Let us engage in some educated guesswork.
Hitherto giventhe multiple failures of the Musudan IRBM, and North Korea’s own announcement following the successful testing of the Pukugsong 2 MRBM that it was shifting to solid fuels for longer range missiles, we understandably tended to assume that North Korea had closed off the liquid fuel path to an ICBM indeed the liquid fuelled path to MRBMs and IRBMs.
That supposition is well captured by this analysis by Young-Keung Chang at 38North.
The test of the Hwasong-12 IRBM on May 14 now reopens the liquid propellant path to an ICBM. Notice that the Musudan was the key intermediate step toward developing a liquid fuelled ICBM, according to Chang and just about everybody else. The Musudan was based on reverse engineering the Soviet 4D10 engine for the SS-N-6 sea launched ballistic missile.
The Hwasong-12, however, uses an indigenously designed and developed engine, and we saw that engine statically tested in March 2017. That engine was a second stage liquid fuelled engine for North Korea’s space programme, but it now powers the Hwasong-12 IRBM. This engine is being referred to as the Paektusan (don’t confuse this with Taepodong engines). The Hwasong-12 is a modification of the KN-08, with the third stage removed and the two remaining stages shortened.
This leaves open the possibility that the Hwasong-12 was developed quite recently in the piece.The interesting thing you notice from the static engine test and the Hwasong 12 test is the distance of the vernier engine plumes. They are more apart in the static engine test than the Hwasong test. The image above is of the Hwasong-12 engine. The picture below is of the March static engine test, and the Hwasong-12 IRBM test.
In short, the Hwasong-12 IRBM test again opens the road to a liquid fuelled ICBM and that based on indigenously designed missile engines. North Korea has achieved, thereby, a type of epistemic breakout in its missile programme.
That said the two TELs with ICBM size canisters paraded at the Kim Il-sung birthday bash were for solid fuelled missiles. So we have to be careful. Saying that North Korea *could* further develop the KN-08 modified Hwasong-12 as the basis for a liquid fuelled ICBM is not the same as saying that it *will.*
But it is interesting to observe that North Korea has tested an IRBM with a 4500km range capability using a liquid propelled engine, and so far as we know they don’t have a solid fuelled engine that can do the same. A solid fuelled IRBM makes more strategic-military sense than a liquid fuelled one, so if you had both capabilities you’d rather use the solid fuelled engine to develop an operational IRBM.
From the photographs of the Hwasong-12 test you can see that it took them a while to fuel and ready the missile for launch. That’s not ideal from an operational perspective. It certainly isn’t ideal for strategic stability; the Hwasong-12, if not a test bed for an ICBM, can hit US strategic assets in Guam but if you’re a North Korean planner you’d want to fire them before anything got off the ground from Guam in a crisis.
If nuclear nonproliferation were a US policy objective on the Korean peninsula things would not have come this far without concerted high level US diplomatic initiatives. I can only conclude that it hasn’t been a priority since Bush and remains a relatively low priority now.