Everybody and his brother is using the announced splash down zones to try to model the North Korean satellite launcher scheduled to be test fired sometime between April 4th and 8th. So I thought I’d put in my two cents worth to get the ball rolling. First, however, I’d like to contrast this launch, which not only has notified stay clear zones, but will take place during daylight hours. I think North Korea is definitely suffering from publicity envy of Iran’s successful launch!
Now on to the missile design. I’m going to proceed along a slightly different path than I think most of my friends are taking and use the sorts of preliminary design techniques I discussed for designing a hypothetical Iranian ICBM well before the successful Safir launch. While these stay clear zones mentioned in my previous post presumably reflect larger areas than North Korea really expects for its uncertainty on where the first and second stages should splash down, they can be used to estimate some of the characteristics of both the flight path and the rocket. We do, however, have to first make some educated guesses on where the first and second stages burn out and the angles of the rocket at those positions.
The most interesting result was for the first stage. If we factor in atmospheric and gravity losses, the first stage has to deliver a total of almost 3.8 km/s and burnout with an angle of roughly 65 degrees (from the horizontal, which is very consistent with the design that finally emerges). This corresponds to a total mass ratio (the total launch mass of the rocket divided by the empty first stage plus fueled second and third plus the payload, which I assume to be 278 kg) of 5.4. It’s going to be a big rocket! I also assumed a modest increase in first stage power with an Isp=250 s and a thrust to weight ratio of 1.5. Then, optimizing for a three stage rocket launching a satellite into orbit with the second stage having an Isp=275 and the third having Isp=250, I get an elliptical orbit with an apogee of over 1100 km and a perigee of 300 km. I also had to have a substantial “negative” loft angle for the last third of powered trajectory, something that is fairly common with, among other rockets, the Soyuz rockets.
The entire rocket weighs in at just over 85 tons. Clearly, I could have gotten a larger payload into a more circular orbit. Just as clearly, if I had used SCUD-type technology the rocket would have had to be substantially bigger. (My estimate for that would have been over 118 tons to put a 100 kg satellite into a similar orbit.) Using SSN-6 technology would allow North Korea to substantially reduce the size of the rocket and all the associated problems designing, building, and launching such a large rocket would entail.
So what does this imply for a North Korean ICBM? If I remove the third stage, which I assumed was SCUD-type technology mainly because I didn’t think the North Koreans would bother expending a lot of effort on it, then the two-stage missile could loft a 1000 kg warhead over the pole a distance of almost 12,000 km. You can download my model of the North Korean satellite launcher here but you have to use the most recent version of my missile simulation program to run it because of changes to the pitch program I had to introduce to simulate satellite launches. (Sorry, there isn’t an updated help file yet, but hopefully if you explore around a bit it should be self explanatory.)
I know of a several people who are working on “designing” the North Korean launcher using these splash-down zones to mine important design information. I expect that their models will be of a higher fidelity. But until we at least see pictures of the launch vehicle I think my method is sufficient for these purposes.