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China and MIRVed Warheads

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A recent Pentagon report on Chinese Military Power reported that for the first time China has apparently begun to put multiple warheads on some of its ballistic missiles. This would mean that China can use its existing missile force to launch more nuclear warheads. This change was also reported in a New York Times article over the weekend.

The Pentagon report does not give details, but refers to the “multiple independently-targetable re-entry vehicle (MIRV)-equipped Mod 3 (DF-5)” missile.

China has long had the technical capability to deploy multiple warheads, but was previously not thought to have done so. It’s not clear what evidence is behind the Pentagon’s announcement that China has now deployed MIRVs, or what China’s motivation might be. Some speculate that the change is motivated by Chinese concerns about maintaining a credible deterrent force in the face of the expansion of U.S. missile defenses.

In any event, it’s important to be realistic about what this step means.

It would result in a modest change in China’s nuclear forces because it affects a small number of silo-based missiles. China is thought to deploy roughly 10 of the DF-5 Mod 3 missiles mentioned in the Pentagon report. So if these went from carrying one warhead to three warheads (see below), that would add about 20 warheads to the Chinese force. (In comparison, recall that the U.S. and Russia each currently deploy 1,800-2,000 warheads.)

And contrary to some reports, China’s fleet of 30 mobile missiles appears able to carry at most one or perhaps two warheads anytime soon, for reasons I discuss below.

Since the New York Times article is bound to lead to Chicken-Little analyses warning that China can quickly build up its long-range nuclear forces to many hundreds of weapons, it’s worth looking at some of the technical issues to ground the story in reality.

What did China MIRV?

The DF-5 is China’s largest missile, and is based in silos. It uses liquid fuel and is believed to be stored without fuel (because it is highly corrosive and can damage missile components over time) and without warheads mated to the missile. So it is not that the U.S. has seen DF-5s in their silos with multiple warheads on them. Instead, the U.S. assessment may be based on seeing a test of a missile that released multiple objects, for example.

What does it take to MIRV …

The key to MIRVing is the “I”, which stands for “independently targetable.” This means that each warhead on a missile can be put on a different trajectory to reach a different target. To do that, in addition to adding warheads to the missile you also need to add a small maneuvering stage that the warheads sit on. This stage is called a “post-boost vehicle,” or a “bus” since it carries warheads and drops them off at different locations on different trajectories.

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Two of three reentry vehicles on a MIRV bus carried by the U.S. Minuteman missile (Source: U.S. Air Force)

When the missile stops burning, it releases the warheads and the bus. The bus then drops off the first warhead on its trajectory, maneuvers to a slightly different trajectory and drops off the second warhead, and so on. To do this, the bus needs to carry enough fuel to allow it to maneuver sufficiently to drop all the warheads off on their desired trajectories. As a result, U.S. and Russian buses and fuel typically weigh as much as the total weight of the warheads they carry. Perhaps China would not require as much maneuverability, but this is probably still the right scale.

… and why does this matter?

The large mass of the bus (and its fuel) is important since it reduces the payload the missile can use to carry warheads, because the missile must lift both the warheads and the bus. If a warhead has mass M, then a missile with a single warhead will carry just the mass M. But adding a second and third warhead increases the payload by the mass 2M of those warheads plus the mass 3M of the bus. As a result, the missile must now carry a mass of roughly 6M.

China is known to have had the technology to build a MIRV bus for a couple of decades since it has been able to launch multiple satellites from the same launch vehicle, which is essentially the same technology. But until it had a light enough warhead, it could not put multiple warheads and a bus on even its most powerful ballistic missile.

In the 1990s China developed a lighter warhead before the negotiation of the Comprehensive Test Ban ended nuclear testing. Unclassified U.S. estimates put the mass of this warhead at about 500 kg.

With this mass, three warheads and a bus would have a total mass of roughly 3,000 kg. This is compatible with the payload capability of the DF-5, which is believed to be 3,000-3,200 kg. But it means China couldn’t add additional warheads without a using a lighter warhead—which China doesn’t appear to have and is unlikely to develop and deploy under a test ban—or a significantly smaller bus with much less ability to independently target the warheads.

Given the large payload capacity of the DF-5 missile, it might not be surprising if China—as it replaces its heavy, older generation warheads with the new, lighter one—decided to put more than one on the missile, even if it didn’t have a specific reason in mind.

Can China MIRV its other missiles?

As noted above, the DF-5 missile is liquid-fueled and silo-based. Most of China’s long-range missiles instead have solid fuel and are small enough to allow them to be mobile. These include the DF-31 and DF-31A (with a total of about 30 currently deployed), and the DF-41 in development. In contrast to the DF-5, which has a launch mass of over 180 tons, the launch mass of these missiles is thought to be 40 to 60 tons.

Given the size of these missiles and the fact that they need to be built rugged enough to withstand transport on mobile launchers (which increases their structural mass relative to silo-based missiles), they are believed to carry much smaller payloads than the DF-5. Estimates I’ve seen range from less than 1,000 kg up to 1,800 kg. To check this, it’s useful to look at a real-world example we know more about.

A relevant comparison is with the Russian SS-25 (Topol) and SS-27 (Topol-M) road mobile missiles. The Topol-M was commissioned in 2000, so it’s a modern missile. The Topol has a reported launch mass of 45 tons, a range of 10,500 km, and a payload of 1,000 kg. The Topol-M has a launch mass of 47 tons, and a range greater than 10,000 km with a payload of 1,200 kg. These two missiles make up the bulk of Russia’s current land-based missile force, and would seem to put a rough upper bound on the capabilities of China’s current and near-future land-mobile missiles, including the DF-41.

But with China’s 500-kg warhead, even just two warheads would take up most of the 1,000 kg payload. With two warheads on the missile, the missile would put the first warhead on the proper trajectory and you would only need the bus to maneuver the second warhead. In this case you could likely have a relatively light bus, but the total mass would still likely be 1,300-1,500 kg, which the Topol missiles could not carry without a decrease in range. (These missiles could carry two warheads with no bus, but the warheads would not be independently targetable in that case.)

I’ve seen some reports that say the DF-41 might be able to carry up to 10 warheads. But with China’s current warhead, that would lead to a total mass of about 10,000 kg, which is well above the payload of even the DF-5. In order to increase the number of warheads its mobile missiles could carry beyond one or two, China would need to significantly increase the payload capacity of its missiles well beyond that of the Topol-M, and/or develop and test a new warhead with a much smaller mass than its current small warhead.

If, for example, China designed, tested, and deployed a warhead with a mass of 200 kg—less than half of the mass of its current warhead—deploying five of these warheads and a bus would require a missile with a payload of about 2,000 kg. Neither step appears likely to happen soon.

Bottom line

The recent announcement that China has equipped some of its large DF-5 missiles to carry multiple warheads does not imply that China will be able to significantly expand the number of warheads it can deploy on missiles any time soon.

However, China might find it useful to add a small maneuvering bus to its mobile missiles to release decoys and other countermeasures along with the warhead, to defeat missile defenses. In this case the added mass of lightweight decoys and a small bus could be within the payload capacity of China’s mobile missiles. If the United States has observed flight tests of Chinese missiles that release multiple objects, these objects may be decoys rather than additional warheads.

 

Posted in: Missiles and Missile Defense, Nuclear Weapons Tags: , , , ,

About the author: Dr. Wright received his PhD in physics from Cornell University in 1983, and worked for five years as a research physicist. He was an SSRC-MacArthur Foundation Fellow in International Peace and Security in the Center for Science and International Affairs in the Kennedy School of Government at Harvard, and a Senior Analyst at the Federation of American Scientists. He is a Fellow of the American Physics Society (APS) and a recipient of APS Joseph A. Burton Forum Award in 2001. He has been at UCS since 1992. Areas of expertise: Space weapons and security, ballistic missile proliferation, ballistic missile defense, U.S. nuclear weapons and nuclear weapons policy. David also blogs on the Equation.

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