Learn about ballistic and cruise missile propulsion, staging, guidance, and warhead considerations for a WMD payload.
|Table of Contents|
|2. Ballistic missiles|
|3. Warhead considerations|
|4. Cruise missiles|
The long-range ballistic missile is rightly considered the ultimate strategic weapon. It can deliver a payload more than ten thousand miles; its high speed makes it virtually invulnerable to defense; its accuracy when armed with a nuclear warhead suffices to destroy almost any target. This same speed, however, makes it unable to deliver a chemical or biological agent effectively, and its high cost makes it impractical for delivering conventional explosives. Thus, an effort to build such a missile should be taken as evidence that the builder has a nuclear weapon in mind. Indeed, only the nuclear weapon powers have built such a missile.
A cruise missile flies at a much slower speed than a ballistic missile, and usually hugs the earth. Such a missile is therefore practical for dispersing chemical and biological agents. A short-range ballistic missile, unlike its long-range cousin, is comparatively easy to produce and is practical for any type of warhead, including conventional explosives.
In 1987, the world took its first step toward curbing the spread of strategic missiles. A group of concerned countries formed the Missile Technology Control Regime (MTCR), which aims to restrict the proliferation of missiles capable of carrying a 500 kilogram warhead at least 300 kilometers, and of missiles intended to deliver a weapon of mass destruction. In order to achieve these goals, the MTCR established export control guidelines. The missiles considered here are those that fall under those guidelines.
1.1 General considerations
To develop a strategic missile, one must master several technologies. These tend to fall into four categories: a propulsion system, which accelerates the missile to the required speed, a guidance and control system, which directs the missile to its destination, a warhead (payload), which is adapted to fit atop the missile, and an overall structure, called an airframe, to hold everything together.
The propulsion system is usually the first step a country tackles. Either a liquid or solid fuel rocket engine can be used to achieve this, but the fuel must be able to lift not only the mass of the warhead, airframe and guidance system, but also its own mass. Long-range missiles typically consist of several rocket engines arranged in stages. After the ability to launch is acquired, the missile developer must move to the second task, which is being able to accurately place the missile on the intended target. Accurate guidance is less important for a nuclear warhead, due to its large radius of destruction. The third step is production of the actual warhead, be it conventional, chemical, biological, or nuclear, in a format that a missile can carry. Finally, the previous three systems must be integrated with the airframe to form the final product.
The guidance system is usually the main challenge, followed in order of difficulty by propellant manufacturing, and by mating the warhead to the missile in such a way as to shield it from the heat of reentry and from vibration during launch.
The missile has now become a potent military force, primarily because no system of missile defense has proved itself reliable. The U.S. Missile Defense Agency, for example, achieved only a 50 percent success rate in intercepting target missiles during test flights in 2003.
2.1. Ballistic missile propulsion
A ballistic missile is so-called because after the payload is accelerated to the desired speed and direction, the payload is released and allowed to fall in an unpowered, ballistic trajectory down to earth. Such a missile does not breathe the ambient air, so it must carry its own fuel, as well as the oxygen to burn it, which is called an oxidizer. The fuel can be either liquid or solid, the latter being preferred for missiles and the former for space launchers. In fact, the same rocket engine can be used for either a missile or a launcher, the main difference being the payload. Many existing space launchers have been developed from, and share components with, ballistic missiles.
2.11. Liquid fuel
A liquid propulsion rocket engine works by feeding the liquid fuel and the oxidizer from storage tanks into a combustion chamber, where it arrives through a complex series of pipes, valves, and pumps. As the fuel burns in the chamber, the hot exhaust gas ejects through a nozzle, giving the rocket its thrust. The liquid-fueled rocket engine produces more thrust per pound of fuel than the solid-fueled engine, but is also more complex and can require many precision-machined and moving parts. The liquid-fueled engine is therefore more difficult to manufacture, requires more maintenance, and takes longer to prepare for launch than the solid-fueled engine. Fuel and oxidizer can be difficult to handle and store because they are toxic, corrosive, or cryogenic.
Hydrazine, monomethylhydrazine (MMH), and unsymmetric dimethylhydrazine (UDMH) are the common liquid rocket fuels. Hydrazine is often used as a monopropellant (without an oxidizer) by decomposing it into a hot gas with a catalyst. Up to 50 percent hydrazine is often mixed with MMH or UDMH fuels in order to improve performance.
If a country wishes to increase the range of an existing liquid-fueled rocket motor, it must either increase the flow rate of the propellant and oxidizer, or allow the motor to burn longer. Either option requires adding more propellant, which usually requires a modification to the airframe and a re-consideration of the missile’s structural integrity and stability. With a longer burn time, the surfaces that are exposed to the combustion process, such as jet vanes in the exhaust flow, or components of the thrust chamber, may need to be modified to protect them from the increased thermal exposure. Alternatively, if the thrust is to be increased, the combustion chamber may need to be modified to withstand the increased pressure, or the nozzle enlarged for increased flow. In addition, structural modifications may have to compensate for higher aerodynamic loads and a new flight profile.
2.12. Solid fuel
Solid propellant rocket motors contain both the fuel and the oxidizer inside a single motor casing. No tanks, pipes, pumps, or valves are needed because the fuel and oxidizer are premixed in the proper ratio and cast into a solid form, which is ignited on the inside. Once ignited, the propellant burns inside a hollow chamber running down the center of the motor; the hot expanding gas rushes through the nozzle and creates thrust. The thrust terminates when the propellant is exhausted. Ammonium perchlorate (AP) is the oxidizer of choice for most solid propellant rocket motors. It is routinely mixed with powdered aluminum fuel.
The outer casing of the solid rocket motor confines the combustion gases, and transmits the thrust to the warhead. It can also serve as the form in which the propellant is cast. Solid rocket motors are relatively economical and easy to maintain; they can be stored for many years; they are capable of rapid launch. Thus, they are preferred for missiles.
The manufacture of solid propellant, however, is difficult. Each rocket grain must be made free of flaws to avoid the possibility of internal burning and breakup. Minor variations may produce significant changes in performance. The most common method for producing these grains is to cast a mixture of ingredients into a mold and cause it to solidify. To do this, one must convert five or six chemicals into a single material. The six chemicals consist of an oxidizer (ammonium perchlorate), a fuel (powdered aluminum), a binder (a polymeric material), a curing agent, a plasticizer (to make the mix more fluid) and a catalyst to control or enhance the burning rate. To process these unstable ingredients successfully, one needs special techniques and equipment.
2.13. Multiple stages
A long-range missile needs more than one stage. It has been proven mathematically that a serially staged missile is the best design for propelling a warhead long distances. By discarding the lower stages as they burn up their propellant, a multi-stage missile progressively loses weight, which enables it to fly farther than a comparably sized, single-stage missile.
The stage of a solid-propellant missile usually consists of a cylinder made either of high-strength steel or fibers wound in a resin matrix, the cylinder being filled with a rubber-like propellant. Liquid-propellant stages are also cylinders, but they are filled with propellant tanks, pressure tanks, pipes and valves. At launch, a signal either fires an igniter inside the solid propellant rocket motor, or triggers pressure that forces liquid propellants into the combustion chamber of a liquid rocket motor, where they react. Hot expanding gas then escapes through a rear nozzle, providing thrust.
Rocket stages must be separated quickly and cleanly for the successful flight of a missile. To accomplish this task, missile designers have used explosive bolts and flexible linear shaped charges (FLSC). Explosive bolts attach the missile stages together through specially constructed, load-carrying interstages. These interstages have flanges at the ends that, on signal, explode to separate the stages. A built-in FLSC works by making a circumferential cut through the interstage skin and structure, which allows the stages to separate.
The trajectory of a long-range ballistic missile extends above the earth’s atmosphere. From what is essentially the beginning of outer space, the warhead must re-enter the atmosphere before it can reach a target. When it does re-enter, it will be traveling at speeds ranging from Mach 2 to Mach 20, which generate high heat and vibration. To house the warhead and protect it from these effects, a special re-entry vehicle (RV) is required.
RVs are in the shape of cones, ranging from sharp- to blunt-tipped, that carry not only the warhead but the arming and fusing gear needed to detonate it over the target. Some RVs, known as maneuvering reentry vehicles (MARV), also carry guidance and control equipment that allows them to home in on targets or avoid defenses.
The conic surface of the RV is usually coated with heat shield material, typically carbon-based, to withstand the high heat of re-entry. The shield can be made by infusing carbon into a porous carbon “preform” under great pressure by means of an isostatic press. This same manufacturing method can also make inserts for rocket nozzles.
2.2. Ballistic missile guidance
To put a missile payload on target, a country must master three techniques: guidance, control and navigation. To guide a missile, one must measure the position of the missile as it flies, and send signals to equipment on the missile that can correct its course if the missile strays from it. To control a missile, it is necessary to manipulate the equipment (valves, motors and actuators) on the missile that can alter its flight and keep it on course. To navigate, it is necessary to locate the target, locate the launch point, and locate the path that connects them in three dimensional space.
A missile’s position during flight is measured by what is called a guidance set, at the heart of which are gyroscopes and accelerometers that sense motion and changes in orientation. After launch, these instruments measure missile acceleration and rotation, and convert these measurements to electrical signals. A flight computer then converts the signals into deviations from the programmed flight path and issues commands to the flight control system to steer the missile back on course. In response to the computer’s commands, the control system can aim nozzles, move aerodynamic control surfaces, control flows, and activate thrusters. It is this constant process of measuring and correcting the missile’s position that allows it to follow its flight path and reach its target. It has been estimated that the response time for the control system to produce corrective steering forces is about 50 milliseconds for an unfinned missile. When the missile has fins, the time is longer. The overall accuracy of the system depends largely on the accuracy of the gyroscopes and accelerometers that sense the missile’s position.
The longer the distance over which the missile must fly, the more will be the inaccuracy produced by small errors in measuring the missile’s position. A degree of error acceptable in a theater-range missile would not acceptable be in a missile flying thousands of miles. The inaccuracy compounded over distance will become too great. The U.S. government defines a theater ballistic missile (TBM) as a ballistic missile with a range of less than 3,500 km. An intercontinental ballistic missile (ICBM) is defined as a ballistic missile with a range of greater than 5,500 km.
Overcoming guidance errors poses a considerable challenge. To be confident that a long-range missile will hit its target, one must test it repeatedly to see where it will land. Each test flight will be observed, and may trigger a diplomatic reaction. Thus, a long-range testing program has a political cost, as well as a financial one.
2.3. Ballistic missile basing
Launch capability is of particular importance for an ICBM. A fixed launch site is vulnerable to preemptive attack (although such a site can be hardened) but if a fixed site is not used, a country must go to the considerable expense of building a fleet of mobile transporter-erector launchers (TEL) sufficient to carry a heavy missile. Sea-based launching, typically by submarine, provides maximum mobility and has been employed by the more advanced nuclear countries.
Even though grouped as “weapons of mass destruction,” nuclear, biological, and chemical warheads have widely differing requirements for delivery by a missile.
A chemical or biological agent must be spread in a diffuse cloud over a large area to be effective. The high speed of a ballistic missile makes it difficult to distribute the agent in this manner. Moreover, a ballistic missile has very little ability to deviate from its predetermined flight path, which means that it cannot spread the agent over the target if the axis of the target does not happen to line up with the missile’s flight path. In addition, the heat from reentry or detonation can degrade the quality of the agent. Thus, a subsonic cruise missile is far more useful than a ballistic missile for disseminating a chemical or biological payload, provided the agent is released outside the aerodynamically disturbed field of flow around the missile. Supersonic cruise missiles generally cannot dispense chemical or biological agents effectively because the air stream will destroy the agent by heating or shock. Biological and chemical agents are relatively lightweight and are flexible enough to be packed in most warhead configurations.
Nuclear explosives, on the other hand, are heavy and need a warhead with a special shape. This poses problems in weight balancing and aerodynamics. The comparatively large kill radius of a nuclear weapon does mean that one can sacrifice some accuracy. Nor are nuclear weapons as susceptible to the heat of reentry. All these considerations point to one conclusion: that long-range ballistic missiles are primarily suited to delivering nuclear warheads.
A cruise missile is essentially the same as a pilotless aircraft. It breathes the ambient air, is powered by a small jet engine, flies slower and does not travel as far as a ballistic missile, and is guided on its entire flight path. The warhead stays attached to the missile until it reaches the target. Payloads for cruise missiles can vary from 200 to 500 kilograms. Ranges can vary from 300 to 5,000 kilometers. These missiles can be launched from aircraft, trucks, ships and submarines.
Cruise missiles are much less expensive than ballistic missiles. The airframe of a cruise missile can be made by adapting virtually any airframe that is adequate for an ordinary aircraft. Thus, the primary proliferation concern is technology that enables mass production of cruise missile airframes. Besides being cheaper to develop than a ballistic missile, the testing required for a cruise missile is easier to conceal. Because of its shorter range, a cruise missile can often be tested inside a country’s borders or quietly at sea.
Previously, cruise missile guidance relied on complex mapping systems such as Terrain Contour Matching (TERCOM). However, the advent of the Global Position System (GPS) and the Global Navigation Satellite System (GLONASS) has provided the missile developer with an inexpensive and widely available system that is sufficiently accurate. An Inertial Measurement Unit (IMU) can also be used for guidance but does not perform nearly as well as GPS. Regardless of the guidance system employed, a low flying cruise missile’s on-board map must be provided with the latest changes in terrain and physical features. Otherwise, the missile risks colliding with newly formed obstacles.
Cruise missiles pose a serious proliferation threat because they are inexpensive to build and can overwhelm defenses by their sheer numbers. Although, as stated above, a supersonic cruise missile is ill-suited to the delivery of chemical or biological agents, it can deliver a nuclear warhead quite well. From the standpoint of missile defense, a stealthy cruise missile poses an especially grave threat, regardless of the type of warhead it carries.
Note: The above material was derived from The Militarily Critical Technologies List, Office of the Under Secretary of Defense for Acquisition and Technology, Washington, D.C., 1998, and from the Missile Technology Control Regime Annex Handbook, United States Department of State, Bureau of Nonproliferation, 1998.