Status of North Korea's Missile Program
The longest range missile that North Korea currently deploys is the Nodong, which is believed to have a range of up to 1,300 km with a 700 kg payload. While there has been speculation about a longer range theater missile based on the Soviet R-7 missile (called SS-N-6 in the United States), which was first deployed as a sea-based weapon by the Soviet Union in 1968, there have been no flight tests of this weapon.
The Unha-2, which North Korea used in an unsuccessful satellite launch in April 2009, represents a significant advance over its past launchers. This is in large part due to the technology used in the second stage, which is considerably more advanced than that seen in previous tests.
The Unha-2 appears to be constructed from components, such as the second stage, that may not have been manufactured in North Korea. If true, this would mean that North Korea's indigenous missile capability could be significantly constrained if it can be denied further access to such components. An important factor in understanding the North's program is therefore to clarify this issue.
While North Korea has twice demonstrated an ability to successfully use staging in a launch-in the Unha-2 and the TaepoDong-1 (TD-1) in 1998-neither test was fully successful since not all of the missile components worked. For example, the third stage of the Unha-2 reportedly separated from the second stage but did not ignite. Had the third stage operated properly it may have been able to place a small satellite (with a mass of up to a couple hundred kilograms) into orbit at an altitude of approximately 500 km.
The Unha-2 launcher represents a significant increase in capability over the TD-1. It is considerably larger than the TaepoDong, with a first-stage diameter of 2.4 meters compared to 1.3 meters. The Unha is also more than three-times larger in mass than the TD-1 (roughly 80 vs. 25 metric tons). This is an important consideration since rockets consist mainly of fuel and the amount of payload they can lift is roughly proportional to their overall mass.
The first stage appears to use a cluster of four Nodong engines housed in a single missile casing and sharing a common fuel tank; the first stage of the TD-1 contained only a single Nodong engine.
The second stage appears identical to the Soviet SS-N-6. There have been reports for years that North Korea had acquired some number of SS-N-6 missiles in the 1990s.
The SS-N-6 utilizes liquid fuels (UDMH and nitrogen tetroxide) that are more advanced than those used in the Scud-B, a 300 km range Soviet missile from the 1960s, and therefore has a high thrust. Since the SS-N-6 was deployed on submarines, the missile has a compact design with a lightweight aluminum casing.
The third stage of the Unha-2 appears to be very similar if not identical to the upper stage of the Iranian Safir-2 launch vehicle, which placed a small satellite in orbit in February 2009. Both appear to use the small steering motors from the SS-N-6 for propulsion. This appears to be a concrete indication of cooperation between the Iranian and North Korean programs.
Capability as a Ballistic Missile
North Korea has conducted two nuclear tests, but is not thought to have designed a nuclear warhead that can be delivered by a missile. A device built for an underground test is not constrained by size and weight limits. Turning it into a deliverable warhead has proved to be difficult for countries in the past, requiring a number of years and multiple nuclear tests. A first generation plutonium warhead could have a mass of 1,000 to 1,500 kg. North Korea is currently thought to have enough separated plutonium for fewer than 10 weapons.
While some experts refer to a possible North Korean long-range missile as the TaepoDong-2 (TD-2), that name does not appear to refer to a specific missile configuration. Any long-range missile North Korea might develop in the near term would probably be a variant of the Unha-2 launcher. Analysis suggests that if the Unha-2 could carry a 1,000 kg warhead, it would have a range of 10,000 to 10,500 km, allowing the missile to reach Alaska, Hawaii, and roughly half of the lower 48 states.
However, since the Unha-2 was designed to launch a relatively lightweight satellite, its structure is unlikely to be strong enough to allow it to carry a warhead of that weight. The upper stage would probably have to be redesigned, and the additional structure would add mass that would reduce the missile's range to less than 10,000 km. Similarly, the range would be shorter if the warhead mass was greater than 1,000 kg.
If a 1,000-kg payload were instead launched by the first two stages of the Unha-2, its range could be 7,000-7,500 km, allowing the delivery system to reach Alaska and parts of Hawaii, but not the lower 48 U.S. states.
There are a number of hurdles North Korea would have to overcome in order to field such a missile. First, since Pyongyang is not believed to have a nuclear weapon light enough to deliver on such a missile, it would need to conduct further nuclear tests to reduce the warhead's size and weight.
Second, in order to deliver a warhead on a long-range missile, North Korea would need to develop a reentry heat shield. Reentry heating increases rapidly with the reentry speed of a missile, so a 10,000 km range missile would require a significantly better heat shield than that developed for the much shorter range Nodong missile. Because heat shield techniques and materials have been known for many years, North Korea should be able to develop an adequate shield, but that shield could still be a major source of missile inaccuracy.
Developing a heat shield that gives relatively high accuracy is a very difficult engineering task. For example, making the warhead very blunt rather than pointed would cause it to lose most of its speed at high altitude, which would significantly reduce the total heating. But during the slower descent the warhead would be subject to high-altitude winds for a longer time, reducing the accuracy. Streamlining the warhead so that it passes through the atmosphere quickly, on the other hand, leads to high heating rates, which would require more sophisticated heat shielding, such as an ablative coating. Asymmetric ablation can cause strong lateral forces on the warhead during reentry, which would reduce the accuracy. As a result, a missile based on the Unha-2 would likely have an inaccuracy of 10 km or more. Of course, such low accuracy could still be sufficient for a terror weapon aimed at a large population center.
Third, the Unha-2 was test fired from a known, visible site and requires days to prepare for launch. During that time, the missile would be highly vulnerable to attack. Reducing this vulnerability would require launch sites that were concealed from view. For example, missiles could be stored on transporters in caves, and could be rolled out, erected, and fueled. North Korea might try to develop such a capability in the future, but at this point it is still trying to get its missiles to function properly.
Finally, ballistic missiles are complex systems. Given its limited supply of fissile material, North Korea may well be reluctant to place a warhead on a missile that is likely to fail. Understanding and improving launcher reliability would require a series of additional flight tests.