total yield of a thermonuclear yield comes from the “pusher” that encases the fusion secondary. U-235, as Bohr and Wheeler first showed, is fissile however U-238 is fissionable and a U-238 based pusher fissions because of the high energy neutrons and high neutron flux produced by the fusion reactions of the secondary. A U-238 pusher would produce a higher yield than a U-238 pusher of otherwise the same design, for example.

North Korea has indicated in various public statements that it may airburst a thermonuclear warhead fired from a long range missile on a depressed trajectory over the Pacific, or conduct a megatonne yield test underground.

I rather suspect that the Hwasong-15 ICBM both is capable and is designed to be able to deliver a thermonuclear warhead into the megatonne class so that North Korea can credibly threaten the major population centres of the United States. The blunt body of the Hwasong-15 RV is indeed less accurate than a sharp nosed cone RV, such as those associated with a MIRV which also have relatively lower yields, but that is beside the point. It only needs to be accurate enough to bring a high yield thermonuclear weapon to bear upon a major city for credible deterrence.

In a way this is a type of “window of vulnerability” not too dissimilar to the mythical variants of the Cold War. By presenting Washington with such a window of vulnerability Pyongyang might seek to control escalation, should deterrence fail as it were, on the Korean peninsula by deterring US military support for South Korea and Japan in the event of a conflict or crisis. Would the United States be prepared to lose Los Angeles, New York and Washington DC on account of Seoul and Tokyo? These are the types of dilemmas that rocked NATO more than once during the Cold War.

I tend to think that a battlefield nuclear weapons capability, say launched from Scud variant shorter range missiles, which could deny South Korea the ability to mass and concentrate large combined arms ground formations in support of operational art, whilst also destroying South Korean, Japanese and Guam air bases supporting deep strikes in the KPA’s rear, in addition to “the window of vulnerability” deterring US intervention as discussed above would make for an interesting mix. If there are any Keith Payne or Colin Gray or Curtis LeMay types in the Korean Peoples’ Army strategic rocket forces they might even call this a “theory of victory.” I hope that the way North Korea plays football isn’t an accurate guide to its strategic culture. I speak of this in the context of contingency planning, rather than pursuit of a capability supporting a preplanned aggression.

Recall the point about the stability of the RV given the large diameter. That diameter is supported by a relatively large second stage, and that large diameter second stage also means North Korea could employ complex countermeasures to mitigate ballistic missile defences. That’s important because the large blunt body RV is more vulnerable to BMD than a smaller sharper nose coned RV.

The Hwasong-15, we might *infer*, is intended to deliver a stable, survivable, high yield thermonuclear warhead to the major cities of the continental United States. After all these words that sounds like a kind of trivial conclusion, but in the context of the debate swirling on North Korea it isn’t I submit. This isn’t a first strike weapon. This isn’t about MIRVs. This is a city busting weapon of deterrence, but because it presents a “window of vulnerability” through which North Korea might want to control escalation that is precisely the problem.

A small and poor country deterring the United States is a novel development in the nuclear age, I contend. That is a real dilemma for planners in Washington, and it should not be underestimated.