Share This!
Text SizeAAA Share Email

The Predicted Path of North Korea's Launch Vehicle

A comparison of North Korea's Unha-2 and Unha-3 Launchers

North Korea's Unha-3 awaiting launch. Photo: Sungwon Baik, Voice of America

In mid-March, North Korea announced plans to attempt to launch a satellite into orbit sometime between April 11-15 (U.S. time). This announcement has drawn international attention, since the technology for space-launch vehicles can also be used for long-range missiles. The concern is that Pyongyang will learn things that would assist its program to build ballistic missiles for military purposes.

North Korea has not successfully launched a satellite. In its last attempt, in April 2009, the first two stages of its Unha-2 launcher appeared to work as intended, but the third stage did not, and the satellite fell into the ocean. That launch brought an angry response from Japan, since the rocket flew over the main Japanese island. This time, North Korea will be launching from a new location and the rocket will fly south to avoid flying over major land areas.

The 2009 launch also led to a UN Security Council resolution demanding that North Korea not conduct “and launch using ballistic missile technology.” This has led to international condemnation of the planned launch.

North Korea argues that it has the right to a peaceful space program and appears ready to launch. It has announced splashdown zones where the first two stages of the three-stage launcher will fall into the ocean, and has invited the international press to observe the launch.

Photos released on Sunday, April 8, after reporters visited the North Korean launch site, confirm that the Unha-3 launcher currently on the pad is very similar to the Unha-2 that North Korea launched in April 2009. The viewing angle of the two photos is somewhat different, but despite that they show that the stages of the two rockets appear to be the same length and diameter. From a closeup of the third stage, it appears that this stage is longer by 0.3-0.4 meters than the third stage on the Unha-2 in 2009.

This lengthening is consistent with the suggestion that North Korea has increased the amount of propellant in this stage for this launch. It is also consistent with our conclusion that the third stage uses liquid fuel rather than solid fuel, since lengthening the fuel tanks would allow additional fuel to be added.

Computer modeling shows that an Unha-3 that is a slightly modified Unha-2 is compatible with the announced locations of the splashdown zones for the first two stages. These zones differ significantly from those announced for the 2009 launch of the Unha-2. Here’s why:

When North Korea launched eastward in 2009, its rocket gained speed due to the rotation of earth. This extra speed is 0.35 km/s at the latitude of North Korea, compared to the required burnout speed of about 7.8 km/s to place a satellite in orbit at about 500 km altitude (which Pyongyang announced as the planned orbital altitude).

Since North Korea will instead launch in a southern direction this time, it does not get that boost from the earth’s rotation and needs to get more speed out of the launcher to place the satellite in a 500-km orbit.

A natural way to do that would be to increase the amount of fuel in the third stage so that it can accelerate to a higher speed. Adding that extra mass to the upper stage would decrease the speeds at burnout of the first two stages, since they have to lift more mass. These two stages would therefore be expected to splash down somewhat closer to the launch site.

And in fact the numbers work out (see graph below). A computer model of the Unha-2 (red curves), based on the technology and known trajectory parameters for the 2009 launch, can place a satellite into orbit at an altitude of about 500 km, with the stages falling to earth at the locations reported for the 2009 launch (dotted lines). The solid red bars along the horizontal axis show the size of the 2009 splashdown zones that North Korea announced prior to that launch.

The Unha-3 can then be modeled by keeping the first two stages the same as those for the Unha-2, and increasing the fuel in the upper stage as well as its burntime. To place the satellite in a 500-kilometer orbit, the launcher must fly on a somewhat steeper trajectory, as shown by the solid blue line. This configuration can lift the satellite to about 500 km—the altitude North Korea announced—and give it the additional speed it needs to stay in orbit. At the same time, the stages fall in the new splashdown zones (shown by the blue bars under the horizontal axis).

There may be other ways to get the splashdown zones, etc., to work, but this may be the simplest.

This trajectory differs in two ways from that of a typical long-range ballistic missile test (below). First, early in flight, the launcher will fly on a steeper trajectory than would a ballistic missile on an optimum trajectory, and will then flatten out at the altitude at which the satellite is to be released. Second, the burntime will be considerably longer than for a ballistic missile, since the launcher needs a long third-stage burntime to get the satellite to high altitude with high speed.

The launch will still teach North Korea about rocket technology it could use in a missile, but it should be clear from the trajectory whether this was actually an attempt to launch a satellite.

Powered by Convio
nonprofit software