LEARNING OBJECTIVE: Identify the components of a basic fire control system and discuss the function of each.
A basic fire control system consists of a computer, a director and radar, and a stable element. The following sections will provide a brief description of each of these.
The definition of a computer is any device capable of accepting data, applying mathematical operations to that data, and obtaining useful information from those operations. A fire control computer accepts target and own-ship's data, processes it, and provides a solution to the fire control problem. Own-ship's data includes course, speed, pitch, and roll. Also included are other variables, such as wind direction and, in the case of guns, projectile initial velocity. This data will be discussed further as we describe the fire control problem later in this section.
The fire control solution for a gun engagement consists of gun mount train and elevation orders, fuze setting orders, and in some cases, gun sight orders. For a missile engagement, the computer supplies launcher train and elevation orders and missile prelaunch programming.
A guided missile, unlike a gun projectile, can change course in flight. Therefore, the missile fire control computer continuously updates the solution for target intercept after the missile is fired. The updated solution is transmitted to the missile that then corrects its course to intercept the target. These actions are occurring during the midcourse phase of an SM-2 missile engagement. The SM-2 engagement also requires target illumination to be scheduled and ordered (during the terminal phase of missile flight). These two actions are also accomplished by the fire control computer.
DIRECTOR AND RADAR
The fire control system's director and radar are discussed as a single unit since, once assigned, their combined outputs are the primary source of target information for the fire control computer.
The radar antenna is mounted to the director. Once assigned by the WDS, the director "slews" to the ordered bearing and elevation where the radar conducts a search for the target. The search is controlled by a subsystem that moves the director in a predetermined pattern around the ordered position until the radar "acquires" the target. Once acquired by the radar, a subsystem of the radar unit controls the director to keep the radar "locked on" the target. The system then begins to track the target. While locked on and tracking, the radar and director continuously provide precise target range, bearing, and elevation data to the computer. Radar provides target range, and the director, based on its train angle and the elevation angle of the radar antenna, provides target bearing and elevation data.
A ship, by its very nature, is in constant motion. The weapon systems, especially gun systems, require a stable platform to deliver accurate fire. Since it is impossible to build a ship that is not subject to constant movement, the stable element input is added to the fire control computer. A stable element is a gyroscope mounted to gimbals. Its output provides the computer with a stable horizontal reference from which to compute a fire control solution. Some older systems have their own dedicated stable element, while most newer systems use an input from the gyro of the ship.
THE FIRE CONTROL PROBLEM
LEARNING OBJECTIVE: Discuss the characteristics of the fire control problem and the components involved.
To deliver accurate fire, a fire control system must consider and compensate for own-ship's movement, gun characteristics, natural forces, and target movement. Each compensation involves a different set of variables. Compensating for these variables is the essence of the fire control problem.
Own-ship's movement and characteristics involve platform stabilization (stable element), parallax, and interior ballistic considerations. Natural forces are compensated for as exterior ballistics. Target movement involves considering exact target position in reference to your ship, then predicting its future position.
If guns were physically located at the reference point (the director), projectiles fired from the guns would hit the target without further correction. The guns are, of course, not located at the reference point but are some distance forward or aft of this point and below it (the director is located high on the superstructure). This difference in location puts the gun at a different angle from the target than the director, giving each unit a different line of sight to the target. Unless corrected, this difference will result in large errors in accuracy. The parallax correction is normally accomplished in the fire control computer.
Ballistics is the science of projectile motion. It is divided into two branches-interior and exterior ballistics. Interior ballistics is the study of projectile motion while inside the gun. Exterior ballistics pertains to the projectile motion after it leaves the gun.
The speed at which a projectile is traveling at the instant it leaves the gun bore is known as initial velocity (IV). The initial velocity of a projectile must be known to predict its trajectory. Initial velocity is determined by the gun, the projectile, and the propelling charge. Projectiles and propelling charges are standardized. This standardization means that all size, weight, and shape variations are predetermined. The only variables left to consider are the condition of the gun and the temperature of the propelling charge. The propelling charge temperature is determined by averaging the powder magazine temperatures for the previous 3 days.
gases from the burning propellant push the projectile through the bore and out the muzzle. As the projectile passes through the bore, the twisted rifling imparts a spin to the projectile that stabilizes it in flight.
Each firing wears on (erodes) the interior surfaces of the gun. This erosion results in a gradual enlargement of the bore. Erosion begins at the rear and extends to the end of the bore as the gun is used. As the bore enlarges, the seal becomes less effective, resulting in a slower initial velocity.
Data from the annual star gauge inspection and from the regular projectile seating and distance gauge (PSDG) tests is used to compute IV The determined IV is then entered into the fire control computer for consideration in the final fire control solution.