I am behind on my laser myths writing and ICBMs seem a more appropriate subject this week. Please bear with me because this rambles.

SECDEF Rumsfeld says that North Korea has missiles capable of reaching the west coast of the United States. That should not be a surprise to anyone who follows international events. About four years ago, NK sent a missile over Japan into the Pacific Ocean. At the time, they asserted that they were attempting to launch a satellite. Which is easier, firing a missile a few hundred miles or launching something into orbit?

Let’s go back to the beginning of projectile weapons. Probably someone picked up a rock and threw it at someone or something. From there, men advanced to spears and other thrown projectiles. These depended on the strength of the thrower for distance and force of impact. Some aids were developed to increase the leverage applied to the projectile. These aids essentially allowed the thrower to transmit his muscular energy to the projectile over a longer time interval.

Later some brilliant research and development guy came up with the longbow which allowed stored energy to be released into an arrow. The concentrated energy transmitted to the arrow meant that it flew farther and impacted with more force than any man could produce by throwing a projectile. Again, the key was increasing the time interval in which the shooter applied his muscular strength. Pulling back on the bowstring over time puts energy into the bow that is released very quickly into the arrow.

Later still, someone invented the crossbow. This really increased the time interval for storing energy in the bow and thus provided vastly increased energy levels to the bolt. A really strong man could shoot the longbow great distances, but the crossbow allowed anyone to shoot a hard-hitting bolt. Medieval catapults worked on the same principle; put the energy in slowly and release it quickly.

With the invention of gunpowder, soldiers no longer had to depend on their muscles to send the projectiles across the battlefield. The energy in the gunpowder meant projectiles could go faster, farther and deliver more energy to the target. Early guns were limited in projectile velocity and range by three things: the energy release characteristics of the propellant, the length of the tube (barrel) and the composition of the gun. Muzzle velocity determines the range of the gun. To increase muzzle velocity you can increase the burn rate of the propellant, increase the amount of propellant, decrease the mass of the projectile or lengthen the tube. There are limits to each, but this is a good rule of thumb.

Problems arise quickly when you improve propellants or increase the amounts used. More and faster burning propellants raise chamber pressures and can result in catastrophic failure of the tube. In other words, it blows up. Exploding guns are an occupational hazard for artillerymen. The number of rounds fired through modern artillery tubes is recorded to ensure that dangerously worn tubes are removed from service.

Lengthening the tube also results in greater muzzle velocities as more of the energy from the burning propellant is imparted to the projectile before it leaves the tube. The maximum velocity of a gun projectile is achieved at or near the end of the barrel. There is a limit to how long an artillery tube can be made before it becomes so unwieldy that it is impractical as anything other than a stationary weapon system. Also, higher velocities in the tube result in faster breakdown of the tube itself. There are artillery rounds that have rocket motors at their base. These rocket-assisted projectiles (RAP) increase the range of tube artillery but sacrifice lethal payload capacity. Accelleration forces on tube artillery projectiles can be in the thousands of Gs meaning that the projectile and any cargo must be extremely rugged to withstand the loads. This ruggedness means less mass can be devoted to the kill mechanism.

Rocket artillery such as the US Multiple Launch Rocket System (MLRS) solves the muzzle velocity limited range problem. Just as the crossbow and longbow allowed for longer time intervals to put energy into the arrows, rocket propellants can burn slower and longer and put energy into the rocket projectile over a longer time interval. G-forces are lower, the projectile can be made less rugged and the kill mechanism more lethal for a given caliber. Ranges can be longer because more energy can be applied to the projectile.

Each type of artillery round is a ballistic missile in that once the propellant burns out, three forces determine the impact point: velocity at burnout, air drag and gravity. The altitude, vertical attack angle and velocity at burnout determine how quickly gravity will bring it back to earth. If the velocity is high enough, the projectile can achieve Earth orbit. If not, it falls to earth short of orbit.

The North Koreans said that they were trying to launch an orbiting satellite. By definition, an orbit is a lap around the Earth. An intercontinental ballistic missile does not have to go around the Earth only a few thousand miles. It requires much more energy to launch a satellite into orbit than it does to send a similar mass payload from Asia to North America. If you can do the first, you can do the second.

Get yourself a small nuclear weapon and a decent launch vehicle and you are a contender.