A New Reentry Vehicle

Continued from:
A New ICBM for North Korea?

Now, on to the differences in the missile itself. Let’s start at the front, which is about a meter and a half shorter than the Mod 1, and much less pointy. That indicates a major change in the design of the reentry vehicle (RV), which is a critical element of ICBM technology that North Korea has never demonstrated. The previous triconic warhead was a compromise between simplicity and performance, something a team of aspiring missile-builders could hope to get right the first time and which would still be fast and accurate enough to reach a small and well-defended target. The new RV is much blunter. That reduces the concentrated heat loads, allowing for a very simple and robust design. It’s probably lighter as well. But it also means the missile will be much less accurate, and much slower at the end of its flight.

Figure 2. US Mark 2 Reentry Vehicle, as used on Jupiter, Thor, and Atlas missiles.
Figure 2. US Mark 2 Reentry Vehicle, as used on Jupiter, Thor and Atlas missiles.

The United States used such reentry vehicles in its first long-range missiles, as shown in figure 2. These were replaced with triconic warheads as soon as we learned to make them work reliably, and the Soviets used triconic designs from the start. Blunt RVs were too inaccurate to be of military use even when equipped with large thermonuclear warheads. Perhaps more importantly, they are slow enough to be engaged by ordinary surface-to-air missiles as they descend toward their targets. If North Korea is planning on fielding such warheads, they are playing it very safe technologically, but they are limiting themselves to a system that can be used only against large, undefended targets.

Strangely, it is not entirely clear which way the reentry vehicle is facing. Some Soviet submarine-launched ballistic missiles (SLBMs) incorporated a backward-mounted warhead section for more efficient packaging in a very tight space, and used thrusters to reorient the RVs after boost. The KN-08 Mod 2 reentry vehicle has two to four visible thruster packs for separation, at least, and a ring of apertures around the base of the spherical cap could be additional thruster ports. The appearance is vaguely similar to a Soviet-style reversed warhead package, and we know North Korea has worked with ex-Soviet SLBM designers. But this would require a more complicated deployment maneuver, and the North Koreans have no reason to do that here—the missile could easily be a bit longer if they need more room for the RV.

A more likely explanation is that the KN-08 uses a forward-facing RV to simplify deployment, and that the spherical cap is just an aerodynamic fairing. Blunt RVs are good for slowing down at the end of a flight, not so much for building up speed at the beginning. The apertures at the base of the cap would then be for separation mechanisms to jettison the fairing when it is no longer needed. Indeed, the ability to remove the fairing may be of value even before launch, allowing access to the warhead section for maintenance and/or launch preparation.[1]

However the RV is oriented, it will have adequate internal volume for the early-generation compact fission warhead North Korea is likely to field. The mass of the warhead is likely to pose a greater challenge than its dimensions—the KN-08 Mod 2 has plenty of space for a large warhead, but may not have much throw-weight. North Korea has conducted three nuclear tests, with the stated goal of miniaturizing its nuclear weapons. According to a North Korean defector, North Korea was attempting to develop a nuclear warhead that weighed approximately 500 kilograms. That is consistent with the experience of Pakistan, which received an early nuclear weapon design from China and worked to reduce its weight. The initial Chinese-design that ended up in Libya reportedly had a mass of approximately 500 kilograms, while other reports suggest that plans for a second Pakistani design were found in Switzerland with a mass of about 200 kilograms. We should probably anticipate that North Korea is capable of building a fission device that weighs between 300-700 kilograms, with another hundred kilograms or so needed for shielding.

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Changes to the Upper Staging

  1. [1]
    Note that some nations prefer to store missiles and warheads separately for security reasons.

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