Fighter Combat

32 FIGHTER WEAPONSMissile PropulsionThe propulsion system of a missile may be of any type suitable for airbornevehicles, but because of the typically high speeds of their targets, AAMsand SAMs are generally rocket or jet powered. Rockets are usually preferredfor shorter-range missiles, since rocket engines provide very highthrust-to-weight, generating great acceleration and high speeds during theshort duration of the flight. Solid-fuel rockets are generally preferred becausesmall engines of this type usually have higher thrust-to-weight, aresimpler, and seldom require throttling.As range requirements for the missile increase, so does the complexityof the motor design. Simply increasing the size of the rocket to providegreater endurance would cause the missile size and weight to grow rapidly,so more propulsive efficiency is required. For medium-range missiles thisis sometimes accomplished by a solid-fuel rocket designed to produce twolevels of thrust: an initial high-thrust booster and a longer-lasting, lowthrustsustainer. As the rocket grows in size to provide greater range,liquid-fuel designs become more competitive in thrust-to-weight whilealso providing convenient thrust control. Ramjet propulsion, however, isusually preferable to liquid-fuel rockets in this application as long as themissiles can remain within jet atmospheric limits. Often, particularlywith SAMs, a solid rocket booster will be provided to assist the missile ininitial acceleration to efficient ramjet operating speed.Missile ControlThe control system causes the missile to maneuver in response to inputsfrom the guidance system. Missiles are often controlled aerodynamically,like conventional aircraft, but they may also use thrust-vector control oran arrangement of fixed control jets. The aerodynamic controls of missilesvary little from aircraft controls. Since anti-air missiles are usually supersonicvehicles, they often use all-moving irreversible control surfaces.They also make frequent use of canard controls for improved maneuverability,as well as sophisticated autopilots to maintain stability. As withaircraft controls, missile aerodynamic controls are subject to the liftlimitations 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 theAppendix for a discussion of aerodynamics and performance.) Aerodynamicallycontrolled missiles, therefore, often have their best turn performanceat their highest speeds. With many rocket-powered missilesthere is a short period of rocket thrust followed by "gliding," or unpoweredflight, for the remainder of their operation. Maximum speed, minimumweight (due to fuel exhaustion), and therefore greatest maneuverability forthis type of missile would generally occur near the time of motor burnout.One of the advantages of aerodynamic controls is that they can providecontrol during the gliding portion of the missile's flight.Thrust-vector control is provided by altering the direction of the exhaustgases to change the thrust line. This may be accomplished by