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Missiles and rockets principle of operation

Numerous types of designs of missiles and rockets have been developed for use against various types of targets. Differ as these designs do all of them have in common at least one characteristic: the principle upon which their engines operate is Isaac Newton’s third law of motion: “Every action has an equal and opposite reaction.”

The “action” in a rocket engine is the pressure exerted by flaming gases when the propellant, either solid or liquid, is ignited in the firing chamber. The “reaction” is the way the chamber that encloses the gases responds to this action.

As shown in Fig. I, when the ignited gases in the chamber press the enclosing surfaces looking for an escape, there is only one escape – through the open end called the discharge nozzle.

Pushing against the closed end, the hot gases build up pressure that thrusts the rocket forward while the gases are ejected through the discharge nozzle, just as a cannon recoils from its powder charge or as a blown-up balloon reacts when released and the air escapes. The resultant forward thrust is the “reaction” which gives its power.

Systems of classification of rockets and missiles

There are in existence a number of classification principles for the rocket engines and the rockets proper (the vehicles). The more general ones will be described below.

The rocket engines are divided into two main classes – the solid propellant engines and the liquid propellant engines. In the solid propellant engine, all the fuel is contained in a high pressure combustion chamber. Once the propellant is ignited, the rate of combustion cannot be controlled nor can it be stopped and refined.

In the liquid propellant rocket engines, the fuel and oxidizer are carried in thanks, and a feed system is used to force the propellant into the combustion chamber. The rate of combustion in this type of engine can be controlled by the feed system.

The rockets proper (the vehicles) are divided into two main classes – the “free” (nonguided) rockets termed “rockets” and the “guided” rockets “missiles.”

Free rockets do not contain any guidance mechanism. The launcher imparts initial direction to the rocket, which then follows a normal ballistic trajectory to the target. Free rockets are characterized by the great weight of ammunition, relatively light weight of launcher, and decrease in accuracy as compared with artillery cannon as shorter ranges.

As opposed to free rockets, missiles are vehicles which move above the earth’s surface and whose trajectory or light path can be altered by a mechanism within the vehicle. Radar, television, heat, or number of other agencies may be adapted to move the steering mechanism of missile to direct it on a collision course. Or it may be steered to its target by remote control by means of radio signals.

The type of frame construction furnishes another principle for classification of rockets and missiles. In accordance with this principle all rockets and missiles are divided into the winged and the unwinged groups. The aerodynamics of the winged rocket is similar to that of the airplane, i.e., its flight is determined by the lifting force of the wings. The unwinged rockets and missiles are often called ballistic for they usually follow a ballistic trajectory.

According to their combat missions all missiles and rockets of the US Armed Forces are designated as Strategic Missiles, Tactical Missiles, Air/ Space Delense Missiles and Antisubmarine Missiles. The strategic missiles are subdivide into intercontinental ballistic missiles (ICBM) and intermediate range ballistic missiles (IRBM), while the tactical missiles include such subcategories as antitank missiles and anti-low-flying aircraft missiles; the are space defense missiles include antiaircraft and antimissiles systems. Missiles designed to penetrate enemy defenses or launched upon approaching targets for diversionary purposes are called “diversionary” missiles. Missiles used to hamper the operation of radar installations are termed “antiradar” missiles. The relative location of the missiles launcher (or the launcher pad) and the target provides the basis for dividing the missiles and rockets into several general categories (or types). In the US Armed Forces there are four of them: surface-to-surface, surface-to-air, air-to-air and air-to-surface.

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The surface-to-surface missile is equivalent to long range artillery that may be fired from land against troop concentrations, important supply depots, communication centers, or industrial areas.

Under the “US Army policy for the integration of rockers and missiles into the Army weapons system”, surface-to-surface missiles are defined by three ranges.

Short-range – assault or demolition guided missiles to be used armor and fortifications; medium-range – missiles to supplement and extend the or firepower of artillery cannon, to provide close or interdiction fire support for ground combat forces, and to compensate for the expanding dimensions of the battle area; long-range – missiles capable of supporting deep penetrations, or airheads, from protected and widely dispersed rear; and of delivering accurate fire on distant targets which are capable of affecting the execution of the Army’s combat mission.

The surface-to-air missile is analogous to an antiaircraft shell but is far more lethal and has far greater speed, range, and accuracy. These missiles are effective in destroying high-speed airplanes at al altitudes.

The air-to-air missile is the principal weapon of air-to-air combat. The speed of jet fighters gives them both an offensive advantage over the larger and slower strategic bombers that they may attack. However, if bombers are armed with missiles that, when launched, automatically steer themselves toward the attacking fighter and explode at contact or in close vicinity, then they have an effective defense against the faster attacking plane.

The air-to-surface missiles are, in affect, a type of controlled bomb. They are very effective against special kinds of ground targets, bridges, ships, etc.

To characterize a rocket or missile fully all the above mentioned principles of classification must be applied. In other words one has to state whether the rocket (missile) is equipped with a solid or a liquid propellant engine, whether the rocket is guided or free, whether the frame is winged or wingless, what the category of the rocket is and what type of mission it fulfills.

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Main components of the combat missile

The Modern rocket is, in effect, a complex mechanism consisting of thousands of intricate details. The complexity of its design can be easily seen from the fact that V-2, a rocket developed by the Germans towards the end of World War II, consisted of approximately 30,000 parts. The V-2 when judged by the present day standards is a crude and primitive job.

An Atlas (a modern intercontinental ballistic missile) contains over 300,000 precision parts. To produce a modern antimissile missile Hawk the coordinated effort of 40 European and 12 US companies is needed, while the production documentation for this missile includes some components which support the systems integral to the missile.

The designs of modern missiles are many and diverse. Any missile however consists of several major parts: a propulsion system, propellants for that system, storage tanks for the propellants, structure which is able to communicate the propulsive thrust to the payload, a guidance system, auxiliary power supplies and payload (in the case of the combat missile the payload is conventional, nuclear or thermonuclear charge). The propellant storage tanks and the general structure are often known as the airframe. The airframe of a ballistic missile is then defined as the assembled structural and aerodynamic areas; and of delivering accurate fire on distant targets which are capable of affecting the execution of Army’s combat mission.

Both the single-stage and each stage of the two-stage vehicle contain a rocket engine, two propellant tanks, one for fuel and the other for oxidizer. A turbopump assembly drives the propellant from the tanks to the engines. These assembles consist of a fuel pump and a propellant pump driven by a turbine. The missile is controlled in flight by a control system which operates thrust vector steering devices. The control signals are generated as a result of commands from a guidance system which determines whether or not the vehicle is on the correct path to reach its target. An auxiliary power supply supplies electrical and hydraulic power for the various devices and sub-systems within the vehicle as opposed to the main propulsion power supplied by the rocket engines. The re-entry body contains the warhead, the fuse, the arming mechanism, and the re-entry guidance equipment. When a two-stage vehicle is used, some but not all of these sub-systems are duplicated in first and second stages.

An overall block diagram of a typical two-stage solid-propellant ballistic vehicle is shown in Fig. The first and second stages contain a solid-propellant rocket motor. This motor consists of a solid-propellant charge and an exhaust nozzle system. It has a thrust termination system to cut off the thrust when the desired velocity has been reached for the payload. In addition, it has a thrust vector control which can direct the thrust for trajectory control.

As with the liquid-propellant missiles, flight control is obtained through the use of a control system which operates the thrust vector steering device. The control signals are generated as a result of commands from a guidance system which determines whether or not the vehicle is on the correct path to reach its target. An auxiliary power supply supplies electrical and hydraulic power for sub-systems within the vehicle. The re-entry body contains the warhead, the fuse, the arming mechanism, the safety mechanism, and the re-entry guidance equipment. In some respects the solid-propellant vehicle is simpler than its liquid-propellant counterpart.

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Propulsion units and propellants

Propulsion for a ballistic vehicle usually relies upon a rocket system which is defined as propulsion by ejection of matter, all of which is originally carried within the vehicle being propelled. A rocket engine produces an unbalanced force on the vehicle and in thus able to move it. The propulsive action of the rocket engine arises from the reaction to the acceleration, relative to the rocket engine, of a mass of propellant originally carried within the rocket-propelled vehicle. In the rocket engines so far employed in ballistic missiles, acceleration of the propellant mass is brought about by:

(a) releasing heat energy by a chemical reaction of propellants within a combustion chamber, and

(b) the use of an expansion nozzle to produce a supersonic exhaust stream by expanding the evolved gas from combustion-chamber pressure to ambient pressure.

The combustion chamber and expansion nozzle are together known as the thrust chamber.

The thrust chamber has an injector plate. The design of the thrust chamber is governed by the propellants used, the thrust required, the permissible pressure within the chamber, the altitudes at which the thrust chamber must operate, the combustion temperature, and the method of cooling. After the dimensions of the thrust chamber are determined the throat area, expansion ratio, and flow rate are established. If a long period of combustion is required, regenerative cooling is nearly always used.

Rocket thrust chambers used in ballistic missiles have been fabricated from nickel-alloy tubes through which the coolant flows. The manufacturing process consisted of assembling the tubes in the configuration of the combustion chamber and expansion nozzle and brazing or welding them together to from the shaped thrust chamber. Originally steel bands were welded around them to give the necessary hoop strength. A considerable weight reduction was later made in thrust chambers by eliminating the steel bands and using-untwisted glass filament tape wound around the cylinder.

The thrust vector control is achieved by gamboling or swiveling the thrust chamber itself as a whole or by deflecting the jet be jet vanes or paddles, or by swiveling the nozzle.

The control system of the ballistic missile rocket engine has to ensure that the engine can be started and shut down at the correct times. It has also to ensure that the thrust is maintained at a predetermined level and that propellants are fed to the combustion chamber at the required pressures and at the correct mixture rations. The control systems must sense any malfunctions and incorrect operations of the start and stop sequences and must shut down the engine if abnormal and dangerous conditions develop. After the shut down of the engine, the control system must arrange for the venting of unused propellants.

Propellants which are used in rocket vehicles can be stored in eight solid or liquid form, and the associated engines are known as solid or liquid-propellant rocket engines respectively.

A rocket engine a basically eats as a chamber containing a high-pressure gas whish is continuously replenished as some of the gas escapes through an orifice into the region of lower pressure outside the chamber. The simplest rocket engine consists of a chamber – the combustion chamber – in which fuel can be burned in an oxidizer. The oxidizer can be carried either separately as in liquid propellants or mixed with the fuel as with solid propellants. The fuel and oxidizer are known as propellants. In a liquid-propellant engine they are injected into the combustion chamber. In a solid-propellant motor, the propellant storage tank and the combustion chamber are one and the same. The propellant mixture is triggered electrically, thermally, or chemically, to produce a heat-releasing chemical reaction, and the molecules of the gas produced by the combustion process large amounts of energy, moving rapidly in all directions within the chamber. Individually these molecules each have a high kinetic energy but their motions are randomly directed. To achieve a propulsive effect, this random motion has to be directed as uniformly as possible in a direction away from the rocket thrust chamber.

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In issuing through the expansion nozzle, the random motion of the gas particles is changed to a more unidirectional motion. The gas emerges as a high-velocity stream from the thrust chamber.

Fuels for a rocket engine can consist of many substances ranging from solid such as asphalt, synthetic rubber, beryllium, to the well-known commercial liquid fuels like gasoline, alcohol, jet fuels, and even liquefied gas such as hydrogen. Oxidizers are more limited. Principal substances used are liquid oxygen, nitric acid, and concentrated hydrogen peroxide. Atlas, Titan, Thor, and Jupiter used liquid oxygen as the oxidizer.

When the propellants are solid, they are already in the combustion chamber. Liquid propellants, on the other hand, have to be displaced from storage tanks into the combustion chamber. In small rockets units, this in done by compressed inert gas acting on the propellants in their storage tanks. For large engines of the type used in ballistic missiles, a pump feed is used which centrifugal pumps are driven by a gas turbine.

Ignition of a ballistic missile rocket engine is the initiation of combustion. During the ignition phase a supporting flame is maintained and a low flow rate of primary propellants is begun. As soon as the propellants are burning properly either an automatic or a manual changeover to maximum performance burning takes place.

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