Saturday, June 10, 2017

Could North Korea Fire An H-Bomb Mounted ICBM At The U.S. ? Not Likely!

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Designs of primitive H-bombs (ca. 1952-55) from U.S. (top) and Soviet Union. The necessary mass to build such  weapons would eliminate any rocket the North Koreans currently have - or would have even ten years from now.

Probably the worst nightmare currently for Americans is whether North Korea has the potential to lob a megaton scale warhead - via ICBM  -   on San Francisco or LA. Especially, as I've already shown, e.g.

the intercepting missile system is of dubious benefit given all the tests have been rigged thus far.

What I hope to show in this post is that there is little concern of any such scenario happening any time in the near future.

Recall in an earlier post I cited the Bulletin of the Atomic Scientists (the same group that operates the ‘doomsday clock’ and has now moved it two and a half minutes to midnight) estimating the North possesses twenty weapons in the 10 – 20 kiloton range or roughly the same magnitude of yield as the atomic bomb that reduced Hiroshima to ashes in 1945.

This was based on North Korea being at the most primitive, early stages of A-bomb development. If this is the case,  then according to  Physics Today (April issue) their existing weapons would deliver a yield of roughly 0.08 kt/ kilogram. That is yield per actual bomb mass.  This means they’d need to be able to launch or carry (via plane) at least 100 kg of bomb mass to get an 8 kiloton A -blast, or 200 kg to get a 16 kt yield.   This is feasible with their existing bombers and might also be with their most advanced (solid fuel) missile and certainly with their Taepodong 2 the closest they have to classic ICBMs like the U.S. Atlas V.
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As shown above, the Taepodong-2  is 95 feet long, as compared with the Atlas 85' 6" in nuclear warhead configuration. (The rocket was also used for Project Gemini launches, e.g. sending John Glenn into orbit in 1962.) As can also be seen, the range could be up to 9,300 miles - depending on payload mass and thrust - but more confidently 2,300 miles. Bottom line, this is the missile - if redesigned sufficiently - that could conceivably deliver an H-bomb warhead.

But there are many formidable obstacles.  Bear in mind a fission bomb, such as dropped on Hiroshima and Nagasaki, is not the same as a fusion weapon. The mass, in general, is much less for the former and this is because the fusion bomb design (see very top design image) requires incorporation of a fission bomb to set off the fusion bomb that delivers a megaton yield (this is typically the threshold for an H-bomb).

Thus, the fission bombs (A-bombs) dropped on Japan are basically implosion weapons consisting of a solid metal, plutonium core which is compressed using high explosive lenses. Now, compare this with the U.S. original H-bomb (Teller-Ulam ) design shown in the top graphic. Here, a fission bomb - massive in itself  - sits at one end of a heavy radiation case. At the other end is the thermonuclear charge, basically a cylinder with a neutron shield at one end (liquid deuterium inside it) and a thin 'spark plug' of plutonium mixed with tritium. At detonation, the radiation from the fission bomb reflects off the radiation casing and compresses the thermonuclear charge to many times its original density. (Recall that in fusion much higher density is needed to overcome Coulomb repulsion, i.e. between like charged protons).

The compression then sparks a fission reaction in the mixed Pu- 3 H spark plug that compresses the fusion fuel simultaneously from the other side. The fusion material is now primed for fusion reactions which yield megaton scale release. Andrei Sakharov's design (lower graphic) adopts  similar design except the neutron shield is integrated over the overall design. and the secondary is stripped of its high-explosive components.

The primary limiting factor for all such weapons is the 1.5 m radius (maximum) dictated by the size of delivery vehicles which the North would also need to attend to.

The Physics Today article notes the transition to megaton scale explosive yield is at least two orders of magnitude higher than the 0.08 kt/ kg for the Nagasaki bomb. This means we are talking about 8 kt/ kg . The U.S.  early 50s' 'Mike' design, once refined, came in at 1,360 kg and for the yield to mass ratio given translates into just over a 1 megaton explosion (1.08 Mt). The thrust needed to hurl this warhead mass over even a 5,500 mile range (say to reach California)  is simply too much for the North Koreans to achieve right now, even with their Taepodong -2. We are talking about a total thrust of 360,000 foot-pounds, or 1, 600 kilo newtons. (kN).   This level of thrust required to hurl a 1,200 - 1,500 kg H-warhead is at least ten years down the line and may require total re-design of the Taepodong-2,  say using "strap on" boosters. (E.g. boosters attached to the side of the main rocket).

In the April Pyongyang parade of military wares there were displayed rocket casings and what might be a new, long range missile.  But in any event worries over this becoming an H-bomb - bearing ICBM are premature. The North simply hasn't demonstrated any long range capacity for an ICBM say to match the U.S. Atlas missiles of the 1960s.

Another primary impediment to ICBM deployment is the design of a re-entry vehicle capable of withstanding the fall through the atmosphere as the warhead zeroes in on the target.  After two successive missile tests with unusually steep trajectories, the North asserted the Hwasong -12 "verified the homing feature of the warhead under the worst re-entry situation". Maybe, but it remains to be seen whether that is feasible with a genuine long range missile like the Taepodong-2. If it can master these technological re-entry problems, then according to Lt. Gen. Vince Stewart it will be "on a pathway where this capability is inevitable." In other words, an H-bomb bearing ICBM. (WSJ Editorial, 'North Korea's Missile Advances')

Note the warhead of the Atlas D was originally paired to a  re-entry vehicle (RV) with a W49 thermonuclear weapon, for a combined weight 3,700 lb (1,680 kg) and yield of 1.44 megatons (Mt). The W-49 was later placed in a higher ablative RV, for a combined weight 2,420 lb (1,100 kg) The maximum explosive yield delivered with these later designs was 3.75 megatons - or more than ample to reduce Moscow to radioactive ashes along with the surrounding 150 square miles.

The bigger threat from the North remains its solid fuel Hwasong-12 missile which experts say is capable of flying 2,800 miles or more than enough to reach Guam. (WSJ, 'Pyongyang Revs Up Missile Work',  May 23, p. A10).  The missile is capable of carrying a nuclear warhead, say in the A-bomb regime (0.08 kt/ kg),  but clearly not an H-bomb. The mass constraints are simply too great for that achievement, but one can't discount the possibility the North's rocket and bomb wizards will try working on it.

Right now, though, the danger is that they mount the less massive, A-bomb yield warheads, say onto the Hwasong -12 (or their KN-15) and use them to strike South Korea or Japan - say if provoked by the U.S.  Right now, the North Koreans are estimated to have 1,000 missiles - according to Daniel Pinkston, a North Korean ballistics experts at Troy University in S. Korea. (Ibid.)

Let us hope that when the next crisis unfolds with Kim Jong Un,  Trump doesn't go too far in his bombast, threats,  or worse - doing something really, really stupid to trigger a catastrophe on the Korean peninsula

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