The Dawn of a New Military Age
Author: Major General Bengt Anderberg, (Swedish Army)Dr. Myron L. Wolbarsht, (Duke University)
Laser Weapons The Dawn of a New Military Age
Plenum Press, 1992
ISBN 0-306-44329-5 [ Amazon ]
This web page was scanned from pages 107-137
High-Energy Laser (HEL) Weapons
Laser weapon projects have always been shrouded by very tight security. In spite of this, it is possible to follow the general lines, at least, of the high-energy laser (HEL) weapon research field through the open literature. This is especially the case with the development of different laser projects in the United States. Most of the following discussion and evaluation refers to well-publicized U.S. programs. However, this should not lead anyone to believe that the laser weapon business is mainly an American affair. Much work is going on not only in other Western countries but also in the Eastern bloc. Even though our information from the Russian part of the former Soviet Union on the state of their laser technology is very sparse, there can be no doubt that they are working very hard to actualize laser weapon ideas. According to David Isby, author of Weapons and Tactics of the Soviet Army, in 1988 the Soviets were at about the same level as, or even more advanced than, the West in the development of offensive laser weapons. He has also stated that the Soviets had begun practicing weapon applications with a variety of laser technologies that are still in the realm of pure scientific research in the United States.The efforts to develop and field high-energy laser weapons were initiated as soon as the first lasers capable of delivering high energy were introduced in the late sixties. The gas dynamic carbon dioxide (CO2) laser was the earliest truly promising high-energy laser concept and was developed in the United States by AVCO Everett in 1968. This was soon followed by the hydrogen fluoride (HF) and the deuterium fluoride (DF) chemical lasers developed by the United Technology Research Center (UTRC) in 1969. In the early 1970s, all three military services in the United States started research programs largely based on theoretical considerations aimed at investigating the vulnerability of relevant military targets to high-energy lasers.
One of the most important events fostering the development of HEL weapon technology was the Strategic Defense Initiative (SDI) established by the US. President in March 1983. The aim of this program is to use different weapons (many are beam weapons, including various laser types) which will kill incoming intercontinental missiles and warheads mainly outside of the Earth's atmosphere. High-energy lasers represent one of several possible classes of weapons that have been intensively discussed and researched for use in this program. The lack of atmospheric influences is, of course, a significant advantage for all high-energy lasers at the enormous ranges that are involved. Although the conditions in space and within this entire strategic warfare concept are quite different from those on the conventional battlefield, the enormous resources in money and manpower that have been allocated to the development of laser weapon technology within the SDI program have certainly speeded up the progress of conventional battlefield laser weapon programs.
Despite all of the SDI efforts to date, no high-energy laser weapons have even been fielded in space or on the ground. This gives an indication of the magnitude of the difficulties involved and indicates that, for the near future at least, large-scale fielding of HEL weapons designed to destroy relatively hard structures such as aircraft seems unlikely.
HEL TARGETS
The main use of HEL weapons will be for air defense, and vigorous efforts have been made by some countries to investigate seriously the use of HEL weapons for this purpose. Defense staff military planners, scientists, and engineers at industrial research institutes worldwide have worked hard at trying to design and field HEL weapons that will meet the growing threat from increasingly sophisticated attack aircraft, armed helicopters, and a growing number of different missiles, including sea skimmers. In theory, the military requirements are quite simple; the HEL weapon must be able to destroy the airborne targets at night as well as in bad weather before they deliver their load of munitions on a protected facility. If the aircraft release their payload outside the range of the HEL weapon, then it must also be capable of destroying the incoming munitions before they can accomplish their mission.The air defense environment is usually complicated by a high degree of atmospheric pollution, yet, despite this problem, a very high standard of performance will still be required from any laser weapon system. When important targets need to be protected, it is necessary to take into account the enemy's probable use of a large number of attacking aircraft or helicopters equipped with the most modern weapons. In most cases, this could mean four to eight aircraft attacking simultaneously from several directions. Modern technology allows an aircraft to fly toward the target area dose to the ground and deliver its munitions from a very low altitude. In some cases, long-range weapons will be used whose missiles can be launched at the final target from the attacking aircraft well outside the range of the defending laser weapons. Of course, the greater the distance from the target that the launching takes place, the lower is the probability of a successful hit.
Aircraft, helicopters, and missiles are becoming increasingly faster, more intelligent, and much more versatile, which means that all types of air attackers will have to be destroyed or countered before they have time to use their weapons. It is no longer sufficient to neutralize the majority of the attackers; it is now necessary to shoot down or counter almost every one of them. This will become even more difficult as future combat aircraft and helicopters will be harder both to detect by radar and to hit, as they will be protected to some extent against air defense weapons. The attacking enemy aircraft will also be supported by airborne electronic countermeasures, will presumably be well informed, and will be guided from airborne command posts. The attackers may even use smart, almost jam-proof missiles or remotely piloted vehicles.
The air defense of today is composed of a combination of interceptor and fighter aircraft, antiaircraft guns, and missiles directed by trained command control and intelligence systems. Even if, in most situations, the combined effect of all air defense units is sufficient to cope with the present threat, it will not be so in the future. When an important target is to be protected properly, it will be necessary to stop virtually all attacking airborne weapons.
Although modern antiaircraft guns have a high rate of fire and use high-speed projectiles or, at least, ones that leave the gun barrel with a high muzzle velocity, they still need the advance knowledge of the target path or trajectory. The presence of electronic countermeasures aboard enemy aircraft in many cases will give an unacceptable kill probability Furthermore, an artillery system often needs several hits or near misses to down a single target depending on the caliber of the gun in question.
Also, contrary to what is sometimes believed, missile weapon systems do not have a 90% or more hit probability. Even if antiaircraft missiles are properly handled, the very hard and sometimes unpredictable realities of the battlefield have been shown to necessitate devastating revisions of peacetime data and calculations of weapon efficiency. When the enemy countermeasures and evasive actions are added to the quite normal difficulties resulting from a very stressful and life-threatening situation, air defense missile units are often very satisfied if the kill probability exceeds 50% for each missile fired. The true figure is usually less than that. A missile system has a relatively long reaction time from target detection until missile launch, often more than five seconds. If these crucial seconds are added to the five to ten seconds it will take the missile to fly to the target, the possibility of engaging any given target successfully becomes somewhat limited. If the target moves at a very low altitude-"tree skimming"-with a speed of 300 yards per second, it will cover at least 1.5 miles before the missile can possibly hit it. This may not be too problematic if the number of targets is the same or nearly the same as the number of guns or missiles and the enemy is flying at an altitude that makes it possible to engage him. If there are multiple targets for every gun and missile, however, there is a good possibility that a substantial number of them will get through. Any enemy will certainly be aware of these facts and will try to attack important targets with as many weapons and aircraft as possible. The most difficult and extreme case will be when the attacker can launch multiple missiles or bombs at extreme ranges. Thus, it is already very difficult and will become even more complicated in the future to defend high-value targets effectively against airborne attacks with conventional missile and gun system technology.
Some military scientists and staff members have advocated the introduction of the HEL air defense weapons on the battlefield as the only solution to these problems. According to one of the individuals involved in the present development of a German air defense HEL weapon, the following essential military requirements must be fulfilled in order for a future HEL system to cope with even a present-day threat. The air defense laser must have multiple target detection and tracking ability with a target detection time of less than 1.5 seconds. The aiming time should be less than 0.5 seconds for the first target and 0.1 seconds for each additional target in a group. In addition to these extremely short reaction times, an additional requirement is that there be a sufficient amount of fuel to allow ten or more laser shots to be fired within very short time limits. Finally, the tracking and fire control systems must provide for a very high hit probability.
Approaching missiles, due to their small size, high speed, and possible large numbers, are usually more difficult targets than aircraft or helicopters. Only if the missile is dependent upon and equipped with a sensor that is sensitive to laser light does it become easy to disable before it reaches the target. One very interesting application of the HEL weapons is to protect ships against sea-skimming missiles, which have a long flat flight path. That such missiles pose a very serious threat to any surface warship has been recently demonstrated in the Falkland Islands conflict, where EXOCET sea-skimming missiles launched by the Argentineans sank both a destroyer, the HMS Sheffield, and a commercial container ship, the HMS Atlantic Conveyor.
One of the advantages of using a large ship as a base for an HEL weapon is the possibility of using the ship's main drive engines as an electrical generator to power the laser. This means that there will not be any shortage of ammunition and that it will be possible to fire many laser shots within a very short time interval. The laser weapon with its direct line of sight in both elevation and azimuth, its almost zero time of flight, which also eliminates the need for a lead, and an almost unlimited supply of energy seems to be an ideal weapon for the protection of valuable ships. However, there are still many problems; the humid atmosphere and often severe weather conditions surrounding vessels at sea may be the most difficult issues to solve.
If the primary goal for a given HEL weapon is not to destroy the target itself but rather to attack battlefield sensors, night fighting equipment, fire control systems, and other electro-optical devices, less energy will be required, and, thus, it will be much easier to cripple or blind the target. An anti-sensor HEL weapon may thus be used at much longer ranges than a laser weapon designed to burn holes and destroy hard targets, or it can be used at the same range with much less energy. Since most aircraft and helicopters are equipped with several sensor systems, which are a necessary part of any attack against targets on the battlefield, in the air, and at sea, the attack may very well be neutralized, for some time at least, just by destroying or blinding the sensors. This indirect application of the HEL may be more cost-effective than more ambitious attempts to shoot down the aircraft itself.
The HEL weapon can also be given to combat units as a very efficient flamethrower, since it can set fire to flammable objects on the battlefield at very long ranges. The enemy soldiers may be burned out of buildings, grassy areas, brush, and forests. Human beings are afraid of fire, and this application may very well be used as a psychological weapon to terrify the enemy infantry. The risk of setting a soldier's uniform on fire may also have a devastating effect upon his morale and will to go on fighting. However, the high economic cost of this application will almost certainly limit its use to a few very strategic enemy positions.
ENERGY LEVELS AT THE TARGET
One of the basic questions facing the laser weapon designer is what energy level must be absorbed by the target in order to get the desired result. The absorbed energy (E) is some fraction (A) of the product of the power density or intensity (I) present in the laser beam and the emission duration (t). E is measured in energy units, joules (J) or watt seconds per area, usually expressed in square centimeters, I in power units, watts (W) per square centimeter, and the time in seconds in the following equation:
The effectiveness of a laser beam in causing mechanical damage is, thus, dependent on beam power, pulse duration, wavelength, air pressure, the material, and the finish of the target surface. For example, a painted area has a considerably increased energy absorption when compared to an unpainted aluminum plate. The absorption varies widely between different materials and at different wavelengths. The absorption of a ruby laser at 694 nanometers is 11% for aluminum, 35% for light-colored human skin and 20% for white paint. The corresponding figures for a CO2 laser at 10,600 nanometers are 1.9, 95, and 90%. This also indicates that one way to counter a HEL weapon is to choose a very reflective material for the target surface. On the other hand, longer wavelengths emitted by the laser can reduce the effects of highly reflective materials and increase the absorption. Every factor in this very difficult pattern combines to determine the degree of target destruction as well as the final energy level that will be needed to produce the desired effect.
It is obvious that the level of energy required to destroy a target varies considerably depending on the circumstances. Therefore, it is not surprising that the required energy level figures quoted in the open literature also show rather large variations. In spite of this, some numbers may be given which indicate the general range of energy levels.
An aircraft, helicopter, or missile could be hit with an HEL weapon in many different ways that in the end would nullify it. Fuel tanks could be ruptured, or the fuel itself could be caused to explode. Windshields could be shattered, and parts of the control surfaces such as elevators or rudders could be destroyed or disturbed enough to make it impossible to continue fighting. The rotor head of a helicopter or the wing of an airplane or missile could be made to fail, resulting in a crash. Sensors, radars, and other navigation aids could be destroyed; if this destruction occurs during a sensitive and crucial moment in the last phase of an attack, it could result in a crash or an aborted mission. Also, in some situations, an HEL weapon could even explode the ammunition carried by an airborne attacker.
To punch through the metal skin of an airplane requires about 700 joules per square centimeter, although it should be noted that a hole burned in the skin of an airplane may not be sufficient to destroy it in the air or even to make it crash. A more realistic energy level to disable an aircraft may be five to ten times higher, which means that a successful HEL weapon will have to be able to deliver at least 5,000-10,000 joules per square centimeter on the target.
Optical sensors and radomes (plastic radar domes) are much easier to damage; no more than 10 joules per square centimeter needs to be delivered directly on the target. Furthermore, if the laser wavelength is within the sensitive wavelength region of the sensor in question, the energy needed could be extremely low. If the HEL weapon is used as an antipersonnel weapon, that is, as a long-range flamethrower, the energy necessary to burn exposed skin is merely 15 joules per square centimeter, and damage to the cornea, the clear window into the eye, requires only 1 joule per square centimeter.
An air defense HEL weapon designed to shoot down airplanes, helicopters, and missiles successfully must have the ability to keep a very powerful beam at one point on the target for a long enough time to deliver at least 5,000 joules per square centimeter. This requires a laser in the megawatt range. If the shot is to be successful, it must be directed to a certain part of the target that is limited in size and very sensitive and then kept there until the desired effect is reached. Thus, the laser beam must track and follow a target if any great length of time is needed to achieve the desired effect.
Many parts of an aircraft or helicopter are highly resistant to an HEL weapon, but there are still enough thin-skin parts and sensitive areas to produce a devastating effect or destruction if hit precisely. On the other hand, it is obvious that at battlefield ranges even an extremely high energy laser weapon cannot penetrate the heavy armor on a tank or other armored vehicles and thus an HEL weapon is of no use for destroying resistant ground targets in the battlefield. However, sensors, optics, and related devices are still valid targets wherever they appear on the battlefield, even in a tank.
CHARACTERISTICS OF HIGH-ENERGY LASER (HEL) SUITABLE FOR USE IN WEAPON SYSTEMS
The rapid development of laser technology has led to hundreds of different kinds of lasers, but only very few of them may be scaled up into the high-energy field. Carbon dioxide (CO2) lasers are the most obvious possibilities for use in HEL weapon applications. Carbon monoxide (CO), hydrogen fluoride (HF), deuterium fluoride (DF), and iodine:oxygen (I2:O2), as well as the free-electron (FEL) and X-ray lasers, along with argon fluoride, xenon fluoride, and many other types of ultraviolet excimer lasers are also candidates. HEL weapons produce a huge internal amount of heat, and prolonged operation at very high powers requires an effective system for the disposal of this wasted heat. In a gas laser, the high fuel flow serves to remove the excess heat, as the fuel is warmed by the laser reaction chamber and, in the process, cools the laser. Most high-energy lasers now under development are gas lasers working in this way. Such a laser will sound and, to some extent, look like a jet engine. Indeed, in the HEL field today, only the X-ray and free-electron lasers are not gas flow systems.The laser in an HEL weapon system has to emit an average beam power of several megawatts during the required exposure time. This power level is two or three orders of magnitude higher than that used by the most powerful industrial processing lasers. This power requirement together with the adverse environment in outdoor use under battlefield conditions makes the design task even tougher. When all aspects of the HEL weapon problem have been considered, very few real possibilities remain.
The gas dynamic CO2 laser is one of the few lasers that shows promise in the HEL weapons field. The fuel may be a common hydrocarbon, for example, benzene (C6H6), which is burned together with an oxidizer such as nitrous oxide (N2O). The fuel can easily be carried in liquid tanks, and the waste gas mixture is nontoxic. The wavelength is between 9,350 and 10,600 nanometers, and, theoretically at least, it is possible to have an average beam power of over five megawatts. The technology for operating this laser is rather well known and highly developed. Of course, there are some disadvantages. The very high output gas temperature has a bright IR signature. That is, the temperature is easily detected by enemy sensors. Also, there is a high risk of causing fire in the surrounding environment because of the hot exhaust gases. This laser will be rather bulky, of comparable size to a battlefield tank. As will be described later, much research is going on to solve the technological problems of high-pressure combustion and adverse changes in beam quality while the atmosphere is being traversed. The use of the gas dynamic CO2 laser seems to be one of the more realistic HEL weapon concepts, and this type of laser has already been used in quite a few military developmental programs but as yet has not become an operational field weapon.
The CO laser operates at several wavelengths within the spectral range between 4,700 and 6,200 nanometers, but poor atmospheric transmission, mainly as a result of water vapor absorption, effectively limits its usefulness to wavelengths shorter than 5,000 nanometers. Electrically excited versions of both the CO2 and CO lasers are not as promising as the gas dynamic versions. Both require a relatively large energy supply with a poor overall efficiency. Even so, electrically excited versions have been tested in some experimental HEL weapons.
The HF laser, operating in the spectral range between 2,500 and 3,000 nanometers, is not the best laser to use within the atmosphere because of very strong atmospheric absorption in that part of the spectrum, but it is relatively cheap and has a simple design. It is probably more useful in military space programs. The DF laser with the same design uses a wavelength of 3,800 nanometers, where the atmospheric transmission is fairly good. The DF technology is mature, and the laser has a low infrared signature and high efficiency with sufficiently good beam quality In spite of the high price of deuterium and difficulties with the chemical pump technology, the DF laser is still a realistic option for a battlefield laser weapon.
The chemical I2:O2 laser is a new and still somewhat unknown high-energy system. A chemical reaction excites oxygen molecules, which transfer their energy to iodine atoms. The wavelength is 1,300 nanometers, which is transmitted rather well through the atmosphere. There is a developmental program for a 50,000-watt iodine laser in the United States, and several reports indicate the construction of Soviet iodine lasers. The information available gives no indication of the future prospects of this laser.
The free-electron laser (FEL) has the potential of generating very high powers and is, therefore, considered very suitable for use as a laser weapon. The SDI program proposes to have an FEL operating on the top of a high mountain directing its beam toward an orbiting relay mirror which will then deliver the energy to a target in space. The big advantage of using the FEL as a battlefield weapon is the capability of selecting a wavelength that is appropriate to the military target requirements and optimizing atmospheric transmittance. However, the possibility of scaling down the present FEL size to one that is useful and practical on the conventional battlefield still seems far away. One of the main centers of research on FEL weapons is located at the Los Alamos National Laboratory, where, in 1989, an existing FEL was adapted for tests within the SDI program. A photo-injector device replaced the cumbersome and expensive electron gun previously used for the creation of the laser beam. The electron gun was truck-sized, while the photo-injector is close to the size of a bread box. Furthermore, the laser beam may be 100 times brighter than those of FELs using an electron gun. However, even the rebuilt FEL with its electron wiggler, all high-voltage accelerators, and the photo-injector will still be a very large non-mobile indoor machine.
Another alterative is under investigation at Stanford University, where the development of a superconductor FEL could lead to very efficient and compact models. In a superconductor system, the magnets are cooled to such low temperatures that the electric currents travel with almost no loss of energy. It will certainly be several years before the FEL technology is mature enough to be used for active service on the battlefield, but if the problems of size and technology can be solved, the frequency-agile FEL will be a prime candidate for tactical HEL weapon applications.
In principle, an X-ray laser beam could destroy electrical circuitry, possibly trigger some types of munitions, set off a nuclear bomb or render it inoperable, and make humans sick or even kill them. The preferred energy source for a very high power X-ray laser is a small nuclear explosion. This makes it almost impossible to contemplate a battlefield HEL X-ray laser weapon. Some research has been done by the Livermore Laboratories in the United States with optical laser-driven X-ray lasers. So far, the output power is modest compared to the input power. Thus, with the present technology, X-ray lasers are not candidates for battlefield HEL weapons.
Two excimer laser systems may be considered HEL weapon candidates-the krypton fluoride laser (KrF) emitting at 249 nanometers and the xenon fluoride laser (XeF) at 350 nanometers. The interest in using excimer lasers for weapons in a manner similar to the FELs has emerged out of the SDI program. Initially, the excimer work concentrated on the use of an HEL weapon mounted on a satellite to be used against nuclear ballistic missiles and warheads in outer space. Later stages of the program have placed the laser in a ground or underground station and reflected the laser beam by an orbiting mirror to the target in much the same way as with the FEL. While the FEL has the possibility of selecting an optimal wavelength, excimer lasers operate at only a few well defined wavelengths. The basic problem is still to overcome atmospheric absorption and scattering. As the atmospheric effects are more severe at shorter wavelengths, the XeF laser at 350 nanometers should be a better choice than the KrF laser operating at 249 nanometers. A high-energy, Raman-shifted excimer laser at 353 nanometers was fired into space in March 1988 with a reported pulse energy of 400 joules, a duration of 0.5 seconds, and a beam width of 20 centimeters. This is believed to be the highest power laser pulse ever sent into space. Other recent experiments at the Los Alamos National Laboratory within the experimental AURORA program, which uses a KrF laser, show that some progress may be possible. The 249-nanometer AURORA laser delivered 1,300 joules to a 500-nanometer spot in pulses lasting 3 to 0.007 microseconds, corresponding to a total peak power on target of 1014 watts. However, this may be compared to the experimental solid-state NOVA Nd:glass laser at the Lawrence Livermore National Laboratory, which, during 1989, delivered pulses of 125,000 joules at 1050 nanometers and 10,000-20,000 joules in the third harmonic at 350 nanometers. Experiments with the NOVA at 350 nanometers are Planned for the 70,000-joule region. The KrF excimer laser cannot presently compete with the solid-state NOVA Nd:glass laser, as the short wavelength of the KrF laser makes penetration of the atmosphere difficult, and this problem remains unsolved. Although the excimer lasers are the most powerful types in the ultraviolet spectral region, the problems with the very short pulses, the short wavelengths, and the special optics required for UV operation make the increase of output power to the same levels possible with the infrared chemical lasers a very difficult task. The highest average powers from excimer lasers are still much lower than can be obtained from infrared chemical lasers.
If we compare all of the alternative laser types for NEL weapon applications, a few remain as feasible short-term possibilities, but it is still doubtful if any cost-effective HEL weapons can be realistically fielded within the next 10 or even 15 years. If any idea of a battlefield HEL weapon still seems valid to staff planners, it will certainly be one that is based on the gas dynamic CO2 laser, the electrically pumped CO or CO2 laser, or the DF laser.
Both the iodine:oxygen and excimer lasers must be considered dark horses. It is questionable if any military requirement now includes plans for the destruction of hard targets. It may be that the really cost-effective solution for HEL weapons on the battlefield is to concentrate the R&D work on the more realistic and limited requirements of attacking sensors, many of which are extremely vulnerable to laser energy.
PREVIOUS HEL WEAPON PROJECTS
There are public reports that a target drone was shot down in experiments by the U.S. Air Force as early as 1969 using a primitive gas dynamic CO2 laser. What has been more widely reported, and even shown on a film in public in 1982 at the annual Conference on Lasers and Electro-Optics (CLEO), is the shooting down of small, winged, propelled target drones as part of some 1973 vintage experiments conducted by laser scientists from the Air Force Weapons Laboratory at the Kirtland Air Force Base in New Mexico. They used a gas CO2 laser of a few hundred kilowatts. The target drones were destroyed by detonating their fuel tanks and by cutting control wires. These experiments were certainly made under almost ideal conditions and only served the purpose of getting a basic knowledge of what could be done with an HEL weapon and what problems were involved. Detailed data and conclusions are still a well-kept secret, but it may be surmised in the end that these experiments simply proved that, in principle, laser weapons could work.One of the first efforts to develop a prototype laser weapon was the Mobile Test Unit (MTU) by the U.S. Army in the mid-1970s. A 30-kilowatt electrically excited CO2 laser was literally squeezed into a Marine Corps LVTP-7 tracked landing vehicle. In 1975, at Redstone Arsenal in Alabama, the MTU destroyed U.S. winged target drones as well as helicopter target drones. No real data are available to the public, but the experiments came to an end rather soon and have been reported as inconclusive. In the late 1970s, a German company, Diehl, worked on a concept for a laser weapon carried by a 28-ton armored tracked vehicle. It was based on a self-contained electrically excited CO2 laser and may very well have been something similar to the weapon employed in the U.S. project MTU. The MTU was followed by the Close-Combat Laser Weapon (C-CLAW), dubbed ROADRUNNER by the U.S. Army. This was designed to attack enemy sensors, night vision equipment, and helicopter cockpits with a combination of rather low-powered Nd:YAG and CO2 lasers. The restricted energy level and the military requirement to support combat units on the battlefield by attacking sensors both place this project in the category of lowenergy laser (LEL) weapons, which will be described in more detail in the next chapter.
In 1978, the US. Navy conducted a series of tests as part of the Unified Navy Field Test Program at San Juan Capistrano in California, in which a chemical DF laser in the 400-kilowatt range destroyed some TOW wire-guided antitank missiles in flight. To direct the laser to this target, which was comparatively small and fast, a Hughes aircraft aiming and tracking system was used. In 1980, a captive UH-1 helicopter was destroyed by this laser system.
The US. Air Force placed a gas dynamic CO2 laser in a Boeing NKC-135 cargo aircraft, dubbed the Airborne Laser Laboratory, and in 1981 tried to shoot down air-to-air AIM-9L Sidewinder missiles while airborne. These tests, performed at the Naval Weapons Center in China Lake, California, were a failure, and, as the planning had been made public in advance, the media could criticize the failure openly The testing continued without any more media coverage, and finally, in May 1983, the 400-kilowatt laser shot down a number of Sidewinder missiles. The program was terminated in 1984, and the Airborne Laser Laboratory ended up in a museum. The aim, to prove that air-to-air and ground-to-air missiles can be destroyed in flight by airborne HEL weapons, had been validated, at least in principle. However, it must be remembered that this laser weapon completely filled a four-engine cargo airplane, and the experiment did not seem to offer any possibility of a weapon that could be carried as add-on equipment on a relatively small fighter to protect it from missiles. In any case, the results must have provided some clues as to how the problems of tracking a target and aiming the laser could be solved.
In 1981, the U.S. Army designed a Mobile Army Demonstrator (MAD), which was based on a small, compact DF laser. The demonstrator was used as a prototype for an air defense weapon against missiles which started at 100 kilowatts but was to be scaled up to 1.4 megawatts. The use of a DF laser poses some difficult problems. The exhaust gases are very poisonous and cannot be vented in the vicinity of friendly forces. The designers tried to solve this by using a closed system which collected the waste gases in a special tank. The tests ran until the project was omitted from the SDI program in the 1983-84 budget. However, development of the laser itself, renamed the Multi-Purpose Chemical Laser (MPCL), continued with U.S. Army funding of Bell Aerospace Textron.
One very interesting HEL development which has been the cause for much debate in the US. Congress is the Mid Infra-Red Advanced Chemical Laser (MIRACL) coupled with the Sea Lite Beam Director (SLBD). MIRACL is a DF laser with a 2.2-megawatt output at 3,800 nanometers. Sea Lite, later called Sky Lite, is the beam steering device for the laser. In the 1988 Strategic Defense Initiative Organization (SDIO) report to Congress, the MIRACL/Sky Lite was described as "the highest power HEL system in the free world." Some rather speculative thought-provoking demonstrations have been made with this system at the High Energy Laser Test Facility at White Sands Missile Range, New Mexico. On September 18, 1987, several vital components were destroyed on a Northrop BQM-74 airborne target drone, which then crashed. The laser test crew had to find, lock on to, and shoot down the drone, which was flying at a speed of 500 knots at an altitude of 1,500 feet. According to the press report, the system downed a Teledyne Ryan Aeronautical Firebee BQM-34S target drone at twice the range in November 1987. Two years later, a Vandal supersonic missile simulating a sea-launched cruise missile was forced down while flying at low altitude and at a range that was "representative of a real tactical scenario." According to the US. Navy, the test demonstrated that "HELs can be a real option for tactical warfare missions." However, the laser is presently too large for a ship, and, therefore, a smaller prototype system may follow, aimed at giving the Navy a shipboard laser weapon that would be able to destroy numerous antiship missiles at operationally effective ranges. Such a prototype is necessary to find out if it is really possible to solve the atmospheric problems in the humid environment at sea. When SDIO reported on the MIRACL/Sky Lite program in 1988, the objectives were given as the
- development and demonstration of a high-power local loop adaptive optics system for improvement of the beam quality of a multi-line infrared high energy laser; development and demonstration of a high power target loop adaptive optics system for ground to space atmospheric compensation in the presence of turbulence and strong thermal blooming; and performance of atmospheric propagation experiments to explore the conditions under which stable correction can be achieved and the degree of correction possible.
The future funding of the MIRACL/Sky Lite Program was heavily debated in the United States because of concern over the size (and cost) of the laser so that continuation of the program seemed in doubt for a while. Some statements made during the debate may be of interest. At one stage, when deletion of the beam director was suggested, the SDIO declared that it needed the MIRACL for its own missile vulnerability tests, similar to the test in which a laser beam destroyed the second stage of a pressurized Titan I rocket in 1985. Even if the SDIO may have had little real use for the MIRACL/Sky Lite as it wanted to explore the FEL, the US. Army had a growing interest in MIRACL for use in short-range missile defense experiments. Some military people urged the continuation of the program with three aims in mind: continuation of Navy anti-cruise missile tests, continuation of experiments on satellite vulnerability, and the tests and experiments cited by the SDIO. Finally, the project got funded for 1989.
Officials from TRW Inc.'s Space and Technology Group at Redondo Beach, California, have been urging the US. Navy ever since to fund a shipboard test program as the next step in the evolution of the HEL technology used in the MIRACL/Sky Lite program. The Navy presently has no funds for such a project, but, according to the Navy's Space and Naval Warfare Command, lasers could play a significant role in naval warfare in the future. As missiles approach supersonic speeds and incorporate stealth-like capabilities, the Navy will need the near-instantaneous targeting and killing abilities inherent in laser technology. According to TRW, their program is ready to move to tests at sea, because the laser already has been tested extensively and modified through development under the SDI program. The problem is the cost, amounting to several hundred million dollars, to deploy the laser system aboard a Navy test ship. Whatever the future of the MIRACL/Sky Lite program, it has certainly managed to create a heated debate in the United States over the viability of HEL weapon systems.
Research and development of high-energy laser weapon systems is proceeding also in France. The system named LATEX (Laser Associe' A une Tourelle Experimentale) consists of a laser in the 10-megawatt range coupled to an advanced aiming system commercially developed by Laserdot. The program was started by the General Delegation for Armament in 1986 and has advanced to a preliminary test carried out at Marcoussis in France over a range of 200 yards against a missile head and an aircraft fuselage panel. It has been reported that trials will now proceed in Landes, in southwest France, against a target flying at 300 yards per second at a range of 1.25 miles. LATEX may be similar in concept to the German air defense laser, HELEX, and the French Ministry of Defense has indicated an interest in cooperation with Germany This kind of cooperation on other systems has probably been going on between these two countries for some time.
GERMANY'S AIR DEFENSE LASER (HELEX)
One of the most interesting HEL weapon projects is the German air defense system called HELEX, which is an industrial joint project between Diehl, Gmb., in Nuremberg and MBB in Munich. HELEX stands for High Energy Laser Experimental. The project is still in its early stages, although the initial work started in the late 1970s. MBB together with Diehl have been commissioned by the Federal Ministry of Defense in Germany to implement and study this experimental system as a continuation of the work done previously In the following discussion, the term HELEX refers to the industrial conception of the final weapon to be delivered to combat units if the experiments are successful. The project is interesting, not only because a comparatively large amount of information has been made public so far, but also because it tries to meet a precise military requirement. Since this is not only a research program but also a very extensive development program aimed at producing a well-defined laser weapon for a future battlefield, it will be described in detail. The idealized conceptualization. is given in Fig. 5.1.Germany has a long common border with Poland and Czechoslovakia, which were Warsaw Pact (WP) countries, and the distances from important targets inside Germany to WP air bases and missile sites were very short. The time between an airborne attack launched from the WP air bases across the border could be extremely short, lasting only minutes. Thus, Germany was very vulnerable to low-level air attacks by combat aircraft missiles and standby weapons with the capability of engaging targets automatically. However, the distances are still comparatively short, and, even though the warning time is slightly longer, this limited distance will still be a problem for Germany's air defense. The present-day German air defense is heavily dependent on ground to-air missiles, fighter planes, and sophisticated chains of radar stations which feed the command and control system with information. In spite of all the money spent so far on this very complicated air defense system, it may be insufficient to counter future threatening situations in which the other side will use an increasing amount of more and more sophisticated electronic countermeasures. Air defense laser weapons could be one way to achieve the extremely short warning and engagement times that Germany will eventually require.
FIGURE 5.1. High-energy CO2 laser system. The laser energy is directed
toward the target by a highly controllable large mirror, which, on its
scaffolding, can go over buildings, trees, and other ground
obstructions. Photograph courtesy of MBB/ Diehl.
The main component of the HELEX is a gas dynamic carbon dioxide laser which emits an average beam power of several megawatts over the specified mission time. To carry the laser and all of its accessories, the basic chassis from a German tank, Leopard 2, has been suggested. The supply tanks for gas, water, etc. are used for the laser fuel, while the laser itself and its coolant water are carried in the chassis. As laser weapons have a direct-line-of-sight action, it is important to position the laser beam above the tops of surrounding trees and buildings. This problem is solved by using an elevator platform to carry a focusing mirror of more than one yard in diameter along with the passive surveillance and target acquisition system. The area of coverage of the HELEX will also be greatly increased by the elevated platform, since the time between the identification of a target and the laser hit is very short, and it may be possible to engage very low flying targets that quickly appear and disappear out of the immediate field of view.
A relatively simple technical principle has been used for the HELEX. The high-energy gas dynamic laser employed does not need a heavy and complicated gas pump or flow system nor does it require sophisticated cooling. The fuel is a common hydrocarbon burned together with a nitrogen compound oxidizer, both of which can be easily carried in the liquid storage containers. The hot gas flows at supersonic velocity through a comb of very fine nozzles, expands, and is transformed into the population inversion state required to amplify the laser energy. The gas then flows at supersonic speeds through an optical resonator (mirrored cavity), where stimulated emission occurs, and the laser beam is finally created. The beam leaves transverse to the gas flow direction. The used, nontoxic gas is vented into the atmosphere through a diffuser. At the same time, the exhaust gas carries off most of the waste heat. Overall, the function of the laser is similar to that of a rocket engine.
The emitted power of the high-energy gas dynamic laser is proportional to the amount of fuel used. The research to date indicates that the dimensions of even very high energy laser equipment will remain within acceptable limits from a technical point of view. The fuel consumption per laser shot corresponds roughly to the weight of a guided missile, but the fuel consumption of future-generation systems should be lower. If these estimates are correct, an HEL weapon like the HELEX should be able to fire something like 50 laser shots with the amount of fuel (5-10 tons) carried in the tank.
The wavelength of the HELEX system may be either 9,350 or 10,600 nanometers. Most reports on the system indicate a wavelength of 10,600 nanometers. However, the shorter wavelength may be a more appropriate choice, since the larger the focusing mirror is relative to the wavelength, the smaller the focal spot and the higher the energy density will be. Obviously, the desired effect requires as high an energy density as possible.
The optics of the HELEX Must cope with the difficult task of focusing enough laser energy on the target to destroy it in the air or cause it to crash. This has to be done on the battlefield even when the atmospheric conditions are unfavorable and at a combat range of at least five to ten kilometers if the HELEX is to be cost effective within the air defense concept.
Only mirrors suitable for use at the wavelength and high power levels of this system can be used to direct and focus the beam. The use of transmission optics such as lenses is not very feasible due to their high cost and fragility, and, in any case, the HELEX Will probably damage any lenses to some extent. The reflector at the top of the elevated platform is a concave mirror with a diameter of more than one meter. To achieve a sufficient effect at the target range, compensations for atmospheric turbulence, blooming, and other disturbances to the laser beam inside and outside the system are planned with an adaptive mirror. The mirror surface can assume the required shape and the correct axial angles with the aid of numerous piezoelectric (small electronic) actuators exerting mechanical forces on the mirror back. To enable the mirrors to withstand the HEL beam, a cooling liquid flows through fine channels on the rear of the mirror. Compensation by adaptive optics may double the range possible with a rigid mirror system.
The information necessary to control the mirror surface shape is furnished by the laser beam reflected by the target; thus, the beam itself becomes a sensor element in the closed control loop by which the target is tracked. It is a difficult problem to achieve a really high precision laser beam, and it is necessary to keep a focused beam on a single location of an extensive target for a considerable time. If a target moves at the speed of sound (Mach 1) and the beam must be coupled with it for, at least, a half a second, during this time the target will move nearly 105 yards. Keeping the laser on the same spot may be done either by using the variable reflection characteristics of the target or by a procedure where the deviation of the beam center from reference marks on the target is used as the control signal. Diehl has demonstrated this procedure by means of a rotating aircraft model.
It is not only necessary to keep the beam directed to the same spot on the target, but it is also a prerequisite for the HEL system that the beam can be focused correctly. Basically, the mirror at the top of the elevated platform functions just like a burning glass which concentrates the energy of the sun to such an extent that combustible material catches fire. The advantage of the coherent laser energy is that it can be focused sufficiently over distances of many kilometers to produce thermal effects at the final site. Adaptive optics can be used to focus the beam continuously, even as the target changes its position.
The HELEX will have some type of a passive surveillance and target acquisition system, such as satellite monitoring, which will probably cover the entire hemispherical air space of the protected zone and permit tracking of numerous targets simultaneously This is also the prerequisite for sequential engagement of targets by the laser weapon without any delay. The passive target acquisition makes radar surveillance and tracking unnecessary, and, as a passive surveillance system is used, it may be very difficult for an airborne attacker to find and counter the system beforehand by any electronic countermeasure activity. The HELEX will make it possible to carry out identification, threat analysis, and target selection and finally to hold the beam on the target on automatic, or, if desired, part of the sequence can involve a human operator to select target priority. However, the choice of the best or, at least, a suitable spot to hit on the target has to be done automatically to cope with the time constraints.
If the research and development of the HELEX air defense laser weapon is successful, battlefield commanders will have a powerful tool to cope with highly threatening situations. One air defense HELEX could effectively control an area against multiple low level, high-speed attackers with comparatively low operating costs. The effective range will be dependent on atmospheric conditions. Under very favorable conditions, the range against aircraft, helicopters, and missiles would be up to 6 miles; this would be reduced to 3 to 4 miles in the normally heavily polluted atmosphere over a battlefield. Due to the extremely short time for target detection, tracking, slaving, and firing, it would be possible to engage many targets in rapid succession. If one HEL weapon is defending a facility that is attacked by a squadron-sized enemy force, the laser weapon may very well shoot down all aircraft during their first attack. Reloading is simple; there is no minimum range, and different types of targets do not require the use of different types of ammunition. The main limitation of such a weapon as the HELEX is the reduction in range of the system under very poor weather conditions or when the pollution on the battlefield is extremely heavy. It is difficult to quantify these limitations, but it is obvious that the HELEX will not replace conventional gun and missile systems, not even at distances well within its range. Such HEL weapon systems will only be able to complement existing air defense systems. However, the survivability on the battlefield of a HELEX type system compared to a system dependent on radar technology will be very high, since the passive localizer will not reveal itself. Also, the mobility of a 20-40-ton tracked HELEX system will be high, and it will be possible after terminating one firing action sequence to change the location of the weapon quickly.
Many problems still must be solved before it is even possible to decide if the HELEX concept is a valid one. To date, tests have only been done in the laboratory The scaled-down experimental weapon paid for by the German Ministry of Defense will not be available until 1993 or 1994. If this weapon is a success, and if it is possible to solve all of the very difficult problems, the development of a final air defense high-energy laser weapon based on the HELEX concept may start in the mid-nineties and should be completed about ten years later. This means that theoretically such a weapon could be produced and handed over to the combat units at the beginning of 2005. Due to the technological difficulties involved in this concept, even such a distant delivery date may be overly optimistic.
Other countries have begun developmental work on possible laser weapons along similar lines. In France, several companies together with the French National Aerospace Research Agency (ONERA) are working on a HELEX-like experimental HEL weapon. There have also been some reports on a possible collaboration between France and Germany. In the United States, a similar idea is currently under investigation in the JAGUAR project.
The military specifications for the HELEX weapon are really very ambitious, and this, along with the technological difficulties, is the main reason for the high costs and the very long time necessary for research and development. It is debatable whether or not it would be more cost-effective to limit the requirements to simply damaging some very sensitive parts of the target such as sensors, canopies, and radomes and leave the actual destruction of the platform itself to conventional antiaircraft guns and missiles. This would mean that, up to 6 miles, much less energy would be required, and the sensitivity to the atmospheric conditions should be less. Such a weapon could possibly be fielded much earlier and at a significantly lower cost. Of course, there are even some limitations to this less demanding military requirement. Some of the targets are not all that dependent on their sensors, and, even if they are, it may be possible in the future to make the most crucial sensors insensitive to the effects of laser energy- Whatever the future holds for the HELEX, the fact remains that a high-energy air defense laser weapon capable of outright destruction will be expensive to develop and manufacture, and it will take many years before such a weapon can be successfully fielded. It is very possible that the whole idea will be abandoned because it simply proves technically impossible or just too expensive to implement.
HEL WEAPONS IN THE SOVIET UNION AND THE NASCENT REPUBLICS
Very few facts are known to the public about the research and development of HEL weapons in the Soviet Union or its surviving constituents. Some official reports and statements are available as well as some material by independent writers, but most are of a very general nature. This, of course, is not surprising; all work on laser weapons in the West is shrouded in security, and very few facts are made public. This is even truer in the Soviet Union.However, the fact that so many papers on high-energy lasers and their effects have been published in Soviet scientific journals is an indication of the amount of work done in this field and, thus, reveals the strong interest of the Soviet Union in this technology. The papers, of course, deal only with basic laser technology and not with the details of developing laser weapons. There have been unconfirmed reports of the installation of a high-energy chemical laser on a Kirov-class cruiser. The HEL weapon was said to be successfully used against the sensors of sea-skimming missiles out to a range of 10 miles. If this is true, it may have been some kind of experimental installation, as the existence of such a weapon has not yet been confirmed. There have also been descriptions of the Soviet research facility at Sary Shagan in Kazakhstan following a visit by a delegation of U.S. scientists in 1989. A beam director with a diameter of approximately 1 yard was connected to a ruby laser and to a carbon dioxide laser and, according to US. analysts, had been used in tests against both aircraft and satellite targets.
The most powerful laser at Sary Shagan was reported to be a 20-kilowatt CO2 laser. Another U.S. delegate from the US. House Armed Services Committee in 1989 reported the existence of a previously unknown high-energy megawatt-range Soviet laser, seen when he visited the Kurchatov Institute of Atomic Energy in Troisk, a center of scientific research south of Moscow. It was a 1-megawatt CO2 laser, and the Soviet officials claimed that it was unique in the country and that they had been operating it for several years.
In January 1987, the Pentagon published an edition of Soviet Military Power including a photo which was identified as a laser device on a Soviet destroyer that has been used in the past against Western patrol aircraft (US. Department of Defense, 1987). If the picture really shows such a laser, it could only be a low-energy laser with no capability of destroying aircraft or missiles but rather with a blinding effect for sensors and eyes. The same publication states that
- the Soviets have built high energy laser devices up to the 10 megawatt level and generally place more emphasis on weapon application of lasers than does the West. in doing so, the Soviets have concentrated on gas dynamic and electric discharge lasers. They have not attained a high power output for chemical lasers as the West.
ASSESSMENT OF LASERS FOR FUTURE HEL WEAPONS
The development of HEL weapons will certainly continue in both the East and the West based on the present as well as new concepts of HEL weapons as long as there seems to be a reasonable possibility of solving the problems involved in fielding a cost effective system. Military staffs and research centers will probably stick to the concept of air defense of important targets and of attacks on sensor systems as the main area of use. As long as the Eastern bloc and the Western SDI programs allocate substantial financial resources to laser technology, there will continue to be spin-offs to tactical HEL weapon projects for the conventional battlefield.Besides the CO2, CO, and chemical DF HEL weapons exemplified by the German HELEX and the U.S. MIRACL systems, research and development of HEL weapons will also progress based on other laser concepts such as the U.S. ground-based free electron laser (GBFEL). The FEL may be a future choice in HEL weapon applications for anti-sensor and air defense tasks. According to some reports, very efficient FELs are on the horizon, and, with the new superconductor technology, very compact and efficient FELs may soon be possible. Another re ort gives the impression that low-power tunable FELs are under development as short range high-energy weapons. Some work will certainly proceed, although the statement that "in the weapons field, contemporary FELs promise to become the germ of the ray gun of the future which hurls powerful bolts of energy at the enemy" seems a bit premature.
Another laser test program of the US. Air Force involves a moderate-power Raman-shifted excimer laser device (EMRLD). The work is performed at the Kirtland Air Base Weapons Laboratory in New Mexico, and the stated goal is to produce more than 5,000 watts at 100 pulses per second. Some of the technology developed in this way may finally be used in real battlefield systems.
A development project is also under way, or, at least, planned, with an iodine:oxygen laser. This is indicated by the worries of some administrators that MIRACL would compete for funding with other beam projects such as iodine:oxygen, chemical, and excimer lasers.
Thus, although there are some promising lasers that may form the basis for HEL weapon systems usable on the conventional battlefield, there are still some unsolved problems. Even if the laser can achieve a sufficient energy output, the atmospheric conditions still severely limit the practical use of HEL weapons. So far, no HEL weapon program seems to have solved this problem, and it is still somewhat uncertain whether or not it is really possible to do so. Techniques such as the use of an active mirror that can adapt instantly to varying conditions have still only been demonstrated in small-scale models.
CONCLUSIONS
The interest in HEL weapons is high in many countries due to the great advantages that they theoretically offer. The HEL energy travels at the speed of light with a flat trajectory and acts almost instantly on the target. These are qualities which, if they can be used in an air defense situation, for example, can neutralize even the smallest and fastest missiles. Therefore, it is no wonder that HEL weapons continue to fascinate military staffs and military laboratories sufficiently to fund further research and development. Also, in some countries, spin-offs from SDI programs have contributed heavily to knowledge about and interest in HEL weapons.However, the use of HEL weapons within the atmosphere presents severe problems. Thermal blooming, turbulence, scattering, and absorption all have very negative effects on the laser beam, and these difficulties grow rapidly with increasing range. Battlefield conditions mostly imply a heavily polluted environment, which adds still more problems. We conclude that, even if adaptive optics and other technological problems can be solved in the laboratory, still no operational weapon is within sight. To date, it is not even known if a usable, cost-effective solution is possible in the distant future. Assuming that all the problems could be solved satisfactorily in the near future, the earliest HEL weapons could not be handed over to combat units until the beginning of the next century.
The difficulties associated with the development of HEL weapons are, to a great extent, generated by the very tough military specifications to literally burn holes in airborne targets at long ranges. This requires megawatt lasers with a range of at least 3 to 6 miles in a hazy atmosphere. If the requirements could be limited to attacking sensors and other highly laser-energy-sensitive parts of the target instead, it would be possible to field an HEL weapon earlier and at much lower cost. Since most important airborne targets are highly dependent on electro-optical devices and other items sensitive to laser damage, a medium-sized HEL weapon could very well turn out to be a cost effective weapon.
