Tuesday, 28 February 2012


The flight control system is a key element that allows the missile to meet its system performance requirements. The objective of the flight control system is to force the missile to achieve the steering commands developed by the guidance system. The types of steering commands vary depending on the phase of flight and the type of interceptor. In the boost phase the flight control system may be designed to force the missile to track a desired flight-path angle or attitude. In the mid-course and terminal phases the system may be designed to track acceleration commands to effect an intercept of the target. 


 This article explores several aspects of the missile flight control system, including its role in the overall missile system, its subsystems, types of flight control systems, design objectives, and design challenges. Also discussed are some of APL’s contributions to the field, which have come primarily through our role as Technical Direction Agent on a variety of Navy missile programs.

Figure 1

The inertial missile motion controlled by the flight control system combines with the target motion to form the relative geometry between the missile and target. The terminal sensor measures the missile-to-target LOS angle. The state estimator forms an estimate of the LOS angle rate, which in turn is input to the guidance law. The output of the guidance law is the steering command, typically a translational acceleration. The flight control system uses the missile control effectors, such as aerodynamic tail surfaces, to force the missile to track steering commands to achieve a target intercept.
The missile flight control system is one element of the overall homing loop. Figure 1 is a simplified block diagram of the missile homing loop configured for the terminal phase of flight when the missile is approaching intercept with the target. The missile and target motion relative to inertial space can be combined mathematically to obtain the relative motion between the missile and the target. The terminal sensor, typically an RF or IR seeker, measures the angle between an inertial reference and the missile-to-target line-of-sight (LOS) vector, which is called the LOS angle. The state estimator, e.g., a Kalman filter, uses LOS angle measurements to estimate LOS angle rate and perhaps other quantities such as target acceleration. The state estimates feed a guidance law that develops the flight control commands required to intercept the target. The flight control system forces the missile to track the guidance commands, resulting in the achieved missile motion. The achieved missile motion alters the relative geometry, which then is sensed and used to determine the next set of flight control commands, and so on. This loop continues to operate until the missile intercepts the target. In the parlance of feedback control, the homing loop is a feedback control system that regulates the LOS angle rate to zero. As such, the overall stability and performance of this control system are determined by the dynamics of each element in the loop. Consequently, the flight control system cannot be designed in a vacuum. Instead, it must be designed in concert with the other elements to meet overall homing-loop performance requirements in the presence of target maneuvers and other disturbances in the system, e.g., terminal sensor noise (not shown in Fig. 1), which can negatively impact missile performance. The remainder of this article is divided into six sections. The first section discusses the specific elements of the flight control system. Particular emphasis is placed on understanding the dynamics of the missile and how they affect the flight control system designer. The next three sections describe different types of flight control systems, objectives to be considered in their design, and a brief design example. The last two sections discuss some of the challenges that need to be addressed in the future and APL’s contributions to Navy systems and the field in general.
Figure 2
 The four basic elements of the flight control system are shown in the gray box. The IMU senses the inertial motion of the missile. Its outputs and the inputs from the guidance law are combined in the autopilot to form a command input to the control effector, such as the commanded deflection angle to an aerodynamic control surface. The actuator turns the autopilot command into the physical motion of the control effector, which in turn influences the airframe dynamics to track the guidance command.

As noted above, the flight control system is one element of the overall homing loop. Figure 2 shows the basic elements of the flight control system, which itself is another feedback control loop within the overall homing loop depicted in Fig. 1. An inertial measurement unit (IMU) measures the missile translational acceleration and angular velocity. The outputs of the IMU are combined with the guidance commands in the autopilot to compute the commanded control input, such as a desired tail-surface deflection or thrust-vector angle. An actuator, usually an electromechanical system, forces the physical control input to follow the commanded control input. The airframe dynamics respond to the control input. The basic objective of the flight control system is to force the achieved missile dynamics to track the guidance commands in a well-controlled manner. The figures of merit (FOMs) used to assess how well the flight control system works are discussed in Flight Control System Design Objectives. This section provides an overview of each element of the flight control loop.

Guidance Inputs
The inputs to the flight control system are outputs from the guidance law that need to be followed to ultimately effect a target intercept. The specific form of the flight control system inputs (acceleration commands, attitude commands, etc.) depends on the specific application (discussed later). In general, the flight control system must be designed based on the expected characteristics of the commands, which are determined by the other elements of the homing loop and overall system requirements.

The dynamics of the airframe are governed by fundamental equations of motion, with their specific characteristics determined by the missile aerodynamic response, propulsion, and mass properties.
The missile actuator converts the desired control command developed by the autopilot into physical motion, such as rotation of a tail fin, that will effect the desired missile motion. Actuators for endo-atmospheric missiles typically need to be high-bandwidth devices (significantly higher than the desired bandwidth of the flight control loop itself) that can overcome sig­nificant loads. Most actuators are electromechanical, with hydraulic actuators being an option in certain applications.
The IMU measures the missile dynamics for feedback to the autopilot. In most flight control applications, the IMU is composed of accelerometers and gyroscopes to measure three components of the missile translational acceleration and three components of missile angular velocity.


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