If unchecked, North Korea is increasingly likely to resume strategic weapons testing in the months ahead—including tests of the technologies necessary for its missiles to carry multiple nuclear-capable reentry vehicles (RVs). Such missiles would, at a minimum, increase Pyongyang’s ability to challenge U.S. missile defenses, to make the most of its limited resources for strategic weapons production, and to threaten U.S. targets more credibly despite its limited flight testing of RVs.

It is, unfortunately, too late to prevent North Korea from obtaining the necessary know-how and materials for such a capability. The technologies involved are over a half-century old—already well within reach for North Korea’s advancing strategic weapons programs. Fortunately, however, a North Korean multiple RV capability would lack reliability and face international skepticism if not flight tested. Therefore, the United States should seek to prevent, or at least delay, the flight testing of such systems, because such tests could lead to a dramatic increase in the threats to stability and to U.S. interests posed by North Korea’s ballistic missile and nuclear weapons programs.

A Brief History of Multiple Reentry Vehicle Technology

Enhancing the capability of a ballistic missile by fitting it with more than a single RV is neither a particularly new nor sophisticated concept. Both the Soviet Union and the United States began developing this capability in the 1960s—after they had developed Intercontinental Ballistic Missiles (ICBMs) with individual RVs that could reliably carry a nuclear warhead through the shock and heat of atmospheric reentry to detonate reasonably near the intended target. The technologies they developed for fitting missiles with more than one RV take two general forms. Either method can be used with nuclear-armed RVs alone or with a combination of nuclear-armed RVs and decoy RVs that do not actually carry a nuclear warhead—along with other “penetration aids” to assist in overcoming missile defenses, such as radar jammers.

The more advanced MIRV (multiple independently targetable reentry vehicle) capability is far better known and more flexible. As the name implies, it allows one missile to direct RVs independently at different targets. This is accomplished by means of a maneuvering “post-boost vehicle” or “bus,” essentially a small spacecraft carrying the RVs that detaches from the last stage of the missile. The bus maneuvers in space to direct each RV on its own trajectory, which gives a MIRV the option to direct more than one RV or decoy toward the same target area or to direct each to an entirely different target.

The earlier, less sophisticated MRV (multiple reentry vehicle) design uses a much simpler approach. It does not require a maneuvering bus, and instead simply releases the nuclear-armed RVs and any decoy RVs into a pattern on a path to a single target area, similar to a cluster bomb or a shotgun shell. Though in a relatively tight pattern at time of separation, the RVs disperse far enough as they travel that they spread the damage over a wider area surrounding the target point.

The first missiles with MRVs were improvements of the Polaris submarine-launched ballistic missile (SLBM) fielded by the United States in 1964. These “A3” versions of the Polaris traded a single RV with a one-megaton warhead for three 200-kiloton warheads. Though the MRVs of the A3 were intended to overwhelm potential Soviet missile defenses, the U.S. Navy also assessed that the spread of smaller warheads would actually inflict a larger “footprint” of destruction in the target area than the single larger warhead they replaced. These were followed by U.S. MIRVs for both SLBMs and ICBMs by 1970.

The Soviets began work on an MRV version of their large liquid-propellant R-36 (SS-9) ICBM in 1967, which completed flight testing and was deployed in 1970. According to Russian sources, it carried three RVs with warheads of over two-megaton yield each instead of a single RV with a 5- or 10-megaton warhead.

Earlier in the 1960s, the Soviets had also developed a Fractional Orbit Bombardment System (FOBS) version of this missile, which launched a single warhead into a low orbital path. This vehicle could approach its target from an unexpected direction, very different than would be typical for a direct ICBM attack—complicating an attempt to track or intercept it.

Despite this early experience with a type of post-boost vehicle, it took the Soviets until the mid-1970s before their own MIRV ICBMs completed testing, and they did not field a MIRV SLBM until 1977. A key limitation that delayed a Soviet MIRV SLBM was a lack of sufficient “throw weight” for its smaller SLBMs, combined with a lag in developing smaller and lighter warheads. This underscores that first and foremost, MRVs and MIRVs must be based on missile airframes with sufficient payload capacity.

North Korean Missiles with Capacity for Multiple Reentry Vehicles

North Korea has already test-fired two different types of missiles that at least some analysts assess may have sufficient payload capacity to employ multiple RVs in the future—the Pukkuksong-3 SLBM and the larger Hwasong-15 ICBM. The shape of the nose of each of these missiles suggests that it is just a shroud designed to cover one or more RVs inside. It is an open question, however, whether or not North Korea has viable RVs and nuclear warheads small and light enough for these missiles to carry multiple nuclear-armed RVs. A single nuclear-armed RV with one or more lighter decoys or other countermeasures might be a more credible possibility for these missiles, at least for now. As a result, for its initial test of a missile with multiple RVs, North Korea is likely to choose a missile with a larger payload.

In its military parade on October 10, 2020, North Korea displayed two missiles even larger than these predecessors: the Pukkuksong-4 SLBM and a massive ICBM dubbed by some Western experts as the “monster missile” because its probable name of Hwasong-16 was not officially announced by Pyongyang. If the new “monster missile” is ready for flight testing, its dimensions strongly suggest it would have the capacity to loft multiple RVs with warheads of the size consistent with the designs previously displayed by North Korea—meaning it would be a good candidate for North Korea’s first test of an MRV. This payload capacity could also be useful for lofting a maneuvering post-boost vehicle to experiment with the requirements for a MIRV. It could even potentially be used for a FOBS-type design that would allow North Korea to test RVs on varying trajectories while avoiding a particularly provocative target area or flight path, besides the challenge this could pose for U.S. missile defenses. (The potential logic behind a North Korean FOBS program was explained by Joshua Pollack in his recent analysis of the “monster missile.”)

The Importance of Preventing Flight Testing

If North Korea were to successfully test an ICBM with MRV capability—even if such a test were on a “lofted” trajectory into nearby waters, as were the ICBM tests of 2017—this would immediately increase the credibility of the North Korean threat to the continental United States for at least three reasons:

1. Optimistic assessments of the capability of U.S. ground-based anti-missile interceptors to defeat a North Korean ICBM attack would be called into question. MRVs would increase North Korea’s potential ability to saturate the system with too many targets at once, or to simply exhaust the limited supply of interceptors.
2. The perceived effectiveness of UN sanctions in constraining North Korea’s strategic weapons programs would be further undermined. In particular, the apparent challenges that North Korea has faced in obtaining or building large transporter erector launchers (TELs) for ICBMs would become far less meaningful if each North Korean ICBM TEL could carry a missile that can deliver multiple warheads.
3. Questions raised by the limited flight testing of North Korea’s ICBM RVs—one of the key factors hampering the credibility of North Korea’s ICBM force—could be overcome. From a single missile launch, North Korea would be able to gain more RV test data if multiple RVs were employed and would even have the option to conduct a comparative test of multiple variations on an RV design simultaneously. In addition, even if observers ultimately continued to question North Korean ICBM RV reliability and accuracy, multiple chances for each missile would increase the odds of at least one RV striking a target successfully.

Given the value that such testing could have for North Korea, efforts to prevent it will probably have to meet a very high bar to affect Kim Jong-un’s calculus on testing—something that has proven difficult, but not impossible. Kim did completely halt ballistic missile flight testing in 2018 as part of his pivot to diplomacy, and also pledged to refrain from both ICBM and nuclear testing on several occasions that year.

Over the course of 2019, however, Kim resumed ballistic missile flight testing—though short of ICBM-class missiles—and warned that his patience for a change in U.S. policy would only last until the end of 2019. Kim then proclaimed at the year-end ruling party meeting that he no longer felt bound by these pledges, warning that the world would soon witness a new strategic weapon (presumably the “monster missile” but possibly the Pukkuksong-4) and that he would take a “shocking actual action.” Combine those statements with the elevation of Ri Pyong-chol—who formerly presided over North Korea’s missile tests—to Kim’s “right hand man” for military matters and the parade of the new ICBM, and it seems that Kim has all but explicitly committed to ICBM test launches in the months ahead. We cannot be certain the first such launch would include an attempt at testing multiple RV technology, but history and logic suggest such a test is just a matter of time if North Korea does resume ICBM launches.

Ideally, an explicit agreement by North Korea to refrain entirely from ballistic missile testing could be achieved through multilateral diplomacy underpinned by a combination of incentives and implicit threats of consequences. Failing that, it is worth some political and economic costs—and even accepting some risks—for the United States, along with willing allies and partners, to present Kim with a combination of incentives and potential punishments to influence him to delay such testing, given what it could mean for dealing with Pyongyang in the years ahead.

If multiple-warhead technologies are tested, even if North Korea fails in its first attempts, North Korea’s weapons developers will learn from the experience—probably increasing both Kim’s appetite for such testing and the prospects for success when they try again. Once North Korea conducts its first flight test of multiple warhead capability, it will be uncorking a new nuclear “genie”—presenting a whole new level of challenge from North Korea—and that genie would be very difficult, even impossible, to put back in the bottle.