Continued from “A New ICBM for North Korea?”
The KN-08 Mod 1 was a three-stage missile, with a third stage of substantially smaller diameter than the first and second, substantially similar to (but shorter than) the upper stage used successfully on Iranian and North Korean satellite launch vehicles (SLVs). The first and second stages of the Mod 1 were of the same diameter, with a clear interstage region or engine bay between them. For the KN-08 Mod 2, the third stage appears to have been either eliminated or replaced by something that maintains the same diameter over most of the missile’s length. What isn’t clear is what may have replaced it.
One possibility is simply that the third stage has been reshaped to fit inside the new forebody. Conveniently for this interpretation, the cylindrical section of the KN-08 Mod 2 is just long enough for the unmodified first and second stages of the Mod 1 with a reasonable interstage section beneath the third stage. In this hypothesis, the third-stage fuel and oxidizer tanks would fit neatly in the conical section, almost to the base of the RV, holding almost as much propellant as the original. Conical tanks are slightly harder to manufacture than cylindrical tanks, but they would be more compact and efficient in this geometry.
However, it seems more likely that the third stage has been removed entirely, with the first two stages lengthened to compensate. Two-stage missiles will generally offer inferior performance to three-stage designs, especially with light warheads, but two stages mean fewer chances for things to go wrong. Stage separation is one of the more common failure points for large rockets, and North Korea’s track record in three-stage rocketry is only one success out of four attempts.
To turn the KN-08 into a two-stage missile, the first-stage propellant tanks would have to be stretched by three to four meters, with the second stage being similarly stretched and pushed forward until its oxidizer tank fills the conical region. This also would be a very good match for the observed mock-up. It probably would not deliver quite the same performance as the Mod 1, but if it can still reach targets in the contiguous United States, the improved reliability might be seen as a worthwhile trade.