North Korea recently showed images of a large solid-fuel rocket motor test that appears to have been both real and successful. The motor, much bigger than any solid-fuel motor previously seen in the North, is not appropriately sized to be used on any existing missile in Pyongyang’s stockpile. Rather, a more likely role for the motor is as the upper stage of a solid-fuel replacement for the liquid-fuel Nodong medium-range ballistic missile (MRBM). Such a missile would be more operationally robust, capable of being transported off-road without damage and launched on very short notice. With no indication that such a missile yet exists, and with substantial testing yet to be done, deployment is likely to be five years or more in the future if the program proves to be successful. This new development could serve as a stepping stone to the development of a solid-fuel intercontinental ballistic missile (ICBM), but even if Pyongyang pursues the development of such a system, it would not become operational until after 2030.

Upping the Ante

In recent months, the North Koreans have displayed mocked up missiles in parades, television footage of “new” missile tests that faked success and given us a look at ground test activities. Ground test activities are not as sexy as rockets on parade or in flight, but are more revealing—and harder to fake. Moreover, coming soon after a satellite launch and an underground nuclear test, it is increasingly difficult to credit the hypothesis some analysts have advanced that the North’s missile related-activities are a giant hoax.

The most recent episode is a series of photographs of a ground test of a solid-fuel rocket motor. Video would have perhaps been more convincing, but the pictures are not only detailed and specific but also real. They show a solid-fuel rocket motor—much larger and more powerful than anything we had suspected North Korea possessed—both while firing and as one slightly-charred piece afterwards. There’s some grandiose text to go with the pictures, but these pictures are worth far more than the words that accompany them.

A bit of technical background: Aside from short-ranged battlefield weapons, North Korea’s rockets to date have all used liquid-propellant. A liquid-propellant rocket engine is a machine for turning fuel into thrust at a prodigious rate. They are more powerful and efficient than any other self-contained propulsion system that can be built; they put men on the Moon. And if you are desperately struggling to find a way to deliver warheads to an enemy beyond your reach, that’s where you start. But the engines are complex pieces of machinery. The propellant tanks are simple but fragile. And the propellants tend to be highly corrosive and toxic. Putting that combination in the hands of frightened conscript soldiers working in a remote location under extreme stress is a recipe for disaster. The United States retired its last liquid-fuel missile in 1987 after several lethal fires and explosions were caused by simple mistakes.

For weapons, solid propellant is the preferred fuel. The simplest solid-fuel motors have no moving parts, just a strong hollow case with an igniter on one end and a nozzle on the other. Inside is a full load of propellant, cast in place from a tough rubbery compound that burns like thermite’s more exuberant cousin. Small solid-fuel motors can be made simple and safe enough for amateurs to manufacture from scratch and launch even to the edge of space. At the scale the North Koreans demonstrated, there are technical challenges. Slight cracks or voids in casting the propellant can lead to explosions. Prolonged exposure to intense heat can destroy the nozzle. And if you’re hoping to hit a specific target, solid motors can be tricky to steer and even harder to turn off at the right time.

While the performance of a solid rocket will never be as good as a liquid-fuel system, it is far more likely to work on the battlefield. They require little maintenance, can survive rough handling and off-road transport, are less prone to leaking toxic, corrosive vapor at the slightest provocation, and even the largest solid-fuel missiles can be launched on a few minutes’ notice. That last characteristic is going to be particularly important for North Korea, as South Korea’s missiles can reach targets anywhere in the North in the fifteen minutes or so it would take to fuel and ready a liquid-fuel missile for launch.

The North Koreans have always been able to build small solid-fuel rocket motors suitable for battlefield weapons. About ten years ago they introduced the KN-02 “Toksa,” a short-ranged ballistic missile based on an old Soviet design probably provided by Syria. Weighing about two tons, the Toksa can be transported and launched from a six-wheeled off-road vehicle and deliver a roughly 500 kg warhead to a range of 120 kilometers. Subsequent improvements have increased the range to maybe 200 kilometers, but this is still a tactical weapon suitable for use against targets not far behind the front line. To reach military bases and logistics targets deep in South Korea or, perhaps more importantly to North Korean strategists, to threaten Japan or the United States, would require much larger rocket motors. The key question is whether the new solid-fuel motor just displayed is able to fly that far?

The answer is that, as part of a two-stage missile, it could probably reach Japan but not the United States. The North Koreans didn’t give us the dimensions of the rocket they tested, but they did photograph two cheerful figures posing in front of the test article. With some margin for error due to an uncertain camera angle, the motor case looks to be about 1.25 meters in diameter and 3 meters long. Since the Toksa is longer, but only 65 centimeters in diameter, this is something new. If the motor uses a steel case and is filled with the usual sort of propellants, it would weight about 800 kg empty and hold about four tons of propellant—twice the weight of an entire Toksa missile

Pictures of the test in progress look like what a successful solid-fuel rocket test should look like. We haven’t seen any indication of photoshopping or other doctoring of the pictures, nor any indication that the test failed just off-screen. The rocket plume has an intensity and color indicating the likely use of powdered aluminum in the propellant blend, a very powerful but hot-burning additive used in the best-performing Western solid-fuel motors. A glancing view of a telemetry screen suggests the engine ran for about 57 seconds, which is about right. Such an engine would likely produce 15-20 tonnes of thrust. And, just to make it clear the rocket didn’t blow up, we get a view of the aft end after the test. Charred, as expected, but intact. There are four thrust vanes and their actuators at the base of the nozzle, chewed up a bit by the intense heat, not the most efficient way to steer a missile but one North Korea has used effectively in the past. A photo of a telemetry screen in the control room shows data that might indicate a test of these vanes, but we can’t be certain. There also is no indication that the North Koreans tested a thrust termination system, which would be necessary for an accurate missile.[1]

Based on the evidence of these pictures, North Korea appears to have conducted a successful test of a large solid-fuel motor of 15-20 tonnes thrust and about one minute of burn duration. This is roughly three times more powerful than the biggest solid motor North Korea has previously demonstrated. Further testing, including a functional steering and thrust termination mechanism, would be needed to use this motor in an operational system. Still, it represents a step forward in North Korea’s capabilities.

What will the North Koreans do with this new larger motor?

A set of barely-visible drawings indicates that this motor is meant to be used as the upper stage of a larger missile or rocket, as does its relatively squat geometry. The motor’s 1.25 meter diameter is the same as North Korea’s Nodong missile, a medium-range ballistic missile (MRBM), and the upper stage of its Unha space launch vehicle. But this motor is too heavy to simply be stacked on top of a Nodong carrying a 4.8-tonne upper stage in addition to its usual warhead; it would barely be able to get off the ground.

While the solid-fuel motor is almost a perfect fit in terms of size, it would also have distinct disadvantages when compared to the Unha’s current liquid fueled third stage. An Unha would launch easily enough, but using this motor as a third stage would reduce overall performance. The liquid-propellant third stage, while limited in thrust, is lightweight and efficient while the powerful solid motor would be heavier and less efficient. And the low thrust of the liquid upper stage has a hidden benefit—by requiring a very long burn, it allows the Unha to reach its target orbit under continuous powered flight and active control. A powerful but fast-burning solid-fuel motor would have to coast through space for several minutes before being ignited. During that time, the rocket would be in danger of tumbling out of control. Even if North Korea is having difficulty producing the current Unha third stage, a solid-fuel motor like this wouldn’t be the best solution.[2]

Similar objections apply to using the solid motor as the third stage of the road-mobile KN-08 ICBM seen in Pyongyang parades and in a recent photo op with Kim Jong Un. The substitution would reduce the missile’s range, payload and accuracy, and with the large first and second stages still dominated by fragile tanks of toxic, corrosive propellants, the missile would still be limited in mobility and responsiveness.

Since there are clear disadvantages to using this motor in any existing North Korean missile systems,[3] could a new missile be under development? One possibility is a two-stage solid fuel replacement for the Nodong. Both Iran and Pakistan, once major customers for the Nodong, have developed two-stage solid-fuel missiles of similar size and performance but much greater robustness and operational flexibility. Pakistan’s Shaheen and Iran’s Sejil missiles would be well-suited to North Korea’s regional deterrence and perhaps warfighting requirements. But in spite of past cooperation between all three nations, neither of them seems willing to sell these systems to Pyongyang. And the dimensions of the new North Korean motor don’t seem to match the Pakistani or Iranian systems, though the diameter and maybe thrust vanes are a match for the Sejil.

So if North Korea wants a solid-fuel MRBM, it will have to design its own. And the new motor is ideally suited for such a missile. It would need to be mated with a new first stage motor, likely of the same diameter but almost three times the length. Developing and building such a motor would be no trivial matter, but stretching a proven 1.25-meter solid-fuel motor to that extent should be within North Korea’s capabilities. Then the North Koreans would need to design, build, and test the missile. While there is no sign that they are pursuing such a weapon, it would be premature at this early stage of the development process to do anything more than the most basic design work for a missile that doesn’t yet have an engine. After the recent test, presumably North Korea knows enough to start designing whatever missile will use the new motor.

The 1.25 meter diameter of the new motor may mean the North Koreans want to match the dimensions of the Nodong, perhaps with the idea of using the existing transporter-erecter-launchers (TELs) and other infrastructure. If that is the case, a new two stage missile could match the Nodong in overall length, weigh 18 tonnes at launch and deliver a 1000 kg warhead to a range of roughly 1800 km. With a lighter 500 kg warhead, the range could be as high as 2600 km. Such a missile could be transported both on and off road, and launched on no more than five minutes’ notice.

Since, as far as we know, this missile does not yet exist, North Korea has a lot more work to do before it becomes operational. It will have to conduct more tests of this motor that will probably last over the next year or so if things go well. The design will then need to be stretched to make a first-stage motor. That will probably take another year. Design of the missile could, to some extent, proceed in parallel with the motor tests, but it still took Iran three years from its first ground motor firings to the first flight test of the Sejil missile, and Iran has greater industrial resources than North Korea. Even if North Korea can match Iran’s performance in this respect, we would not expect to see a solid-fuel MRBM fly before 2019, or to enter operational service before 2020.

A Stepping Stone to Solid-Fuel ICBMs?

Could the KN-08 ICBM, which has undergone one major design change already, be transformed into a solid-fuel missile? A solid-fuel KN-08 would have significant advantages over the current liquid-fuel version, which has very limited mobility and could require an hour or more to prepare for launch. But such a transformation would require a completely new, larger missile. The first-stage engine would need to be almost an order of magnitude bigger than the one recently tested. Moreover, solid rocket motors get trickier as they get bigger; that level of up-scaling has traditionally called for at least a decade of steady work, often accompanied by one or two catastrophic explosions on the test stand, the aftermath of which would probably be visible in satellite imagery. We haven’t seen any sign that the North Koreans have begun to test such a system.

The Bottom Line

If this analysis proves correct, this development is certainly a step forward in building solid-fuel motors. However, rather than moving North Korea closer to building an advanced ICBM, a development that is still far in the future—perhaps 2030 or later—it is only likely to result in an improvement in the regional-range ballistic missiles already in North Korea’s inventory. A new solid-fuel Nodong, possibly ready for deployment after 2020 if the development program succeeds, will have greater off-road mobility and take less time to prepare for launch.

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[1] When it is important that a solid motor shut off at a precise time, as in a ballistic missile aimed at a specific target, this is usually accomplished by using explosive charges to either sever the nozzle or cut holes in the motor case—either method is a violent process that leaves obvious signs. We can’t rule out that thrust termination ports were located at the front of the motor and that all of the photos were carefully framed to hide this. Thrust termination is in any event usually demonstrated late in a motor development program.

[2] A liquid-fuel third stage using engines from obsolete SA-2 or SA-5 surface-to-air-missiles would deliver better performance as an ICBM or a satellite launch vehicle, would be within North Korea’s demonstrated capabilities, and would better match the current, flight-proven Unha design.

[3] North Korea can almost certainly launch a salvo of Nodong missiles in a surprise attack, but if they were to use them as a weapon or a deterrent during an escalating crisis or prolonged war they would risk having them destroyed on the ground while they were being readied for launch. A solid-fuel replacement could be kept safely hidden in underground tunnels or bunkers until perhaps five minutes before launch.