After weeks of escalating tension, North Korea seems to have backed down a bit from its warlike rhetoric and threats of missile strikes against the United States. But even if Pyongyang wanted to attack, its military would be hard-pressed to do much damage beyond the country's own peninsula. That's because it's very difficult to build a reliable, working intercontinental ballistic missile, and North Korea doesn't have one, according to nuclear proliferation experts. Here's why.
The rocket equation
North Korea was able to launch a questionably successful satellite into orbit late last year, but there are some key differences between ICMBs and satellite-launching rockets, according to David Wright, co-director of the Global Security Program of the Union of Concerned Scientists. Imagine throwing a baseball; missiles work in a similar way, he said. A baseball accelerates with the force of your arm, but that's only a quick power burst, and then it free-falls. That's the nature of ballistics — after launch (or a pitch) an object's trajectory is controlled by gravity, friction and the other laws of classical mechanics. "The ball arcs based on gravitational forces. The faster it leaves your hand, the farther it’s going to go," Wright said. "So by the time you get up to a long-range missile, you need to have much higher speeds."
Higher speeds mean more fuel, which means a bigger rocket to carry the fuel, which means more fuel to launch the rocket — and therein lies the problem. The Tsiolkovsky rocket equation explains this relationship between mass and weight, and it's the key problem for ballistic missiles. Basically, payloads that are big and heavy require rockets that are even bigger and heavier, and at some point, the rocket simply becomes unable to lift itself: "At same time you are trying to scale this whole thing up, make it hold more propellant and make it more robust to withstand more forces, you have to make the propellant a smaller fraction of the total," Wright said. "The amount of propellant you need goes up exponentially with the speed that you want, so you are getting into the realm of diminishing returns."
So, for a long-range ballistic missile, weighing 80 to 100 tons, only about 1 percent of the weight is the actual payload — the warhead. Another 10 percent is the structure of the rocket, and the other 89 percent is fuel.
As missiles grow in size, they also grow in complexity. It's more efficient to use several stages of boosters and thrusters, rather than one giant structure the whole way; this is how most heavy-lift rockets work. Staging allows rockets to drop dead weight once fuel is consumed. But this just means more components must work in perfect harmony, Wright noted.
North Korea has tried to launch several multi-stage rockets, but many of their staging processes haven't worked. Last summer, for instance, a rocket blew up shortly after launch. The December launch of a multistage Unha-3 rocket, which lofted a weather satellite into orbit, doesn't necessarily mean future success, Wright said.
"Suppose they try the same launch tomorrow — what is the probability it will be successful? It's well under 10 percent, just based on all the things that have to go right," Wright said, calculating odds based on previous launch failures. "Doing it once right means you have the components and basic technology, but a big part of it is just getting everything to work at the same time."
Only a missile's launch energy and initial trajectory can be controlled; after the rocket runs out of fuel, gravity and other forces take over. A ballistic missile only burns for three to five minutes, and coasts for nearly half an hour, Wright said. A satellite launcher, on the other hand, needs to burn fuel for much longer to reach higher altitudes and speeds. While a ballistic missile would send a projectile into a parabolic arc back toward Earth, a satellite launcher has to accelerate the satellite to orbital speeds. A satellite launcher could conceivably loft a missile, but bombs are much heavier than satellites, Wright noted — so a rocket capable of launching a satellite would not necessarily be capable of launching a warhead. "It’s not even clear that what (North Korea) launched in December is even physically capable of carrying a 1-ton payload. It would be silly if you had over-designed it to carry a much heavier warhead,” he said.
After its initial boost, an ICBM flies through suborbital space for several minutes, reaching altitudes 200 to 700 miles above the surface. As the payload re-enters the atmosphere, it is subject to continually thickening air, changing atmospheric densities, winds and other forces. If the warhead is tumbling, or if it's not perfectly symmetrical, it could be thrown off balance. All these factors can widen the missile's range of inaccuracy, Wright said. For North Korea right now, a large warhead would probably have a target range of about five kilometers, he said, referring to its target area's uncertainty. In other words, if the warhead was aimed at one place, it could be reasonably expected to land somewhere within five kilometers of that place, which is an area of nearly 80 square kilometers. "That’s not very useful as a military system," Wright points out. "If what you are trying to do is a terror attack on a large population center, that could work, but it's not very useful unless you use a high-explosive warhead. If your point is to launch a missile in the first place, then you want to put something useful on it."
What Does North Korea Have?
North Korea has just two missile launchers known to be tested and operational. The Hwasong, which is essentially a Soviet-era Scud missile, can carry a 700 to 1,000-kilogram warhead across 300 to 500 kilometers. The Nodong missile can carry a 700 to 1,000 kg warhead to a range of 1,000 to 1,300 kilometers, which would allow it to reach most of Japan, Wright notes. The Musudan missile, which could hypothetically reach 3,000 kilometers, has not been tested and is thought not to be operational. Finally, the Taepo-Dong -2 is the name given to a missile based on the Unha satellite launcher, and it’s neither been tested nor operated, Wright said.
It could be years before North Korea possesses that level of technology. Soviet and US rocket development took decades, but those scientists were discovering the physics for the first time. And China used a more systematic approach, building smaller-scale designs, testing them and improving on them, Wright said. North Korea never launched a single-stage version of its satellite launcher. "It seems like they are trying to bootstrap their way up with as few tests as possible," he said. "For one thing, that is hard, and for another, you have no idea what the reliability is."
Wright said if he was building a missile launch system, it would be designed very differently than a system used for launching satellites into orbit, since those systems have such different goals. He would also test it in increments, like Chinese, Indian, American and former Soviet engineers did. "I think you would start over and redesign the second two stages to work as a ballistic missile,” he said. “I’m not saying they can’t do it; I’m saying that it is interesting that what they launched in December was not how you would build a ballistic missile."