The term cruise missile covers several vehicles and their capabilities, from the Chinese Silkworm (HY-2), which has a range of less than 105 km, to the U.S. Advanced Cruise Missile (ACM), which can fly to ranges of up to 3,000 km. These vehicles vary greatly in their speed and ability to penetrate defenses. All, however, meet the definition of a cruise missile: "an unmanned self-propelled guided vehicle that sustains flight through aerodynamic lift for most of its flight path and whose primary mission is to place an ordnance or special payload on a target." This definition can include unmanned air vehicles (UAVs) and unmanned control-guided helicopters or aircraft.
Cruise missiles pose perhaps the gravest delivery system proliferation threat. They are inexpensive to build and can, therefore, overwhelm current defenses by sheer numbers. They can be designed to be small with low-thrust engines and can penetrate radar and infrared-detection networks. The technology to build them is simple and available to any country that builds even rudimentary aircraft. Finally, since cruise missiles are unmanned, they require no flight crew training, expensive upkeep programs, special hangars for housing, or large air bases for basing. These factors make it especially difficult to collect intelligence on the development of indigenous cruise missiles and to anticipate the developing threat.
Countries can achieve a cruise missile capability by simply buying existing cruise missiles from supplier states and modifying them to meet a particular need, or they can make a complete system from readily available parts. European aerospace firms, the FSU, and the Chinese have all sold many cruise missiles of one description or another to customers in proliferant and industrialized countries. In most cases, the performance of missiles is range limited and, in some cases, even payload limited, and their use as a carrier of WMD is probably confined to tactical applications. With the introduction of new guidance technologies, particularly the GPS, future cruise missiles will be more accurate and attractive to proliferants.
The United States introduced cruise missiles into its inventory when a combination of technologies reached a critical point in their development. Taken together, these same technologies can easily form the underpinnings for a capable unmanned aerial system. Except for Terrain Contour Matching (TERCOM), the 1990's have seen these technologies, or the knowledge of how to reproduce them, become widespread among industrialized and newly industrializing nations. The introduction of GPS and GLONASS eliminates the need for a country to rely on TERCOM navigation. A proliferator is not forced to seek out any other technologies to build a cruise missile, though many, such as rocket-assisted take-off units, may give a combatant more flexibility in using a cruise missile for a variety of combat operations. Many proliferants have the scientific and research base to design airframes and build them to meet the needs of a cruise missile program. Arms control officials in the U.S. State Department and many of its overseas counterparts are attempting to reduce high volume serial production of cruise missiles, particularly ones that support a chemical or biological weapons infrastructure. Consequently, the tables identify technologies that assist the mass production of cruise missiles. Once a country has an assured supply of engines and guidance components, the path to a capable cruise missile fleet becomes easier.
Of the four major subsystems that compose a cruise missile -- airframe, propulsion, guidance, control, and navigation, and weapons integration -- none is expensive in and of itself, and a steady supply of each is available. In the late 1960's, the United States first introduced turbine propulsion systems that weighed less than 100 lb and produced many hundreds of pounds of thrust. These turbine engines, or their lineal descendants, powered most of the early U.S. cruise missile designs and were one of the least costly items. Depending upon the range a proliferant desires for its cruise missile, the powerplant may even be as prosaic as a reciprocating engine with a propeller. The latter, of course, has little hope of disguising its signature from defenses, but the mission profile may allow it to disguise itself as another platform. Even if no signature modification is considered, this type of missile has applications in regional wars where the technology of the defense is not as important as it is to an attacking proliferant.
Currently, GPS receivers provide more capability and accuracy than any targeting strategy requires of the guidance, control, and navigation subsystem. Cruise missiles, being aerodynamic vehicles, do not need the rapid response cycle time that ballistic missiles must have to keep the vehicle under control and on an appropriate track. Avionics systems available for first-generation commercial aircraft are both light enough and accurate enough to keep a cruise missile under control for long periods of time. For navigation, civilian code GPS is priced for the civilian hobbyist market, so purchasing an off-the-shelf navigation unit capable of obtaining 20 m of CEP is within the range of the common pocketbook. This level of accuracy is better than that of the early TERCOM systems installed on U.S. cruise missiles, which made them practical for the first time in the late 1970's.
For long cruise missile flight paths, a country without access to GPS systems must develop a mapping guidance logic for its cruise missile or accept highly degraded performance from an inertial measurement unit (IMU). A proliferant using one or two cruise missiles in an isolated attack from a standoff platform can achieve all of its targeting aims with an IMU, but long flight paths allow errors in the IMU to become so great that the missile may stray far from its target. Also, without an updated mapping system, the cruise missile must fly at an altitude high enough to avoid all manmade obstacles, thereby exposing itself to detection.
Even with GPS, the autonomous cruise missile carrying an on-board map must be supplied with the latest terrain and physical feature changes that have occurred along its course if it flies near the ground. Updated autonomous map guidance systems require large computer storage memories aboard the aircraft with units that can withstand the flight vibrations and possible thermal extremes of the missile over a long-duration flight. These units must be supplied with the latest maps that the delivering nation can obtain. Few nations have the space flight vehicles or high-altitude aircraft to build radar maps from overflights alone. Consequently, these maps will have to be purchased, or the proliferant will have to accept the attrition from missiles lost because of outdated information. The United States and Russia understand the key position that radar maps play in cruise missile guidance and are unlikely to allow the information stored in these maps to be released on the world market. Even if these maps are sold through some clandestine channel, they will quickly become outdated since cultural features change rather rapidly. As an alternative, a country may try to develop another guidance scheme, but the costs for developing a new infrastructure to support a map-based guidance system probably rivals that of the original TERCOM or a GPS constellation itself.
In the absence of GPS, the reliability of the cruise missile targeting philosophy becomes increasingly more problematic. As an alternative, a country may attempt to fly its cruise missile with radio guidance or other commands. Usually radio guidance uses frequencies high enough to operate only on line-of-sight reception. If the country expects to operate in hostile territory or attack at very long ranges, it must control the intervening repeater station to contact these missiles by real-time transmission of flight controls signals and position information.
Since cruise missiles fly relatively slowly and with only gentle accelerations, at the entry level, the airframes of these delivery systems can be built out of inexpensive aluminum of a grade as simple as 2024 - T1. Most proliferants with a basic metal production facility and an access to textbooks on metallurgy have a ready supply of this grade of aluminum. As proliferants design and build more sophisticated cruise missiles, they will undoubtedly substitute composite materials and other more elaborate structural elements in the airframe, but, for the most part, these materials are not needed.
A cruise missile airframe does not undergo particularly severe stress on its flight to a target, it does not pull any high "g" maneuvers, and it does not experience propulsion accelerations associated with gun or ballistic missile launches. Virtually any airframe that is structurally sound enough to be used in an ordinary airplane is adequate for a cruise missile. A designer can use factors of safety of 1.5 or 2 in the design to ensure structural integrity under all dynamic conditions without recourse to structural finite element computer codes, which generally only assist a designer to shave four or five percent from the weight of a design. Still, these technologies are included in the tables because their use does allow a proliferant to build a more capable cruise missile. Technologies that advance the large serial production of inexpensive cruise missiles threaten current defenses built against missile attacks. These technologies include sheet metal processing machines that could form complex shapes, such as those found on the airframe or leading edge of cruise missiles; hydraulic presses or stamping mills that shape the nose cones or turbine inlets; and numerically controlled machines for parts production.
If a country wants to increase the penetrability of its cruise missiles, it must identify technologies that aid in signature reduction, signature masking, or other means to confuse detection systems. Some of these technologies include radar jamming and spoofing technologies; infrared suppression of engine exhaust; paints and coatings that disguise the thermal signature of leading edges; computer routines that predict the flow field around aerodynamic surfaces and the methods to change those surfaces to reduce heat transfer and turbulent flow fields; wind tunnel technology that supports the computer prediction; and computer routines that predict the RCS from a given geometry and predict redesign methods to achieve certain design specifications. The cruise missile is suited for the delivery of chemical or biological agents if it does not fly at supersonic or transonic speeds. Most cruise missiles designed to fly at high speeds are not similarly able to fly at slow speeds without dramatic changes in the wing planform in flight. These changes in wing planform are generally not consistent with cruise missile geometries or packing volumes in the same way they might be in manned aircraft, such as the FB-111. Supersonic missiles generally cannot dispense chemical and biological agents from sprayers since the airstream itself will destroy the agent by heating or shock, but they do deliver nuclear weapons with great efficiency.
None of these considerations are exclusive impediments to a proliferant's cruise missile development program. It is only a general guideline that high-speed cruise missiles make sense as a means to deliver nuclear weapons and low-speed cruise missiles are better suited for chemical and biological weapons.
Bomblets can also be included on transonic or supersonic missiles. These bomblets can be released over a target to ameliorate the airstream problem. After release, the bomblets decelerate, float to the target, and spray their agent into the air. Bomblets reduce the packing fraction of agent within the cruise missile airframe and, therefore, reduce the overall payload of a cruise missile. A subsonic cruise missile equipped with a sprayer dispensing agent from a single tank onboard the missile may simply release the agent into the airstream. In most cases, a large fraction of this agent will be destroyed before it reaches its target. To be more effective, the sprayer must dispense the agent so that it avoids the vortex from the tips of the wings and the disturbed airflow from the fuselage. Technologies that are required to develop bomblets, predict their flight path, or enhance the capabilities of sprayers as a means for a proliferant to deliver WMD from a cruise missile are highlighted.
Three key concerns of the cruise missile threat are (1) range extension to ranges greater than 500 km, (2) the ability to penetrate defenses, and (3) any technologies that reduce the cost of manufacture and therefore increase the size of a cruise missile inventory. In order of priority, the tables first list technologies that assist a country in building long-range cruise missiles. The tables then cover technologies that reduce the signature of a cruise missile and list those technologies that decrease the per unit cost or increase the total serial production of cruise missiles for a fixed price. Finally, the tables include support technologies that may make cruise missiles easier to use, package, or launch. As with each of the other delivery systems subsections, the tables are organized by specific subsystem of the aircraft: airframe, propulsion, guidance, control, and navigation, and weapons integration.
Cruise missiles differ from ballistic missiles as a potential threat because they share so many common technologies with existing vehicles that have been designed for other purposes. As a consequence, a proliferant can obtain much of the hardware to construct a cruise missile by cannibalizing existing commercial aircraft or by purchasing parts and components for the missile from legitimate suppliers. The technology tables serve only as a guideline to alert and inform export control regulators of general categories of technologies as opposed to specific performance specifications.
At least 12 exporting countries-Great Britain, the United States, China, France, Germany, Israel, Italy, Japan, Norway, Russia, Sweden, and Taiwan-have developed cruise missiles with some capability in the hands of proliferants to threaten U.S. worldwide interests. Generally, these cruise missiles are small and have a limited range. While it is possible that they can be converted to deliver WMD, their short range limits their possible targets of interest. They may deliver biological or chemical agents against ports and airfields in regions of concern such as the Persian Gulf, but are not able to attack longer range targets. In addition, cruise missiles, such as the Chinese Silkworm, have many other limitations besides short range that restrict their utility as a WMD delivery system. The missiles leave a turbulent airflow in their wake, which makes it difficult to deliver a sprayed pathogen or chemical agent cloud. They fly along a predictable path towards the target rather than one that can realign itself to match the geometry of the target.
The following cruise missiles are a sample of missiles that are available on the world market and pose less threat as possible candidates for conversion to WMD delivery: the British Sea Eagle, the Chinese Seersucker and Silkworm, the French Exocet, the German Kormoran, the Israeli Gabriel, the Italian Otomat, the Japanese SSM-1, the Norwegian Penguin, the Soviet SSN-2C and its derivatives, the Swedish RBS-15, the Taiwanese Hsiung Feng 2, and the U.S. Harpoon. Older missiles, such as the Silkworm, have cumbersome and slow-moving control surfaces that do not readily adapt to the improvement in position calculation that GPS provides. Moreover, their guidance systems are intended mostly for the missiles in which they are placed and have little transference to a new airframe if they should be cannibalized. In most cases, the ease with which a cruise missile can be built leads a proliferant to build a new missile from scratch rather than attempting to adapt these older missiles for WMD delivery.
Even if the missiles do not pose a significant threat, some aspects of their manufacturing base may migrate to more capable missiles and require close scrutiny. Missiles that contain small turbojet engines can be cannibalized, and the engines can be used in more threatening applications. A proliferant can also glean the knowledge to build these turbojets by reverse engineering the engines or setting up indigenous co-production facilities. Examples of exported missiles with small turbojet engines include the British Sea Eagle and the Chinese HY-4. Israel is offering an upgraded Gabriel, which features the latest in propulsion technology, to overseas customers. Other missiles in this class include the U.S. Harpoon, the Swedish RBS-15, the Soviet SS-N-3, the Soviet SS-N-21, and the Otomat Mark-II. Cruise missiles that have immediate application to nuclear, chemical, and biological delivery include the U.S. Tomahawk and ACM, the Russian SSN-21, the AS-15, and the French Apache.
Harpoons have been exported to 19 countries, including Egypt, Iran, Pakistan, South Korea, and Saudi Arabia. India has received Sea Eagles, while Egypt, Iraq, Iran, Pakistan, and North Korea have Silkworms and Seersuckers, a version of which North Korea now manufactures. Italy has Kormorans, and Taiwan, South Africa, Chile, Ecuador, Kenya, Singapore, and Thailand have Gabriel Mark-IIs. Italy has exported turbojet powered Otomats to Egypt, Iraq, Kenya, Libya, Nigeria, Peru, Saudi Arabia, and Venezuela, while the Swedes exported the RBS-15 to Yugoslavia and Finland. In addition, the Soviets sold the long-range (500 km, 850 kg) turbojet powered "Shaddock" to Syria and Yugoslavia. At the next notch down in technological capability, the Soviets have flooded the world market with 1960's-generation liquid-fueled "Styx" (SS-N-2C) missiles. Algeria, Angola, Cuba, Egypt, Ethiopia, Finland, India, Iraq, Libya, North Korea, Somalia, Syria, Vietnam, Yemen, and the former Yugoslavia have the Styx missile in their inventories.
As the list of customers for the Styx demonstrates, the cost of a cruise missile is within the financial resources of even the most basic defense budgets. Even highly capable cruise missiles such as the Tomahawk only cost around $1.5 million per copy. This cost reflects the most advanced avionics systems and TERCOM guidance. At least one congressional study has shown that with the substitution of GPS, a proliferant could build a cruise missile with a range and payload capability roughly equivalent to the Tomahawk, for about $250,000. Unlike production of the heavy bomber, many countries have the economic resources and technical base to produce this kind of delivery system indigenously.
Though the sale of complete systems on the world market is a concern, that threat is much smaller than the possibility that a country could indigenously design and build a capable cruise missile by cannibalizing other systems for parts it cannot build on its own. Of particular concern are components and parts that reduce the cost of the missile in serial production, reduce the cost of position mapping navigation systems, and increase the range of these missiles.
Navigation and guidance continues to be the pacing item in threatening cruise missile development. The Standoff Land Attack Missile (SLAM) is a derivative of the Harpoon and contains in its nose a video camera that acts as a terminal guidance system. If a proliferant adopts this technology and can position a transmitter and receiver within line-of-sight to the missile from anywhere in the theater, it can dispense with the need for any other kind of guidance system. Israel has developed a capable guidance system that can be used in this application.
The next major subsystem component that enhances the capability of a cruise missile is the powerplant. The United States pursued the cruise missile long before the development of the first lightweight engine technology, so this is not a critical path item towards developing a cruise missile. Still, more capable engines increase the threat of a cruise missile. First, they reduce the RCS of the missile. Next, they increase the range by reducing the drag and power required for control surface actuation. Finally, they reduce other flight signatures, such as infrared cross-section and acoustic emission, that might be exploited in a defense network.
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