Friday, September 09, 2016

What have the North Koreans really tested?

This part had me scratching my head. The problem with the North Koreans is that they have a very different sense of what some of these terms mean, and it is hard to understand what they are really saying.

Based on the seismic estimates they have boosted their yields by a factor of 1.5 - so they have something like 15kT-20kT depending on what you assume as the yield for the last test. That much the seismic data tells us but beyond that...

It is fair to assume that at least some more work has happened on the core physics package. Some people had speculated in the past that DPRK had gone for a levitated pit design with some Fusion-Boosted-Fission capability. The original package that was tested in January probably was powered off electronics that wasn't particularly compact.

A warhead basically consists of a package, associated electronics and a casing.  The exact choice of propulsion system usually limits the payload of the missile. The warhead has to match geometrically and weight-wise the payload limits.

In order to run the large number of detonators in their explosive lenses the North Koreans need electronics that can pump a significant amount of current at the exact time of the detonation. This kind of thing needs some hefty batteries and that is a problem.

Usually the warhead has to have a relatively thick and heavy shell to damp out the vibrations that couple from the launch platform to the physics package. Additionally depending on how efficient the fission-fusion coupling is you may need different amounts of Plutonium and Uranium. Both of those metals are heavy.

A sensible design approach for warheads is to set a budget (size and weight) for casing, physics package and electronics. When the North Koreans first tested in January - I think it was a proof of principle test of boosting at scale. They had smaller scale boosting experiments but the January test was the first time they had pushed the envelope on package yield. But it likely that their electronics was outside the budget set for size and weight. That is probably what they have revised for this test.

My guess is that the North Koreans have managed to construct a lighter weight electronics - specifically something with a more acceptable battery weight. The reason why this is important - from a threat estimation p.o.v - is that this points an increased North Korean ability to make a more compact weapons. For a deep dive on the issue of explosive lens detonators and associated power supply requirements look here (note the last paragraph about the need for redundancy - this part is actually more complicated than described here because you also have to design failsafe mechanism which prevent accidental detonation). I don't know what if any failsafes the North Korean devices have.

It is important to note the time spacing of the tests. The difference in time reflects the difficulty the North Koreans have getting their hands on boosting materials (LiD or DT boost gas).  I realize a DT boost gas device would likely produce a non-ideal package weight and dimension. but I am leaving the door open in case the entire North Korean claim about a warhead capable package is pure marketing. (Again not entirely implausible).

What this does not mean  - the recent test does not mean that North Korea actually possess a warhead capable of surviving the rigors of launch, ballistic trajectory and re-entry. There is no evidence of that at this time.

And for reference - the only thing that the DPRK has proven with repeated missile tests (including the "SLBM" tests) is they seem to have some success at ganging rocket motors together, at popping a missile out of a submerged tube, and some success injecting the missile into a ballistic trajectory. While I believe Dave that the last KN-11 test gives them the ability to study the physics of re-entry, I feel North Korea has not yet demonstrated the ability to launch a missile from a submerged platform and inject the missile into a specific trajectory. Being able to hit the sea of Japan isn't really the same as making a certain CEP.

I want to add one more note about the issue of cold launch and then injecting into a trajectory. Please look at the images of the KN-11 launch - the missile is coming out of the water at very different angles. Dave has brought the issue of fins being added to the KN-11 in the latest photos, that tells me the DPRK guys were having issues with the behavior as it breached the surface, Usually when a missile pops out of its canister under water it sits in a bubble of air. That bubble can introduce a small amount of rotation. One needs to sense that rotation and correct it, It is a significant amount of work to ensure that the missile pops out at exactly the right angle to inject into a particular trajectory at a particular speed.  I am not saying DPRK can't do it - only that it is more work than can easily be squeezed into a few weeks of work.


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