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WEAPON PROPULSION AND ARCHITECTURE
17.1 OBJECTIVES AND INTRODUCTION
Objectives
1. Know the different types of propulsion systems.
2. Understand the factors that control propellant burning rate.
3. Understand the definitions of the following terms: degressive, neutral, and progressive burning.
4. Be able to describe propellant action inside a gun.
5. Know the different types of reaction propulsion systems.
6. Understand the classification of jet-propulsion engines.
7. Understand the basic principles of operation of a rocket motor.
8. Understand the basic functions of a jet nozzle.
9. Understand the basic principles of operation of a turbojet, fanjet, and ramjet engine.
Introduction
Every weapon requires some type of propulsion to deliver its warhead to the intended target. This chapter will be a study of the propulsion systems used to propel weapons to their targets and the design requirements for the vehicles themselves. The underlying principle of propulsive movement has been stated by Newton in his Third Law of Motion: To every action there is an equal and opposite reaction. Every forward acceleration or charge in motion is a result of a reactive force acting in the opposing direction. A person walks forward by pushing backwards against the ground. In a propeller-type airplane, the air through which it is moving is driven backward to move the airplane forward. In a jet-propelled plane or a rocket, a mass of gas is emitted rearward at high speed, and the forward motion of the plane is a reaction to the motion of the gas. Matter in the form of a liquid, a gas, or a solid may be discharged as a propellant force, expending its energy in a direction opposite to the desired path of motion, resulting in a predetermined acceleration of the propelled body along a desired trajectory.
17.2 TYPES OF PROPULSION
The power required to propel a warhead to its target is obtained through the controlled release of stored energy. Usually types of propulsion are considered from two viewpoints: the energy source or method of launch. The energy source may be the end product of:
(1) a chemical reaction
(2) a compression of gases or liquids
(3) the effect of gravity
By method of launch, weapon propulsion systems are normally classified as:
(1) impulse-propulsion or gun-type--a projectile
(2) reaction--a missile or torpedo
(3) gravity--a bomb
17.3 IMPULSE PROPULSION
Impulse propulsion systems include all weapons systems in which a projectile is ejected from a container (usually a long tube) by means of an initial impulse. Also included in this classification are systems that employ compressed gases (torpedo and Polaris/Poseidon launching systems) to provide the initial impulse propulsion until they have cleared the launching vehicle.
The expulsion of a projectile at the high velocities demanded by modern warfare requires tremendous forces. The study of these forces and of the action within a gun at the time of fire is referred to as interior ballistics. It comprises a study of a chemical energy source, a working substance (high-pressure gas), and the equipment to release and direct the working substance. To provide the high speed of response needed in a gun, the propellant must transfer its energy of reaction to the projectile by means of the expanding gaseous products of combustion, which are considered the working substances.
17.3.1 Explosive Propellant Train.
The gas that causes the pressure that propels the projectile is generated by the ignition of an explosive train. This explosive train is termed a propellant train and is similar to the high-explosive train discussed previously. The difference is that the propellant train consists primarily of low explosives instead of high explosives and has a primer, an igniter or igniting charge, and a propelling charge. Ignition of a small quantity of sensitive explosive, the primer (lead azide), is initiated by a blow from the firing pin and is transmitted and intensified by the igniter so that the large, relatively insensitive propelling charge burns in the proper manner and launches the projectile. Figure 17-1 shows the elements just mentioned.
17.3.2 Propellants.
All guns and most rocket-powered weapons use solid propellants to provide their propulsion. The first solid propellant used by man was black powder in the thirteenth century. Black powder is no longer considered suitable for use as a propellant for several reasons: it burns incompletely, leaving large amounts of residue; it creates high temperatures resulting in rapid bore erosion; it creates great billows of black smoke; and it detonates rather than burns.
Gunpowders or smokeless powders are the propellants in use today. This substance is produced by combining nitrocellulose (nitric acid and cotton) with ether and alcohol to produce a low explosive. Although called smokeless powders, they are neither smokeless nor in powder form, but in granule form. Smokeless powders may be considered to be classed as either single or multibase powders.
In single-base powders, nitrocellulose is the only explosive present. Other ingredients and additives are added to obtain suitable form, desired burning characteristics, and stability. The standard singlebase smokeless powder used by the Navy is a uniform colloid of ether-alcohol and purified nitrocellulose to which, for purposes of chemical stability, is added a small quantity of diphenylamine.
The multibase powders may be divided into double-base and triple-base powders, both of which contain nitroglycerin to facilitate the dissolving of the nitrocellulose and enhance its explosive qualities. The nitroglycerin also increases the sensitivity, the flame temperature, burning rate, and tendency to detonate. The higher flame temperature serves to decrease the smoke and residue, but increases flash and gun-tube erosion. Double-base propellants have limited use in artillery weapons in the United States due to excessive gun-tube erosion, but are the standard propellants in most other countries. Double-base propellants are used in the United States for mortar propellants, small rocket engines, shotgun shells, the 7.62-mm NATO rifle cartridge, recoilless rifles, and the Navy's 5"/54-caliber gun.
Triple-base propellants are double-base propellants with the addition of nitroguandine to lower the flame temperature, which produces less tube erosion and flash. The major drawback is the limited supply of the raw material nitroguandine. At present, triple-base propellants are used in tank rounds and are being tested for new long-range artillery rounds.
17.3.3 Burn Rates.
Solid propellants are designed to produce a large volume of gases at a controlled rate. Gun barrels and some rocket casings are designed to withstand a fixed maximum gas pressure. The pressure generated can be limited to this maximum value by controlling the rate of burning of the propellant. The burning rate is controlled by varying the following factors:
(1) The size and shape of the grain, including perforations. (2) The web thickness or amount of solid propellant between burning surfaces; the thicker the web, the longer the burning time.
(3) The burning-rate constant, which depends on the chemical composition of the propellant.
(4) The percentages of volatile materials, inert matter, and moisture present. A 1% change in volatilesin material a low-volatile-content propellant may cause as much as a 10% change in the rate of burning.
When a propellant burns in a confined space, the rate of burning increases as both temperature and pressure rise. Since propellants burn only on exposed surfaces, the rate of gas evolution or changes in pressure will also depend upon the area of propellant surface ignited. Propellants are classified according to variations in the rate of burning due to changes in the quantity of ignited surface; for example, grains are:
(1) Degressive or regressive burning--as burning proceeds, the total burning surface decreases. Propellants formed in pellets, balls, sheets, strips, or cord burn degressively. (See figure 17-2.) Degressive grains are used in weapons with short tubes.
(2) Neutral burning--as burning proceeds the total burning surface remains approximately constant. Single perforated grains and star perforations result in neutral burning. (See figure 17-3.)
(3) Progressive burning--as burning proceeds, the total burning surface increases. Multi-perforated and rosette grains burn progressively. (See Figure 17-4.)
17.3.4 Compressed Air Gas.
One final propellant that needs mentioning is compressed air or gas. Compressed air or a compressed gas is used for the ejection of torpedoes and missiles. Particularly useful in submarines, compressed air has the advantage of being available and easily controllable without the obvious disadvantages of explosives. In addition, the high pressures obtainable from gunpowder are not needed in these applications, since the expanding gas serves primarily as a booster, with a sustaining propulsion system taking over once the missile is clear of the submarine or torpedo launcher. Its disadvantage is the requirement to maintain a high-pressure air compressor with its associated high noise level.
17.3.5 Interior Ballistics.
When the charge is ignited, gases are evolved from the surface of each grain of propellant, and the pressure in the chamber rises rapidly. Due to friction and resistance of the rotating band, the projectile does not move until the pressure behind it reaches several hundred pounds per square inch. For a time after the projectile starts to move, gases are generated more rapidly than the rate at which the volume behind the projectile in increased, and the pressure continues to rise. As the propellant burning area decreases, the pressure falls; however, the projectile continues to accelerate as long as there is a net force acting on it. When the projectile reaches the muzzle, the pressure inside the tube has fallen to about a tenth to a third of the maximum pressure. Gas pressure continues to act on the projectile for a short distance beyond the muzzle, and the projectile continues to accelerate for a short time. Figure 17-6 shows the relationship of pressure and velocity versus distance of projectile travel.
Since the work performed on the projectile is a function of the area under the pressure-travel curve, the muzzle velocity produced by the propellant can be determined.
Work = KE = 1mv2, if the Initial Velocity is zero.
2
Should it be desired to increase the muzzle velocity of a projectile, the work performed or the area under some new curve must be greater than the area under a curve giving a lower muzzle velocity. Such an increase in velocity is indicated by curve B, whose maximum pressure is equal to that of curve A, but whose area is greater than that under A. It appears that the ideal pressure-travel curve would be one that coincided with the curve of permissible pressure. If it were possible to design such a propellant, it would have undesirable characteristics. In addition to producing excessive erosion (which would materially decrease the accurate life of the gun), brilliant flashes and non-uniform velocities due to high muzzle pressure would result. The powder chamber would also have to be enlarged, thus increasing the weight and decreasing the mobility of the gun. Curve C is the gun strength curve, which indicates maximum permissible pressure and is a function of gun tube thickness. (See Chapter 20).
Note: Often the pressure-travel curve is the given variable, and the gun tube thickness is then dictated by the pressure-travel curve.
There are several methods available for changing the pressure-travel curves for a given gun. The easiest method is to change the type of propellant grain. Figure 17-7 illustrates the effect that the three types of powder grains have on the pressure-travel curve. As can be seen, a progressive powder reaches maximum pressure much later than a degressive powder and loses pressure much more gradually. The degressive powder reaches a much larger maximum pressure as shown. Thus, it stands to reason that degressive powders are used in weapons with short tubes, and increasingly progressive powders are used in weapons with longer tubes.
Other methods of changing the pressure-travel curve are by altering the burning rate of the powder, changing the loading density of the propellant, or by changing the grain size.
The energy developed by the burning of the propellant in a typical medium-caliber gun, assuming complete combustion, is distributed as follows:
Energy Absorbed % of Total
Translation of projectile 32.00
Rotation of projectile 00.14
Frictional work on projectile 2.17
(Due to engraving of rotating 34.31
bands, wall friction, and
effects of increasing twist)
Translation of recoiling parts 00.12
Translation of propellant gases 3.14
Heat loss to gun and projectile 20.17
Sensible and latent heat losses in 42.46
propellant gases
Propellant potential 100.00%
(Reflected in the area generated under a pressure-travel curve for the cannon, figure 17-6.)
17.4 REACTION PROPULSION
17.4.1 Introduction.
Contrary to popular belief, reaction motors do not obtain thrust by "pushing" against the medium in which they are operating. Thrust is developed by increasing the momentum of the working fluid and creating a pressure differential therein. Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This principle forms the basis for the motion of all self-propelled objects. Pressure differentials are employed to move propeller-driven aircraft as well as rockets and jet-propelled missiles. By definition, a reaction-propelled vehicle is one containing within its structure its own source of propulsion, i.e., a reaction-type motor. The mediums in which reaction motors operate and the types that have been used in weapon systems are:
air: propeller engines, turbojets, ramjets, rockets
vacuum: rockets
water: screws, hydrojets, rockets
The performance required of modern propulsion systems, insofar as range, velocity, and control are concerned, is considerably in excess of that which can be accomplished by heretofore conventional methods. Until the late 50's, the reciprocating engine-propeller combination was considered satisfactory for the propulsion of aircraft. But the generation of shock waves as the speed of sound is approached limits the development of thrust and thus the speed of propeller-driven craft. Although future developments may overcome the limitations of propeller-driven vehicles, it is necessary at the present time to use jet propulsion for missiles traveling at high subsonic and supersonic speeds. A minor disadvantage of jet propulsion is that tremendous quantities of fuel are consumed per second of flight time. The efficiency ratings of jet engine types, however, are so superior to those of propeller-driven types that the disadvantage as a measure of degree is minimal. Also, the only means of self-propulsion that will operate in a vacuum is jet propulsion with rockets.
17.4.2 Classification of Jet Propulsion Engines.
Jet propulsion is a means of locomotion obtained from the momentum of matter ejected from within the propelled body in the form of a fluid jet. The fluids used for producing the jet include water, steam, heated air, and gases produced by chemical reaction. The fluid may be drawn into the body from the very medium in which the body is moving, such as air in a jet engine or water in a hydrojet engine, or it may be self-contained, as in the case of rocket engines, where the working fluid is composed of the gaseous products of rocket fuel combustion. A jet-propulsion engine consists essentially of an air inlet section (diffuser) for air-breathing jets, a propellant supply system, a combustion chamber, and an exhaust nozzle. The purpose of the propellant system and the combustion chamber is to produce large volumes of high-temperature, high-pressure gases. The exhaust nozzle then converts the heat energy into kinetic energy as effeciently as possible. In jet engines and in liquid-fuel rockets, the fuel is pumped into the combustion chamber to be burned. In a solid-propellant rocket, the combustion chamber already contains the fuel to be burned.
Popular terminology makes a distinction between jets and rockets: a jet takes in air from the atmosphere; a rocket needs no air supply, as it carries its own supply of oxygen. Both types of engines operate by expelling a stream of gas at high speed from a nozzle at the after end of the vehicle. For purposes here, a rocket can be considered a type of jet engine. Jet-propulsion systems used in missiles may be divided into two classes: thermal jet engines and rockets.
In a thermal jet engine (air breathing), the missile breathes in a quantity of air at its input end and compresses it. Liquid fuel is then injected into the compresses air and the mixture is ignited in a combustion chamber. As the resulting hot gases are expelled through a nozzle at the rear of the vehicle, heat energy is transformed into kinetic energy, and thrust is created. Missiles using air-breathing propulsion systems are incapable of operating in a vacuum.
As previously mentioned, the basic difference between a rocket and a thermal jet engine is that the rocket carries its own oxidizer and needs no outside air for the combustion process. In addition, the thrust developed by an air-breathing engine is dependent on the momentum change between the fluid (air) entering the engine and the fluid (hot gases) exiting the engine, whereas in a rocket the thrust depends only on the momentum or velocity of the jet of exhaust gases. Furthermore, the thrust developed by a rocket per unit-of-engine frontal area and per unit-of-engine weight is the greatest of any known type of engine. Rockets are distinguished by the means used to produce exhaust material. The most common type of rocket engine obtains its high-pressure gases by burning a propellant. This propellant consists of both fuel and oxidizer and may be solid or liquid.
17.4.3 Propellants.
The fuels and oxidizers used to power a jet/rocket engine are called propellants. The chemical reaction between fuel and oxidizer in the combustion chamber of the jet engine produces high-pressure, high-temperature gases. These gases, when channeled through an exhaust nozzle, are converted into kinetic energy creating a force acting in a direction opposite to the flow of the exhaust gases from the nozzle. This propulsive force, termed thrust, is a function primarily of the velocity at which the gases leave the exhaust nozzle and the mass flow rate of the gases.
In order to develop a high thrust with a solid propellant, grains or charges of propellant are employed with large burning surfaces so that a high rate of mass flow is developed. The duration of burning of a propellant charge is determined by the web of the grain and the burning rate. Since the combustion chamber has fixed dimensions and capacity for propellant, the thrust may be either great, but of short duration, or low, but of long duration. The thrust developed by a reaction motor is the resultant of static pressure forces acting on the interior and exterior surfaces of the combustion chamber. This force imbalance is shown graphically in the figure 17-9.
The static pressures acting upon the interior surfaces depend upon the rate at which propellants are burned, the thermochemical characteristics of the gases produced by their combustion, and the area of the throat of the dispersing nozzle. As the internal forces are several times greater than the external forces, and as the forces acting normal to the longitudinal axis of the combustion chamber do not contribute to the thrust, the thrust developed is primarily the resultant axial component of the pressure forces.
If the thrust-time curve (figure 17-10) obtained from firing a rocket is integrated over the burning duration, the result is called total impulse and is measure in Newton-seconds. Then
to
It = Tdt = Tamtb (17-1)
o
where
It is the total impulse Newton-sec (N-sec)
tb is burn time(sec)
Tav is average thrust in Newtons (N)
The performance characteristics of a specific propellant is called the specific impulse of the propellant (solid-fuel) or specific thrust of the propellant (liquid-fuel). The specific impulse is a measure of the "quality" or "merit" of the fuel and is defined as:
Isp = Tavtb = It
mpg mpg (17-2)
where
Isp is specific impulse (sec)
mp is mass of solid propellant (kg)
g is the force of gravity (9.8 M/sec2)
Although the units of specific impulse are seconds, the Isp of a fuel is actually the amount of impulse per kg of fuel. In other words it reflects the specific energy of the fuel. Thus, the fuel with the highest Isp will produce the greatest performance.
Propellants are classified as either solid propellants or liquid propellants. Nearly all of the rocket-powered weapons in use by the United States use solid propellants. Liquid propellants are still used in some of the older ICBMS and will be used in future cruise missiles. Of course, all thermal jet engines burn a liquid fuel.
17.4.3.1. Solid propellants consist of a fuel (usually a hydrocarbon) and an oxidizer, so combined as to produce a solid of desired chemical potential. The burning rate, which directly affects the amount of thrust produced, is the velocity in meters per second at which the grain is consumed. Burning rates of propellants depend upon the chemical composition of the propellant, the combustion chamber temperature and pressure gradient, gas velocity adjacent to the burning surface, and the size and shape of the grain or the geometry of the charge. The propellant charges are cast to obtain the types of burning previously discussed: neutral, progressive, or degressive burning. Propulsive specifications will determine which type is to be used. Normally a uniform burning rate is desired so that a constant thrust is produced. Most Navy propellants use either a cruciform grain or a cylindrical grain with an axial hole and radial perforations. (Figure 17-11) The cruciform grain in cross section is a symmetrical cross. If all of the exterior surface of this grain were permitted to burn, there would be a gradual decrease of area, and the burning rate would be regressive. Since a uniform burning rate is desired, a number of slower-burning plastic strips or inhibitors are bonded to certain parts of the area exposed on the outer curved ends of the arms. These control or slow the initial burning rate, and gas-production rate is approximately uniform over burning time. Other cylinder grain types are shown in figure 17-11.
17.4.3.2 Liquid propellants are normally stored in tanks outside the combustion chamber and are injected into the combustion chamber. The injector vaporizes and mixes the fuel and oxidizers in the proper proportions for efficient burning. Liquid propellants are classified as either monopropellant or bipropellant. The monopropellant has the fuel and oxidizer mixed, and the bipropellant has the fuel and oxidizer separated until firing.
Liquid fuels are more powerful than solid fuels; but other than this advantage, a liquid-fuel rocket is not ideally suited as a weapon-propulsion system. Because of their high volatility and corrosive nature, liquid fuels cannot be stored for long periods of time, which usually means the system must be fueled just prior to launch. This negates its ability to be a quick-reaction weapon, which is usually required in combat situations.
Solid-fuel rocket engines do not require complex plumbing systems and are fairly simple systems as illustrated in figure 17-12. Storage problems are minimized, and the propellant is usually very stable. For these reasons, solid-fuel rockets are almost in exclusive use.
17.4.4 Elements of Propulsion Subsystems.
A jet/rocket-propelled engine is basically a device for converting a portion of the thermochemical energy developed in its combustion chamber into kinetic energy associated with a high-speed gaseous exhaust jet. The basic elements of the engine are: 1) the combustion chamber, wherein the transformation of energy from potential to heat occurs; 2) the exhaust nozzle, wherein thermochemical energy is converted into the kinetic energy necessary to produce an exhaust jet of propulsive potential; and 3) the diffuser (for air-breathing jets only) or intake duct, wherein the high-speed air intake is converted into low-speed, high-pressure air for entry into the combustion chamber as an oxidizing agent.
The combustion chamber is the enclosure within which high-temperature, high-pressure gases are produced and potential energy is converted to kinetic energy. For liquid-fuel engines, injectors bring the fuel and oxidizers together in the chamber, and for solid-fuel engines the propellant is already contained in the chamber. Also contained in the combustion chamber is some type os ignition device to initiate the burning of the propellant.
An exhaust nozzle is a mechanically designed orifice through which the gases generated in the combustion chamber flow to the outside. The function of the nozzle is to increase the exit velocity of the hot gases flowing out of the engine so that maximum thrust is extracted from the fuel. A nozzle consists of a mouth, throat, and exit as illustrated in figure 17-13.
Under conditions of steady flow, the mass flow rate, by Bernoulli's Theorem, remains constant. Thus, in subsonic flow, where the density of the gas/fluid is considered to remain constant, the velocity must increase at any point where the cross-sectional area decreases in order to accommodate a constant mass flow rate. If flow remains subsonic, then as the cross-sectional area increases, the velocity will decrease.
It is possible to reduce the cross-sectional area of the nozzle throat to the point that the velocity will reach a maximum and become sonic. Then, with proper design of the divergent section of the nozzle, the flow velocity can be expanded supersonically as the cross-sectional area increases.
Since thrust from a rocket motor is proportional to the momentum of the exhaust gases ejected per second and momentum is equal to mass times velocity, the thrust efficiency can be increased at no extra cost in fuel consumption if the exhaust velocity is maximized by proper nozzle design.
One of the more efficient designs in common use is the De Laval nozzle. The De Laval nozzle converges to a throat at which the velocity becomes sonic, and then the nozzle diverges such that the flow is expanded supersonically.
17.4.4.1 Turbojet Engines. A turbojet engine derives its name from the fact that a portion of its exhaust output is diverted to operate a turbine, which in turn drives the air compressor used to compress the input air flow. Modern turbojets are of the axial-flow design as depicted in figure 17-14. An axial flow compressor is similar in operation to a propeller. As the rotor of the axial compressor turns, the blades impart energy of motion in both a tangential and axial direction to the air entering the front of the engine from the diffuser. The function of the diffuser or intake duct is to decelerate the velocity of the air from its free stream velocity to a desired velocity at the entrance of the compressor section or combustion chamber, with a minimum of pressure loss. For both subsonic and supersonic diffusers, the flow is decelerated from inlet to the engine compressor section. It is important to note that the air flow must be subsonic before it enters the engine. The stator is set in a fixed position with its blades preset at an angle such that the air thrown off the first-stage rotor blades is redirected into the path of the second-stage rotor blades. The added velocity compresses the air, increasing its density and, as a result, its pressure. This cycle of events is repeated in each stage of the compressor. Therefore, by increasing the number of stages, the final pressure can be increased to almost any desired value.
The combination of the air-intake system, air compressor, combustion system, and turbine is essentially an open-cycle gas turbine combined with a jet stream. In operation, the compressor, driven by the gas turbine, supplies air under high pressure to the combustion chamber, where the fuel and air are mixed and burned. The turbine absorbs only part of the energy, while the remainder is employed for the production of thrust. Once started, combustion is continuous.
Without proper intake or diffuser design, the turbojet is limited to less than the speed of sound. As velocity approaches Mach 1, shock waves develop on the compressor blades and severely interfere with its operation. However, with variable-geometry inlets (the shape of inlet can be changed with changes in velocity) and improved diffuser designs, the turbojet engine's capabilities can be extended into the supersonic region. A turbojet can develop large static thrust, carry its own fuel, and its thrust is practically independent of speed.
The turbojet engine can be modified into three other major engine types: turboprop, turboshaft, and turbofan. These engines extract additional energy from the exhaust gases aft of the gas compressor turbine by adding a second stage of turbine blades. This second stage, the power turbine, propels a shaft attached to a propeller (turboprop), helicopter rotors (turboshaft), or fan blades (turbofan). The direct thrust produced by the flow of exhaust gases rearward is converted to a torque that drives the propeller, rotors, or fan. These types of engines are more efficient than the turbojet engine.
Currently several missiles use turbojets and turbofans for main propulsion plants. These are generally longer-range missiles and include the Harpoon, Tomahawk, and the Air-Launched Cruise Missile (ALCM).
17.4.4.2 Ramjet Engines. The most promising jet engine, from the standpoint of simplicity and efficiency at supersonic speed, is the ramjet--so called because of the ram action that makes possible its operation.
The ramjet, as illustrated in figures 17-15, 17-16, and 17-17, has no moving parts. It consists of a cylindrical tube open at both ends, with an interior fuel-injection system. As the ramjet moves through the atmosphere, air is taken in through the front of the diffuser section, which is designed to convert high-speed, low-pressure air flow into low-speed, high=pressure air flow. Thus, a pressure barrier is formed and the escape of combustion gases out of the front of the engine is prevented.
The high-pressure air then mixes with the fuel, which is being sprayed continuously into the engine by the fuel injectors. Burning is initiated by spark plug action, after which it is uninterrupted and self-supporting--i.e., no further spark plug assistance is required. The flame front is kept from extending too far toward the rear of the engine by a device called the flameholder. By restricting burning to the combustion chamber, the flameholder maintains the combustion chamber temperature at a point high enough to support combustion. The combustion gases bombard the sides of the diffuser and the ram air barrier, exerting a force in the forward direction. Since the gases are allowed to escape out of the rear through the exhaust nozzle, the force in the forward direction is unbalanced. The degree to which this force is unbalanced depends on the efficiency with which the exhaust nozzle can dispose of the rearward-moving combustion gases by converting their high-pressure energy into velocity energy.
Ramjets operate in the subsonic, supersonic, and hypersonic ranges. Theoretically, there is no limit to the speed they can attain. However, because of the intense heat generated by air friction, present-day materials are unable to withstand speeds in excess of Mach 5.0 in the atmosphere.
The main disadvantage of ramjet engines lies in the fact that they are, by the nature of their operation, unable to develop thrust at rest (static thrust). If fired at rest, high-pressure combustion gases would escape from the front as well as the rear. Consequently, before a missile with a ramjet engine can function properly, it must first be boosted by some other propulsion system to a speed approximately equal to that at which it was designed to operate. Ramjet engines are also restricted to use in altitude below approximately 90,000 feet, since they must have air to operate.
A new variant of the ramjet, an integral rocket-ramjet, is under development for advanced cruise missiles. The rocket propellant is carried in the combustion chamber of the ramjet and provides the boost to get the ramjet up to high speed. When the propellant has burned leaving the chamber empty, the ramjet begins operation.
By burning a solid fuel containing insufficient oxidizer and combining the oxygen-deficient combustion products with air from outside the missile, engineers have been able to build a solid-fuel ramjet. This is especially promising when the advantages of solid-fuel are combined with the increased range potential realized when using ambient air rather than an onboard oxidizer. The solid-fuel ramjet (figure 17-17) shows great potential to increase both the speed and the range of surface-to-air and cruise missiles.
17.5 GRAVITY-TYPE PROPULSION
Gravity-type propulsion is the propulsion system used for free fall and glide bombs, and for missiles once their reaction propulsion system has ceased functioning. In either case, the missile or bomb has some initial velocity given to it from its launching platform (e.g., for a bomb it would be the aircraft velocity). Once it has been released, the only forces acting on it are gravity, drag, and lift in the case of a glide bomb. The potential energy of the weapon (its altitude) is converted to kinetic energy (velocity) as it falls due to the force of gravity.
Table 17-1. Reaction Motor Advantages
Solid-fuel Liquid-fuel
Rocket Rocket Ramjet Turbojet
Very simple Relatively simple Very simple Develops
large
static thrust
Unlimited speed Practically un- No wearing Consumes fuel
limited speed parts. Gets only; gets
oxygen from oxygen from
air. air.
Operates in any Operates in any Lightweight Thrust prac-
medium or in a medium or in a tically in-
vacuum vacuum. dependent of
speed.
No moving parts. Relatively few Relatively Uses common
moving parts. inexpensive fuels(liquid)
to construct
and operate.
Full thrust on Develops full Easy to build.
thrust on take off.
Requires no Has less need for Uses common
booster. a booster than air- fuels. Can be
breathing engine. solid-fueled.
Can be used in Can be staged in Efficient at
stages or combination high speeds
clusters. with liquid or and altitudes.
solid rockets.
Can be stored Supersonic
fully fueled
Ready to fire
anytime Source: NAVORD OP 3000
Table 17-2. Reaction Motor Disadvantages
Solid-fuel Liquid-fuel
Rocket Rocket Ramjet Turbojet
High rate of High rate of Must be boosted Limited to
fuel consumption fuel consump- to high speed low super-
tion. before it can sonic speeds
operate. usually less
than Mach 3. Short burning Short burning Limited to Complicated
time time operation in engine with
atmosphere. many moving
parts.
Comparatively Comparatively Speed presently Power
short range un- short range un- limited to about limited by
less staged. less staged. 3,600 mph. stresses on
turbine
blades.
Fragile and Cannot be stored Limited to
sensitive to fully fueled for operation in
environmental long periods of earth's at-
conditions. mosphere.
Long checkout
procedure at
launching.
Source: NAVORD OF 3000
17.6 WEAPON ARCHITECTURE
The structure of a missile or torpedo is designed to support and protect the warhead, guidance, and propulsion subsystems. It must withstand environmental forces as well as accelerations imposed by motion of the weapon. It must be lightweight in order to attain useful velocity easily and have adequate range, while having sufficient strength to resist bending or outright structural failure during high-speed turns. Additionally, its physical shape must be aerodynamically or hydrodynamically sound to ensure efficiency, while being compatible with storage facilities, launch mechanisms, and platforms. The architecture of the weapon can, to a large measure, determine its success or failure.
17.7 MISSILES
The size and type of a missile selected for a particular function are based on the target, the launch vehicle or platform, range and maneuverability requirements, altitude envelope, and storage requirements. Minimum size and weight may not be the most efficient architecture, and it is often best to employ various types of structures for different sections of the missile to obtain certain design or maintenance advantages. Before addressing missile architecture, consideration must be given to forces experienced by a missile in flight (figure 17-18).
17.7.1 Basic Requirements
17.7.1.1 Drag and Lift. These aerodynamic forces are handled analytically be resolving them into two planar components--parallel to, and at right angles to, the direction of flow of the medium being traversed. The lift and drag forces are considered to act at a point called the Center of Pressure, C.P. (figure 17-18).
Lift is supplied by airfoils due to pressure distributions caused by their shape and inclination to the passing medium (angle of attack). The entire body of the missile supplies lift when inclined to the airstream and sometimes provides a major contribution to this force at high velocities. The amount of lift provided is directly related to the density of the medium in addition to the missile's velocity. Thus, because the size of missile airfoils cannot be increased at high altitudes, loss of lift due to lower air density must be compensated for by an increased angle of attack.
Parasitic drag is a force that opposes motion of the missile, due to displacement of the air in front of it, and to friction from the flow of air over its surface. All exposed missile surfaces resist this flow and contribute to drag. Drag is directly proportional to the velocity squared, thus streamlining of the missile becomes paramount as velocity increases. If the surface is rough or irregular, turbulence in the flow will increase and counteract the effect of streamlining. By minimizing these irregularities, the layers of air close to the surface of the missile flow smoothly (laminar flow), and drag is minimized for that particular configuration. Induced drag is caused by inclination of the wings or control surfaces to the airstream thus increasing the amount of air displaced in front of the missile.
17.7.1.2 Stability. While the weight of the missile is considered to act at the center of gravity (C.G. in figure 17-18), the lift and drag forces are considered to act at a point called the center of pressure (C.P. above). Lift and drag-force vectors intersect at this point, and if the center of gravity and the center of pressure are not located at the same point, as is often the case, the resulting moment of the forces acting at these points may cause unstable flight conditions. Therefore, the location of the center of pressure, determined by the configuration of the missile's structure, is an important factor in missile stability. In addition to the moments tending to produce rotation about the center of gravity (roll, pitch, and yaw), there are moments applied about the hinges or axes of the missile control surfaces. When force vectors representing these pressures have their origin at the center of gravity, it is then possible to describe the rotation of the missile in terms of angular displacements about these axes. When all forces acting on the missile are resolved into component forces along the three axes, we can completely specify the resultant motion of the missile with respect to its center of gravity. The angle of attack of the missile is identified as the angle between the longitudinal axis and the direction of flow of the undisturbed medium; it is generally taken to be positive when the direction of rotation is as shown in the illustration of forces acting on a missile in motion. A missile in flight encounters various forces that may be characterized as disturbing forces or moments, inasmuch as they tend to cause undesired deviations from the operational flight path. They may be random (wind, gusts, etc.) or systematic (misalignment of thrust components). When a missile is design-stable, it will return to its former position after being disturbed by an outside force. Guided missiles have the great advantage over unguided missiles in being able to correct, to some degree, both random and systematic disturbances that may occur during flight.
17.7.2 Control Surface Configuration
In guided missiles it is desirable to have a method of altering the direction of motion. By including airfoils with variable angle of attack, forces can be applied to alter the orientation of the missile in a plane at right angles to the plane of the airfoil, just as the rudder of a ship alters its angle to the flow of water. In both cases, it is this change in angle of attack of the body (hull) of the vehicle that results in the turn. Variation of the location of the control surfaces and other airfoils can significantly change the performance characteristics of the missile.
17.7.2.1 Canard Control. The canard type is characterized by small control surfaces well forward on the body, while the main lifting surface is well aft (figure 17-19). These control surfaces are deflected in a positive manner; that is, the leading edge is raised to provide for a positive attack angle. To apply sufficient force, the canards must be positioned at a large angle of attack, causing large loads on hinges and missile structure. Therefore, large amounts of power are required to position these surfaces rapidly in order to reduce the total time that the force and additional drag are experienced. Canards require relatively large fixed surfaces aft on the missile airframe to improve stability.
17.7.2.2 Wing Control. The wing-control type has control surfaces near the center of the airframe, which are also the main lifting surfaces. The entire lift surface is controllable, increasing or decreasing lift in response to control signals. This produces rapid missile maneuvers without requiring large inclinations to the airstream, resulting in minimum drag and minimum change in missile attitude or orientation in space. Maneuvers are initiated at the instant the wings are deflected and are not dependent upon an increase in body angle. Delay in change or missile orientation may be detrimental to some guidance means because the missile seeker may not be able to maintain contact with the target during maneuvers.
17.7.2.3 Tail Control. The control surfaces in this case are at the rear of the body. Lift, supplied by fixed airfoils at the midsection, and deflection of the tail control surfaces are used to alter the missile angle of attack. With this configuration, control surface deflections are in a direction opposite to that of the angle of attack. Deflections are minimized, resulting in low control surface and body-bending loads as well as in little force on control surface pivots or hinges. Since the main lifting airfoils are not deflected, wing-tail interference effects are minimized. Because of the distance from the center of gravity (long lever arm), control surface size can be reduced, resulting in minimum drag. High power is required for the same reason as in canard control. Control-mechanism design problems are compounded by the small volume available and the severe heat conditions that may be encountered when the control surfaces are located aft in the vicinity of the propulsion-system exhaust nozzle.
17.7.3 Missile Components
The components of a missile are located in five major sections: the guidance section, warhead section, autopilot section, and control and propulsion sections. The functional systems of the missile are:
1. The guidance system
2. The warhead section
3. The autopilot
4. The propulsion system
5. The control system
17.7.3.1 The Guidance System. The guidance system for a homing missile consists of an antenna assembly or electro-optical device protected by an optically transparent cover or a radome in the case of the radio frequency system, and electronic components that analyze signals from the target and compute orders for use by the autopilot. The sensor employed is usually a gimbal-mounted automatic tracking sensor (except the interferometer method) that tracks the target line-of-sight (LOS) and sends signals about the target's movement to the guidance electronics.
17.7.3.2 The Warhead. The warhead consists of the fuze assembly, warhead, safety and arming device, and fuze booster. The fuze assembly usually contains a contact and proximity fuze. The contact fuze is enabled at all times, and the proximity fuze is actuated electronically. Its circuitry works in conjunction with the guidance section to ensure that the target detection device (TDD) remains unarmed until just prior to intercept, minimizing vulnerability to jamming. The safety and arming device prevents arming of the warhead until the missile is a safe distance from the firing platform.
17.7.3.3 The Autopilot. The autopilot is a set of electronic instruments and electrical devices that control the electric actuators (motors) of aerodynamic control surfaces (fins). In the absence of signals from the guidance computer, the autopilot maintains the correct missile attitude and maintains the missile flight in a straight line. Called-for-acceleration signals from the guidance computer will cause the autopilot to command corresponding changes in flight path, while continuing to stabilize the missile.
17.7.3.4 The Propulsion System. Any of the methods of propulsion previously described may be used as long as the missile has sufficient speed advantage over the target to intercept it. The propulsion system must accelerate the missile to flying speed rapidly to allow a short minimum range and achieve sufficient velocity to counter target maneuvers. Powered flight may occur for most of the operational range of the weapon or only at the beginning (boost-glide). Boost-glide weapons are limited in their ability to engage at long range targets that have significant altitude difference or perform rapid maneuvers.
17.7.3.5 The Control System. The steering or control unit may be located forward, in the midsection, or aft on the missile, depending on where the control surfaces are located. Movement of control surfaces may be electrical or hydraulic, with electrical actuation becoming the dominant method. Some weapons are limited in allowable locations for the control actuators because of size limitations or difficulty in passing signals from the autopilot to remote points on the airframe.
17.8 ARCHITECTURE OF GUN AMMUNITION
Gun projectiles are similar in function to missiles, but differ in the means of propulsion and stabilization. Their external shape is designed to obtain maximum stability while experiencing minimum drag. The internal structure of the projectile is dictated by the type and strength of the target, gun pressure characteristics, and the type of trajectory desired. The distribution of weight is a matter of considerable importance in that the center of gravity should be on the longitudinal axis and forward of the center of pressure to ensure maximum stability. High standards of accuracy are required in gun manufacture to ensure uniform flight characteristics and therefore predictable hit patterns. A detailed discussion of the forces affecting projectiles and missiles in general is contained in Chapter 20.
17.8.1 Projectiles
Small arms and machine-gun projectiles are made of solid metal; however, projectiles of 20-mm guns and larger have many components. The external features of projectiles are depicted in figure 17-23. The form of the forward end of the projectile is an ogival curve (generated by revolving an arc of a circle about a chord) that is aerodynamically efficient. Behind the ogive, the projectile is cylindrical with the exception of the bourrelet, which is slightly larger than the diameter of the body to reduce the surface area (and thus the friction) of the projectile contacting the gun bore. Near the after end of the projectile is the rotating band, which is actually larger than gun bore diameter to engage the rifling grooves and seal the bore while supporting the aft end of the projectile. The rifling actually engraves the rotating band to ensure a gas-tight seal. Aft of the rotating band the cylindrical shape may continue to the base of the projectile or it may be tapered to a "boat tail." A square aft section is more efficient in sealing propellant gases and converting pressure to kinetic energy; however, the boat tail provides better aerodynamic characteristics. Newer projectiles such as that in figure 17-24 incorporate a plastic rotating band and rear section that discards in flight, leaving a boat tail rear section and reducing air friction due to the rough surface of the rotating band. This projectile has less drag and therefore more range than similar projectiles without this capability. There are three general classes of projectiles
1. Penetrating
2. Fragmenting
3. Special-purpose
17.8.1.1 Penetrating Projectiles. These include armor-piercing (AP) and common (COM). They employ kinetic energy to penetrate, respectively, heavy and light armor. The bursting charge employed must be insensitive enough to permit penetration without premature detonation. Figure 17-25 shows the architecture of AP and COM projectiles. Both have thick steel walls and the AP projectile includes a hardened nose cap to penetrate armor or concrete, thus reducing the amount of space for explosives. Because efficient penetrating shapes are not good aerodynamically, an ogival-shaped thin steel windshield is included to reduce drag. These are the heaviest projectiles and therefore have the lowest initial velocity, but their velocity does not decay as rapidly as other projectiles due to the additional mass (see equation 13-2).
17.8.1.2 Fragmenting Projectiles. As presented in chapter 13 these projectiles inflict damage by high-velocity fragments. They have relatively thin walls that may be scored and have a large cavity for the bursting charge. In use today, fragmenting projectiles are classified as high-capacity (HC) or antiaircraft common (AAC), which differ primarily in the type of fuze employed. As newer types of multi-purpose fuzes are employed, one projectile can serve both of these purposes.
17.8.1.3 Special-Purpose Projectiles. These are designed to support the destruction of targets, such as providing illumination or screening smoke. If the payload includes any explosive, its small charge is designed to expel the contents of the projectile (figure 17-28).
17.8.2 Guided Projectiles.
The idea of guided projectiles came from the outstanding performance of the laser-guided bombs used in Vietnam. They employ a miniature shock resistant semiactive laser seeker in the projectiles for terminal guidance and may or may not include a rocket assist (RAP) motor. The Navy, in conjunction with the Army and Marine Corps, developed a family of 5-inch, 155-mm, and 8-inch projectiles that provide greatly increased hit probability at much less cost than a guided missile in some applications.
17.8.3 Gun Propellant Configuration.
Case guns are those that employ propellant encased in a metal shell, while bag guns are those that employ propellant charges packed in silk bags. The use of bags is confined to large guns where the total propellant powder required to attain the required initial projectiles velocity is too great in weight and volume to be placed in a single rigid container. By packing the powder grains in bags, it is possible to divide the total charge into units that can be handled expeditiously by one man.
17.9 TORPEDO ARCHITECTURE
17.9.1 Basic Requirements
The technology acquired in the development of advanced guided missiles is generally applicable to the torpedo. There is, in fact, a direct analogy between the homing missile and the homing torpedo, which allows a similar approach in their discussion by substituting the theories of hydrodynamics for those of aerodynamics. Air and water are fluids that display some of the same properties when allowances are made for differences in density, mass, and the general lack of compressibility of water. The force diagram for the torpedo as depicted in figure 17-32 is similar to that for the missile, except buoyancy is included.
Torpedoes consist of a large number of components located in four structural sections, the nose section, warhead (or exercise head), center section, and afterbody and propellers (or hydrojet). To understand how the torpedo operates, a study of the functional systems is required; these are:
(1) the propulsion system
(2) the control and guidance system
(3) the warhead and fuze
17.9.2 Components
Some of the major components of liquid fuel and electric torpedoes are shown in figure 17-33.
17.9.2.1 The Propulsion System. The components of the propulsion system are physically located in the torpedo center and afterbody sections. Upon water entry the torpedo propulsion system is activated, allowing liquid fuel to flow to the combustion chamber. The hot gases produced from the burning fuel drive the engine, which turns an alternator to provide electrical power and also propels the torpedo via two counter-rotating propellers. The engine is usually an external combustion (i.e., the fuel is burned outside the cylinders), axial piston reciprocating type that burns liquid fuel. This fuel is a monopropellant; i.e., it contains the oxygen necessary for its combustion. This fuel, as with most liquid propellants, is very toxic, combustible, and corrosive. In some torpedoes a solid fuel similar to that described for rockets is used to supply hot gas to the engine.
Because propulsion power requirements increase as a function of depth, it is necessary to increase engine output as depth increases to maintain exhaust flow due to higher ambient pressure. To accomplish this, a hydrostatic device on the fuel pump increases the quantity of fuel supplied to the combustion chamber, which increases hot gas production and engine output. This increased fuel consumption causes a loss of torpedo running time and range at greater depths. Torpedoes employing electric motors for propulsion merely substitute a battery and motor for the engine and fuel tank described above. They have a shorter range and lower speed than torpedoes propelled by heat engines.
17.9.2.2 Control and Guidance System. The torpedo can employ either active and/or passive homing and in some cases can receive command guidance via a trailing wire. In active homing the torpedo transmits sound and listens for a returning echo. In passive homing the torpedo detects target radiated noise. If no signal is received, the torpedo can shift automatically to active search. After reaching its operational depth, the torpedo can search in a circular pattern or other pre-programmed path. The type of search conducted depends on the launch platform, torpedo type, and target category.
17.9.2.2.1 Transducers. The torpedo detects targets by receiving acoustic signals that can be evaluated to determine the validity of the contact and target bearing and range. The transducer array consists of many elements that operate by employing the electronic scanning principles in Chapter 7 to create a directional beam and change its direction. A narrow vertical beam is used in shallow water and during the attack phase to minimize surface and bottom reverberation, while a broad vertical beam is used in deep water for better target acquisition. The computer (or automatic pilot) decides which beam width is appropriate.
17.9.2.2.1 Transmitter. Whenever the torpedo is searching in the active mode, the transmitter is controlled by the computer and, when directed, generates a pulse of energy that is sent to the transducer through a series of amplifiers. Computer inputs control the characteristics of the pulse, such as its power, duration, and interval.
17.9.2.2 Receiver. The basic functions of the receiver are to receive target information from the transducer, determine if the contact is a valid target, and supply valid target signals to the computer. Self-noise and reverberation are detrimental to target detection, but can be suppressed by special circuits that distinguish targets from the background.
17.9.2.2.4 Computer. The computer provides control signals and a time base for the transmitter, receiver, and automatic pilot. When the torpedo reaches its search depth, the automatic pilot sends a signal, enabling the computer to function. At the same time it informs the computer whether homing is to be active or passive, and the type of search pattern. Upon receipt of a valid signal, it determines range by calculating the time interval between transmission and reception. Next, it sends a range gate back to the receiver that inhibits reception of any echo not at approximately the same range as the previous valid one. After transmission of the range gate, the yaw angle signal from the receiver is converted to right or left commands to the autopilot. This command starts the torpedo steering towards the target bearing. The computer updates its command signals after every echo in order to guide the torpedo to the target. At appropriate times during this attack phase, information is sent to the transmitter to change its power, vertical beam width, and beam axis.
17.9.2.2.5 Autopilot. The automatic pilot receives torpedo programming information before launch, transmits part of this information to other components in the control and guidance system, controls certain depth functions, and combines command signals with existing conditions to control a steering actuator. A course gyro is contained in the autopilot, which senses the torpedo heading in relation to a programmed heading set in at launch.
The autopilot also contains the yaw, pitch, and roll integrating gyros. Steering commands for pitch and yaw, whether generated by the computer or autopilot itself, are combined with the rate gyro's outputs so that correct command signals are sent to the actuator controller. That is, the autopilot combines desired results with existing conditions of motion and orders the fin controllers to reposition the fins.
17.2.2.6 Steering unit. The steering control actuator contains three motors controlled by signals from the power unit. The outputs of the motors move four fins that control the movement of the torpedo; the upper and lower rudder fins are operated individually by separate actuators to control yaw and roll; the port and starboard elevator fins are operated simultaneously by a common actuator to control pitch.
17.9.2.3 Warhead Section. Torpedo warheads are either pure blast devices or special warheads designed to penetrate well-protected targets. Warheads and fuzes are covered in Chapter 15 and will not be further addressed here.
17.10 SUMMARY
The first part of this chapter has presented a study of the propulsion systems used to propel weapons to their targets. The three main types of propulsion systems are: impulse or gun-type, reaction, and gravity. Impulse propulsion includes the study of interior ballistics and the propellants used in today's guns and impulse-propelled weapons. Reaction propulsion deals with systems that internally carry their own propulsion. Rocket engines and thermal jet (air-breathing) engines are two types of reaction propulsion systems. Within the rocket engine category are both liquid and solid-fueled engines, and within the thermal jet engine category fall the turbojet and ramjet engines. Finally, gravity propulsion is the type of propulsion system used for the dropping of bombs and relies on the principle of converting potential energy to kinetic energy.
17.11 REFERENCES/BIBLIOGRAPHY
Commander, Naval Ordnance Systems Command. Weapons Systems Fundamentals. NAVORD OP 3000, vol. 2, 1st Rev. Washington, D.C.: GPO, 1971.
Department of Engineering, U.S. Military Academy. Weapons Systems Engineering. \vol. 1, West Point, N.Y.: U.S. Military Academy, 1975.
Fox, J. Richard, ed. Shipboard Weapon Systems. Annapolis, Md.: U.S. Naval Academy, 1975.
Mason, L., L. Devan, F.G. Moore, and D. McMillan. Aerodynamic Design Manual for Tactical Weapons. Dahlgren, Va.: Naval Surface Weapons Center, 1981.
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