The laws of physics dictate that, to reach targets beyond roughly 100 miles, ballistic missiles must fly through outer space. That is because a typical ballistic trajectory is an arch about a fourth as high as it is long. So, even a missile with a range of 100 miles has to climb to over 125,000 feet. That means going beyond earth’s atmosphere into space. Intercontinental missiles can climb 1000 miles high and stay out of the atmosphere for almost a half hour. To reach long distances, ballistic missiles have to fly high for two reasons. First, if the top of their trajectory were too low, the warhead would not have time to go very far before gravity pulled it to the ground. Flying into space is essential to gain time. Second, if a missile were to try to fly across continents and oceans in a half hour without leaving the atmosphere, it would burn up through air friction long before it reached the necessary speed. Besides, the very effort to plow through air would use so much fuel that the missile would run out of fuel even before burning itself up. Because missiles can reach high speeds only in outer space, flying mostly through space is essential for the missile to go fast, far, and survive. Hence ballistic missiles are necessarily outer space weapons.
To achieve maximum results from propulsion and a high thrust to fuel weight ratio, propulsion must be slower at first, and increase only as air resistance decreases. There is no use expending twice as much fuel to achieve a particular speed at a low altitude when by waiting until a higher altitude will achieve that speed with much less fuel. Engineers who try to make the boost phase as short as possible are working against the laws of physics. The shortest possible ballistic boost phase is of course that of a gun, in which the propellant that pushes a bullet burns out instantly. But projectiles that come out of guns hit the air at maximum speed, and only slow down after that. That is most inefficient. Air resistance makes long range guns impossible and ballistic missiles necessary. Air resistance also mandates that ballistic missile engines burn until well into outer space. The more time a missile spends boosting, and especially boosting in outer space, the more vulnerable it is to boost phase defenses.
How high missiles fly determines how fast they come down onto their targets. The speed at which they come down is important because the greater the speed the more difficult it is for ground-based defenses to do their job. A warhead’s maximum speed just before reentry is a natural consequence of the range of the missile on which it was launched. The longer the range, the higher the missile must climb to reach it. The higher it climbs, the more time it takes for it to fall to the ground. And the longer time it takes, the more time that gravity has to make it go faster. Since gravity accelerates the warhead downward at 10 meters per second, and an intercontinental missile may be falling from an altitude of 1200 to 1600 kilometers, it may attain a speed of between 6 and 8 kilometers per second before it hits the atmosphere. For the same reason, a short-range missile like the Iraqi version of the Scud used in the Gulf War of 1991 reaches only heights of some 160 kilometers and hence arrives at a speed of less than 2 km per second.
The speed of a warhead is not solely dependent on the distance a missile travels. First, those who fire any given missile at any given range may choose to make its warheads arrive faster than the range alone would indicate. They can do this by using missiles that, were they fired at angles of 45 degrees, would go farther than the intended range — but instead they fire them at steeper angles. Thus the missiles would climb higher on a steeper arch and come down farther and faster. Or they could use a missile for which the desired range would normally be maximum, reduce its payload, and send it on a steeper, higher arch to reach the same point. Second, each warhead’s speed will be reduced differently by atmosphere. The atmosphere acts differently to slow each kind of warhead according to the warhead’s shape. Thus an intercontinental warhead from an old US Poseidon missile would hit the ground slower than one from a newer model medium range US Pershing II missile because the Poseidon’s is blunt while the Pershing’s is sharp. The point is that missile designers can increase the speed of warheads both by adjusting launch angles and by making warheads aerodynamic, thus complicating the task of defenses.
The speed of a warhead’s arrival is important because it affects how much time the defender at the receiving end of the attack has to see it and shoot at it before it hits him. The faster the warhead, the faster it will close the distance between the point where it is first seen, and the target. This means that the faster the warhead the closer to the target it will be before it may be intercepted. A faster interceptor may hit it farther away than a slower one. But the distance between the bull’s eye and the intercept is determined chiefly by the warhead’s speed, and by its distance from the bull’s eye when it is first seen and the interceptor is first launched. Time is the key to how far away the warhead can be kept.
The speed of the warhead (along with the timeliness of the interceptor’s launch) also determines the size of the area that can be protected against it. A slower warhead (or, conversely, the earlier launch of an interceptor) means that the warhead may be intercepted at a higher altitude. A faster warhead (and/or the later launch of an interceptor) means that the warhead may be intercepted at a lower altitude if it is even intercepted at all. Since any given warhead may be heading anywhere within a broad area, the laws of geometry dictate that the closer to its origin that interceptors launched from any given point destroy that warhead, the larger the area they can protect from that warhead.
The above is a modified excerpt from a forthcoming book by the Claremont Institute on the science of missile defense.
