SELF-CONTAINED GUIDANCE SYSTEMS
Certain guided missiles have self-contained guidance systems. All guidance and control functions are performed totally within the missile. They neither transmit nor receive any signals during flight. Therefore, jamming or other electronic countermeasures are ineffective against them. Generally, self-contained guidance systems are used in surface-to-surface or shore applications.
The term preset completely describes this method of guidance. Before the missile is launched, all the information relative to target location and the required missile trajectory must be calculated. The data is then locked into the guidance system so the missile will fly at correct altitude and speed. Also programmed into the system are the data required for the missile to start its terminal phase of flight and dive on the target.
One disadvantage of preset guidance is that once the missile is launched, its trajectory cannot be changed. Therefore, preset guidance is really only used against large stationary targets, such as cities.
Navigational Guidance Systems
When targets are at very great distances from the launch site (beyond the effective range of radar, for example), some form of navigational guidance must be used. Accuracy at these distances requires exacting calculations and many complicated factors must be considered. Three types of navigational guidance systems that may be used by long-range missiles are inertial, celestial, and terrestrial.
INERTIAL GUIDANCE.- The inertial guidance method is similar to the preset guidance method. Inertial guided missiles also receive preprogrammed information before launch. After launch, there is no electromagnetic contact between the missile and its launch point (the ship, in our case). However, unlike preset guidance, the missile can make corrections to its flight path and does so with amazing accuracy.
Flight control is accomplished by using special sensors, called accelerometers, mounted on a gyro-stabilized platform. All in-flight accelerations are measured continuously and the guidance and control systems generate steering orders to maintain the proper trajectory. The unpredictable outside forces (e.g., wind) are also monitored by the sensors. Correction orders are generated to maintain proper flight attitude.
The use of an inertial guidance system takes much of the guesswork out of the long-range fire control problem. It has proven to be extremely reliable and, above all, very accurate.
CELESTIAL GUIDANCE.- A celestial guidance system uses stars or other celestial bodies as known References (or fixes) in determining a flight path. This guidance method is rather complex and cumbersome. However, celestial guidance is quite accurate for the longer ranged missiles.
TERRESTRIAL GUIDANCE.- Terrestrial guidance is also a complicated arrangement. Instead of celestial bodies as reference points, this guidance system uses map or picture images of the terrain which it flies over as a reference. Terrestrial and celestial guidance systems are obviously better suited for large, long-range land targets.
LEARNING OBJECTIVE: Recall the types of missile propulsion, engines, and fuels, the affects of acceleration, and the four associated speed regions.
"Propulsion" is defined as the act of driving forward or onward by means of a force that imparts motion. Considering all the different types of weapons, there are three methods of propulsion:
1. Gun or impulse
Any weapon that uses an internal source of propulsive power to carry it to a target is said to be a reaction-propelled weapon. Guided missiles are reaction-propelled weapons. The propelled power is obtained from the combustion of a fuel in a reaction motor.
The basic principle of reaction propulsion can be summarized by the old law of physics that states, "for every action, there is an equal and opposite reaction." A person walks forward by pushing backward against the ground. A missile moves forward when a mass of gas (a jet) is expelled rearward at high speed.
Jet propulsion is another term that describes reaction propulsion. Jet propulsion is a means of locomotion obtained from the momentum of matter ejected from within a body. This matter must be in the form of a fluid jet. The fluid can be water, steam, heated air, or gaseous products produced from burning a fuel. For our purposes, jet propulsion systems used in guided missiles may be divided into two types-thermal jet engines and rocket engines. Both types operate by expelling a stream of high-speed gas from an exhaust nozzle.