US Korea Institute

Tuesday March 28th 2017
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No More Rivets

Continued from “A New ICBM for North Korea?

The KN-08 Mod 1, especially in its 2013 parade incarnation, had an extensively riveted skin. So too did the Unha SLV, North Korea’s only unambiguously real and successful long-range rocket. For the missiles we get to see parade mock-ups, but for the Unha we got to fish an entire rocket stage out of the ocean. And at least in 2012, it looked like the state of the art in North Korean metallurgy wasn’t up to building an all-welded or intricately machined rocket body; they needed the rivets.

Which makes it somewhat surprising that the KN-08 doesn’t seem to have any. It is also annoying to those of us trying to figure out how the missile is put together; the rivet lines are what told us where the engine bays, interstage regions and propellant tanks were. Indeed, there is less external detail all around. As noted, there is a bit of external hardware visible on the reentry vehicle, like propellant discharge ports for the first and second stages, and a few features that look like stage-separation thrusters. There might be a stage separation joint, partially hidden by the clamps holding the missile to the transporter. Even the cable raceway is now a single unbroken run, unless there is a break we can’t see hidden under the clamp.

Figure 3. Comparison of surface detail on KN-08 Mod 1, Unha-3 and KN-08 Mod 2 (left to right).

Left and middle: 38 North; Right: Korean Central Television footage crop.

Left and middle: 38 North; Right: Korean Central Television footage crop.

One possibility is that this mock-up simply was not built to show that level of detail. That would be surprising, as the 2013 mock-ups revealed great detail. Perhaps the North Koreans are concerned we are learning more than they wish to reveal. Another possibility is that their engineers have started using flush rivets, which would not be visible through a layer of camouflage paint. That would make more sense for a jet plane, spending its career flying at high speeds through the atmosphere, than for a rocket that ascends as quickly as possible into the airless void above, but even such a small gain as a smooth skin might appeal to engineers trying to increase the performance of a marginal ICBM. The same logic may apply to other features—a smooth and featureless skin means less drag. Stage separation, for example, might be accomplished using linear shaped charges inside the missile’s fuselage, a technique the US mastered in the 1960s.

The most worrisome prospect is that the new mock-ups don’t show any rivets because North Korean engineers have improved their machining and welding skills to the point where they no longer need them. If the KN-08 Mod 2 uses the sort of machined isogrid structure common on more modern US and Russian missiles, the tanks and structures will be significantly lighter and the missile would be able to fly significantly farther. Improved propellant tank design would allow more of the missile’s propellant to be used, rather than left in dead spots. This is a technology North Korea hadn’t mastered in 2012, but they may have learned something new in the past few years. There is evidence they have been looking for technological assistance.

In June 2012, Ukraine reportedly arrested two North Korean diplomats for attempting to photograph secret documents at Ukraine’s Yuzhnoue Design Bureau. The UN Panel of Experts has noted that the documents “would have provided the Democratic People’s Republic of Korea with information on advanced technologies and new forms of technological processes for the design of missile systems, liquid-propellant engines, spacecraft and missile fuel supply systems and associated computer programmes.”[1]

Yuzhnoye is an ex-Soviet rocket builder that wound up on the Ukrainian side of the post-Cold War border, which has had some luck in various joint ventures building satellite launch vehicles for international customers, but has lately fallen on hard times. Yuzhnoye has not built ICBM-class engines in almost 30 years, but they have been the place to go for state-of-the-art integrated rocket fuel tanks and structures at a discount price, and now their engineers and technicians aren’t getting paid regularly.

The names of the two documents being photographed were “Methods for predicting the capability of capillary intakes in fuel tanks of motor assemblies for spacecraft” and “Hydrodynamic processes in fuel tanks of spacecraft.” It is hard to pin down the exact content from translated titles, and we don’t know what else the North Koreans might have had access to before they were caught, but it is probably safe to assume that they have learned a bit about how to build better tanks and airframes for the KN-08: how to make them lighter all around and allow less residual propellant in the tanks, with no need for rivets.

Return to last section:  A New First Stage
Next section:  What’s “Under the Hood”

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[1] “Report of the Panel of Experts established pursuant to resolution 1874 (2009),” 25.

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Credit for photo of young North Korean girl: T.M. All rights reserved, used with permission.