The regime in North Korea keeps launching missiles – or trying to – and threatening its neighbors with nuclear holocaust. On Tuesday, reports emerged that North Korea has successfully developed a miniaturized nuclear weapon it can fit atop one of its rockets.

The Trump administration has threatened to respond to any further tests with "fire and fury" to which North Korea responded, through state media, by floating the possibility of a pre-emptive strike on Guam. But amid these heated exchanges and military posturing, there is a technical discussion that is often overlooked.

For some clarity, we turned to John Schilling, an aerospace engineer specializing in rocket propulsion. As a key contributor to the North Korea-monitoring website 38 North, Schilling is among the best versed in missile technologies outside of the Pentagon.

How would fielding an ICBM change the geopolitical calculus when nations deal with North Korea?

North Korean Musudan intermediate-range ballistic missile during launch, June 2016. Via KCNA.

The short answer is, North Korea wants nuclear-tipped ICBMs to directly deter the United States from attacking North Korea, or otherwise imposing a Gaddafi-esque regime change. Most every credible war plan against North Korea, offensive or defensive, hinges on the alliance of the United States, South Korea, and Japan (even if the Japanese don't contribute combat forces, their ports and airbases are critical for logistical support). Right now, North Korea can directly threaten South Korea and Japan with nuclear attack, but the United States can stand back at a safe distance and promise massive retaliation against the North at essentially no cost or risk. With ICBMs as well as shorter-ranged missiles, North Korea can separately deter each member of the alliance, and cause each member of the alliance to doubt the commitment of the others. Would the United States really risk San Francisco to avenge Tokyo?

Coupled with a bit of diplomacy, this could enable North Korea to break one or two partners loose from the alliance on the grounds that their cities are at risk in a fight that maybe isn't their top priority, and so stop a war that would otherwise topple the North Korean regime.

How has a nation like North Korea, isolated as it is, advanced these technologies? Where does Kim Jong-un's regime get the money for it?

We know that North Korea has collaborated with Iran and Pakistan in developing nuclear and/or missile technology in the past, and to some extent this may be ongoing. They also received assistance from Russian technical and probably military personnel during the Yeltsin era—not as a matter of Russian government policy, but because the Yeltsin administration wasn't paying everyone's salary regularly and wasn't keeping track of what they did on the side to make up for that. That collaboration probably stopped with Putin's ascension. And there have been black-market deals elsewhere: secondhand missiles from Egypt and Syria that could be reverse-engineered, missile transporters from China, maybe some technology from Ukraine.

"Would the United States really risk San Francisco to avenge Tokyo?"

But we must give credit where it is due. Much of this—in recent years, probably most of it—has been North Korea's own doing. They are a minor and relatively backwards industrial power, but they have devoted about 25 percent of their entire gross domestic product to defense, and much of that to missiles. The industrial resources and the general level of technology available for developing weapons in North Korea today, at crippling expense to the civilian economy, are roughly equivalent to the French defense industry in the 1960s. And France in the 1960s was building a diverse assortment of nuclear weapons.

Does anyone really know how far off North Korea is to fielding functional nuclear ICBM? What's the conventional wisdom, accepted range or best guess?

North Korea could probably test an ICBM at any time, but it is highly unlikely that the initial test would be a complete success. Given the amount of testing that will likely be required and the competing demands on North Korean resources (such as the submarine-launched missile program), I and most everyone else I have talked to expect that a North Korean ICBM will most likely enter operational service shortly after 2020. If I had to make specific bets, I would wager on first (unsuccessful) test sometime next year and initial operational capability in late 2021, but there is no way even the North Koreans can predict this with any precision.

Was 2016 a breakthrough year for North Korean missile technology?

It is difficult to be certain how much of what we saw in 2016 was genuinely new or just the first public demonstration of capabilities that had been developed earlier, but there were some clear milestones. Their two successful nuclear tests should end any real doubt that they can produce reliable nuclear (but not thermonuclear) warheads, and the Pukguksong-1 and -2 solid-fuel missiles will provide a much more robust regional warfighting capability than the older Scud derivatives. Their second satellite launch should prove that 2012 was not a fluke and North Korea can build large, powerful, multistage rockets and missiles on demand.

North Korea could probably test an ICBM at any time, but it is highly unlikely that the initial test would be a complete success.

Their increasing openness in revealing ground tests may reflect older work from their point of view, but enhances the credibility of their ICBM program. Similarly, we know they have been conducting increasingly realistic training exercises for nuclear warfighting for many years, but 2016 is when they did their first openly public salvo-firing demonstration.

Still, it wasn't an entirely positive year for the North. The Musudan missile, which may have been a key part of North Korea's strategic deterrent for nearly a decade, was first put to the test in 2016 and proved to be mostly a dud, with only one success in eight launches.

Update: This year is also shaping up as a big one for North Korea's nuclear capability. On September 3, 2017, North Korea successfully detonated what experts believe to be a thermonuclear bomb, the biggest test since the Chinese conducted its own test in 1992.

What is the significance of the North Koreans use a 2-stage rocket, like the KN-14 missile displayed in parades and photo ops, as opposed to a 3-stage version?

NurPhoto Getty Images

The advantage is reliability. North Korean single-stage liquid-fuel rockets, even of well-established design, only work about 80 to 90 percent of the time under combat conditions. Stage separation mechanisms are typically as big a contribution to reliability [or lack thereof] as the rocket stages themselves. They are simpler than rocket stages, but harder to test realistically on the ground. For a single-stage missile, that level of reliability is tolerable, and if it doesn't work you just launch another one. If your missile needs three rocket stages and two separation mechanisms to work, then with typical North Korean engineering you're at about 40 percent reliability for the complete system. Cutting it down to two stages gets that up above 60 percent.

But even with the lighter and more advanced structural design, the two-stage missile won't have the same range. Our best estimate is that the three-stage KN-08 will be able to deliver nuclear warheads to a range of about 12,000 kilometers—enough to reach the U.S. East Coast—while the two-stage KN-14 with a 10,000-kilometer range will be limited to West Coast or Rocky Mountain targets. It is not clear whether they will continue to develop the less reliable KN-08 as a way to have some chance at striking the US leadership in Washington, DC, but that does seem to be an important part of their propaganda at least.

There's a lot of worry that North Korean rockets used for space launch can be adapted to carry weapons. Is that fear valid?

With the exception of the reentry vehicle, the technology required for first-generation ICBMs is very similar for that of space launch. [You need] large liquid-propellant rocket engines … lightweight structures, stage separation mechanisms, and reliable and reasonably precise guidance.

The specific implementations, however, can be quite different. Almost certainly the North Koreans have in the past used their space program as a way to build expertise in long-range missile technologies, but the space and missile programs appear to be moving in different directions at present. The Unha-3 space launch vehicle, which we initially mistook for an ICBM prototype, has design features that make it ill-suited for missile applications—in particular, the low thrust and long burn time of its upper stage works quite well for delivering small satellites, but would be very inefficient for lofting a heavier warhead on a typical ICBM trajectory. Also, the Unha-3 is already inconveniently large for an ICBM. it could never be made mobile nor effectively concealed, which is essential for any system North Korea hopes to survive the opening minutes of a war.

The North Koreans are big on mobile launch systems. What advantage does this give them?

Hardened missile silos are an increasingly obsolete technology in the era of precision-guided conventional weapons capable of delivering a hard-target penetrator directly onto the silo door. They are still of some use for nations like the U.S. and Russia, with the strategic depth to keep them thousands of kilometers from any foe, but North Korean missile silos would be unlikely to survive the first hour of any war.

Putting a 40-ton liquid-propellant missile on a mobile transporter is something nobody has ever done before.

Every strategic missile we have seen North Korea develop, display, or test, has been designed for mobile deployment. And putting a 40-ton liquid-propellant missile on a mobile transporter is something nobody has ever done before. North Korea would not have done that if they didn't think it was vital, and I expect that in the longer term they will want even their ICBMs to be highly mobile solid-propellant systems like those of China.

KN-15 missile transporter. AFP PHOTO / KCNA VIA KNS

How worried should U.S. and South Korean military planners be when they see a new missile on parade?

North Korea definitely parades missiles that it doesn't operate yet, and it may parade missiles that it will never operate. The former Soviet Union did the same, not so much plain hoaxes as parading the mock-ups and leftover prototypes of abandoned development programs. So when North Korea puts something on parade, we have to be careful.

What can we learn from the rocket engine test footage released by the North Korean regime?

The plume of an engine test tells us by its size the approximate thrust level. By its color, opacity, and smokiness, [it tells us] something of its propellants. If we can see it closely enough, the detailed structure can tell us whether the system has separate vernier rockets or jet vanes for steering, whether it uses a separate gas generator to drive its fuel pumps or an integral "staged combustion" cycle. And, at a coarser level, whether it has one thrust chamber and nozzle or several.

The plume of the April 2016 engine test indicated two main nozzles with four vernier rockets, matching the configuration we had seen on the base of an ICBM or ICBM model some time earlier. Most importantly, the clean and translucent orange color indicated a fuel with substantial amounts of carbon but not a long-chain hydrocarbon like kerosene (which almost always produces an opaque flame and copious smoke). The most likely candidate is a compound called Unsymmetrical Dimethylhydrazine.

All of these signatures are consistent with a pair of closely-coupled Isayev 4D10 engines. This is an ex-Soviet design that we had reason to believe North Korea had obtained for the Musudan missile, but is more advanced and efficient than anything we had really expected them to be able to develop for the first stage of their ICBMs.

How hard is it to design a warhead that's small enough to fit on a missile but tough enough to work?

Ahn Young-joon AP

If the reentry vehicle is intended to carry a nuclear warhead, it is necessary to verify that the warhead—which is a fairly precise piece of machinery including potentially sensitive electronic components—can survive the acceleration, shock, and vibration of a missile launch. This is typically done on the ground, placing test articles in centrifuges, on shaker tables, and the like (actual live-fire testing of nuclear weapons has been exceedingly and fortunately rare). The success of North Korea's missile guidance systems suggests that they have adequate test capabilities in this area even if they haven't seen fit to show them to us.

The warhead and the rocket can be developed separately, and the same warhead used on several different types of missile. The United States has often followed that practice, and it is probably vital for a nation like North Korea that cannot test separate warheads for every type of missile. The warhead obviously has to be light enough for the missile to carry and small enough to fit inside the reentry vehicle. Most every North Korean nuclear-capable missile seems to have a payload section about 65 cm in diameter, appropriate for a first-generation nuclear missile warhead and consistent with the theory that they are using the same nuclear payload for all of their missiles.

Since North Korea has many more missiles than it can possibly have nuclear warheads, we expect they have interchangeable high explosive, chemical (nerve gas), and nuclear warheads for most of these missiles.

What challenges remain for North Korea's nuclear program?

If by "miniaturized" you are willing to accept something that will fit into a 65-cm diameter, 500-600 kg reentry vehicle, and if you are willing to accept a simple atomic bomb with a 10-20 kiloton yield rather than a megaton of thermonuclear fire, then there unfortunately aren't many technical challenges remaining. Too much of what was secret 70 years ago has leaked (or been openly released) into the public domain, and for the dedicated nuclear proliferator, there are persistent reports that A.Q. Khan was peddling one or two proven Chinese nuclear warhead designs along with his uranium enrichment technology in the 1990s. We know that North Korea was one of his customers.

There is a great deal of detailed engineering work required, so make no mistake: This is a task for a team of well-funded experts working over a prolonged period. And that work will need interim tests at every stage. But those interim tests can be conducted covertly, and it is likely that the first actual nuclear explosion will be the proof test of a "miniaturized" nuclear weapon. This was basically true of the French nuclear program in 1961, would have been true of the aborted but well-documented Swedish and South African nuclear programs, and is probably true for North Korea. Their first nuclear test seriously underperformed, and it took them a few years to get it right, but they almost certainly have had a proof-tested warhead suitable for missile delivery since 2013 at the latest.

This post has been updated and originally ran on April 20th, 2017.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io