North Korea also unveiled what appears to be its new solid propellant ICBM, presumed to be the Pukguksong-3, enclosed in a canister in the massive military parade on April 15.
Limitations of the Musudan Missile
Throughout 2016, North Korea test launched Musudan missiles employing high-energy liquid propellants eight times, with only one successful attempt. These tests used a lofted, high-angle trajectory, presumably to reduce the range of the missiles and avoid any escalated tensions that might occur from flying over Japan. However, from an operational perspective, a lofted launch can also make the reentry vehicle (RV) descend more quickly during the terminal phase, allowing missile defenses less time to intercept them. It seems that these consecutive test failures exposed the limitations of its engine, which was developed by reverse engineering the Russian R-27 submarine-launched ballistic missile (SLBM). The one successful flight test of the Musudan, conducted in June 2016, was likely aimed at simulating the velocity and environment of ICBM reentry by reaching a higher peak altitude of more than 1,400 km with a decreased range of 400 km.
New Technologies Demonstrated by Pukguksong-2 IRBM Test Launch
The Pukguksong-2 IRBM demonstrated a new solid propellant propulsion system. North Korea officially announced that this missile employed technologies used in the Pukguksong-1 SLBM, claiming it had created “the amazing miracle” of completing a new strategic weapons system in just six months. Solid propellant motors shorten launch preparation times by eliminating the time-consuming process of loading liquid propellant, thus reducing the risk of timely detection by intelligence, surveillance and reconnaissance assets. As a result, they improve the missile’s survivability and its capability for rapid launch from more secure locations, making effective pre-emptive military strikes more problematic. Solid propellant ballistic missiles have other significant military-operational advantages over conventional liquid propellant ballistic missiles. While being safer and easier to handle and maintain, they are also simpler and can provide better range performance than equivalent-sized liquid propellant missiles. Accordingly, solid propellant missiles pose a greater threat to allied forces than liquid propellant missiles.
Additionally, the Pukguksong-2 missile demonstrated other new capacities:
- Greater Mobility: First, the Pukguksong-2 IRBM used a caterpillar-type transporter erector launcher (TEL) that can provide better mobility in harsh terrains than wheeled TELs. This new type of TEL, which was shown in the test launch, is based on a heavily modified tank chassis originally designed and built by North Korea and is similar in layout to the Soviet Union’s SS-14 system.
- Use of a Cold Launch: Second, the Pukguksong-2 was the first road-mobile missile to employ a cold launch system, which uses compressed gas pressure to eject the missile from the erected canister on the ground before igniting the rocket motor as soon as the missile clears the canister. This system does not require additional volume within the launcher for exhaust plume control, making it feasible to launch from a confined space. Unlike hot launch systems, a cold launch also lowers the possibility of damage or destruction of the TEL in the event of missile failures, since they would happen in mid-flight rather than on the ground. This technology was already demonstrated last August by the Pukguksong-1 SLBM, which was successfully launched from a vertical launch tube.
- Use of High-Angle Trajectory and Reduced Maximum Altitude: Third, the missile fired on February 12, 2017 was launched at a high angle with a fixed operational range, similar to the Musudan IRBMs and Pukguksong-1 SLBMs launched last year. As the Pukguksong-2 IRBM was fired in a lofted trajectory (almost vertically), the maximum altitude reached should have exceeded 1,000 km. However, the maximum altitude announced by South Korea’s military was only 550 km. If this claim is accurate, it would appear that North Korea intentionally launched the missile with a heavier warhead to lower its peak altitude. Based on a simulation by the author, it was estimated that the warhead mass loaded on the Pukguksong-2 would be around 1.6-1.7 tons, which is much heavier than previous warheads (Figure 1). The heavier warhead could allow for greater lethality due to higher yields. This simulation also estimated that the operational range of the Pukguksong-2 IRBM using a minimum energy trajectory (MET) would reach approximately 2,300-2,500 km, which is expected to be shorter than normal IRBM range (Figure 2). The difference between Model A and B lies in the assumed dry masses, such as the interstages and fairing in Figure 1 and Figure 2.
Projected Evolution of North Korean Ballistic Missiles
North Korea has supposedly worked for over 20 years to develop a liquid propellant ICBM with the ability to hit targets in the United States. It appears that through trial and error, they have developed a high-thrust first stage liquid rocket engine for an ICBM, as demonstrated by ground firing tests of an 80-ton Paektusan engine conducted during the past six months. While the exact development path is unclear, Pyongyang may have decided to develop ICBMs with solid propellants in parallel with liquid propellants. In March 2016, the North conducted a successful ground test firing of a solid rocket motor that is 3-3.2 m in length and approximately 1.3 m in diameter. Five months later, it tested the Pukguksong-1 two-stage SLBM with solid propellant motors, which was estimated to be approximately 9 m in length and 1.35 m in diameter (Figure 3).
With the development and successful testing of two new types of solid propellant missiles, North Korea has moved closer to a technological breakthrough that is required to develop an effective road-mobile ICBM. Pyongyang might even be developing two different types of road-mobile ICBMs with solid propellant motors; one could be based on the liquid propellant KN-14 configuration and size—which was displayed in a 2015 military parade—to minimize the development time. The other could be a completely new design to meet stricter mission requirements. The operational performance of new solid propellant ICBM, referred to as the Pukguksong-3, has been analyzed under the assumption that its dimensions are based on the KN-14. The length and diameter of the mobile KN-14 ICBM are assumed to be around 17 m and 1.9-2.0 m, respectively (Figure 3).
In the massive military parade in Pyongyang on April 15, 2017, North Korea displayed two different types of Pukguksong-3 ICBMs, both enclosed in launch tubes mounted on trucks (see Figures 4 & 5). Although the exact dimension and size of the new ICBM was not clear, one can estimate from the dimensions of the KN-14 that its range is up to 12,200 km with a 550 kg warhead and 10,300 km with a 750 kg warhead for a missile diameter of 1.9 m. These operational ranges may be enough to reach the US mainland (Figure 6).
North Korea may, in the future, replace all its liquid propellant ballistic missiles with solid propellant missiles. The first to be replaced will most likely be the Scud short-range ballistic missiles (SRBM) and Nodong medium-range ballistic missiles (MRBM). Both use conventional liquid propellant engines and more than 800 units are believed to have been deployed. Considering the North’s economic problems, replacement would happen gradually and possibly not on a one-for-one basis. A rapid replacement of the Musudan IRBM fleet, which also uses a high energy liquid propellant, with the Pukguksong-2 solid propellant IRBM may occur if the Musudan continues to prove unreliable.
The future evolution of the road-mobile ICBM is difficult to predict. However, if North Korea succeeds in the test flights of a solid propellant ICBM and continues to make improvements in its reliability, Pyongyang will likely abandon its liquid propellant ICBM designs.
What Explains North Korea’s Solid Propellant Rocket Technology?
How did North Korea make such significant progress in the solid propulsion technology field? As is well known, the structure of a solid propellant rocket is much simpler than that of a liquid propellant rocket, but the complexity of the design grows exponentially with its size, requiring extensive testing and design iterations for development. Therefore, it is entirely possible that Pyongyang has been working on solid propellant technology for more than a decade while simultaneously developing the indigenous Musudan liquid propellant engine. It is also possible that Pyongyang is concurrently developing the solid propellant Pukguksong-2 IRBM and solid propellant Pukguksong-3 ICBM. As such, it would not be surprising if the North also decides initially to pursue concurrent development of both solid and liquid propellant ICBMs (Figure 7).
Some experts also suspect that North Korea’s accelerated development of solid fuel rockets is connected to Iran’s missile program—more specifically, that the North provided liquid propellant missile technology to Iran in exchange for solid propellant missile technology. However, Iran only started to develop solid propellant missiles in the early 2000s, and its two-stage solid propellant Sajiil missile is still in development. In short, Iran has not yet mastered this technology. Nevertheless, a connection between the two countries is possible, but it is difficult to substantiate such claims due to the lack of solid evidence.
Ultimately, the transition from liquid to solid propellant missiles will bring about a fundamental paradigm shift in North Korean missile systems (Figure 8). A road-mobile ICBM, tentatively named the Pukguksong-3, employing solid propellant rocket motors could easily achieve the range performance required to hit the US mainland in the future, making it a serious potential threat to the United States.
Young-Keun Chang is a Professor of Aerospace and Mechanical Engineering and Director of Global Surveillance Research Center at Korea Aerospace University, Seoul, Republic of Korea.
 KCNA announced on the morning of February 13, 2017: “We developed a surface-to-surface medium-to-long range Pukguksong-2, which is North Korean-style new type strategic weapon system based on Pukguksong-1 SLBM within 6 months.”