While North Korea’s missile test on August 28, reportedly of the intermediate range Hwasong-12 (KN-17), broke up into three pieces during flight only flying 2,700 kilometers—far short of its 4,000 kilometer range—the launch was a disturbing development in Pyongyang’s continuing program to develop long-range missiles.

Why Fly Over Japan?

The test launch had two primary objectives. First, Kim Jong Un likely sought to take measure of the international community’s response to the overflight of Japanese territory. Though North Korea twice before attempted to launch a satellite using a flight path that passed over Japan, the Hwasong-12 is the first ballistic missile to overfly the island nation. If the US response to the test firing is judged to be mild by Pyongyang, North Korea may feel that future flights will be similarly accepted by Washington and its regional allies.

Second, the flight was likely intended to evaluate the missile’s performance and reliability under operational conditions. Before the Hwasong-12 and the intercontinental range Hwasong-14 can be deployed, engineers must demonstrate that they work reliably when flown to maximum range. To date, the two missiles have been launched on steep flight paths that reach high altitudes, ensuring that the mock warheads land in the sea just short of Japan. Much can be learned from the tests, but full-range flights replicating the conditions a combat-capable missile would experience are needed. This flight begins to address that requirement.

Is North Korea Testing a Post-Boost Vehicle (PBV)?

The most recent Hwasong-12 flight ended with the mock warhead landing in the Pacific Ocean about 2,700 km from the launch site, well short of its maximum range. North Korean engineers may have shut the Hwasong-12’s engine down early, resulting in a shorter flight. Turning off the engine roughly five seconds early would yield a range and apogee matching the reported values. Or the payload mass of the first test may have been considerably lighter than that of North Korea’s nuclear warhead, allowing the missile to achieve a much higher altitude, thus exaggerating the derived maximum range when the missile is flown on a standard flight path. For example, if the May flight carried a payload of 250 kg, and the most recent test fitted the Hwasong-12 with a 1,250-kg package, the range is reduced to about 2,700 km, with a peak altitude of roughly 550 km. Finally, the Hwasong-12 may have been flown on a non-optimal trajectory, one that was steeper or flatter than a normal flight path employed to maximize achievable range. However, the reported apogee of the Hwasong-12 flight is inconsistent with this explanation.

An alternative disturbing hypothesis is that tests of the missile have included a small post-boost vehicle (PBV) to provide extra boost to the payload after the main stage is discarded. The May 14 test of the Hwasong-12 was previously modeled, but could not replicate the flight’s reported apogee and range, without the addition of a small PBV. The increased velocity provided by the PBV allows the payload to reach an apogee of 2,000 km and travel about 700 km in range. The hypothesis is consistent with observations made by others, including Norbert Brugge, who identified the existence of fuelling ports located just under the Hwasong-12’s reentry vehicle.

There are logical reasons for employing a PBV. In addition to boosting range, it can be used to make fine adjustments to payload’s velocity after engine shut-down. ICBMs fielded by the US, Russia, France and China all employ PBVs to achieve better accuracy. PBVs also provide a platform to carry a warhead, plus lightweight decoys or other penetration aids capable of impairing effectiveness against missile defenses.

The presence of a PBV on the Hwasong-12 is just a hypothesis for now, although reports that the missile “broke into three pieces” are consistent with PBV engine failure. As the PBV separates from the main booster, its engine should activate and accelerate the reentry vehicle to the pre-defined velocity. If the PBV’s engine fails to initiate, and the reentry vehicle separates from the PBV, all three components (main booster, PBV and warhead) follow an approximately similar trajectory. To a distant radar, the missile appears to separate into three disparate pieces at the end of boost for no obvious reason. Without the added velocity provided by a properly functioning PBV, the reentry vehicle along with the main stage and PBV, land in the ocean 2,700 km from the launch site—well short of the missile’s maximum range. In other words, the dead weight of the non-functioning PBV, if it was fully fuelled, is equivalent to launching the missile with a 1,200 to 1,300 kg payload, as described above.

While it is impossible to know with certainty why the most recent test flew only 2,700 km, if it was the result of a failed PBV, its use on North Korea’s long-range missiles is ominous. It is another sign that Pyongyang is deadly serious about developing and fielding nuclear-tipped missiles capable of striking the US mainland, and critical US military bases in the Pacific Ocean. North Korea has much work remaining—perhaps a year or two—before it matures the technologies and systems needed to credibly threaten the US.