There’s much to write about at the moment regarding North Korea and its growing nuclear capabilities, and thus far most of my attention has been on the technical aspects, kind of narrow at that given the bias toward analysis of Hwasong-14 propulsion elements, with relatively little devoted to questions of nuclear deterrence and strategy.
I have thought deeply about the latter, however and will be getting some analysis together on this in the coming period. That said, I’d like to make some brief remarks regarding reentry vehicle dynamics.
There exists the thesis that North Korea pretty much has an intercontinental nuclear capability now, with reliability testing prior to operational deployment remaining, whereas others argue North Korea still needs more time to demonstrate RV capability for an ICBM, estimates vary regarding how long, and yet others say the Hwasong-14 doesn’t have throw-weight sufficient to deliver its most likely nuclear weapon design to the lower 48 US states.
Some of the images that North Korea previously provided of the KN-14 ICBM prototype, most probably the Hwasong-14 is a variant of the KN-08/14 ICBM prototypes, it featured a blunt body RV, as the image below shows.
However, the Hwasong-12 IRBM and the Hwasong-14 ICBM both feature sharper nose cones. See below for both respectively.
North Korea, we all know, has been testing the Hwasong-12 and Hwasong-14 to high angle lofted trajectories. This is doubtless, in part, a reflection of overflight concerns. When thinking about the RV for the HS-12 and HS-14 one must be mindful of a quantity known as the ballistic coefficient or beta β, which measures how easily an RV descends through the atmosphere. The ballistic coefficient is a function of the mass of the RV, the RV cross section and atmospheric drag.
An RV that descends rapidly has high β and those that descend relatively slowly have low β. The interesting thing here is that sharp nose coned RVs have high β and blunt bodied RVs have low β. This means that sharp nose coned RVs do most of their deceleration in the lower, thicker, layers of the atmosphere whereas blunt body RVs experience most of their deceleration in the higher and thinner layers of the atmosphere.
This means sharp nose coned RVs experience high reentry temperatures but for shorter periods of time than blunt body RVs, which have high total reentry temperatures but that is accrued over a relatively longer time.
The lofted trajectories of the HS-12 and HS-14 tests means that they have high peak reentry temperatures but over a shorter time than if launched on a standard minimum energy trajectory, but don’t forget, however, that the HS-12 and HS-14 RVs have higher β than the blunter bodied RV of the KN-14 prototype.
So, there’s something for the North Koreans to learn, with regard to RV dynamics, from the lofted trajectory of its HS-12 and HS-14 tests. How much have they, remains an outstanding question but a blanket dismissal of their RV capabilities on grounds of lofted trajectory testing needs to be considered carefully. There does exist the view that the apparent HS-12 and HS-14 RVs actually consist of a shroud which conceals a blunt body RV, but that isn’t known with a high degree of certainty and largely follows from a general scepticism regarding North Korea’s scientific capabilities which doesn’t appear warranted, even though it once most certainly was.
Of course, some analysts have argued that amateur footage of the RV off Japan accompanying the July 28 HS-14 test shows the RV breaking up, however others argue that the physical features seen, especially the glow of the RV, are associated with Russian RV tests too so thereby does not support the break up thesis. Don’t forget that high β RVs do most of their deceleration in the lower atmosphere, so the features observed could be a function of the HS-14s sharp nose coned RV.
There have been reports, based on non-attributed statements by US officials, that the July 28 RV did break up however North Korea does have an RV able to withstand reentry dynamics consistent with an MET ICBM. These reports are hard to substantiate. North Korea had threatened to demonstrate the Hwasong-12 IRBM by firing toward a target just off Guam, over flying Japan, which could have been an RV dynamics and accuracy test on an MET using current tensions as cover. Kim Jong Un has withheld from this, at least for the time being.
The interesting thing about blunt RVs is that, given their low β, as they decelerate in the upper atmosphere relatively slowly they create a trail of ionised gas streaming behind the RV body. This is easily picked up by tracking sensors. Now the GMD Ballistic Missile Defense system is all about midcourse interception, however THAAD is designed to intercept RVs in the terminal phase of their flight.
The HS-12 IRBM likely has a sharp nose coned RV because it comes up against THAAD. So one would expect that North Korea has an interesting developing sharp nose coned RVs for the HS-12, which kind of cuts against the concealing shroud thesis.