The North Korea ballistic missile question
A question haunting the West is whether North Korea has the engineering and scientific capability to launch a ballistic missile that can carry a nuclear bomb and accurately guide it to a target several thousand kilometres away.
A ballistic missile is one that is powered by a rocket engine until it reaches a predetermined peak suborbital altitude, at least 100km above the Earth, then it returns to Earth, without power, under the influence of gravitational and aerodynamic forces only.
The front of the missile, the warhead, contains a bomb which in the case of a nuclear bomb weighs well over half a tonne. It requires at least 20 tonnes of rocket fuel to launch this payload into space. Usually this is achieved by dividing the missile into two or more stages – each stage has its own rocket engine.
By jettisoning a stage as each engine runs out of fuel, the missile mass can be decreased en route and the target altitude can be reached more efficiently. The challenging engineering problems start when the missile has attained its sub-orbital altitude.
As it returns to Earth the missile will be travelling about 15 times the speed of sound. Three competing factors have to be managed - deceleration, heating and accuracy.
If on re-entry it plunges at too deep an angle into the Earth’s atmosphere, the missile will break up because at these speeds it would be like hitting a brick wall.
Even with a shallower re-entry angle, the increasing density of air as the missile descends causes violent deceleration. At the same time, the heat generated by the friction of the air molecules on the surface of the missile is enough to melt iron.
But, if the angle of approach into the atmosphere is too shallow, the accuracy of the missile is significantly affected and it can end up hundreds of kilometres off target. The deeper the angle into the Earth’s atmosphere, the more direct and hence the less perturbed the trajectory is – this means greater accuracy.
A careful compromise has to be achieved between the angle of reentry to the atmosphere, the amount of heating allowed and the accuracy required.
The design of the missile, particularly the cone, has to be such that excessive temperatures are not allowed to build up on its surface – even so the cone will reach well over 800C.
This means exotic materials are required to keep the missile and its electronics from melting. In addition, its contents have to be robust enough to deal with the buffeting the missile will experience.
Currently the key issue for the North Koreans is the bomb itself. The North Koreans initially acquired their nuclear know how from the Soviet Union who in 1963 supplied them with a research reactor.
There are three types of nuclear bomb - a fission bomb (same type as the Hiroshima bomb), a hydrogen bomb and a hybrid bomb. A hydrogen bomb is the most devastating, it can be 1,000 times more powerful than a fission bomb.
However hydrogen bombs are much more difficult to engineer and miniaturising them to fit on the end of a missile is a major challenge.
There is no evidence the North Koreans have miniaturised a hydrogen bomb yet. They appear to be well on the way to detonating a hydrogen bomb underground, although their most recent test may have been a hybrid bomb – that’s a fission bomb with some hydrogen in the nuclear mix.
The West is still faced with two serious problems. Firstly, the West consistently underestimates North Korea’s technological prowess. The inconvenient truth is that North Korea will eventually be able to launch hydrogen bombs half way around the world.
Secondly and equally worrying, is that the much heralded US defence missiles are inadequate. The tests to date which show these killer missiles, shown hitting incoming enemy missiles, are carried out under carefully scripted unrealistic conditions.
At least two reports from US government audits have painted a sorry picture about the operational effectiveness of these missiles.
The only real deterrent is that North Korea is aware that the first nuclear strike on any target in the West will be its last.