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Theater Ballistic Missiles

Theater Ballistic Missiles (TBMs) are ballistic missile with a range of less than 3,500 km. Except where noted, this document will use that definition. TBMs can carry a conflict outside of the immediate theater of fighting and can usually penetrate to their targets. Iraq’s limited capability missiles made an impact by tying up allied air assets on seek-and-destroy missions against mobile launchers and in the other steps taken to calm Israeli and Saudi populations. Extant whole missile systems, such as the SCUD and SS-21, can satisfy the targeting needs for many proliferators. A proliferator’s potential ability to upgrade existing, outmoded missiles (e.g., short-range SCUDs) is quite real. Much of the hardware and technology to support many of the modifications are readily available or can be produced indigenously. However, some of the hardware and technology (those requiring more advanced technology, special materials, and/or precise manufacturing) are not readily available and may require special design and production efforts by more advanced countries. A proliferator can achieve an understanding of the most efficient and costeffective methods to extend the range of a missile by using finite element structural and fluid dynamic computer routines and automated codes to predict missile performance and aerodynamic properties. A proliferator can also test and validate the com-puter routines in wind tunnels and structural laboratories. Since these computer routines reduce the number of engineers needed to modify missiles, they are particularly key to reducing both the unit and system costs. Automated engineering computer routines are ranked at the same level of importance in the technology tables as hardware items.

The type of propulsion system selected also affects launch strategy, the second important proliferant capability. Liquid-propellant missiles generally create less of a military threat than solid-propellant missiles. Solid-propellant missiles are stable and storable and do not require fueling before launch, a time when the missile is particularly vulnerable because of its exposure. In addition, solid-fueled missiles have a shorter launch support train than liquid-fueled missiles. Fewer vehicles and less activity associated with the vehicles limits exploitation of acoustic, seismic, and other signatures. The enormous progress made in guidance and navigation with the GPS, particularly in automated design with computer routines such as finite element codes and in materials science with the introduction of composite materials, has further reduced the design burden on proliferants seeking TBMs. Transferred to proliferant nations, these advances streamline the manufacturing processes, which accelerate and expand the potential for a missile arsenal.

When a proliferant seeks a range extension from an existing airframe, it may need to strengthen the airframe if the original missile had a low factor of safety. This is necessary so the missile can withstand higher aerodynamic loads; change the propulsion subsystem by altering either the burning rate or the duration of propellant flow or by selecting a high-energy propellant; adapt the guidance system to accommodate the new acceleration loads and the higher cutoff velocities; and weaponize the warhead by including thermal protection on the nosetip or modifying the reentry strategy of the missile to withstand the higher aerodynamic heating on reentry.

Proliferants can modify or manufacture longer range ballistic missile airframes in several ways. Iraq extended its missile range by reducing the payload and lengthening existing airframes to hold more fuel and oxidizer. Iraq also introduced the concept of “strap-ons” to extend a missile’s range when it launched the “al Abid” in December 1990. To manufacture the “al Abid” missile, Iraq strapped five SCUDs together to form a single large missile, theoretically capable of a 2,200-km range.

Proliferants can also stage missiles in parallel or serial. The United States used a concept known as “parallel staging” to extend the range of its Atlas missile. Parallel staging fires several component engines simultaneously at launch. Then, as the missile accelerates, it drops these extra engines. When a nation possesses the technical capability to support extra range, the most efficient way to achieve it is through conventional “serial” staging, in which a missile’s stages fire one at a time in sequence. Some Chinese TBMs, such as the M-11, which may have originally been designed as a multiple-stage missile (and, therefore, has sufficient thrust-to-weight ratio), can be converted to two-stage missiles with minor modifications and modest assistance from technical experts if they are aware of certain design limitations.

But some constraints, such as avoiding maximum dynamic pressure at staging and timing the staging event precisely enough to maintain control over the missile, are solved when multi-stage missiles are built derived from components which originally came from a multi-stage missile.

To extend the range of liquid-fueled and solid-fueled missiles, these missiles require different adaptations to the propulsion subsystem. Liquid-fueled missiles supply fuel to the thrust chamber by turbopumps. To increase the range of an existing liquid-fueled missile, the proliferant must either increase the flow rate of the propellant and oxidizer or allow the missile to burn for a longer period of time. This can be accomplished by adding more propellant, which usually requires a modification to the airframe, and consideration of other factors such as structural integrity, stability, and thermal integrity. If a longer burn time is chosen, many 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 missile thrust is to be increased, the combustion chamber must be designed or modified to withstand the increased pressures, or the nozzle must be redesigned with a larger throat area to accommodate the increased mass flow rates. In addition, structural modifications may be required to compensate for the higher aerodynamic loads and torques and for the different flight profile that will be required to place the warhead on the proper ballistic phase trajectory. Usually a country will design a completely new missile if new turbopumps are available. A proliferant that wishes to increase its liquid-fueled missile’s range may need to consider upgrading all the valving and associated fluidic lines to support higher flow rates. The proliferant will seek lightweight valves and gauges that operate with sub-millisecond time cycles and have a reliable and reproducible operation time. These valves must also accept electrical signals from standard computer interfaces and require little if any ancillary electrical equipment. A country may use higher energy propellant combinations in existing missile designs with relatively minor structural, material, and turbopump modifications. Technology requirements would focus on thermal protection for the thrust chamber and improved injector design.

A solid-propellant missile differs in overall operation because it simply burns propellant from an integral motor chamber. A proliferant seeking to make longer range solid missiles generally has to stage the missile (either in parallel or serial); strap on additional whole motors or motor segments; improve the stage fraction; or improve the propellant. When a nation chooses to stage an existing missile, it may be able to procure the first stage of a serially staged design, which is larger and more difficult to manufacture, and simply add an indigenous smaller upper stage of its own. A key determinant of a missile’s utility as a first stage is the performance specification of thrust-to-weight ratio. Whole missile systems used as a first stage must produce a thrust-to-weight ratio greater than one for the entire assembled multi-stage missile. Missiles that may fall below the Missile Technology Control Regime (MTCR) guidelines are still of interest because they might be used by proliferants as upper stages of serial staged missiles or as strap-ons.

Once a country can indigenously produce a solid rocket motor, few, if any, components do not automatically scale from more basic designs. If a proliferant desires a more advanced solid rocket fleet, it may choose to build the missile case from carbon graphite or more advanced organic matrix materials. To support this, it will need to import either filament winding machines, an equivalent manufacturing process, or the finished motor cases. A proliferant might import the finished filament wound cases without propellant if it chooses to use a manufacturing technique pioneered in the former Soviet Union known as “cartridge loading.” Cartridge loading allows the propellant to be inserted into the case after it is manufactured. The competing manufac-turing procedure, known as “case bonding,” usually requires the case, propellant, and insulating liner to be assembled in close proximity at the same site, though it is still possible to import empty cases for case bonding. Designs employing propellants with higher burning temperatures require many supporting components, including better insulating material to line the inside of the rocket case and stronger or larger thrust vector control actuators to direct the increased thrust.

The three separate flight functions performed by the guidance, control, and navigation subsystem generally require separate technical considerations. Guidance refers to the process of determining a course to a target and maintaining that course by measuring position and attitude as the missile flies (while, at the same time, steering the missile along the course). Control generally encompasses the hardware and software used during the missile’s burn phase to change the missile’s attitude and course in response to guidance inputs and to maintain the missile in a stable attitude. Navigation concerns locating a target and launch point and the path that connects them in three-dimensional space. An effective design requires that all three functions operate in concert before and during flight for the missile to reach its target. Some of the hardware and software in each feature overlaps functions.

The aerodynamic and inertial properties of the missile and the nature of the atmospheric conditions through which it flies determine the speed with which guidance commands need to be sent to the control system. First generation TBMs, such as the SCUD and the Redstone, have fins to damp out in-flight perturbations. The rudimentary guidance systems used in these missiles do not support rapid calculations of position changes. When a missile’s thrust vector control system becomes responsive enough to overcome these perturbations without aerodynamic control surfaces, these fins are usually removed from the design because their added weight and aerodynamic drag diminish the missile’s range.

Most TBM designs have a resonance around 10 Hz (cycle time of 100 millisec-onds). Calculations to correct disturbances must occur within this cycle time. Guid-ance and control engineers generally add a factor of safety of two to their cycle time or, in other words, half the cycle time. When thrust vectoring is the exclusive control standard of a missile, the system must respond or have a major cycle time of 50 milli-seconds or less. When fins are used, the control cycle time for a missile may be much longer than a second.

As the guidance and control subsystems work together to keep a missile stable and flying on its trajectory, all the components of these subsystems must operate within the major cycle time. Guidance computers, for instance, have to accept acceleration, angular position, and position rate measurements; determine if these positions are proper for the missile’s course; and correct any deviations that have occurred in the flight profile. Computers of the i8086 class, and later, are capable of making these calculations in the times required. In addition to the calculation procedures, all the control hardware must reliably and repeatedly accept the control signals generated by the flight computer and effect the commands within the cycle time. Since some of these operations must occur in a specific sequence, the sum of all operational times in the sequence must be much shorter than the major cycle time. Therefore, valves, electric motors, and other actuators must produce steering forces within 50 milliseconds to support an unfinned ballistic missile control system. When the missile has fins, the allowable response times increase, permitting the hardware operational specifications to be greatly reduced.

In addition to the cycle time, the control subsystem must also hold the missile within acceptable physical deviations from specified attitude and velocity during its short burning period. Missiles with autonomous control systems generally rely on acceleration measurements rather than position measurements to determine attitude and position rates. However, positional indications can be substituted if the positional variables can be determined quickly and accurately enough. Position measurements reduce the control system cycle time by generally reducing the computer integration of accelerations that are required to determine position. Positional measurements also do not suffer the degradation in performance that occurs with time, acceleration force, and vibrations on measurement instrumentation that supports acceleration measurements. Multi-source radio signals that allow a triangulation of position offer an alterna-tive to acceleration measurements. Advanced missile powers dropped radio guidance in the 1960’s and switched to autonomous inertial measuring units, which are carried onboard the missile. The United States considered radio guidance again in the late 1980’s for mobile missiles but dropped the idea in favor of a Global Positioning Sys-tem (GPS). Nonetheless, if a proliferant chose to build a radio guidance system, it could transmit signals from the launch site, or it may build an accurate transmitter array near the launch site to create the signals. Guidance engineers often refer to this latter technique as using pseudolites. However, radio command and control schemes, because of the immediate presence of a radio signal when the system is turned on, alert defenses that a missile launch is about to occur. However, performance for these systems degrades because of the rocket plume and radio noise. Also, these systems are very much subject to the effects of jamming or false signals.

On the other hand, GPS and the Global Navigation Satellite System (GLONASS) are unlikely ever to be used in the control function of a ballistic missile. The best military grade GPS receivers produce positions with an uncertainty of tens of centimeters. If a missile has two of these receivers in its airframe spaced 10 meters apart, the best angular resolution is roughly in the centi-radian range. TBMs require milliradian range angular accuracy to maintain control. However, GPS has significant application for an TBM outfitted with a post-boost vehicle (bus) or attitude control module that navigates a reentry vehicle to a more accurate trajectory.

Older, less-sophisticated guidance systems perform less navigation than modern TBMs. In the older TBMs, a launch crew sets the azimuth to the target at a mobile site and the control computer determines when the missile is traveling at the proper velocity and velocity attitude angle to achieve the desired range. These three properties, in addition to random winds at the target and errors that accrue in the guidance instruments, uniquely determine where the missiles land. Any technologies that allow a proliferant to position and target its missiles in the field quickly reduces the time defending forces have to target and destroy the missile. GPS allows a mobile launch crew to operate more quickly in the field when not launching the missile from a pre-surveyed launch site.

When no in-flight update of position is given, a crew must set a reasonably accurate azimuth before the missile is launched. To be consistent with the overall accuracy of an older missile, such as the SCUD, which has a non-separating warhead, the crew must strike an azimuth line within 1 milliradian of the actual azimuth to maintain a satisfactory cross range accuracy. With military grade GPS receivers of 1–3 meter accuracy, the launch crew must survey no further than 1 km from the actual launch point to support a 1-milliradian azimuth. Pseudolites or differential GPS will either reduce survey distance required or increase accuracy—whether using military or civil-ian GPS signals.

Any technologies that allow for the separation of a reentry vehicle after the boost phase assist the proliferator in two ways. First, a separating warhead is often more accurate than a warhead that reenters while still attached to the main missile body. Secondly, the separated warhead produces a much smaller radar cross section (RCS), thus making the warhead harder to locate.

Technologies that assist a country in separating its warheads and producing a clean aerodynamic shape for reentry include computer aerodynamic prediction routines, nosetip materials that can withstand higher aerodynamic heating, and space-qualified small missile motors that can steer out accumulated error. Hardware that assists in separating a warhead from a booster includes timing circuits, squibs, and other cutting charges, and if accuracy is an issue, an alignment mechanism. This mechanism might be as simple as aerodynamic fins that unfold upon reentry.

Systems

Several countries purchased SCUDs up to the end of the Cold War, and many of these countries still have arsenals of varying size and threat. These countries include Afghanistan, Egypt, Iraq, Iran, Libya, Syria, and Yemen. The Soviets also sold Syria, Yemen, and possibly Libya, the shorter range SS-21 missile. Egypt, Iraq, Iran, and North Korea all display the manufacturing base and technical prowess to make range extension modifications similar to those that Iraq accomplished before the Gulf War.

In addition to these countries, several nations have built or attempted to build their own TBMs. An inherent capability to produce unique and totally indigenous missiles exists in these countries: Argentina, Brazil, India, Iran, Iraq, Israel, North Korea, Pakistan, South Africa, and Taiwan, and nearing production in Syria. Iran and Iraq must import the guidance and control systems of these missiles; however, beyond those constraints imposed on Iraq by UN sanctions, it has no limitations on its ability to produce 600-km range TBMs.

Both China and North Korea continue to sell missile technology and missile systems. Also, North Korea continues to sell missiles abroad. North Korea has offered the 1,000-km-range No Dong missile, and the Chinese sold between 30 and 50 CSS-2’s, a 2,200-km-range missile, to Saudi Arabia in the late 1980’s. Apparently, the Israeli government acted as an intermediary for shipping Lance missiles to the Taiwanese. Lances are a short-range nuclear delivery system that the United States based in Europe. They can be reverse engineered to serve as strap-ons for existing missiles. Each TBM may cost as little as $1.5 million dollars, so a proliferator with even modest resources can afford to build a sizable missile force. If a country seeks au-tonomy from the world market and wishes to build its missile indigenously, it can purchase a manufacturing plant from the North Koreans or Chinese for about $200 million and purchase critical parts, such as guidance systems, elsewhere. To develop complete autonomy requires a capital investment of about $1 billion dollars.

Technical Assistance

Besides whole systems, many corporations and nations have offered technical assistance during the last 10 years to some emerging missile powers. German firms reportedly assisted the missile programs of Argentina, Brazil, Egypt, India, Iraq, and Libya. Italians have offered assistance to Argentina, Egypt, and India, and the French have participated in missile programs in Iraq and Pakistan.

Most European countries can lend technical assistance to emerging missile powers. The French have a long history of developing missiles not only to support the Ariane space launch capability but to launch the force de frappe nuclear arsenal. The Italians have participated in the European Union space program that helped design and prototype the Hermes missile. While the British relied on American missile programs to supply their TBM needs in the 1960’s, a technical exchange program between Britain and the United States has trained and educated a sizable pool of missile talent from the British Isles. Many Western European nations and Russia are in the process of downsizing their defense industries. As many as 2 million physicists and engineers may become available over the course of the next decade.

As of 1997, the U.S. Government lists at least 11 countries outside of the Former Soviet Union (FSU) and China with programs for producing an indigenous missile. Most of these programs are technologically sophisticated enough to produce a militarily threatening system in a relatively short time. Guidance systems are the principal impediment to most countries in developing their own missile, followed by propellant manufacturing and warhead mating to prevent failure caused by the heat of reentry and vibration during boost.



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