North Korea launched a missile from its east coast into the Sea of Japan at 6:12 am local time on April 5 (5:42 pm on April 4 US eastern time).
US Pacific Command initially identified it as a KN-15 missile, called Pukguksong-2 in North Korea, which is a two stage solid-fueled missile with an estimated range of 1,200 km based on its previous test in February.
Subsequently, however, Pacific Command said it believed the missile was instead an older Scud, and that it may have tumbled, or “pinwheeled,” during flight.
South Korean sources reported the missile flew only about 60 km before splashing down, and reached a maximum altitude of 189 km. And based on Pacific Command’s statement, the flight time was eight to nine minutes.
I used those numbers to investigate the trajectory with a computer model I have of several missiles.
Short-range Scud missile
I found that a Scud missile, with a nominal range of 300 km, could roughly match these numbers if the warhead was lightened somewhat (from 1,000 kg to about 700 kg) and if it was launched on a very lofted trajectory, with a burnout angle only about 5 degrees from vertical. On a 300-km range trajectory, this angle would be roughly 45 degrees (see Fig. 1).
If the missile did not tumble during reentry, I calculate the flight time would be about 7.5 minutes. However, taking account of the additional atmospheric drag due to the tumbling body can increase the flight to about 9 minutes.
In the calculation above, the Scud burns to completion and then begins to pinwheel (the short-range Scud does not separate the warhead from the missile body at burnout).
Longer range missiles could also follow this trajectory if the engines failed partway through powered flight, as long as the missile was on a highly lofted trajectory (5 degrees from vertical) and stopped accelerating after reaching a speed of 1.7-1.8 km/s. It may have been an engine failure that caused the missile to tumble. If the engines did not burn to completion, the warhead may have remained attached to the missile body even for a longer range missile that would separate the warhead under normal operation.
The fact that the missile flew on a nearly vertical trajectory suggests there may have been a problem with the guidance system. If the missile was liquid fueled, North Korea may have shut down the engine when it realized there was a problem. A solid fueled engine could not be shut down in the same way.
If the missile had been a KN-15, the engine would have had to fail about halfway through the burn of the second stage engine. It seems surprising that initial reports identified the missile as a KN-15, since I would have expected sensors could tell whether or not the missile had undergone staging. In addition, the plumes from liquid and solid missiles are different in appearance, so depending on what sensors viewed the launch they should have been able to differentiate a liquid from solid missile. Analyzing these issues may have been what led Pacific Command to change its mind about what type of missile was launched.
Why fire a Scud?
If Pyongyang decided to launch a missile to attract attention in advance of the Trump-Xi summit that starts tomorrow, it may have decided to launch some type of Scud because these are well tested and it could be relatively assured the launch would be successful. The missile may have been a Scud-ER like the four it launched simultaneously in early March.
That fact that it appears to have failed illustrates how uncertain the missile business can be.