Fighter Weapons as a Drone Pilot

Fighter aircraft exist to destroy other aircraft. The airplane itself may be considered only a weapons platform designed to bring the weapons system into position for firing. Fighter weapons have varied greatly over the years, and each weapon has had unique requirements for successful employment. The requirements might include effective ranges, aiming, relative position of fighter and target, or any number of other factors. All of the requirements of a particular weapon must be satisfied simultaneously in order for the weapon to be used successfully. Meeting these weapons-firing requirements, while frustrating those of the enemy, must therefore be the goal of all fighter tactics and maneuvering.

Before fighter tactics can be discussed effectively, an understanding of weapons systems must be developed, since these weapons are the driving forces behind tactics. This chapter discussed the major classes of weapons which have been used by and against fighter aircraft. Included in the discussion are operating characteristics, operating limitations, and countermeasures associated with these weapons.

Air-to-Air Guns

The gun is by far the most widely used and important air-to-air weapon ever devised. The story of the adaptation of this weapon for aircraft use is very interesting and has been the subject of several other works, so it will only be treated in summary fashion here.

Aircraft guns may be classified as “fixed” or “flexible.” Fixed guns installed in a stationary position relative to the aircraft, usually forward firing, and are aimed by pointing the entire fighter. Flexible guns, although fixed to the aircraft, may be aimed up, down, and from side to side by the operator to cover a certain field of fire, which may be in any direction relative to the aircraft. Such guns may be manually operated or installed in power turrets.

Fixed, forward-firing guns have many advantages for small, maneuverable fighters. Their installation is generally lighter and produces less drag, so they have less negative impact on performance. Flexible guns usually require a dedicated operator in addition to the pilot, which further adds to aircraft size and weight. Maneuvering relative to another aircraft is also much simpler when the opponents can be kept in front of the attacker, which essentially requires a forward field of fire. For these and other reasons, fixed forward-firing guns have been found to be superior for small, offensive aircraft (fighters), while flexible guns are generally preferred for the defense of larger, less maneuverable aircraft.

By trial and error, fighter armament in World War 1 progressed from personal side arms to flexible machine guns, and, eventually, to fixed machine guns. The standard fighter at the end of this conflict has two .30-cal-class fixed forward-firing machine guns, which often were equipped with synchronizers to allow fire through the propeller disc.

The tremendous progress in aircraft performance during the 1920s and 1930s was in large measure the result of the intense interest generated by the many international speed competitions of those years. Aircraft structural methods were also revolutionized, as essentially all-metal construction became standard. These developments, as well as the lessons of World War 1 on the value of firepower, led to significantly increased fighter armament by the outbreak of World War 2.

The reasoning behind these developments is fairly clear. First, increased aircraft performance allowed the wight of greater armament to be carried. Second, designers recognized that the higher closure weights resulting from faster aircraft speeds would, in general, lead to shorter firing times, so more destructive power was necessary in a shorter period of time. Third, metal aircraft, particularly the bombers, were much tougher targets, and increased performance enabled the planes to carry additional armor that could be used to protect vital areas of the aircraft (armor for World War 1 fighters sometimes was an iron stove lid in the pilot’s seat).

These developments created a need for greater firepower, which could be achieved with more guns, larger projectiles, higher rates of fire, greater muzzle velocities, or explosive bullets. Some pairs of these factors, however, are related in such a way that neither member of the pair can be increased independently. Probably the most important of these relationships is that between the projectile weight and rate of fire. In general, the greater the weight of the shell (including bullet, charge, and casing), the slower the rate of fire, owing primarily to the inertia of the heavier moving parts required to handle this ammunition. Obviously, depending on the gun technology at a given time, there should be an optimum balance between these two factors. As guns and ammunition are made lighter for a given projectile weight, the optimum balance shifts toward heavier bullets. Another factor in this equation involves target vulnerability. The greater rate of fire possible with smaller bullets results in an increased probability of registering a hit, but greater projectile weight generally leads to more target damage given that a hit occurs.

Some of the armament variations of the combatants during World War 2 can be explained by this factor. For instance , bombers generally are relatively large, poor maneuvering aircraft that are fairly easy to hit but hard to destroy because of the armoring of vital areas and greater redundancy of important systems. Such a target may best be destroyed by fewer numbers of more destructive projectiles. The opposite may be true of smaller, highly maneuverable fighters, which are usually harder to hit.

The search for more destructive projectiles led to the development of the aircraft cannon. A cannon is essentially a gun that fires explosive bullets. In general, these explosive charges are armed by the firing accelerator of the shell, and they explode on contact with the target. Although some use was made of the single-shot cannon in World War 1, truly-effective automatic cannon were developed between the wars. These were generally 20- to 40-mm weapons and had projectiles significantly larger than those of the .30- and .50-cal-class machine guns in common use, with correspondingly lower rates of fire. The cannon themselves were also larger and heavier, leading to further tradeoffs in usable aircraft space and in performance.

The many variations and exceptions of aircraft armament used in World War 2 cannot be discussed in detail here, but some general trends deserve mention. The firepower of the earlier fighters was invariable increased in later versions of the same aircraft, as in new fighters introduced during the war; increased projectile / target specialization also was apparent as the conflict progressed through its various stages. For instance, U.S. fighter designers, primarily concerned with German and Japanese fighter opposition, tended to stay with high rate-of-fire machine guns. The standard armament of the more important U.S. fighters (P-51, P-47, F4U, F6F) at war’s end was six or eight .50-cal Browning machine guns. These were usually mounted on the wings, where there was more room and no requirement for synchronization, so that the full rate of fire could be developed. German designers generally employed a combination of cowl-mounted (often synchronized) and wing guns, and they tended to use cannon for more potency against heavy bombers that were their prime concern. Late in the war the Me 262 (jet) and the Me 163 (rocket) fighters, primarily used as bomber interceptors, employed four 30-mm cannon and / or 50-mm unguided rockets. Even larger guns were successfully used by both sides in an air-to-ground role, as were unguided rockets.

The advent of wing-mounted guns led to increased problems with bullet dispersion. When all guns were cowl mounted, they were simply bore-sighted to fire essentially straight ahead (the sight might be aligned to allow for the normal gravity drop of the bullets at a selected range). But when guns spread out over much of the span of the wings, bullet dispersion became excessive, leaving large holes in the bullet pattern at some firing ranges. The “lethal bullet density” was increased by a method known as “harmonization,” which generally involved one of two techniques.

“Point harmonization” aligned the outboard guns slightly towards the aircraft centerline so that the bullets met at a point that was assumed to be the optimum combat firing range (normally 700 to 800 ft). This method resulted in maximum lethal density near this particular range but led to wide dispersion at much longer ranges. Point harmonization was often preferred by the pilots who had the best marksmanship and were confident they could place this maximum density point on target.

For most pilots, another method, known as "pattern harmonization," yielded better results. This involved adjusting each gun individually slightly up, down, left, or right to produce fairly uniform bullet pattern of a certain diameter at the harmonization range. Although maximal lethal density is not achieved in this manner, the average fighter pilot had a better chance of getting hits. The advantages of this method were much like those of a shotgun over those of a rifle. More lethal projectiles also favored this technique, as maximal density is not necessary.

Mounting guns such that their line of fire does not extend through the aircraft center of gravity (CG) introduces other problems. Particularly when wing-mounted guns are located large distances from the CG, failure of a gun to fire on one side can cause the aircraft to yaw significantly, greatly complicating aim. Aircraft designed with asymmetrical gun mounts ofter require some automatic aerodynamic control coordination, such as rudder deflection, to compensate for these effects.

The recoil action of heavy, rapid-fire guns can be considerable and can often cause significant speed loss for the firing aircraft. At slow speeds, especially under asymmetrical firing conditions, this recoil can cause stall and subsequenct loss of control.

With the advent of jet aircraft, one further complication has arisen to the mounting of guns. The gun gases produced must be exhausted in such a manner that they are not ingested by the engine, as this can cause compressor stalls and flameouts.

The next significant technical breakthrough in air-to-air guns appeared World War 2. This was a new cannon, modeled from an experimental German gun and built around a rotating cylinder similar to a "revolver" handgun. This design, known as the M39 in the United States, resulted in a great increase in rate of fire.

Even greater performance was obtained in the late 1950s with the introduction of the "Gatling-gun" cannon. Rather than a revolving cylinder, this weapon employed multiple rotating barrels. Designated the M61 in the United States, this gun could develop a tremendous rate of fire with less barrel overheating and erosion. Additionally, this gun was usually electrically, hydraulically, or pneumatically propelled; because it was not dependent on the residual energy of the expended round, problems associated with duds were eliminated; a good indication of the technological development of a gun is the weight of the projectiles that it can fire in one minute (assuming barrel limitations and ammunition supply allows). In this table weight of fire is measured by the factor WF. Tremendous progress can easily be seen here by comparing the post-World War 1 Browning .30-cal M2 machine gun with the 20-mm M61 Catling gun of the 1950s. Improvement in this area has been one of the leading factors in the lethality increase of airborne gun systems.

The lethality of a gun can be measured by multiplying the destructive power of its projectile and the number of hits. For nonexplosive bullets, destructive qualities are generally proportional to kinetic energy: half the mass of the projectile times the square of its velocity. To be more technically correct, the velocity used should be the relative impact velocity, but for comparison purposes, muzzle velocity will do. The factor FL in Table 1-1, a measure of the lethality of the gun, is proportional to the kinetic energy of each projectile and the rate of fire.

FL should be roughly indicative of the lethality of a nonexplosive bullet fired at the specified rate from a given gun. Cannon are a somewhat different case, since much of the lethality of these weapons is derived from their explosive shells. Therefore FL is a fairly accurate relative assessment of the destructiveness of machine guns, but it underrates the cannon in comparison. Likewise, it can be used to compare cannon of the same projectile size, but it would slight larger guns in comparison with smaller ones.

Even with its limitations, FL can give a qualitative feel for the incredible increase in fighter gun-system lethality over the years. For example, the combined FL of the two .30-cal-class synchronized machine guns typical of fighters at the end of World War I would be on the order of FL = 2, while the six wing-mounted .50-cal guns of the World War II P-51D fighter would rate about FL = 38. In addition, a much better gunsight on the P-51 and many other fighters of its day greatly increased the probability that hits would be scored. A further lethality increase can be seen in the gun systems of some present-day fighters, such as the F-14, F-15, F-16, and F-18, which mount a single M61 Catling gun. Ignoring the increased lethality of the explosive shell and even better gunsights, these aircraft would rate about FL = 145. Such technological advances, combined with inherent reliability, cost-effectiveness, simplicity, and flexibility in comparison with many other weapons systems, make the gun a formidable asset of the modern fighter.

Regardless of the lethality of a given gun system, it is of little value unless it can be brought to bear on the target. The fact that even the relatively benign systems of World War I were effective in their time demonstrates that lethality is certainly not the only factor, and probably not even the most important factor, in gun effectiveness. The ability to achieve a hit initially is probably more relevant. By this reasoning, a simple comparison of rates of fire among the various guns and gun installations is likely to be a better measure of their effectiveness, since this factor is more closely related to the probability of a hit. Lethality and target vulnerability are still important, however, since they determine the number of hits required for a kill. Additionally, for the guns to be placed in a reasonable firing position, aircraft peformance and pilot ability must be adequate. The location of this position is very much dependent on the effectiveness of the gunsight, as is discussed later.

Air-to-Air Gunnery Principles

The air-to-air gunnery problem is a difficult one; it involves hitting a moving target from a moving platform with projectiles that follow curved paths at varying speeds. This complicated problem can be better under stood if each part of it is isolated in turn.

Most people who have fired a gun or an arrow or have thrown a rock at a stationary target realize that the projectile takes a finite length of time to reach that target. During this period the projectile is acted on by gravity, which causes it to curve downward. The longer the projectile time of flight (TOP), the farther the projectile drops. In the first second this gravity drop is about 16 ft. During its flight the projectile is also subjected to aerody namic drag, which causes it to decelerate at a rate dependent on its shape, size, weight, and speed, as well as the density of the air. In general, the greater the muzzle velocity of a bullet, the shorter the TOP and the smaller the gravity drop at a given range. As range, and therefore TOP, increases, however, the rate of gravity drop also increases. Gravity drop may be negligible at very short ranges, but it becomes increasingly important as TOP increases.

This finite TOP also poses a problem if the target happens to be moving, since the target's position will change somewhat from firing of the projec tile to its impact; thus lead is required for the projectile and target to arrive at the same point in space at the same instant. This will come as no surprise to anyone who has ever shot at flying birds or skeet. The lead required is roughly proportional to the crossing speed of the target, so if its track is directly toward or away from the shooter, no lead is necessary, but maximum lead is called for when the target's track is 90° to the line of sight (LOS) from shooter to target.

As shown in Figure 1-1, lead usually is described as a "lead angle." Lead angle is sensitive to target crossing speed and average bullet speed. Range is also a factor, since average bullet speed decreases with greater TOP. Lead angle is also dependent on the geometry of the firing situation because of the influence of this factor on target crossing speed and TOP. This geome try can be described as "target-aspect angle" (TAA), which is denned as the angle between the target's velocity vector (flight path) and the LOS betwen the target and shooter. When the target is moving directly toward the shooter, TAA is zero. The shooter would have a 180° TAA when he is situated directly behind the target, and a 90° TAA on the target's beam (i.e., "abeam" the target). As TAA varies, so does target crossing speed, changing the lead angle required.

To this point only nonmaneuvering targets (i.e., those traveling in a straight line at constant speed) have been discussed. To gain an apprecia tion of the effects of target maneuvering on lead angle, assume that the shooter is directly behind the target at the moment of firing, but before the bullet TOF the target begins a turn to left or right. If the shooter applied no lead angle (because target crossing speed was zero at the time of firing), the bullet might pass behind the target. The target's lateral acceleration (radial G) has generated an average crossing velocity that requires a lead correc tion. The amount of this lead correction is very sensitive to target G near nose or tail TAAs, but it is less dependent on target maneuver (and more dependent on target speed) near beam aspects when the target turns directly toward or away from the shooter.

Target movement and maneuver also affect range. If TOF, gravity drop, angle, etc., are calculated based on target range at the time of firing (position "1" in Figure 1-1), any movement or maneuver during projectile TOF could change the range, invalidating all calculations and causing a miss.

The final complication in air-to-air gunnery is the motion of the shooter aircraft itself. Accurate ballistics calculations depend on knowing the true velocity of the projectile as it leaves the barrel. The true airspeed of the shooter must be added to the muzzle velocity to determine launch speed. Shooter aircraft maneuvering will also have several important effects. For example, as the shooter maneuvers, the gun-bore line (GBL) may be dis placed somewhat from the firing aircraft's direction of motion because of "angle of attack," sideslip, etc. (Angle of attack is discussed in the Appen dix.) The actual trajectory at the instant the bullet leaves the muzzle will not, therefore, generally be aligned with the GBL. Motion imparted to the projectile by rotating barrels (Catling gun), as well as aircraft flexing under maneuvering loads, may be factors. These and some other factors are usually grouped together under the term "trajectory jump," which in cludes any angular difference between the GBL and the initial trajectory.

Given all the foregoing factors that come into play, it's amazing that an air-to-air gun kill is ever recorded, especially when many of these factors are unknown quantities for the pilot. Little wonder that the most effective technique often is to "fill the windscreen with target and let 'er rip." Effective air-to-air gunsights have done much to aid the fighter pilot in this difficult task.

Tracer bullets, introduced during World War I, were also a great aid to the pilot, since he could see the trajectory of his bullet stream and make corrections. Small pyrotechnic charges located in the rear of tracer bullets burn during the TOF, making the projectile visible. Although this feature can be an aid in placing bullets on the target, the benefits can work both ways. The pilots of many target aircraft do not realize they are under attack until the first shots are fired. Any tracer that misses the target will defi nitely get the target pilot's attention and cause him to maneuver defen sively. Without tracers, attacking pilot normally gets a few extra seconds' chance at a steady target, greatly increasing the probability of a kill. For this reason it is recommended that tracer ammunition be used only for gunnery practice, to allow the student to develop a feel for bullet trajectories and dispersion.

The usual practice with tracers is to intersperse these rounds among the normal ammunition (every fifth bullet, for example), since rate of fire is usually such that several will be in the air simultaneously anyway. Since the ballistics of tracer ammunition generally varies slightly from the ballistics of the nontracer rounds, the trajectories also are likely to differ slightly, which can be misleading, especially when the pilot is firing at long range. Difficulties in depth perception can also make assessment of tracer trajectories ambiguous. With the advent of effective air-to-air gun sights, the disadvantages of tracers in combat probably began to outweigh the benefits.

In the absence of an ammo-remaining indicator, tracers have been used to warn the pilot that his ammo is nearly spent. For this purpose, the last few rounds in the can might include some tracers. It doesn't take long for an observant enemy to pick up on this practice, however, and it may give him the advantage of knowing which fighters are low on ammo. Some other indicator of rounds remaining is, therefore, preferable.

Guns — Defense

In discussing defenses against any weapon it is useful to look at the weapon as a system. Each component of this system must work effectively if it is to succeed in its mission. Defeating any one component will defeat the system, and the more subsystems degraded, the less the chances of system success. The components of a gun system are the gun and ammunition, the gun platform (aircraft), the sight, and the aircrewman firing the guns.

The gun/ammunition combination largely determines the maximum effective range of the system at various aspects about the target. Some of the factors involved are muzzle velocity, rate of fire, dispersion, bullet aerodynamics, and fuzing characteristics. Probably the best defense against a gun is to remain outside its effective range. This may be accom plished if the defending aircraft has speed capability greater than that of the attacker and the attacker is detected far enough away (depending on aspect and overtake) to allow the defender to turn away and outrun him. When this situation exists and the defender does not wish to engage, he can make a maximum-performance turn away from the attacker to place him as close to dead astern as possible, accelerate to maximum speed, and fly as straight a line as possible until he is no longer threatened. If the defender does not put the opponent close to the six o'clock position, the attacker may continue to close to guns range because of the geometry. Turning during the run-out (arcing) allows even a slower fighter to close the range by flying across the circle. Under some circumstances it may be desirable to keep the attacker in sight during this maneuver or to change the direction of the run-out after it has begun. To maintain sight and to reduce geometric closure to a minimum, the attacker should be kept near the defender's aft visibility limit. A series of small, hard turns can be made in the desired direction (allowing the attacker to be kept in sight), and each turn can be followed by a period of straight-line flight until the attacker drifts back to the aft visibility limit; this process can be repeated until the desired heading is reached. Sight can be maintained after this point by making a series of these small turns alternately left and right of the desired course. This technique is often called an "extension maneuver."

The next best thing to denying the attacker any shot at all is to deny him a good shot. This can be accomplished by complicating the task of any of the gun subsystems. Looking a little deeper into the requirements for a good gun shot will clarify the discussion that follows. Figure 1-4 is a representative guns "envelope," looking down on the target located in the center, which is heading toward the top of the page. It can be seen that the effective guns envelope is defined by the min-range boundary (primarily a function of closure) and the max-range boundary (primarily a function of gun/ammunition characteristics, dispersion, lethality, gunsight, closure, apparent target size, and vulnerability). Note that min-range is much greater in the target's forward hemisphere because of higher closure. Max range is also generally greater in the forward hemisphere for the same reason. This relates to shorter bullet TOP, smaller dispersion radius, and greater bullet density on the target. Lethality is also improved in the forward hemisphere since greater bullet kinetic energy is provided by the closure. Maximum effective range increases in the target's beam because of larger apparent target size and better fuzing of the shells (cannon) resulting from a higher "grazing angle" with the target. Low grazing angles in the forward and rear quarters may allow shells to bounce off the target without penetrating or exploding.

Guided Missiles

When discussing missiles in relation to air combat this section refers to the guided variety that change their flight paths in response to target maneu vers. Unguided rockets may be thought of as big bullets, and essentially the same tactics and techniques may be applied to these weapons as to guns. Guided missiles are broadly categorized according to their mission, which is generally stated in terms of their launching platform and intended target: air-to-air, air-to-surface, surface-to-surface, and surf ace-to-air. This section deals primarily with air-to-air missiles (AAMs), but much of the discussion is also relevant to other types, particularly surface-to-air missiles (SAMs).

Missile Propulsion

The propulsion system of a missile may be of any type suitable for airborne vehicles, but because of the typically high speeds of their targets, AAMs and SAMs are generally rocket or jet powered. Rockets are usually pre ferred for shorter-range missiles, since rocket engines provide very high thrust-to-weight, generating great acceleration and high speeds during the short duration of the flight. Solid-fuel rockets are generally preferred be cause small engines of this type usually have higher thrust-to-weight, are simpler, and seldom require throttling.

As range requirements for the missile increase, so does the complexity of the motor design. Simply increasing the size of the rocket to provide greater endurance would cause the missile size and weight to grow rapidly, so more propulsive efficiency is required. For medium-range missiles this is sometimes accomplished by a solid-fuel rocket designed to produce two levels of thrust: an initial high-thrust booster and a longer-lasting, low thrust sustainer. As the rocket grows in size to provide greater range, liquid-fuel designs become more competitive in thrust-to-weight while also providing convenient thrust control. Ramjet propulsion, however, is usually preferable to liquid-fuel rockets in this application as long as the missiles can remain within jet atmospheric limits. Often, particularly with SAMs, a solid rocket booster will be provided to assist the missile in initial acceleration to efficient ramjet operating speed.

Missile Control

The control system causes the missile to maneuver in response to inputs from the guidance system. Missiles are often controlled aerodynamically, like conventional aircraft, but they may also use thrust-vector control or an arrangement of fixed control jets. The aerodynamic controls of missiles vary little from aircraft controls. Since anti-air missiles are usually super sonic vehicles, they often use all-moving irreversible control surfaces. They also make frequent use of canard controls for improved maneuver ability, as well as sophisticated autopilots to maintain stability. As with aircraft controls, missile aerodynamic controls are subject to the lift limitations of airfoils and the results of induced drag. Unlike fighters, however, missiles are seldom restricted to a limiting structural load factor, i.e., they generally operate at speeds below their corner velocities. (See the Appendix for a discussion of aerodynamics and performance.) Aerody namically controlled missiles, therefore, often have their best turn per formance at their highest speeds. With many rocket-powered missiles there is a short period of rocket thrust followed by "gliding," or unpowered flight, for the remainder of their operation. Maximum speed, minimum weight (due to fuel exhaustion), and therefore greatest maneuverability for this type of missile would generally occur near the time of motor burnout. One of the advantages of aerodynamic controls is that they can provide control during the gliding portion of the missile's flight.

Thrust-vector control is provided by altering the direction of the ex haust gases to change the thrust line. This may be accomplished by swiveling the nozzles, by installing deflector vanes in the exhaust, or by other means to cause the missile to pivot about its CG in a severe sideslip. The thrust is then vectored to stop the body rotation at the proper heading, and, finally, it is centered to send the missile off in the desired new direction. Such a system is highly unstable and requires an extremely fast and sophisticated autopilot, but it has the potential for great maneuver ability, such as the ability to turn nearly square corners at low speed. One obvious disadvantage of thrust-vector control is that the motor must be burning, making it inoperable during a gliding flight segment. This would tend to make the missile bigger for a given range and may limit its application to fairly short-range weapons.

Most thrust-vector-controlled vehicles are inherently more maneuver able at very low speeds, since there is less inertia in the missile to be overcome by the thrust in producing a change in flight direction. There are many other factors involved, however, including vehicle weight, moment of inertia about the vehicle's CG, and CG location. These factors generally tend to increase maneuverability near the point of motor burnout, so such a missile should remain very agile throughout its powered flight. This type of control is quite useful for very high-altitude missiles, since, unlike aerodynamic controls, it is not dependent on the atmosphere.

Fixed control jets, arranged around the missile body to pivot the vehicle about its CG, are just another method of thrust-vector control; in this case the thrust line is changed by rotating the entire missile rather than just the nozzle or exhaust gases. A system of fixed control jets may be lighter than a straight thrust-vector control system, since no large actuators are required. Some maneuverability may be lost, however, since greater control power is usually available from the main engine, but maneuverability characteristics are essentially the same.

Almost any control system requires actuators of some sort for move ment of control surfaces, nozzles, valves, etc. The power source and the design of these actuators also have an effect on the maneuverability of the missile. These power sources are usually pneumatic, electric, or hydraulic, or some combination thereof. Pneumatic power may be provided by bot tles of compressed gas or by a gas generator. Such systems are lightweight and simple, but they are generally fairly slow in reacting, particularly when heavy control loads are involved, and they have a rather limited endurance. Pneumatic control systems, therefore, are usually found only in small, short-range missiles.

Electric actuators are generally faster than pneumatic ones. Also, since virtually all guided missiles already have electrical systems, electric actuators may simplify the missile by eliminating additional systems. Electric actuators, however, are expensive and tend to be heavy when great amounts of control power are required.

Hydraulic actuators usually provide the fastest reaction time of these three methods, and they can produce great control forces efficiently. Mis sile hydraulic systems may be either "open" or "closed." In an open system used hydraulic fluid is vented overboard. In a closed system the used fluid is returned to the reservoir for reuse.

Missile Guidance

The guidance system provides inputs to the missile control system, which in turn maneuvers the missile to intercept the target. Guidance for AAMs and SAMs can be classified as one of the following: preset, command, beam-rider, and homing.

Preset guidance means that a prelaunch determination is made of the missile-target intercept point in space. Prior to missile launch the guid ance system is provided with this information and the trajectory to be followed (by dead reckoning, inertial, or some other form of navigation) to the missile's destination. Since this information cannot be changed after the missile is fired, any inherent system inaccuracy or postlaunch target maneuver may result in a wide miss. Preset guidance is therefore closely related to the unguided rocket, and it is applicable to the anti-air mission only in conjunction with very large warheads (nuclear) or as an initial guidance mode in combination with more accurate terminal guidance techniques.

Command guidance may be likened to classic remote control. During missile flight the positions of both the target and the missile are monitored at the launching platform, and commands are sent to the missile to fly a course that will result in target interception. Tracking of target and missile is usually accomplished by radar, through electro-optics (television), or by sight. Of these three methods, only radar generally provides target/missile range information sufficiently accurate to allow computing of a lead intercept trajectory for the missile, but since two tracking radars are usually required, this technique largely has been limited to SAM systems. Without range data the missile is ordinarily guided along the LOS between the target and the launcher. This technique, known as "command-to LOS," can be accomplished with no range information at all and is applica ble to visual and electro-optical systems as well as to radar and combination systems.

The guidance instructions to the missile are generally transmitted by radio data link, which is susceptible to jamming, as are most radar track ers. Trailing wires (wires connecting the missile and the launch platform) have been used for transmitting guidance commands with much success in several short-range air-to-surface and surface-to-surface applications. Such a system is highly resistant to jamming, and was employed by the first AAM. This was a German X-4, designed and tested late in World War II for use by the Me 262 and Fw 190. The X-4 was a command-to-LOS trailing-wire system that was controlled manually by the launching pilot long the visual LOS to the target aircraft. Apparently it was never used operationally.

Beam-rider guidance is somewhat similar to command-to-LOS guid ance, except that the missile guidance system is designed to seek and follow the center of the guidance beam automatically, without specific correction instructions from the launching platform. The guidance beam may be provided by a target-tracking radar, by electro-optics, or by a visual system. Like radar-enhanced command guidance systems, radar beam rider systems are not limited to daylight, good-weather conditions, but they are more susceptible to electronic countermeasures than are electro-optical and visual trackers.

One problem with beam-rider systems, as with command-to-LOS, is that the missile must have high maneuverability in order to intercept an evasive target. As they approach the target, beam-rider missiles often must tighten their turns continually to keep up. At high speeds tight turns may exceed the missile's capabilities. Using two radars, one for target tracking and a second for missile tracking and guidance, can reduce this problem somewhat by providing a more efficient lead trajectory, but such systems are more complex and their use is generally limited to SAMs. Beam-rider guidance, however, is usually more accurate and faster-reacting than com mand guidance systems, and it can be quite effective against even evasive aircraft targets.

The most effective type of guidance against evasive targets is homing. Within this broad category are three subtypes: passive, semi-active, and active. The simplest of these, passive homing, relies on emissions given off by the target itself (e.g., sound, radio, radar, heat, light) for its guidance information. Semi-active homing systems guide on energy reflecting off the target. This energy, usually radar or laser, is provided by a source external to the missile, often the launching platform. For active homing guidance the missile itself illuminates and tracks the target.

The various forms of horning guidance generally offer improved capabil ity against airborne targets, especially highly maneuverable targets. More efficient trajectories and better guidance accuracy in the critical terminal phase of the intercept are often available. Each guidance method, however, has some advantage over the others in certain situations, so combination systems are sometimes employed. An example is the use of preset or command guidance during the early portion of a long-range shot to get the missile close enough to the target to allow passive or active homing. Advances in solid-state electronics technology have made it practical to place more sophisticated guidance and sensor capability in small, light weight missile packages.

Missile Seekers

The seeker system of a missile is responsible for sensing and tracking the target and providing the information necessary for performance of the guidance system. Preset and command guidance do not require a seeker in the missile, since the tracking function is accomplished by the launch ing/guidance platform. Beam-rider missiles usually have a receiver in the tail to collect information from the host guidance/tracking beam. Passive missiles generally require a sensor receiver in the nose, as do semi-active homers; but semi-active homers may also include a rear receiver for interception of information directly from the illuminating platform which can be compared to the reflected energy received by the forward sensor to derive additional guidance data. Active homers require both a transmitter and a receiver, generally located forward.

The maximum range of its seeker operation often limits the effective range of a missile system. Passive seekers have an inherent advantage here, because their received power is inversely proportional to the square of the target range, while the max-range of active and semi-active systems varies inversely with the fourth power. Several other factors also are involved. For passive systems these include the intensity of the target radiation in the direction of the sensor, the type of radiation (which determines the rate of signal attenuation by the atmosphere), and the seeker sensitivity. For active and semi-active systems maximum range depends on, in addition to transmitted power and receiver sensitivity, the reflective characteristics of the target relative to the type of illumination used. These reflective charac teristics are usually sensitive to target size, and also to the target's con struction material, shape, surface contours, and aspect, all of which may combine to increase or decrease reflectivity.

The most common passive seeker now in use is the heat seeker. This device contains a material (the detector) which is sensitive to heat (in frared—IR—radiation) that is produced primarily by the target's pro pulsion system. The detector is often cryogenically cooled to eliminate internally generated thermal "noise" and allow detection of even very small amounts of IR energy coming from an external source. The seeker must still have the capability to discriminate between target radiation and background radiation, however. Such differentiation is essential for all sensor systems, which normally require that the strength of the target signal exceed that of the background (i.e., the signal-to-noise ratio must be greater than one).

Background IR radiation is generated by the sun, by reflections off water, snow, etc., and also by clouds and hot terrain such as deserts. If the temperature of the background is within the band of sensitivity of the sensor material and is of sufficient intensity, it will be detected along with the target heat. When sensors are made sensitive to cooler targets for improved detection, the seeker becomes more susceptible to background noise also. This problem is partially resolved by designing the seeker to track only small, "point-source" radiations, usually associated with air craft targets, rather than the broader areas of IR energy common to many background sources. In general, the seeker tends to track the most intense point-source target within its band of temperature sensitivity. The greater the background radiation within the band of temperature sensitivity of the seeker, the stronger the IR signal received from the target must be if it is to be detected and tracked. This fact may limit the detection range for a target of given IR intensity.

A hot object emits IR energy in a rather wide band of frequencies. As the object becomes hotter the radiated power increases very sharply (pro portional to the fourth power of absolute temperature), and the frequency of the most intense IR radiation is shifted higher. The hot metal of jet tailpipes can be expected to emit IR energy of greater intensity and higher frequency than that of the hot exhaust gases, which begin to cool rapidly. Depending on the sensor material used, a heat-seeking missile may detect only the tailpipe, or it may also be sensitive to the cooler exhaust gas and even to the heat generated by air friction on a very fast aircraft. One disadvantage of tailpipe guidance is the likelihood that the hot metal may in some views be shielded by part of the aircraft structure. Hot exhaust gas is usually more difficult to shield, and this fact has led to heat seekers with "all-aspect" capability. However, the pilot of the target aircraft can reduce substantially the IR signature of his exhaust gases easier and faster (by power reduction) than he can his metal tailpipe, which tends to retain heat longer. The physical size of an exhaust plume may also cause problems for hot-gas seekers, as they may become "saturated" at close range. Rather sophisticated guidance techniques are required to cause such missiles to aim forward of the heat source in order to hit the target. Discrimination between this cooler target and the background radiation may also be a problem, as explained previously.