Defending America: The Case for Limited National Missile Defense

Chapter Two: Missile Defense: Concepts and Systems

29 The basic ideas behind how missile defense systems operate are not particularly complex. But it is important to have a clear mental picture of how ballistic missiles, and technologies designed to counter them, function. This chapter provides that background information, with a number of graphics, illustrating the main concepts. Basic Elements of Ballistic Missiles Ballistic missiles are rockets designed to accelerate to fast enough speeds so that they can fly relatively long distances before falling back to earth. They are first accelerated by the combustion of some type of fuel, after which they simply follow an unpowered—or ballistic—trajectory. They consist, most basically, of rocket engines, fuel chambers, guidance systems , and warheads, though the specifics vary a great deal depending on the range and sophistication of the missile. Missile Parts and Types For shorter-range missiles, the entire weapons system is generally simple . The missile usually consists of a single stage rocket, which fires until its fuel is exhausted or shut off by a flight-control computer and then ceases functioning for the duration of the flight. The missile body and Missile Defense: Concepts and Systems CHAPTER TWO warhead often never separate from each other, flying a full trajectory as a large, single object. For longer-range missiles or rockets, the system consists of two or three stages, or separate booster rockets, each with its own fuel and rocket engines. The rationale for this staging is to improve boosting efficiency and thereby maximize the speed of the reentry vehicle or vehicles. Putting all the fuel for a long-range rocket in one stage would make for a very heavy fuel chamber and mean that the rocket would have to carry along a great deal of structural weight throughout the entire phase of boosted flight. That would lower the ultimate speed of the warhead or warheads, reducing their range. With staging, by contrast, much of the structural weight is discarded as fuel is consumed. That makes it possible to accelerate the payload to speeds sufficient to put it on an intercontinental trajectory. Long-range warheads must reach speeds of about 4.5 miles a second (roughly 7 kilometers a second), or almost two-thirds of the speed any object would need to escape the earth’s gravitational field entirely (roughly 7 miles, or 11 kilometers, a second). To reach such speeds with existing rocket fuels, efficiency in design—including rocket staging—is essential. On long-range rockets, warheads are designed so that they can be released from the missile body during flight. Generally, warheads and any decoys are released after boosting but while the rocket is still going up— that is, in the ascent phase of flight.1 Releasing warheads from the missile is clearly necessary if multiple warheads with multiple aim points are to be used. It is also desirable since large missile bodies are subject to extreme forces on atmospheric reentry that could throw them, and any warheads still attached to them, badly off course. In fact, warheads do not fly free and exposed. They are instead encased within reentry vehicles. These objects provide heat shields and aerodynamic stability for the eventual return into earth’s atmosphere. They protect the warheads from melting or otherwise being damaged by air upon reentry and also maximize the accuracy with which they approach their targets. Missiles may be powered by solid fuel or liquid fuel. If liquid fuels are used, it is usually considered desirable that they be storable and not require cooling or other special treatment that would involve extensive preparation before launch. Advanced intercontinental ballistic missiles (ICBMs) can use either type of fuel; Russian SS-18s use liquid fuel, for example, whereas modern U.S. missiles use solid fuel.2 30 MISSILE DEFENSE: CONCEPTS AND SYSTEMS Missile guidance must be exquisitely accurate. Warhead trajectories are determined by the boost phase, meaning that their course is set hundreds or thousands of miles before they reach their targets. To land within a few hundred feet of a target—or even a couple miles—requires considerable care in how long the rocket motors are fired and in what direction the rocket is directed to take by their firing. Generally, rockets use inertial guidance systems to measure the acceleration provided by the boosters at each and every stage of their burning. Computers then integrate those measurements to plot out a trajectory for the warheads; a feedback...