Fighter Combat

FIGHTER WEAPONS 33swiveling the nozzles, by installing deflector vanes in the exhaust, or byother 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 newdirection. Such a system is highly unstable and requires an extremely fastand sophisticated autopilot, but it has the potential for great maneuverability,such as the ability to turn nearly square corners at low speed. Oneobvious disadvantage of thrust-vector control is that the motor must beburning, making it inoperable during a gliding flight segment. This wouldtend to make the missile bigger for a given range and may limit itsapplication to fairly short-range weapons.Most thrust-vector-controlled vehicles are inherently more maneuverableat very low speeds, since there is less inertia in the missile to beovercome by the thrust in producing a change in flight direction. There aremany other factors involved, however, including vehicle weight, momentof inertia about the vehicle's CG, and CG location. These factors generallytend to increase maneuverability near the point of motor burnout, so sucha missile should remain very agile throughout its powered flight. This typeof control is quite useful for very high-altitude missiles, since, unlikeaerodynamic controls, it is not dependent on the atmosphere.Fixed control jets, arranged around the missile body to pivot the vehicleabout its CG, are just another method of thrust-vector control; in this casethe thrust line is changed by rotating the entire missile rather than just thenozzle or exhaust gases. A system of fixed control jets may be lighter than astraight thrust-vector control system, since no large actuators are required.Some maneuverability may be lost, however, since greater control poweris usually available from the main engine, but maneuverability characteristicsare essentially the same.Almost any control system requires actuators of some sort for movementof control surfaces, nozzles, valves, etc. The power source and thedesign of these actuators also have an effect on the maneuverability of themissile. These power sources are usually pneumatic, electric, or hydraulic,or some combination thereof. Pneumatic power may be provided by bottlesof compressed gas or by a gas generator. Such systems are lightweightand simple, but they are generally fairly slow in reacting, particularlywhen heavy control loads are involved, and they have a rather limitedendurance. Pneumatic control systems, therefore, are usually found onlyin small, short-range missiles.Electric actuators are generally faster than pneumatic ones. Also, sincevirtually all guided missiles already have electrical systems, electricactuators may simplify the missile by eliminating additional systems.Electric actuators, however, are expensive and tend to be heavy when greatamounts of control power are required.Hydraulic actuators usually provide the fastest reaction time of thesethree methods, and they can produce great control forces efficiently. Missilehydraulic systems may be either "open" or "closed." In an opensystem used hydraulic fluid is vented overboard. In a closed system theused fluid is returned to the reservoir for reuse.