UCS Blog - All Things Nuclear, Missile Defense

Reentry of North Korea’s Hwasong-15 Missile

Photos of the Hwasong-15 missile North Korea launched on its November 29 test suggest it is considerably more capable than the long-range missiles it tested in July. This missile’s length and diameter appear to be larger by about 10 percent than July’s Hwasong-14. It has a significantly larger second stage and a new engine in the first stage that appears to be much more powerful.

While we are still working through the details, this strongly implies that North Korea could use this missile to carry a nuclear warhead to cities throughout the United States. A final possible barrier people are discussing is whether Pyongyang has been able to develop a reentry vehicle that can successfully carry a warhead through the atmosphere to its target, while protecting the warhead from the very high stresses and heat of reentry.

Here are my general conclusions, which I discuss below:

  1. North Korea has not yet demonstrated a working reentry vehicle (RV) on a trajectory that its missiles would fly if used against the United States.
  2. However, there doesn’t appear to be a technical barrier to building a working RV, and doing so is not likely to be a significant challenge compared to what North Korea has already accomplished in its missile program.
  3. From its lofted tests, North Korea can learn significant information needed for this development, if it is able to collect this information.
  4. While the United States put very significant resources into developing sophisticated RVs and heatshields, as well as extensive monitoring equipment to test them, that effort was to develop highly accurate missiles, and is not indicative of the effort required by North Korea to develop an adequate RV to deliver a nuclear weapon to a city.

The Hwasong-15 RV

When the photos appeared after North Korea’s November 29 missile launch, I was particularly interested to see the reentry vehicle (RV) on the top of this missile. The RV contains the warhead and protects it on its way to the ground. It is significant that the Hwasong-15 RV is considerably wider and blunter than that on the Hwasong-14 (Fig. 1).

Fig. 1. The RVs for the Hwasong-14 (left) and Hwasong-15 (right), roughly to scale. (Source: KCNA)

This fact has several implications. The new RV can clearly accommodate a larger diameter warhead, and the warhead can sit farther forward toward the nose of the RV. This moves the center of mass forward and makes the RV more stable during reentry. (This drawing shows how the cylindrical nuclear weapon in the US Trident II RV, which was roughly the same size and shape, although much heavier, than the Hwasong-15 RV.)

But the blunter nose on the Hwasong-15 RV also helps protect it from high atmospheric forces and heating during reentry. Here’s why:

As the RV enters the atmosphere, drag due to the air acts as a braking force to slow it down, and that braking force puts stress on the warhead. At the same time, much of the kinetic energy the RV loses as it slows down shows up as heating of the air around the RV. Some of that heat is transferred from the air to the RV, and therefore heats up the warhead. If the stress and/or heating are too great they can damage the RV and the warhead inside it.

A blunter RV has higher drag and slows down in the thin upper parts of the atmosphere more than does a slender RV, which continues at high speed into the thick lower parts of the atmosphere. This results in significantly less intense stress and heating on the blunter RV. In addition to that, a blunt nose creates a broad shock wave in front of the RV that also helps keep the hot air from transferring its heat to the RV.

Fig. 2. This shows two low-drag RVs being placed on a Minuteman III missile, which can carry three RVs. (Source: US Air Force).

A rough estimate shows that if the RVs had the same mass and flew on the same trajectory, the peak atmospheric forces and heating experienced by the Hwasong-14 RV in Fig. 1 would be roughly four or more times as great as that experienced by the Hwasong-15 RV; those on a modern US RV, like that on the Minuteman III missile (Fig. 2), might be 20 times as large as on the Hwasong-15 RV.

The tradeoff of having a blunt warhead is that when the RV travels more slowly through the atmosphere it reduces its accuracy. In order to get very high accuracy with its missiles, the United States spent a tremendous amount of effort developing highly sophisticated heatshields that could withstand the heating experienced by a slender, low-drag RV.

For North Korea, the decrease in accuracy due to a blunt RV is not particularly important. The accuracy of its long-range missiles will likely be tens of kilometers. That means that it would not use its missiles to strike small military targets, but would instead strike large targets like cities. For a large target like that, the reduction in accuracy due to a blunt RV is not significant.

What could North Korea learn from its recent test?

Press stories report US officials as saying that the reentry vehicle on North Korea’s November 29 test “had problems” and “likely broke up” during reentry. If true, this implies that the RV used on this flight could not withstand the strong drag forces as the RV reached low altitudes.

It’s worth noting that the drag forces on the RV during reentry on the lofted trajectory would be more than twice as great as they would be on a standard trajectory of 13,000 km range flown by the same missile (Fig. 3). This is because on the flatter trajectory, the RV flies through a longer path of thin air and therefore slows down more gently than on the lofted trajectory. It is therefore possible the RV might survive if flown on a standard trajectory, but North Korea has not yet demonstrated that it would.

However, given the estimated capability of the Hwasong-15 missile, North Korea appears to have the option of strengthening the RV, which would increase its mass somewhat, and still be able to deliver a warhead to long distances.

Fig. 3. This figure shows the atmospheric forces on the RV with altitude as it reenters, for the highly lofted test on November 29 (black curve) compared to the same missile flying a 13,000 km standard  trajectory (a minimum-energy trajectory, MET). The horizontal axis plots the product of the atmospheric density and square of the RV speed along its trajectory, which is proportional to the drag force on the RV. The calculations in all these figures assume a ballistic coefficient of the RV of 100 lb/ft2 (5 kN/m2). Increasing the ballistic coefficient will increase the magnitude of the forces and move the peaks to somewhat lower altitudes, but the comparative size of the curves will remain similar.

The situation is similar with heating of the RV. The last three columns of Fig. 4 compare several measures of the heating experienced by the RV on the lofted November 29 test to what would be experienced by the same RV on a 13,000 km-range missile on a standard trajectory (MET).

Fig. 4. A comparison of RV forces and heating on the November 29 test and on a 13,000 km-range trajectory, assuming both missiles have the same RV and payload. A discussion of these quantities is given in the “Details” section below.

These estimates show that the maximum heating experienced on the lofted trajectory would be about twice that on a standard trajectory, but that total heat absorbed by the RV on the two trajectories would be roughly the same. Because the heating occurs earlier on the RV on the standard trajectory than on the lofted trajectory, that heat has about 130 seconds to diffuse through the insulation of the RV to the warhead, while the heat on the lofted trajectory diffuses for about 80 seconds (Fig. 5). This somewhat longer time for “heat soak” can increase the amount of heat reaching the warhead, but North Korea would put insulation around the warhead inside the RV, and the heat transfer through insulators that North Korea should have access to is low enough that this time difference is probably not significant.

Fig. 5: This figure shows how the heating rate of the RV surface varies with time before impact on the lofted and standard trajectory. The areas under the curves are proportional to the total heat absorbed by the RV, and is only about 20% larger for the MET. The vertical axis plots the product of the atmospheric density and the cube of the RV speed along its trajectory, which is proportional to the heating rate on the RV.

Fig. 6 shows heating on the two trajectories with altitude.

Fig. 6. This figure shows the heating of the RV with altitude as it reenters.

These results show that if North Korea were able to demonstrate that its RV could survive the peak drag forces and heating on a lofted trajectory, it should also be able to survive those on a standard trajectory. As noted above, the estimated capability of the Hwasong-15 missile suggests North Korea would be able to increase the structural strength of the RV and its heat shielding and still be able to deliver a warhead to long distances.

There is still some question about what information North Korea may actually be getting from its tests. One advantage of testing on highly lofted trajectories that fall in the Sea of Japan is that the RV can presumably radio back data to antennae in North Korea for most of the flight. However, because of the curvature of the Earth, an antenna on the ground in North Korea would not be able to receive signals once the RV dropped below about 80 km altitude at a distance of 1000 km. To be able to track the missile down to low altitudes it would likely need a boat or plane in the vicinity of the reentry point.

Some details

The rate of heat transfer per area (q) is roughly proportional to ρV3, where ρ is the atmospheric density and V is the velocity of the RV through the atmosphere. Since longer range missiles reenter at higher speeds, the heating rate increases rapidly with missile range. The total heat absorbed (Q) is the integral of q over time during reentry. Similarly, forces due to atmospheric drag are proportional to ρV2, and also increase rapidly with missile range.

The calculations above assume a ballistic coefficient of the RV equal to 100 lb/ft2 (5 kN/m2). The ballistic coefficient β = W/CdA (where W is the weight of the RV, Cd is its drag coefficient, and A is its cross-sectional area perpendicular to the air flow) is the combination of parameters that determines how atmospheric drag reduces the RV’s speed during reentry. The drag and heating values in the tables roughly scale with β. A large value of β means less atmospheric drag so the RV travels through the atmosphere at higher speed. That increases the accuracy of the missile but also increases the heating. The United States worked for many years to develop RVs with special coatings that allowed them to have high β and therefore high accuracy, but could also withstand the heating under these conditions.

Based on the shape of the Hwasong-15 RV, I estimate that its drag coefficient Cd is 0.35-0.4. That value gives β in the range of 100-150 lb/ft2 (5-7 kN/m2) for an RV mass of 500-750 kg. The drag coefficient of the Hwasong-14 RV is roughly 0.15.

Did Pilots See North Korea’s Missile Fail during Reentry?

News reports say that a Cathay Airlines flight crew on November 29 reported seeing North Korea’s missile “blow up and fall apart” during its recent flight test. Since reports also refer to this as happening during “reentry,” they have suggested problems with North Korea’s reentry technology.

But the details suggest the crew instead saw the missile early in flight, and probably did not see an explosion.

One report of the sighting by the Cathay CX893 crew gives the time as about 2:18 am Hong Kong time, which is 3:18 am Japan time (18:18 UTC). According to the Pentagon, the launch occurred at 3:17 am Japanese time (18:17 UTC), which would put the Cathay sighting shortly after the launch of the missile from a location near Pyongyang, North Korea.

Since the missile flew for more than 50 minutes, it would not have reentered until after 4 am Japanese time. Given the timing, it seems likely the crew might have seen the first stage burn out and separate from the rest of the missile. This would have happened a few minutes after launch, so is roughly consistent with the 3:18 time.

The New York Times posted a map that shows the track of flight CX893. It shows that the flight was over northern Japan at 6:18 pm UTC (Fig. 1) and the pilots would have had a good view of the launch. By the time reentry occurred around 7:11 pm UTC, the plane would have been over mid-Japan and reentry would have occurred somewhat behind them (Fig. 1).

Fig. 1. Maps showing the location of flight CX893 shortly after launch of North Korea’s missile near the red dot on the left map, and at the time of reentry of North Korea’s missile, which took place near the red dot on the right map. (Source: NYT with UCS addition)

Burnout of the first stage would have taken place at an altitude about 100 km higher than the plane, but at a lateral distance of some 1,600 km from the plane. As a result, it would have only been about 4 degrees above horizontal to their view—so it would not have appeared particularly high to them. Ignition of the second stage rocket engine and separation of the first stage may have looked like an explosion that caused the missile to fall apart.

There are also reports of two Korean pilots apparently seeing a “flash” about an hour after the missile’s launch, which would be consistent with the warhead heating up during reentry, since the missile flew for 53-54 minutes. Neither reported seeing an explosion, according to the stories.

Chinese Military Strategy: A Work in Progress

Chinese President Xi Jinping, also general secretary of the Communist Party of China (CPC) Central Committee and chairman of the Central Military Commission (CMC), presents the heads of the People’s Liberation Army (PLA) Academy of Military Science with the military flag in Beijing, capital of China, July 19, 2017. (Xinhua/Li Gang)

Several years ago UCS reported China could put its nuclear weapons on high alert so they could be launched on warning of an incoming attack. Last week I had the opportunity to speak with some of the authors of The Science of Military Strategy: the authoritative Chinese military publication that was the source of the information in our report.

In a lively discussion, most of which took place between the authors themselves, I was able to confirm our original report is accurate. But I also learned more about how and why The Science of Military Strategy was written and what that can tell US observers about the broader context of how military thinking is evolving in China.

What it means to say China “can” launch on warning.

As of today, China keeps its nuclear forces off alert. The warheads and the missiles are separated and controlled by different commands. The operators are trained to bring them together and prepare them for launch after being attacked first.

China’s nuclear arsenal is small. Reliable estimates of the amount of weapons-grade plutonium China produced and the amount of plutonium China uses in its warheads tell us China has, at most, several hundred nuclear warheads. It has even fewer long-range missiles that could deliver those warheads to targets in the United States.

Because China’s nuclear arsenal is small and kept off alert some Chinese military strategists worry it could be completely wiped out in a single attack. Their US counterparts have told them, in person, that the United States will not rule out attempting a preemptive strike at the beginning of a war. The question for Chinese strategists is whether or not they should do something to mitigate this vulnerability. Many believe the risk of a major war with the United States is low and the risk of a nuclear war is even lower.

For Chinese strategists who don’t share that optimism, there are two basic ways to address their vulnerability. The first would be to significantly increase the size of China’s forces. Chinese nuclear weapons experts told me that would require a lot of time and considerable effort. They would need to resume producing plutonium for weapons and may also need to resume nuclear testing. The economic costs would be considerable. The diplomatic costs would be even greater.

The second way to avoid the risk of allowing an adversary to think they can wipe out China’s nuclear force with a preemptive strike is for China to put its forces on alert and enable them to be launched on warning of an incoming attack. That would require the development of an early warning system. It may also require upgrading China’s nuclear-capable missiles. One Chinese missile engineer explained that China’s existing missiles are not designed to be kept on continuous alert.

Either option would significantly alter China’s nuclear posture. But the latter may also require a consequential change in China’s nuclear doctrine.

China’s political leaders promised the world they would never, under any circumstances, be the first to use nuclear weapons. Wouldn’t launching on warning of attack, before any damage is done, violate that promise? The answer is not as obvious to Chinese policy-makers as it probably seems to their American counterparts, who don’t believe in the efficacy or credibility of a no first use pledge in the first place.

What I learned in my conversation with the authors of The Science of Military Strategy is that when they wrote that China “can” launch on warning of an incoming attack they were not saying China has the technical capability to do so,  nor were they announcing the intention to implement a launch on warning policy. They were simply declaring that, in their view, China could launch on warning—before their missiles were destroyed—without violating China’s no first use pledge.

Shouldn’t they have made that more explicit?

The authors told me, in response to a direct question, that they did not consider the impact of what they were writing on external audiences. That does not mean they were unaware non-Chinese might read it, just that they weren’t writing for them. The Science of Military Strategy is  an institutional assessment of China’s current strategic situation prepared for the consideration of the rest of China’s defense establishment and its political leadership. Those two audiences wouldn’t need to be told what the “can” in an Academy of Military Science (AMS) statement on launch on warning was referencing. They would already understand the context. As the authors explained, AMS is not responsible for making technical assessments of China’s capabilities, nor does it make public announcements about Chinese military policies or the intentions of China’s political leadership.

It’s difficult for many US observers to imagine that Chinese open source publications like The Science of Military Strategy aren’t just another form of Chinese Communist Party (CCP) propaganda. That’s understandable given Chinese government controls on speech and publication. But even in a relatively closed and tightly controlled polity like China’s, professionals still need to engage in meaningful discussion, including military professionals. Understanding that internal discussion from abroad requires more than parsing the language in Chinese publications. It also requires a sufficient degree of familiarity with the social, institutional and sometimes even the personal factors that define the context within which Chinese discussions of controversial topics – like nuclear weapons policy – take place.

Regular interaction with Chinese counterparts is the only way to acquire this familiarity. Unfortunately, both governments make that much more difficult than it needs to be. And language is still a significant barrier, especially on the US side.

Pessimism on US-China Relations

Most of my Chinese colleagues believe the intergovernmental relationship between China and the United States is deteriorating. The cooperative relationship of the 1980s and 1990s gradually gave way to an increasingly competitive relationship over the past two US administrations. The new edition of The Science of Military Strategy, composed over an 18-month period prior to its publication in 2013, addresses new issues that might emerge if this trend continues, and the relationship moves from competition toward conflict.

There is no fixed schedule for putting out a new edition. According to a general who was also involved the production of two prior editions, the first addressed concerns related to China-USSR relations. The second responded to the so-called “revolution in military affairs” exemplified by the new technologies used in the 1991 Gulf War. The current edition had no equally specific point of origin. It was, in the Chinese general’s words, more “forward-looking.” And as the Chinese military looks forward, its relationship with the United States looms large on the horizon.

None of the authors felt China’s overall military capabilities were remotely comparable to those of the United States. One of the more interesting barometers they used was the average annual salary of an ordinary soldier. All of the authors agreed this gap is unlikely to be closed in the foreseeable future. China still needs to focus its military development in select areas. Having a clearer understanding of what China’s future military challenges might be—an understanding AMS is charged with articulating—can help Chinese decision-makers set priorities.

That one of those priorities is addressing the vulnerability of China’s nuclear forces to a US preemptive attack is a troubling indicator of deteriorating relations.


North Korea’s Longest Missile Test Yet

After more than two months without a missile launch, North Korea did a middle-of-the-night test (3:17 am local time) today that appears to be its longest yet.

Reports are saying that the missile test was highly lofted and landed in the Sea of Japan some 960 km (600 miles) from the launch site. They are also saying the missile reached a maximum altitude of 4,500 km. This would mean that it flew for about 54 minutes, which is consistent with reports from Japan.

If these numbers are correct, then if flown on a standard trajectory rather than this lofted trajectory, this missile would have a range of more than 13,000 km (8,100 miles). This is significantly longer than North Korea’s previous long range tests, which flew on lofted trajectories for 37 minutes (July 4) and 47 minutes (July 28). Such a missile would have more than enough range to reach Washington, DC, and in fact any part of the continental United States.

We do not know how heavy a payload this missile carried, but given the increase in range it seems likely that it carried a very light mock warhead. If true, that means it would not be capable of carrying a nuclear warhead to this long distance, since such a warhead would be much heavier.

No, Missile Defense Will Not Work 97% of the Time

In an October 11 interview on Fox News, President Trump claimed:

We have missiles that can knock out a missile in the air 97 percent of the time. If you send two of them, they are going to get knocked down.

This is not true. At least not in any relevant way.

The only homeland missile defense system is the Ground-based Midcourse Defense (GMD) system, which I’ve written plenty about here in these pages, and have co-authored a recent report about. If you’ve been following along, you’ll know the president’s statement was clearly untrue.  I’ll explain why.

What does the actual test record show?

The GMD interceptors have succeeded in destroying the target in nine out of 18 tests since 1999 (50%).  They have destroyed their target in four out of 10 tries (40%) since the GMD system was nominally deployed in 2004. They have destroyed their target in two of the last five tests (40%).

So there is no basis to expect it to work any better than 40 to 50% of the time even under the most generous and easiest conditions—former Pentagon testing agency director Phil Coyle calls the test conditions so far as “scripted for success.”

While the test record says something about the GMD’s capabilities under scripted conditions, the real world will be more complex and challenging. The Pentagon’s highest testing official assessed in 2014 that the test program was “insufficient to demonstrate that an operationally useful capability exists.” More on this later.

But for sake of argument, say the “single shot kill probability” has been determined via tests to be 40 to 50% in those optimistic conditions. Because reliability is low, the US would fire multiple interceptors at the missile to try to boost the system’s effectiveness. Using four-on-one targeting, and a 40 to 50% chance that a given interceptor would work, this leads to a 6 to 13% chance that the warhead gets through.

Real-world conditions

But this isn’t the right question. If it came down to a nuclear attack, would North Korea send just a single missile, and choose the most convenient conditions? That seems unlikely. Let’s say the salvo is five incoming missiles. In that case, with an interceptor kill probability of 40 to 50%, using four interceptors on each missile, the probability that one warhead gets through is 28 to 50%. Uncomfortably high.

I could not stress more that this is a best-case scenario. It assumes that:

1) Failures are uncorrelated and not, e.g., a design flaw common to all interceptors, such as the guidance system issues that took nearly a decade to diagnose and fix,

2) The intercept attempts take place under simplified conditions and that the system is not being stressed as it would in a real-world situation, and

3) The system successfully identified the five real targets from among decoys. If the system cannot distinguish decoys from the real targets, it will have to engage them all, quickly depleting the interceptor inventory. These do not need to be the Ferraris of decoys to be an issue. Some of the GMD intercept tests have included decoys, but all of those have been designed to be easily distinguished from the target warhead.

In short, one can construct situations under which missile defense might destroy missiles: a small salvo of missiles sent without countermeasures and under the limited range of conditions under which the system has been tested. The problem is that these are not by any stretch the most *likely* situations. A potential adversary has every incentive to make the attack as difficult as possible to intercept if he is going to initiate World War Three.

Note that even if the president were instead talking about one of the missile defense systems that has a better and more complete test record, such as THAAD, the issues with not having been tested in operationally realistic conditions is the same. And because THAAD defends against shorter-range missiles from North Korea, which are cheaper and more plentiful, it has the additional issue that it may be overwhelmed even if it is able to discriminate between decoys and real targets. There just may be too many targets.

Why is this dangerous?

The best-case scenario is that President Trump is trying to avoid a confrontation by allowing himself to save face: he has declared that North Korea must not be able to threaten the US mainland with nuclear-armed missiles. Or that he hopes such statements would help dissuade North Korea from considering an attack.

Certainly worse than this is the possibility that Trump actually believes that strategic missile defense provides credible protection and he has not been advised correctly. One hopes he is provided accurate information by stewards of these programs, although at least in public, government official often describe the GMD system as much more capable than it has been demonstrated to be.

This is dangerous, because common sense would say that if we have spent $40 billion on a missile defense system that the US has claimed has been “operational” for going on fifteen years, it must “work.” But it doesn’t. Look at the test record.

The problem is that believing missile defense works when it doesn’t can lead you to take actions that make you need it, and then it can’t help you.

North Korea’s Sept. 15 Missile Launch over Japan

North Korea conducted another missile test at 6:30 am September 15 Korean time (early evening on September 14 in the US). Like the August 28 test, this test appears to have been a Hwasong-12 missile launched from a site near the Pyongyang airport. The missile followed a standard trajectory—rather than the highly lofted trajectories North Korea used earlier this year—and it flew over part of the northern Japanese island of Hokkaido (Fig. 1).

Fig. 1. Approximate path of the launch.

The missile reportedly flew 3,700 kilometers (km) (2,300 miles) and reached a maximum altitude of 770 km (480 miles). It was at an altitude of 650 to 700 km (400 to 430 miles) when it passed over Hokkaido (Fig. 2).

Fig. 2. The parts of Hokkaido the missile flew over lie about 1,250 to 1,500 km (780-930 miles) from the missile launch point.

The range of this test was significant since North Korea demonstrated that it could reach Guam with this missile, although the payload the missile was carrying is not known. Guam lies 3,400 km from North Korea, and Pyongyang has talked about it as a target because of the presence of US forces at Anderson Air Force Base.

This missile very likely has low enough accuracy that it could be difficult for North Korea to use it to destroy this base, even if the missile was carrying a high-yield warhead. Two significant sources of inaccuracy of an early generation missile like the Hwasong-12 are guidance and control errors early in flight during boost phase, and reentry errors due to the warhead passing through the atmosphere late in flight. I estimate the inaccuracy of the Hwasong-12 flown to this range to be likely 5 to 10 km, although possibly larger.

Even assuming the missile carried a 150 kiloton warhead, which may be the yield of North Korea’s recent nuclear test, a missile of this inaccuracy would still have well under a 10% chance of destroying the air base. (For experts: This estimate assumes the air base would have to fall within the warhead’s 5 psi air blast radius, which is 3.7 km, and that the CEP is 5 to 10 km.)

Heating of the reentry vehicle

As I’ve done with some previous tests, I looked at how the heating experienced by the reentry vehicle (RV) on this test compares to what would be experienced by the same RV on a 10,000 km-range missile on a standard trajectory (MET). My previous calculations were done on North Korea’s highly lofted trajectories, which tended to give high heating rates but relatively short heating times.

Table 1 shows that in this case the duration of heating (τ) would be roughly the same in the two cases. However, not surprisingly because of the difference in ranges and therefore of reentry speeds, the maximum heating rate (q) and the total heat absorbed (Q) by the RV on this trajectory is only about half that of the 10,000 km trajectory.

Table 1. A comparison of RV heating on the September 15 missile test and on a 10,000 km-range trajectory, assuming both missiles have the same RV and payload. A discussion of these quantities can be found in the earlier post.

So while it seems likely that North Korea can develop a heat shield that would be sufficient for a 10,000 km range missile, this test does not demonstrate that.

North Korea’s Missile Test over Japan

Yesterday’s missile launch by North Korea is reported to have been launched from a site near the capitol city of Pyongyang (Sunan) and landed 2,700 kilometers (km) (1,700 miles) to the east after flying over part of the Japanese island of Hokkaido. The missile reportedly flew to a maximum altitude of about 550 km (340 miles), reaching Hokkaido after about eight minutes of flight and splashing down after 14 to 15 minutes.

Fig. 1 shows a possible trajectory for the flight, although it is possible the missile flew somewhat further north and passed over more of Hokkaido.

Fig. 1 (Source: Google Earth)

The launch appears to have been of a Hwasong-12 missile, since it is the only known missile able to reach this distance. The range of this test, however, was much shorter than that of the May 14 test of the Hwasong-12, which would have had a range on a standard trajectory of about 4,800 km (3,000 miles).

What accounts for the shorter range?

One possibility is that the missile was flown with a much larger payload on this flight than on the May 14 test. However, even assuming the May 14 test only carried a payload of 150 kg (corresponding to an empty RV), this launch would have required a payload of about 1,300 kg to give the reported trajectory. That seems unlikely.

A second possibility is that it was flown on a depressed trajectory to reduce the range from 4,800 to 2,700 km. However, that would require a severely depressed trajectory with a burnout angle below nine degrees and a maximum altitude of only 150 km (95 miles). That would also give flight times that were much shorter than those reported.

A more likely reason for a shorter range is a shorter burn time for the engines, either due to North Korea terminating the thrust early to reduce the range, or possibly due to a mechanical problem. In particular, I find if the burn time of the engine is reduced by about eight seconds from the time of about 151 second for the May 14 launch, the missile will fly on the reported trajectory (Fig. 2).

If flown on a standard trajectory (a “minimum-energy trajectory”), a missile with this range would reach a maximum altitude of about 630 km (390 miles) with a burnout angle of 38.1 degrees. The reported altitude of 550 km on yesterday’s launch would mean it was slightly depressed from normal, with a burnout angle of 33.6 degrees. This amount of depression does not seem particularly significant, and may not have been intended.

Fig. 2. The apparent trajectory of yesterday’s launch. Cape Erimo on Hokkaido is at a range of about 1525 km.

A missile on this trajectory would reach the closest part of Hokkaido after eight minutes, which seems to agree with reports. It would pass over Cape Erimo after 9 minutes, and would splash down at 15.5 minutes

Flying over Japan

Yesterday’s launch was the first time North Korea flew a ballistic missile over Japanese territory, although in 1998 and 2009 it launched rockets that overflew Japan on failed attempts to put satellites into orbit. It has gone to some lengths to avoid flying over Japan, by launching its missile tests on highly lofted trajectories so they will land in the Sea of Japan. In addition, it has directed its more recent satellite launches to the south, even though it is preferable to launch to the east—over Japan—since it allows the rocket to gain speed from the rotation of the earth.

After its threats of firing Hwasong-12 missiles near Guam, it is interesting that North Korea fired this missile to the east rather than in the direction of Guam, which might have been interpreted as an attack despite the short range. The missile also appears to have flown in a direction that did not pass over highly populated parts of Japan.

It is not clear what new North Korea would have learned from this launch that is relevant to a long-range missile. It would not have been useful in simulating the reentry forces and heating of a long range missile; in particular, the heating would have been only about half of that on a 10,000 km range missile.

The launch could be useful in getting information about reentry on a standard, non-lofted trajectory with a missile that could reach Guam, although that would require a missile with about 3,400 km range rather than the 2,700 km of this flight.