A North Korean Mobile ICBM?

Press stories appearing in early December 2011 raised the possibility of a North Korean mobile intercontinental range ballistic missile (ICBM). Given what is known publicly about North Korea’s missile program, this is a surprising claim that is worth examining.

The impression these and other stories give is that North Korea is developing a new ballistic missile—leap-frogging its previous efforts at building a long-range missile—and is on the verge of posing a new threat to the United States.

This is almost certainly not the case. As the analysis below shows, these reports do not necessarily imply the development of a new missile different than those which it is already known to be working on. Instead, they may mean that the North is building or acquiring trucks or trailers that can be used to transport missiles currently under development.

Press Reports

The earliest public statement about a possible North Korean mobile ICBM was a comment by Robert Gates just before he stepped down as Secretary of Defense six months after his January 2011 remarks that Pyongyang might have a nuclear-armed intercontinental missile within five years.[1]

In particular, on June 4, 2011,responding to a question about North Korea during a meeting in Singapore, Secretary Gates talked about a looming threat and added the detail about a potential mobile ICBM:

With the continued development of long-range missiles and potentially a road-mobile intercontinental ballistic missile, and their continued development of nuclear weapons, North Korea is in the process of becoming a direct threat to the United States.[2]

Just two and a half weeks later in a June 21 interview in Newsweek, Secretary Gates went beyond his previous statements, expressing much more certainty about both the threat to the United States and the existence of a mobile ICBM program:

North Korea now constitutes a direct threat to the United States. The president told [China’s] President Hu that last year. They are developing a road-mobile ICBM. I never would have dreamed they would go to a road-mobile before testing a static ICBM.[3]

Unfortunately, he was not asked to elaborate in either case. It is odd that in early June Secretary Gates referred to North Korea as “in the process of becoming” a direct threat to the United States—consistent with his statements the previous January—while two weeks later he said that the United States had already considered North Korea to be a direct threat the previous year.

Secretary Gates’ remarks attractedpress attention at the time, and the issue surfaced again—largely because of a December 5 story in the Washington Times written by Bill Gertz.[4] This article was about a November 17 letter sent by five Republican members of the House Armed Services Committee’s subcommittee on strategic forces to the current Secretary of Defense, Leon Panetta, urging stronger administration support for long-range missile defenses.[5] The letter referred to a briefing given by the administration but does not tie that briefing to claims about a North Korean mobile ICBM. The authors also referred to Secretary Gates’ June 4 statement  when discussing the reason they supported an invigorated missile defense program.

Mr. Gertz writes that following the briefing “Congressional aides declined to comment on the intelligence” and that “Administration officials familiar with the missile data said U.S. intelligence analysts have some disagreement over the developments.” Given what is known about North Korean missiles, what might be going on?

What is a “Mobile Missile”?

North Korea has a clear motivation to pursue a missile that is not tied to a fixed launch site. The United States and other countries are watching its launch sites carefully, and an ICBM is large and easy to spot. Once the missile is moved into launch position it becomes a target that an adversary could attack during a crisis.

So it would not be surprising if North Korea was interested in developing the ability to move intermediate- or long-range missiles and launch them from unknown sites. One option might be to hide them in caves, for example, where they could be concealed until they are rolled out to launch, similar to what China is believed to have done with some of its early missiles.

The term “mobile missile” tends to bring to mind an image of a missile carried on a mobile launcher that can move to a desired location, and then raise the missile into launch position and fire it relatively quickly. Current examples of mobile ICBMs of this kind are the Russian SS-25 missile and the Chinese DF-31 and DF-31A missiles.

But these three mobile missiles use solid propellant. As discussed below, North Korea does not have the ability to build large solid-fueled missiles. Liquid-propellant missiles of the same long-range capability tend to be much larger and more fragile. Such missiles cannot be transported, or erected into a vertical launch position from a horizontal traveling position, while fueled.

The main reason that large liquid-propellant missiles cannot be transported while filled with fuel has to do with the missile’s structure. To attain the high burnout speed necessary to propel a warhead long distances, the mass of the metal missile body must be kept as low as possible, and this requirement is especially strict for ICBMs.[6] Such a lightweight structure for the missile body would not be strong or rugged enough to withstand the stress of transporting a fueled missile or attempting to raise it into launch position.

As a result, moving a large liquid-propellant missile by truck or trailer would require the missile to be transported without being filled with fuel. This would reduce the mass to well under 10 tons and reduce the stresses on the missile.

While this would allow the missile to be moved to remote launch sites, it could not be launched quickly once it reached that site. Instead, the missile would have to be erected into a vertical position and then filled with fuel from large tank trucks that accompanied it. This process can take a couple hours.

These missiles may therefore be “moveable” but not “mobile” in the typical usage of that term. In short, North Korea may be trying to acquire “moveable missile” capability.

As an example, Figure 1 shows trucks needed to fuel the liquid-propellant Chinese DF-3 missile prior to a launch. This missile is roughly the same size as the first stage of the North Korean Unha-2 launcher; as we discuss below, a North Korean ICBM is likely to be derived from or similar to the Unha-2. A multi-stage missile would require additional fuel trucks.

What kind of truck or trailer would be needed to transport an unfueled liquid-propellant ICBM?

A North Korean ICBM would be essentially the same size and mass as China’s two-stage DF-4 missile (see below), which was used as the basis of China’s first satellite launch vehicle, the Long March-1 (CZ-1). The DF-4 has a length of 28 m, a mass of 80 tons fueled and less than 10 tons unfueled.[7]

Figure 2 shows a photo of the DF-4 being prepared for launch, with the trailer used to carry it. This trailer allowed the missile to be transported—unfueled—to caves, where it was hidden. It could then be rolled out, erected, fueled, and fired.

The transport system could use a large truck instead of a trailer like that shown in Figure 2. But the picture makes clear that the system needed to transport a missile of this kind can be relatively simple and need not indicate a leap in technology.

Even with such a trailer, there is a question of over how large an area North Korea could move such a missile. Trying to move a missile over the rough terrain of the remote, mountainous parts of North Korea may result in jarring and shaking that could cause damage to the missile body and/or components such as the electronics. There are few large, well-maintained roads in these areas, and building a road suitable for a large trailer of this kind would be evident from satellite observation, providing  clues to where the missiles were stored.

Such a mode of operation would build on experience that North Korea has already had with two missiles. One is North Korea’s version of the short-range Scud missile. With a range of about 300 km, these missiles have low mass and a very rugged body, since they were designed to be transported. They are believed to be fueled at a central location and then can be moved while fueled. The second missile is the Nodong, which is essentially a scaled-up Scud with a range of about 1,300 km. When fueled, the Nodong has nearly four times the mass of the Scud. Little is known about how this missile is operated. The Musudan missile has been shown in parades on a transporter, but this missile is not operational (see below).

A Long-range Missile?

While North Korea can build trucks or trailers for a missile, it is important keep in mind that it does not have an operational ICBM or intermediate-range missile. The longest range operational missile it has successfully flight tested is the Nodong.

As noted above, North Korea does not have the ability to build long-range solid-fueled missiles. Its only solid-fueled delivery system is believed to be the Toksa—a version of the Soviet SS-21 missile—with a range of about 100 km. Producing large, solid-fueled rocket motors is a very significant technical challenge. For example, propellant must have a consistency rigid enough to support its own weight, but must not be so hard and brittle that it forms cracks, which can cause the propellant to explode when it burns. These requirements are especially demanding for a mobile missile, since it must be able to withstand the additional stresses of moving.  China only built a road-mobile solid-fueled ICBM recently after 20 years of development.

As a result, any North Korean ICBM developed in the foreseeable future would almost certainly be liquid fueled.

There have been repeated claims that Pyongyang received either Soviet-made R-27 medium-range missiles (originally a submarine-launched missile called the SS-N-6 in the West), R-27 missile components, and/or production equipment from Russian sources, and may have sold some to Iran. If North Korea has acquired some number of complete R-27 missiles from Russia then it could conceivably deploy them without flight-testing since they would have been previously tested. Even so, their range is only about 2,400 km with a 650 kg payload.[8] . If North Korea instead acquired components or production equipment for the R-27 from Russia, it would need to do a proof flight test of the missiles it produced to gain confidence in the production process and to consider the weapon an operational military system.

The Musudan missile, which has been widely discussed in the press, has not been flight tested. Since this deleivery-system is a significant modification of the R-27 it cannot be considered operational without such tests (more below). It appears to have a range of about 3,200 km with a 650 kg payload.[9]

The Taepodong-2 (TD-2), which is the name given to a long-range missile believed to use the technology of the Unha-2 space launcher, has also not been flight tested. The Unha-2 has not been successfully tested, although the first two stages appeared to operate largely as planned in a April 2009 launch. In addition, North Korea has not tested a reentry vehicle and heat shield for a long-range missile, which it would need for delivering a warhead by a missile like the TD-2.

The most likely scenario, therefore, is that North Korea continues to develop the technology displayed in the Unha-2 launcher, which could be adapted to either a two- or three-stage TD-2 ICBM. A two-stage version of the TD-2 could have a range of more than 9,000 km with a 650 kg payload. A three-stage version, with sufficient structure to carry a heavy warhead and a heat shield instead of a lightweight satellite, could have a range of greater than 11,000 km.

If North Korea had acquired or could produce enough R-27 engines it could get somewhat better performance by replacing the cluster of four Nodong engines in the first stage of the Unha-2 with four clustered R-27 engines. Getting higher performance out of these engines would require North Korea to have large amounts of the advanced propellant the R-27 uses;[10] each missile would require some 60 tons. To reach intercontinental ranges, this missile would also have a fueled mass of roughly 80 tons, similar to the TD-2.

What about the Musudan Missile?

Speculation about a mobile ICBM seems to center on the Musudan missile. For example, the December 5 Washington Times story states:

Officials familiar with the intelligence said government analysts believe the missile could be a variant of North Korea’s new Musudan intermediate-range missile, first disclosed publicly in October 2010.

This statement is misleading since it suggests that a “variant” with some relatively minor modification of the Musudan missile would result in a delivery-system with ICBM range. This is not true.

As noted above, the Musudan (Figure 3[11]) is a single-stage missile believed to be a modified version of the Soviet R-27. No flight tests of the Musudan have been observed.

The second stage of the Unha-2 launcher that North Korea used—unsuccessfully—to try to place a satellite in orbit in April 2009 appears to be an R-27 missile, and the third stage appears to use the steering motors of the R-27. During the launch, the second stage appeared to function properly but the third stage did not.[12]

However, the fact that an R-27 was apparently flown as the second stage of the Unha-2 in 2009 does not mean that the Musudan can be considered to be flight tested, as some reports claim, because the Musudan missile has been substantially modified from the R-27. The missile body has been lengthened by some 2.5 m (more than 25 percent) to carry more fuel, which adds four to five tons of mass and significantly changes the mass distribution of the missile and of the stresses on the body.

As a result, the fact that North Korea has never conducted a test launch of the Musudan is significant. It means that the Musudan cannot be considered operational, and North Korea cannot assume that a launch would be successful, and would therefore be unlikely to mate a nuclear warhead to the missile.

Even if the Musudan were tested, its range is much shorter than an ICBM. As noted above, based on the characteristics of the R-27, modeling shows that the Musudan would have a range of roughly 3,200 km with a 650 kg payload.

Reaching intercontinental range would require a missile that was significantly larger than the Musudan and had multiple stages. For comparison, the two-stage version of the TD-2 discussed above would have a length of some 26 m and a mass of 80 tons. This is more than twice as long as the Musudan (which is about 12 m in length) and more than four times heavier (the Musudan mass is about 18 tons). Moreover, it has a first-stage diameter of 2.4 m, compared to the 1.5 m diameter of the Musudan. A true ICBM, with a range of more than 11,000 km would be even larger.

Even if North Korea had enough R-27 engines to develop a new first stage using a cluster of four of these engines rather than Nodong engines, so that the missile consisted entirely of R-27 technology, it would still be a multi-stage missile similar in size to the TD-2.[13] It makes no sense to call this a “variant” of the Musudan.

Conclusion

Statements about a possible North Korean development of a mobile ICBM do not imply the development of a new missile. Instead they may indicate that North Korea is interested in finding a way to transport a long-range missile like the TD-2 and launch it from locations other than a fixed site, similar to what China did with its DF-4 intermediate-range missile.

Mobile missiles would complicate the conduct of a pre-emptive strike on the missile compared to an attack against weapons launched from a known, fixed site. However, in the situation described above, the missile would be transported unfueled and would need to be accompanied by a set of trucks that would fuel it once placed in launch position. Preparation would probably take several hours. That, combined with the additional infrastructure at the site would remove some of the advantages typically associated with mobile missiles.

In any event, the primary concern for the United States should be stopping the development and flight testing of intermediate- and long-range ballistic missiles, whether or not they are going to eventually be mobile. Currently, North Korea has not successfully flight tested a missile with range longer than 1,300 km. Without such tests, readily detected by U.S. satellites, a delivery system cannot be considered operational. As a result, there would be significant benefits to the United States engaging North Korea to reinstate the missile flight moratorium that it observed from 1998 through 2005—which would keep the Musudan, TD-2, and other missiles from becoming operational—and then to seeking a permanent ban on such tests.

[2] 10th IISS Asia Security Summit, The Shangri-La Dialogue, Singapore, June 4, 2011, http://www.iiss.org/conferences/the-shangri-la-dialogue/shangri-la-dialogue-2011/speeches/first-plenary-session/qa/.

[3] J. Barry, “The Defense Secretary’s Exit Interview,” The Daily Beast, June 21, 2011, http://www.thedailybeast.com/articles/2011/06/21/robert-gates-interview-his-lingering-concerns-about-u-s-supremacy-nuclear-proliferation-and-more.print.html.

[4] B. Gertz, “North Korea making missile able to hit U.S.,” Washington Times, December 5, 2011, http://www.washingtontimes.com/news/2011/dec/5/north-korea-making-missile-able-to-hit-us/print/.

[5] Letter to L. Panetta, November 17, 2011, http://turner.house.gov/UploadedFiles/11-17-2011_SF_Rep._to_Panetta_on_Hedging_Strategy.pdf.

[6] This is a direct consequence of the “rocket equation”—see D. Wright, L. Grego, L. Gronlund, The Physics of Space Security, 2005, p. 75, http://www.ucsusa.org/assets/documents/nwgs/space_weapons_section_7.pdf.

[7] J. Lewis and Hua Di, “China’s Ballistic Missile Programs,” International Security, Vol. 17, No. 2, Fall 1992, pp. 5-40.

[8] See D. Wright, “More on Musudan Range Estimates,” October 12, 2010, http://allthingsnuclear.org/post/1303114897/more-on-musudan-range-estimates.

[9] D. Wright, “More on Musudan Range Estimates;” a U.S. estimate in an October 2009 Wikileaks cable gave a slightly longer range (4,000 km with a 500 kg payload), but details of this estimate are not known, http://wikileaks.org/cable/2009/10/09STATE103755.html.

[10] The R-27 uses UDMH with nitrogen tetroxide as an oxidizer, although using nitric acid as an oxidizer would give similar performance.

[11] Some reports say the missile was first seen in a 2007 parade that was not open to the international press; see “North Korea Rolls Out Ballistic Missiles,” Global Security Newswire, October 13, 2010, http://www.nti.org/gsn/article/north-korea-rolls-out-ballistic-missiles/.

[12] D. Wright and T. Postol, “A Post-launch Examination of the Unha-2,” Bulletin of the Atomic Scientists, June 29, 2009, http://thebulletin.org/web-edition/features/post-launch-examination-of-the-unha-2.

[13] The Chinese DF-4 missile uses lightweight structure and advance propellants, similar to what a missile based on R-27 components might use, and it is essentially the same size the TD-2 discussed here, with a mass of 80 tons, a length of 28 m, and a diameter of 2.25 m (Lewis and Hua, “China’s Ballistic Missile Programs”).