Physically, a ballistic missile is typically between 30 and 100 feet tall. It is powered by liquid fuel such as oxygen and kerosene or solid fuels such as combinations of powdered metals like zinc or magnesium. Because it must accelerate to 15,000 miles per hour while pushing several thousand pounds of payload, the missile is built to be light.
During its flight, the ballistic missile is boosted by its engines at an upward angle. During this boost phase, the engines burn for up to 300 seconds, propelling the missile until it is about 200 miles up. Within 50 seconds, the missile is above almost all of the earth’s atmosphere and really begins to accelerate, reaching its top speed of some 15,000 miles per hour. The engines drop off and the boost phase of the flight ends.
With a single warhead ICBM, after the last stage drops off, the missile directs the warhead to its target. With a multiple warhead ICBM, after the last stage drops off, the missile begins to dispense its warheads one by one, directing them to separate targets. It may also spread decoys designed to defeat an anti-missile system. This is known as the post-boost phase and may last until the missile reaches the top of the trajectory some 800 miles above the earth. At this stage, the missile’s path is affected only by the force of gravity.
The longest stage of the flight, lasting perhaps twenty minutes, begins after the last release maneuver. During this mid-course phase, decoys made of tinfoil travel alongside heavy warheads, undisturbed by air. Radars find it difficult to distinguish between decoy and warhead because their spaces reflect radio waves similarly. Moreover, radars have to contend with clouds of thin metallic foil strips called chaff. Infrared devices, for their part, have some difficulty distinguishing between warhead and decoys. But while tinfoil decoys quickly become cold as the surrounding space, warheads retain and radiate some heat. The Space Based Infra Red System (SBIRS-Low) currently being researched has the ability to distinguish between warhead and decoy with great accuracy.
Roughly speaking, the final five minutes of the flight constitute the terminal phase, during which the atmosphere affects the reentering warheads and decoys. By measuring variations in speed and movement, sophisticated radars and satellites can distinguish the warheads from most decoys.
At this point in the warhead’s flight, there is little time to intercept it.
At this point, the warhead will explode either at a preset altitude, when a proximity fuse tells it that it has come as close to its target as it is going to, or when a salvage fuse tells it that it is being destroyed in the case of terminal phase interception by an anti-missile system. If a warhead explodes roughly twenty miles above the earth, for whatever reason, it will do no harm to life or property below. Below this, there will be damage.
In the best case anti-missile interception, the missile is destroyed during boost phase and its warhead or warheads will never reach terminal phase. Gravity will cause them to fall back, unexploded, either on the area whence they were launched, or on the North Polar regions in the event of a Russian or Chinese attack. Obviously, if we want to defend the United States against the ballistic missiles, the boost phase is the best point at which to do it. The challenge becomes greater the farther into its flight the missile travels.
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Boost Phase
Ballistic missiles are launched straight up. To lift many tons, a missile unleashes a massive, controlled explosion that creates flames hundreds of feet long and thousands of degrees hot. Missiles move relatively slowly through the lower atmosphere to minimize air resistance, and gather speed as they rise into thinner air and then into space. As the missile leaves the lower atmosphere, it tips in the direction of the target and gains speed. If the missile is being launched to maximum range, it will tip at an angle of 45 degrees, half its energy going to gain height (and therefore time aloft), the other to gain distance. The missile’s range depends on the ratio between the thrust generated by the engines and the weight that the thrust must overcome—and of course on the duration of the thrust. This means that any given missile’s range can vary tremendously. To lengthen the range of any given missile, just lighten the load. To shorten the range, either increase the load or fire the missile at an angle steeper than 45 degrees. If the missile has stages, the lower ones will drop off after they have burned their fuel, and lighten the load. At a designated point in space, the last engines shut off or burn out. The time between launch and engine burn out ranges from less than one minute to over five. Engine burn out ends Boost phase. From this point on, the laws of physics will carry what remains of the missile, as well as the payload, onto the vicinity of the target, no matter what.
As the missile floats to the top of its trajectory, the section that carries the payload, called the Post Boost Vehicle or ‘Bus,’ makes final adjustments to the course. During this time, missiles that carry multiple warheads spin off each warhead precisely in the direction of its individual target. They also deploy decoys—thin replicas of warheads, or foil balloons, the purpose of which is to give false targets to enemy radars. This is the Post-Boost phase. Multiple warheads mean that each missile can strike many targets and present many real threats with which ground based defenses must deal. Decoys force ground based defenses to face numerous false threats as well as real warheads. Post-boost vehicles can release large numbers of small sub munitions, instead of warheads and decoys. Each of these is a real threat to the target. High numbers of warheads overwhelm ground based defenses.
Midcourse Phase
As warheads, decoys, and the remains of the missile coast over the top of the ballistic arch, and until they reach the upper edges of the atmosphere above the target, they fall freely. As they do so they gradually spread apart along their individual ballistic paths. This, the longest part of the trajectory, is called Midcourse Phase. The warheads, etc. reach maximum speed at the end of midcourse phase, before atmospheric interference begins.
Terminal Phase
The terminal Phase begins as the first air molecules begin to slow down and then to heat and to burn up the thin decoys and the remains of the missile. The air slows and heats the warheads too. But they are armored against heat and pressure, so they get through the atmosphere. The range of the missile determines the angle at which warheads fall onto the target. Warheads from the longest range missiles arrive at shallow angles of little more than twenty degrees, while shorter range ones can come in at 45 degrees. The reason why warheads from the longest range missiles come in at the shallowest angles is that, because the earth over which they have traveled is curved, faraway targets lie on something like the reverse slope of a hill.
