North Korea’s Test of a Liquid Propelled Rocket Engine at Sohae and the Assumption Pyongyang’s Next ICBM Will Be a Solid Fuel ICBM

North Korea’s announcement of a rocket engine test at Sohae, likely related to its missile programme given the announcement was made by the Academy of Defence Science, gives us an opportunity to question some of our prevailing assumptions about that programme.

It has been widely assumed that North Korea’s next ICBM, to the extent that another ICBM is on the pipeline, would be a solid fuelled ICBM. This would provide strategic planners in Pyongyang greater mobility and so therefore greater survivability.  That would be true irrespective of the fact that North Korea’s long range missiles employ storable liquid propellants. It has been assumed that this would be of interest to North Korea because planners in the United States and South Korea have an interest in using preemptive force (as they do in Pyongyang, by the way) in the event of hostilities.

Many analysts immediately concluded on Twitter that the December 7 engine test at Sohae was a test of a solid motor. Footnote 1 in this article by Michael Elleman at 38North gives you a sense of this. That conclusion was made because of the widespread assumption above, and so it’s reasonable to now question that assumption.

One thing that definitely provided credence to the assumption of a solid fuel ICBM was the string of new, shorter range, missiles flight tested by North Korea this year, for instance the KN-23 Iskander like SRBM and the Pukguksong-3 intermediate range SLBM. The latter especially lent support to it. There also was satellite image analysis showing additional construction activity at facilities known to be associated with solid fuel missile research and development. The second aspect preceded the first. At the time, the satellite imagery analysis was assumed to indicate possible work on a solid fuel ICBM but the subsequent string of shorter range missile tests might retrospectively be seen as an early indication of the string of new shorter range missiles recently tested.

The United States and the Soviet Union during the Cold War both had liquid propelled heavy ICBMs, however the US also had the solid propelled MX-ICBM. The Titan II ICBM, which hauled a 9 megatonne W-53 warhead, was a liquid propelled ICBM. The SS-18 and SS-19, of window of vulnerability fame, also were liquid propelled. The Sarmat ICBM, currently in development, is Russia’s successor to the SS-18 and it too will be liquid propelled. Russia is not developing a solid propelled heavy ICBM.

North Korea’s Hwasog-15 ICBM is often compared to the Titan II. It definitely bears a visual resemblance to the Titan II. The W-53 warhead and the Mark VI Reentry Vehicle had a mass of 3,690kg. Now, that’s throw weight. Compare this to the Minuteman III. The W78 warhead and its associated RV (these figures are Cold War era related i.e. don’t consider modifications made after it nor the lighter W87) had a mass of up to 363kg. The Minuteman III during the Cold War exhibited three MIRVed W78 warheads, giving a payload of 1,089kg excluding the post boost vehicle or bus.

The Titan II booster or first stage was powered by two LR-87 engines. The thrust of a single LR-87 engine (at sea level) was 647 kilo Newtons or 65 tonnes force. The mass of the Titan II was 155,000kg, about twice that estimated for the Hwasong-15 ICBM. The total thrust of the twin booster engines for the Hwasong-15 is 80 tonnes force (130 tf for the Titan II). Additionally, the throw weight of the Hwasong-15 is estimated at 1,000kg.

It could be possible that North Korea’s next ICBM is a liquid propelled heavy ICBM, throw weight larger than 1,000kg, for targeting all of the continental United States with a multi megatonne warhead. The engine test on December 7 could therefore have been a test of a new more powerful liquid propelled engine. There’s been one extra reference to the engine test in the North Korean media on top of the original announcement via KCNA. This emphasised that the engine test at Sohae was “weighty.” It is granted this is slim evidence supportive of the heavy ICBM thesis, but it’s the best we have at the moment.

A very weighty test took place at the Sohae Satellite Launching Ground in the afternoon of December 7, 2019The results of the recent weighty test will have an important effect on changing the strategic position of the DPRK once again in the near future.

Recall also that the Titan II was a space launch vehicle. North Korea has plans to place higher mass satellites to low earth orbit, and also payloads to higher orbits especially GEO orbit. A heavy ICBM would also provide Pyongyang the basis for the booster stage of a hefty launch vehicle for its space programme.

There’s good conservative analysis of all this by Vann Van Diepen at 38North. He could well be right. Notice, however, he assumes North Korea’s nuclear warhead research and development programme has not and will not advance beyond what we’ve already seen. Everything in his analysis is variable, except for the physics package of the warhead which is treated as a constant.

A sold fuel ICBM likely wouldn’t have a throw weight greater than the Minuteman III. That means North Korea would have a solid propelled ICBM with about the same throw weight as the Hwasong-15, therefore it would be developed on grounds of mobility and survivability. However, consider this. Solid propelled ICBMs have three stages. Liquid propelled ICBMs like the Hwasong-15 have two stages. Let us say the probability of a successful launch of a rocket is

P= P^1 . P^2 …P^n

Where n is stage number and P is probability that the stage will operate successfully.

North Korea’s research and development philosophy is not one based on frequent testing of a missile. That means a three stage missile, absent an extensive flight testing programme, would be less reliable than a two stage missile. North Korea’s space launch vehicle series, the Unha series, features three stages. We’ll use this as a proxy, even though we’ll be talking different engines and technology in the case of a solid propelled ICBM. Thus far there have been four launches of the Unha series SLV. Two of these have been failures (the first was a third stage failure, the second a first stage failure). North Korea has a three stage batting average with the Unha series of 50%.

The less reliable a missile the less credible becomes nuclear deterrence. In the absence of an extensive testing programme for a solid propelled ICBM North Korea would be trading reliability for mobility and survivability. That could have no overall effect on the credibility of deterrence, it might even have a negative effect.  A bigger button, a Suryong Bomba let’s say, however might be another matter.

In short, I would not rule out that North Korea has developed a new higher thrust liquid propelled engine for a heavy liquid propelled ICBM. That’s certainly not a conservative assumption, but I submit it is reasonable that it forms part of our hypothesis space pending observation. Therefore, so far as I am concerned, it remains on my line of sight. Should North Korea’s next ICBM not be solid propelled (not the same as saying they’ll never be a solid fuel North Korean ICBM) then we’ll have to ask ourselves why the prevailing assumption was wrong.

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