A clever attitude control (also called reaction control) system with solid propellants is used on some ballistic missiles. Its hot reaction gas has a low enough temperature so that uncooled hardware can be used for long durations. Ammonium nitrate composite propellant (mentioned as gas generator propellants in Tables 12-1 and 12-2) or a propellant consisting of a nitramine (RDX or HMX, described in Chapter 12) with a polymer binding are suitable. The version shown schematically in Fig. 11-27 provides pitch and yaw control; hot gas flows continuously through insulated manifolds, open hot-gas valves, and all four nozzles. When one of these valves is closed, it causes an unbalance of gas flow and produces a side force. To keep things simple, the four roll-control thrusters have been deleted from this figure.
With this type of attitude control system it is possible to achieve variable duration thrust pulsing operations and random pitch, yaw, and roll maneuvers. It is competitive with multi-thruster liquid propellant attitude control systems. The solid propellant versions are usually heavier, because they have heavy insulated hardware and require more propellant (for continuous gas flow), whereas the liquid version is operated only when attitude control motions are required.
A similar approach with hot gas valves applies to upper stages of interceptor vehicles used for missile defense; there is little time available for maneuvers of the upper stage to reach the incoming missile or aircraft and therefore the burning durations are usually short. The solid propellant gas temperatures are higher than with gas generators (typically 1260°C or 2300°F), but lower than with typical composite propellants (3050 K or 5500°F), and this allows the valves and manifolds to be made of high-temperature material (such as rhenium or carbon). In addition to attitude control, the system provides a substantial side force or divert thrust. It displaces the flight path laterally. Figure 11-28 shows such a system. Since all hot-gas valves are normally open, a valve has to be closed to obtain a thrust force as explained in the previous figure. The attitude control system provides pitch, yaw, and roll control to stabilize the vehicle during its flight, to orient the divert nozzle into the desired direction, and sometimes to orient the seeker (at the front of the vehicle) toward the target.
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