Missiles with thermal jet engines "breathe" in a predetermined amount of air and compress it. Liquid fuel is then injected into the compressed air and the mixture is ignited. Combustion takes place within a combustion chamber. The resulting hot gases are expelled through an exhaust nozzle at the rear of the missile. At this point, heat energy is transformed into kinetic energy and the thrust or propulsive motion is created.

An air-breathing jet engine must rely on oxygen obtained from the atmosphere for fuel combustion to take place. That is a disadvantage because the flight altitude (or ceiling) of the missile is thereby limited. However, at lower altitudes, the air-breathing (thermal) jet engine is very efficient.

Rocket Engines

A rocket (jet) engine does not depend on air intake for its operation. Hence it is capable of functioning at very high altitudes and even beyond the atmosphere. A rocket engine carries within it all the materials required for combustion. That usually includes a fuel, either solid or liquid, and an oxidizer. The oxidizer is a substance capable of releasing the oxygen that is necessary to support combustion.

Once the propellant of the rocket engine is ignited, hot gases are expelled from the exhaust nozzle. Heat energy is changed to kinetic energy and thrust is created. The amount of thrust developed by a rocket-type engine generally is rated as extremely high compared to the thrust of a similar sized air-breathing engine.

The more important characteristics of all rocket motors are summarized below.

1. The thrust developed by a rocket motor is very high, nearly constant, and is independent' of missile speed.

2. Rockets will operate in a vacuum.

3. Rockets have relatively few moving parts and simple design.

4. Rockets have a very high rate of propellant consumption.

5. Essentially the burning time of a rocket propellant is short.

6. Rockets need no booster since they develop full thrust at takeoff. If a booster is used, it aids the missile in reaching flight speed in minimum time and can extend range.

We'll now discuss two types of reaction/jet propulsion units used in SMS missiles. First, we'll examine a thermal jet-type engine known as a turbojet. Then we'll cover solid-fuel rocket motors which are classified as rocket engines.

TURBOJET ENGINES

A turbojet engine is an air-breathing, thermal jet propulsion system. It is called a turbojet because a portion of its exhaust is used to operate a turbine. The turbine, in turn, drives an air compressor. The primary function of a compressor is to receive and compress large masses of air. It then distributes this air to the combustion chambers.

Therefore, the major areas of a turbojet engine are an air intake system, an air compressor, a combustion chamber, and a turbine. These components essentially form an open-cycle gas turbine combined with a jet stream. In operation, the compressor is driven by the gas turbine, as shown in figure 9-19. It supplies air under high pressure to the combustion chamber. The turbine absorbs only part of this energy while the rest is used for thrust. Once the engine is started, combustion is continuous.

The turbojet does have one minor disadvantage. Its speed is limited to less than the speed of sound. If it approaches that point, shock waves develop on the compressor blades and interfere with engine operation/efficiency.

SOLID-FUEL ROCKET MOTORS

Although there are solid- and liquid-fuel rockets, the majority of SMS missiles have solid-fuel rocket motors. The major elements of such propulsion units include (1) propellant, (2) combustion chamber, (3) igniter or squib, and (4) exhaust nozzle (fig. 9-20).

The combustion chamber of a solid-fuel rocket has two purposes. First, it acts as a stowage place for the propellant. Second, it serves as the area where burning takes place. Depending on the grain configuration used, this chamber may also contain a device to hold the propellant in a certain position. A trap of some sort may be included to prevent flying particles of propellant from clogging the throat of the exhaust nozzle. Additionally, the chamber may have resonance rods. They absorb vibrations set up in the chamber during burning.

The igniter consists of a small explosive charge, such as black powder or a comparable material. The substance is easily ignited by either a spark discharge

Figure 9-19.-Cross-sectional view of a basic turbojet.

Figure 9-20.-Components of a solid-fuel rocket motor, with end burning grain.

or small electric current. As it bums, the igniter produces a temperature high enough to activate the main propellant charge. The igniter is sometimes known as a primer or a squib.

The exhaust nozzle serves the same purpose as in any other jet-propulsion system. It must be of heavy construction and heat-resistant due to the high pressures and temperatures of the exhaust gases.

Operation of a solid-fuel rocket is simple. To start the combustion process, the igniter or electric squib is "fired to initiate the main propellant. You get a cloud of smoke, a pretty loud roar, and a rail-clear indication!