MACH NUMBERS AND SPEED REGIONS
Missile speeds are expressed in terms of Mach number, rather than miles per hour or knots. A Mach number is the ratio of missile speed to the local speed of sound. If a missile is flying at one half of the local speed of sound, it is traveling at .5 Mach; twice the local speed of sound is Mach 2.
Notice that we have referred to the "local" speed of sound. That is because this quantity is not fixed or constant. The speed of sound in air varies with the temperature of the air. At sea level, with ambient temperature about 60°F, the speed of sound is around 760 miles per hour. If you measure it at the top of the troposphere (about 10 miles up), the speed is only around 660 mph. At higher elevations, it then increases (800 mph+).
Regarding guided missiles, we are concerned with four speed regions.
1. Subsonic- the region in which airflow over all missile surfaces is less than the local speed of sound. The subsonic region starts at Mach 0 and extends to about .75 Mach.
2. Transonic- the region in which airflow over the missile surfaces is mixed; subsonic in some areas, higher in others. The limits of this region are not sharply defined but range between .75 Mach and Mach 1.2.
3. Supersonic- the region in which airflow overall missile surfaces is at speeds greater than the local speed of sound. This region extends from about Mach 1.2 upward.
4. Hypersonic- speeds on the order of Mach 10 and higher.
Most SMS guided missiles are designed for use against supersonic air targets (antiair warfare). These missiles normally travel in the Mach 2 to 2.5 range. Other SMS guided missiles, especially those designed only for use against surface targets (like Harpoon), travel in the subsonic region.
LEARNING OBJECTIVE: Recall the types, purpose, and effectiveness of missile warheads, the types of fuzes, and the purpose of the safe and arming device.
Guided missile warheads are the business end of the missile. The basic warhead section consists of three functional elements-payload, faze, and safety and arming (S&A) device. Variations in warhead design can be obtained by altering any one of the three elements since they are usually separate units. In other types of ammunition, like a gun projectile, the fuze and S&A device are combined into one single unit. The faze function and S&A function are still performed separately but the device is known just as a faze.
In this text, we will refer to an S&A device as a safe and arming device. In other publications, you may see S&A (or S-A) defined as safety and arm, safeing and arming, and so forth. Functionally, all S&A devices are the same, only the name has changed. Don't be confused.
The primary element of the warhead is the payload. It is the destructive portion and accomplishes the end result of the missile. The text will examine the following types of payloads: blast and fragmentation.
A blast-effect warhead consists of a quantity of high explosives in a metal case. The force of the explosion creates a pressure or shock wave in the air or surrounding medium. It is this pressure wave that causes damage to the target.
Blast-effect warheads are most effective against underwater targets. Because water is incompressible and relatively dense, the effect of the blast is essentially magnified. A blast-effect warhead is also fairly successful against a ground or surface target. A blast-effect warhead is least effective against an air target. Air is not that dense and the shock wave dissipates quickly as it expands outward.
In all applications, timing is a predominant factor for blast-effect warheads. Figure 9-23 illustrates this point.
Fragmentation warheads use the force of a high-explosive charge to break up the container or casing of the warhead. These "fragments" are then hurled outward as many high-speed pieces to cause damage to a target. The design and construction of a warhead can control the size, the velocity, and the pattern of fragment dispersion.
Figure 9-23.-A blast-effect warhead.
Fragmentation warheads are most effective against air targets. They can have a greater miss distance than a blast-effect warhead and do not have to make actual contact with the target. There are many design variations of fragmentation warheads. In SMS guided missiles, a popular fragmentation-style warhead is known as the continuous rod warhead. (See fig. 9-24.)
Early experiments with short, straight, unconnected rod warheads (view A of fig. 9-24) had shown that they could inflict serious damage to an air target. They could chop off propeller blades, penetrate engine blocks, slice up wings, and so forth. However, as airplanes got bigger, their structures were designed so they could receive a number of small "hits" and keep on flying, But, a long continuous cut in their structure was "bad news," and that's what a "continuous" rod warhead will do.
The continuous rod warhead is packaged in two bundles inside the missile. At detonation, a high-explosive force (inside the bundles) causes them to expand outward. (See view B of fig. 9-24.) The rods expand radially into a ring pattern which lengthens and increases in diameter. Generally, two semicircles are formed as the rods expand. (See view C of fig. 9-24.) These semicircles prevent disintegration of the pattern when maximum expansion is reached. The expansion action can be likened to unfolding a carpenter's rule. The effect of these metal rods is a cutting action. If you've ever run into a clothesline while riding your bicycle, you can clearly understand this principle !
The fuze is the second element of a warhead of a missile. The primary purpose of the fuze is to initiate detonation of the payload. To be effective, detonation or "fuzing" must occur at a point where maximum damage will be inflicted on the target. This point is often called the "optimum time of detonation." It is the job of the fuze to determine this time or point, which is based on the nature of the target and the attack geometry involved.
A large variety of fuze types is available. Three general classes are contact (impact), proximity, and ambient. The fuze type for a given application depends on the characteristics of the target, the missile, and the warhead. In guided missiles, the fuze is generally referred to as a target detection device (TDD). Some guidance systems produce or gather much or all the
Figure 9-24.-View A.-an unconnected rod warhead; View B.-a continuous rod warhead bundle and its expanding action; View C.-the continuous rod warhead's circular pattern and cutting action.
information required to make the fuze function. In other cases, the TDD itself provides this information.