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CHAPTER - 01
Introduction
1.1 What is a missile?
Basically any object thrown at a target with the aim of hitting it is a missile. Thus, a stone thrown at a bird is a missile. The bird, by using its power of reasoning may evade the missile (the stone) by moving either to the Left, right, top or bottom with respect to the flight path (trajectory) of the missile. Thus, the missile in this case has been ineffective in its objective of hitting the bird (the target). Now, if the stone too is imparted with some intelligence and quick response to move with respect to the bird, to overcome aiming errors and the bird's evasive actions and hit it accurately, the stone now becomes a guided missile.
The incorporation of energy source in a missile to provide the required force for its movement (propulsion), intelligence to go in the correct direction (guidance) and effective manoeuvring (control) are mainly the technologies of guided missiles. They help in making a missile specific to a target, that is, they determine the size, range and state of motion of a missile.
1.2 History of guided missiles
Looking back into the history of rockets and guided missiles, we find that rockets were used in China and India around 1000 AD for fireworks as well as for war purposes. During the 18th century, unguided rocket propelled missiles were used by Hyder Ali and his son Tipu Sultan against the British. There is a reference that two rockets belonging to Tipu's forces were captured during the fourth Mysore war in the siege of Seringapatnam in 1799 by companies of the Bengal and Bombay Artillery of the East India Company.
The current phase in the history of missiles began during the World War I1 with the use of V1 and V2 missiles by Germany. Since then there has been a tremendous and rapid global advancement in this field. It spawned the growth and pushed the frontiers of many new technologies in the areas of materials science, aeronautics, communications, radars and computers. Huge amounts of prime resources have been channelized into this field resulting in the development of sophisticated missiles.
1.3 Missile Aerodynamics versus Airplane Aerodynamics
One of the principal differences between missiles and airplanes is that the former are usually expendable, and, consequently are usually uninhabited. For this reason increased ranges of speed, altitude, and manoeuvring accelerations have been opened up to missile designers, and these increased ranges have brought with them new aerodynamic problems.
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For instance, the higher allowable altitudes, and manoeuvring accelerations permit operation in the nonlinear range of high angles of attack. A missile may be ground-launched or air-launched and in consequence can undergo large longitudinal accelerations, can utilize very high wing loadings, and can dispense with landing gear.
In the absence of a pilot the missile can sometimes be permitted to roll and thereby to introduce new dynamic stability phenomena. The problem of guiding the missile without a pilot introduces considerable complexity into the missile guidance system. The combination of an automatic guidance system and the air frame acting together introduces problems in stability and control not previously encountered. Many missiles tend to be slender, and many utilize more than the usual two wing panels. These trends have brought about the importance of slender-body theory and cruciform aerodynamics for missiles.
1.4 Classification of Missiles
Missiles can be classified on the basis of points of launching and impact, type of guidance system, trajectory, propulsive system, trim and control device, etc. An important classification on the basis of points of launching and impact is given in Table 1-1.
TABLE 1-1 Classification of Missiles
Another source of distinction among missiles is the guidance system. In a command system the missile and the target are continuously tracked from one or more vantage points, and the necessary path for the missile to intercept the target is computed and relayed to the missile by some means such as radio. A beam-riding missile contains a guidance system to constrain it to a beam. The beam is usually radar illuminating the target so that, if the missile stays in the beam, it will move toward the target. A homing missile has a seeker, which sees the target and gives the necessary directions to the missile to intercept the target. The homing missile can be subdivided into classes having active, semi active, and passive guidance systems. In the active class the missile illuminates the target and receives the reflected signals. In the semi active class the missile receives reflected signals from a target illuminated by means external to the missile. The passive type of guidance system depends on a receiver in the missile sensitive to the radiation of the target itself.
AAM Air-to-air missile
ASM Air-to-surface missile
AUM Air-to-underwater missile
SAM Surface-to-air missile
SSM Surface-to-surface missile UUM Underwater-to-underwater missile
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Another method of classifying missiles is with regard to the type of trajectory taken by the missile. A ballistic missile follows the usual ballistic trajectory of a hurled object. A glide missile is launched at a steep angle to an altitude depending on the range, and then glides down on the target. A skip missile is launched to an altitude where the-atmosphere is very rare, and then skips along on the atmospheric shell.
On the basis of propulsive systems missiles fall into the categories of turbojet, ram-jet, rocket, etc. If the missile receives a short burst of power that rapidly accelerates it to top speed and then glides to its target, it is a boost-glide missile. Sometimes a missile is termed single-stage, double single-stage, etc., depending on the number of stages of its propulsive system.
Further differentiation among missiles can be made on the basis of trim and control devices. A canard missile has a small forward lifting surface that can be used for either trim or control similar to a tail-first airplane. A missile controlled by deflecting the wing surfaces is termed a wing control missile, and one controlled by deflecting the tail surfaces is termed a tail-control missile. It is to be noted that these definitions depend on which set of lifting surfaces is taken as the wing and which is taken as the tail. For missiles with two sets of lifting surfaces, we will specify the wing-to be the main lifting surfaces and the tail to be the balancing surfaces, a distinction maintained throughout the book. In a cruciform missile, sets of controls at right angles permit the missile to turn immediately in any plane without the necessity of its banking. On the other hand a bank-to-turn missile, like an airplane, banks into the turn to bring the normal acceleration vector as close to the vertical plane of symmetry as possible.
Fig.1. 1: Missile classification by method of launching
SSMs are common ground-to-ground ones though these may also be launched from a ship to another ship. Underwater weapons which are launched from a submarine also come under this class of missiles.
SAMs are essential complement of modern air defence systems along with anti-aircraft guns which are used against hostile aircraft. AAMs are for airborne battle among fighter/ bomber aircraft. These are usually mounted under the wings or fuselage of the aircraft and are fired at
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enemy airborne targets by the pilot through the press of a button. In his decision to launch a missile at a particular moment, the pilot is aided by a computer and radar network onboard as well as from ground based data link. The missiles in certain types are ignited before release while in others ignition takes place after release.
On the basis of range, missiles can be broadly classified as Short-range missiles;
Medium-range ballistic missiles (MRBM); Intermediate range ballistic missiles (1RBM);
Intercontinental or long-range ballistic missiles (ICBM)
1-5. Glossary of Special Terms
Many special terms occur repeatedly in missile aerodynamics. Some of these terms are now listed for ready reference.
Body axes: a set of Cartesian axes fixed in the missile and parallel to the axes of symmetry of the missile if such symmetry axes exist.
Cross flow plane: a plane normal-to the free-stream velocity.
Cruciform wing: four similar wing panels -mounted together at a common chord and displaced one from the next by π/2 radians of arc.
Fineness ratio: ratio of body length to body diameter (calibres).
Horizontal plane of symmetry: the horizontal plane in which the lower half of the missile is the mirror image of the upper half.
Included angle: angle between free-stream velocity and missile longitudinal axis,
Interdigitation angle: angle between the plane of a lifting surface and the plane of another tandem lifting surface.
Normal plane: a plane normal to the missile longitudinal axis,
Subsonic leading edge: a leading edge such that the component of the free-stream Mach number normal to the edge -is less than one,
Supersonic leading edge: a leading edge such that the component of the free-stream Mach number normal-to the edge is-greater than one,
Symmetrical wing: a wing possessing a horizontal plane of symmetry,
Tangent ogive: a missile nose having constant radius of curvature in all planes through the longitudinal axis from the apex to the circular cylinder to which it is tangent.
Trefftz plane: a fictitious cross flow plane infinitely far behind a missile or lifting surface to which the trailing vortex system extends without viscous dissipation.
Vertical plane of symmetry: the vertical plane, in which the left half of the missile is the mirror image of the right half,
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CHAPTER - 02
The Atmosphere
The atmosphere is a gaseous envelope surrounding the earth to a height of roughly 250 miles. Although there is some difference of opinion as to where the atmosphere ends and space begins, the limit defined above is quite generally accepted. So far as tangible evidence goes, the 750-mile upper limit of the aurora borealis is the "top" of the Atmosphere. The behaviour of the electrical discharges indicates that elements of air are present, although extremely rare.
One of the most important characteristics of the atmosphere is that air density changes with altitude. As altitude increases, air density decreases significantly. At sea level, the density of air is about .076 pound per cubic foot. At 20,000 feet, air density is only about .0405 pound per cubic foot. Because of the decrease in air density with altitude, a missile flying at 35,000 feet encounters less air resistance that is, has less drag-than a missile flying close to sea level.
Air pressure also varies with altitude. The pressure acting on each square inch of the earth's surface at sea level is actually the weight of a column of air one inch square, extending from sea level to the outer limits of the atmosphere. On a mountain top, this column of air would be shorter, and so the weight (pressure) acting on each square inch would be less. Therefore, air pressure decreases as altitude increases.
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Another characteristic of the atmosphere which changes with altitude is temperature (see fig. 2-1). But, unlike density and pressure, temperature does not vary directly with altitude. From sea level to about 35,000 feet, the temperature drops steadily at a rate of approximately 3 1/2°F per thousand feet. It then remains fairly constant at -67°F up to about 105,000 feet. It then increases at a steady rate until another constant- temperature zone is reached.
This zone lasts for several miles. Then the temperature starts decreasing. The procedure repeats itself – that is, a second temperature minimum is reached, and after a short constant-temperature zone, it starts rising again. These constant-temperature minimums mark the boundaries between the four regions in the atmosphere: the troposphere, the stratosphere, the mesosphere, and the thermosphere, shown in figure 2-2. In this figure the regions beyond are simply labelled "outer space," but they may be divided into further layers, according to the findings of space probes and explorations of recent times.
Fig 2-2. Atmosphere Regions around Earth
2.1 Troposphere
The troposphere is the lowest layer of the atmosphere and extends from the surface of the earth to a height of 10 miles. This is not a firm figure, as the height of the troposphere varies over different parts of the earth. Near the poles it may be only 4 miles. It is made up mostly of nitrogen and oxygen, and accounts for three- fourths of the weight of the atmosphere. Within this layer temperature decreases with altitude, and it is here that clouds, snow, rain, and the seasonal changes occur.
Because the troposphere is dense, aerodynamic surfaces can be used efficiently to control missiles. However, this high density causes a large amount of drag. It is known that the dense lower atmosphere slows down a projectile. It will have the same effect on a missile. The German V-2, rocket which was used to bomb England during World War II, was slowed down from 3300 to 1800 mph as it passed through the troposphere. The friction of the air also causes extremely high skin temperatures on missiles or re-entry craft, high enough to melt common metals.
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2.2 Stratosphere
The stratosphere is the layer of air above the troposphere. Its upper limits are around 20 miles above sea level. In this region temperature no longer decreases with altitude, but stays nearly constant and actually begins to increase in the upper levels. Higher temperatures in the upper levels are caused by ozone which is heated by ultraviolet radiation from the sun. (Ozone is a gas which is produced when electricity is discharged through oxygen.)
The composition of the stratosphere is similar to that of the troposphere; however, there is practically no moisture in the stratosphere. There are almost no clouds and no storms, and an almost complete absence of dust. At the earth's surface the air contains about 21 percent oxygen, but this decreases to a small value in the stratosphere, while the percent of hydrogen rises from 0.01 percent to 95 percent.
The air is said to be "thin" and will not support human life nor air breathing engines. Propeller-driven vehicles cannot penetrate this region because of the low air density, and aerodynamic surfaces have greatly reduced effect in controlling missiles. But increases in missile speeds are possible because thrust is used more for acceleration and less to overcome drag.
2.3 Mesosphere
Above the stratosphere is the mesosphere, extending from about 19 miles to 50 miles. Within this region the minimum temperature is reached at about 47 1/2 nautical miles (fig. 2-1), decreasing from about 7°C at its lower edge to - 100°C at its upper limit. Ozone is generated in this region and it is the seat of transformation of most primary into secondary radiation. Intense meteor trains reach down into the mesosphere and their brilliant combustion gave rise to the belief that this was a region of comparatively high temperature.
2.4 Intermediate Layers
In figure 2-1 you find the words "Tropopause," "Stratopause," and "Mesopause." These are applied to rather narrow regions between the layers described above. In each case, they are the regions of transition from one type of atmosphere to the next one, and are likely to have turbulence because of the inversion or reversal characteristics.
2.5 Thermosphere
The thermosphere starts at about 47 1/2 nautical miles (fig. 2-1) and extends to the ionosphere. Sometimes the two areas are considered as the ionosphere as they have a continuing upward trend in temperature. The increase in temperature is due to the presence of ozone, which is formed by the action of ultraviolet rays from the sun on the oxygen in the atmosphere.
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Tremendous fluctuations of wind speed (70 knots or more) occur on a daily basis. These fluctuations are referred to as atmospheric tides.
2.6 Ionosphere
Above the thermosphere, ranging up to about 250 miles above sea level is the ionosphere. This is a region rich in ozone, and consists of a series of electrified layers. The ionosphere is extremely important because it refracts (bends) radio waves (fig. 2-2).
This property enables a radio transmitter to send waves to the opposite side of the world by a series of refractions and reflections taking place in the ionosphere and at the surface of the earth. The characteristics of the ionosphere vary with daylight and darkness, and also with the four seasons. The term "ionosphere" is used particularly in referring to the electrical characteristics of the region, while "exosphere" is the name used when referring to the meteorological aspects.
2.7 Higher Atmospheres
The reaches of space beyond the ionosphere have not been fully explored and much of the information about them is conjecture. Much information about the upper air has been transmitted to earth from orbiting satellites and spacecraft which have carried instruments to measure radiation, temperature, and other information that could be measured.
Weather information and reports of meteorites and micrometeorites are recorded. There is still a great deal more to learn about space. If you read different writers on the subject, you will find variations in statements about the size of atmospheric zones and even differences in names for the regions. As conjecture is replaced by real information, these differences will be resolved.
Several belts of high intensity charged particles, called Van Allen Belts (fig 2-4), surround the earth in space. They were discovered by the early U. S. satellites, Explorer I and Explorer II, in 1958. The zones fluctuate and it is believed solar flares or other phenomena of the sun cause the changes. It is almost certain that the outer zone owes its existence to ionized solar gas ejected from the sun.
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Fig 2-3. Van Allen Radiation Belts around Earth
2.8 Planetary Boundary Layer
It is the part of the troposphere that is closest to Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 meters on clear, calm nights to 3000 m or more during the afternoon in dry regions.
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CHAPTER - 03
Basic Flight Principles
The principles of low speed aerodynamics which underlie the operation of most aircraft also apply to missiles, at least in the first few seconds of flight. Before we discuss high speed missile flight, let us consider the motions and forces that are common to both guided missiles and conventional airplanes flying at low speeds.
3.1 Basic Motions
Like any moving body, the guided missile executes two basic kinds of motions: rotation and translation. In pure rotation all parts of a body pivot about the center of gravity, describing concentric circles around it. In movements of translation, or linear motions, the center of gravity of a body moves along a line, and all the separate parts follow lines parallel to the path of the center of gravity. Any possible motion of the body is composed of one or the other of these motions, or is a combination of the two.
Since missiles are free to move in three dimensions, we describe their motions by using a reference system containing three reference lines, or axes. The missile axes (fig. 3-1A) are mutually perpendicular lines which intersect at the center of gravity of the missile.
A missile can make three kinds of rotary movements: pitch, roll, and yaw. Pitch, or turning up or down is rotation about the lateral or Y axis of the missile (fig. 3-1B).
The missile rolls, or twists, about the longitudinal axis, which is the reference line running through the nose and tail and designated the X axis in figure 3-1C. It yaws, or turns to the right or left, about the Z or vertical axis (fig. 3-1D). Rotary motions about all three of these axes are controlled by devices within the missile. Hereafter, these axes will be referred to as the pitch axis, the roll axis, and the yaw axis.
The second type of movement is called translation. Translation includes any linear movement of the missile. For example, a sudden gust of wind or an air pocket could throw a missile a considerable distance off its trajectory without causing any significant angular movement. If you have ever flown in an airplane, this should be fairly easy to understand. If the plane hits an air pocket, it may drop several hundred feet, but still maintain a straight and level altitude. Any linear movement, regardless of direction, can be resolved into three components: lateral movement, vertical movement and movement in the direction of thrust. So, besides the three angular degrees of movement, we have three linear degrees of movement. A missile in flight can therefore be said to have six degrees of movement, or freedom.
In an airplane the pilot checks his instruments or visually observes angular and linear movement. On the basis of his observations, he repositions the control surfaces as necessary to keep the plane where he wants it. Since we don't have a pilot in a guided missile to note these movements, we install devices that will detect them.
11 Fig 3 -1. R otat ion al M ov e me n ts amon g th e M is si le axe s A : R ol l, P itc h an d Y aw ax e s a r e mu tu a ll y p er p e n d ic u lar ; B : P itc h ax e s an d P lan e; C : R ol l axi s an d p lan e ; D : Y aw axi s an d P lan e
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3.2 Aerodynamic Forces
The principal forces acting on a missile in level flight are thrust, drag, weight, and lift. Like any forces, each of these is a vector quantity; a vector has magnitude (length) and direction. These forces are illustrated in figure 3-2.
Thrust (force) is directed along the longitudinal axis of the missile and is the force. This propels it forward at speeds sufficient to sustain flight. Drag is the resistance air offers to the passage of the missile through air, and is directed rearward. The weight of the missile is the pull of gravity on the missile, and is directed downward toward the center of the earth. Opposed to the force of gravity is lift, an upward force which supports the missile. Lift is directed perpendicular to the direction of drag.
Fig 3-2 Principal forces acting on a moving missile
Lift is produced by means of pressure differences. The primary factor contributing to lift is that the air pressure on the upper surface of an airfoil (wing) must be less than the pressure on the underside. The amount of lifting force provided is dependent to a large extent on the shape of the wing. Additional factors which determine the amount of lift are the wing area, the angle at which the wing surface is inclined to the airstream (angle of attack), and the density and speed of the air passing around it. The airfoil that gives the greatest lift with the least amount of drag in subsonic (less than the speed of sound) flight has a shape similar to the one illustrated in figure 3-3.
Some of the standard terms applied to airfoils are included in the sketch. The foremost edge of the wing is called the leading edge and that at the rear the trailing edge (fig. 3-3A). A straight line between the leading and the trailing edges is called the chord. The distance from one wingtip to the other (not shown) is known as the SPAN. The angle of incidence (fig. 3-3B) is the
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angle between the wing chord and the longitudinal axis of the fuselage. In figure 3-3C, the large arrow indicates the relative wind, the direction of the airflow with reference to the moving airfoil. The angle of attack is the angle between the chord and the direction of the relative wind.
In actual flight, a change in the angle of attack will change the airspeed. But if for test purposes we maintain a constant velocity of the airstream while changing the angle of attack, the results on the nonsymmetrical wing will be as shown in figure 3-4. The sketches show a wing section at various angles of attack, and the effect these different angles have on the resultant force and the position of the center of pressure. The burble point referred to in figure 3-4C and D is the point at which airflow over the upper surface becomes rough, causing an uneven distribution of pressure. Note that the center of pressure changes with the angle of attack.
Fig 3-.3 Nomenclature of parts for subsonic Airfoil
The relative wind strikes the tilted surface, and as the air flows around the wing, different amounts of lifting force are exerted on various points on the airfoil. The sum (resultant) of all these forces is equivalent to a single force acting at a single point and in a particular direction. This point is called the center of pressure. From it, lift can be considered to be directed perpendicular to the direction of the relative wind. The dynamic or impact force of the wind against the lower surface of the airfoil also contributes to lift, but no more than one-third of the total lift effect is provided by this impact force.
3.3 Acceleration
Acceleration is rate of change, either in speed or in direction of motion or both. A missile accelerates in a positive or negative sense as it increases or decreases speed along its line of flight. It also accelerates in a positive or negative sense as it changes direction in turns, dives,
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pullouts, and as a result of gusts of wind. During accelerations a missile is subjected to large forces which tend to keep it flying along its original line of flight. This is in accordance with Newton's first law of motion which states:
“A particle remains at rest or in a state of uniform motion in a straight line unless acted upon by an external force”.
Like gravity, acceleration is measured in terms of g's. The acceleration of a body in free fall is said to be one "g." Missiles making rapid turns or responding to large changes in thrust will experience accelerations many times that of gravity, the ratio being expressed as a number of "g's." The number of "g's" which a missile can withstand is one of the factors which determine its maximum turning rate and the type of launcher suitable for the weapon. The delicate instruments contained in a missile may be damaged if subjected to accelerations in excess of design values.
Fig 3-4. Effect of Angle of Attack on Airflow and Centre of Pressure
3.4 Mach Numbers and Speed Regions
Missile speeds are expressed in terms of Mach numbers rather than in miles per hour or knots. The Mach number is the ratio of missile speed to the local speed of sound. For example, if a missile is flying at a speed equal to one half the local speed of sound, it is said to be flying at Mach 0.5. If it moves at twice the local speed of sound, its speed is then Mach 2.
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3.5 Local Speed of Sound
The speed expressed by the Mach number is not a fixed quantity, because the speed of sound in air varies directly with the square root of air temperature. For example, it decreases from 760 miles per hour (mph) at sea level (for an average day when the air is 59~) to 661 mph at the top of the troposphere. The speed of sound remains constant (with the temperature) between 35,000 feet and 105,000 feet, then rises to 838 mph, reverses, and falls to 693 mph at the top of the stratosphere. Thus, you can see that the speed of sound will vary with locality.
The range of aircraft and missile speed is divided into four regions which are defined with respect to the local speed of sound. These regions are as follows.
Subsonic Flight, in which the airflow over all missile surfaces is less than the speed of sound. The subsonic division starts at Mach 0 and extends to about Mach 0.75. (The upper limit varies with different aircraft, depending on the design of the airfoils.)
Transonic Flight, in which the airflow over the surfaces is mixed, being less than sonic speed in some areas and greater than sonic speed in others. The limits of this region are not sharply defined, but are approximately Mach 0.75 to Mach 1.2.
Supersonic Flight, in which the airflow over all surfaces is at speeds greater than sound velocity. This region extends from about Mach 1.2 upward.
Hypersonic Flight, in which the time of passage of the missile is of the order of relaxation time. (Relaxation time is the time required for molecules of air to adjust themselves after the passage of the body.) Mach numbers on the order of 10 may be considered as hypersonic. Velocities that are not hypersonic at sea level may become so at high altitudes, since relaxation times will be longer where densities are relatively low.
3.6 Subsonic Flight
At subsonic speeds, sustained flight is dependent on forces produced by the motion of the aerodynamic surfaces through the air. If the surfaces of airfoils are well designed, the stream of air flows smoothly over, under, and around them. And the air stream conforms to the shape of the airfoil. If, in addition, the airfoils are set to the proper angle, and if motion is fast enough, the airflow will support the weight of the aircraft or missile.
Since most modern missiles are supersonic, with only a few seconds of flight in the subsonic region of speed, the forces that affect missile flight at supersonic speeds are of more importance to you.
3.7 Missile Speed and Air Flow
A missile thrusting its way through the atmosphere may be compared to a boat pushing its way through the water. You can see the effect on water, so picture the air waves the same
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way. A boat moving slowly through the water gently pushes the water out of the way, but if it speeds up, the water is churned up into rushing waves that require increased thrust to push through.
An object pushing through the air produces small pressure disturbances in the air, and each pressure wave expands equally in all directions, moving at the speed of sound. As long as the object is moving more slowly than the air waves, there is no buildup of pressure waves, but as the speed increases, the air waves begin to pile up in front of the object.
When the speed of the object reaches the speed of sound the pressure waves can no longer outrun it, and the piled up airstream just ahead of the object collides with the unmoved air farther ahead, which a moment before was completely undisturbed. This causes a shock wave at the boundary between the air stream and the undisturbed air. The air stream is reduced in speed very rapidly and at the same time the pressure, density, and temperature increase.
A normal shock wave is usually very strong, and the air passes through without changing direction (fig. 3-5A), but always changes from supersonic to subsonic velocity. On other hand an oblique shock wave (fig. 3-5C), the airstream changes direction upon while passing through the transition marked by the wave front. These waves are produced in supersonic airstreams at the point of entry of wedge- shaped or other sharply pointed bodies. The change in speed, density, pressures, and temperature are generally less severe than with normal perpendicular shock waves.
A bow shock, also called a detached shock, is a curved, stationary shock that is found in a supersonic flow past a finite body. The name comes from the example of a bow wave that forms at the bow of a ship when it moves through the water.
Unlike an oblique shock, the bow shock is not necessarily attached to the tip of the body. Oblique shock angles are limited in formation and are based on the flow deflection angle, upstream Mach number. When these limitations are exceeded (greater deflection angle or lower Mach number), a detached bow shock forms instead of an oblique shock. As bow shocks form for high flow deflection angles, they are often seen forming around blunt objects. In other words, when the needed rotation of the fluid exceeds the maximum achievable rotation angle for an oblique attached shock, the shock detaches from the body. Downstream of the shock, the flow-field is subsonic, and the boundary condition can be respected at the stagnation point.
The bow shock significantly increases the drag in a vehicle traveling at a supersonic speed. This property was utilized in the design of the return capsules during space missions such as the Apollo program, which need a high amount of drag in order to slow down during atmospheric reentry.
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Fig 3-5. Movement of Air over Airfoils; A, Formation of Normal Shock wave;
B, Expansion waves C, Airflow about a supersonic wing
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CHAPTER -04
Missile Aerodynamics
Study of the movement of a body in the presence of air is called aerodynamics and this study is vitally important for the design of aircraft, missiles and rockets. The atmosphere as we know is densest close to earth's surface at sea level. As we go higher it becomes thinner (i.e., the pressure and density are lower). The sensible atmosphere is up to a height of about 80 kilometers. The temperature also varies with height. The layer of atmosphere nearest to earth is called troposphere. Above that is stratosphere which is further subdivided into lower stratosphere and upper stratosphere regions, beyond that, is ionosphere or ozonosphere and the last is exosphere. The very high speed fighter aircraft fly up to altitudes of about 30 km, while transport jets fly up to about 10-11 km.
The aircraft and missiles are bodies that are heavier than air and so can support their weights only if they produce a force to counter it. This force can be either lift force generated by the flow of air over the wings and body or generated by means of an engine in the form of thrust. This is done by helicopters or by aircraft with swing-engines (vertical takeoff type) where main engines can be swivelled. In missiles (most are launched vertically or with an inclination), a part of the weight is countered by the rocket engine thrust.
When we have a body with wings or without wings moving through air, there are forces generated which act on the body to oppose its motion (drag). In other words, this force must also be countered by the engine's thrust. The drag force depends upon the fineness or bluntness and size of the body. To minimize the drag force one has to choose the aerodynamic shape such that functional requirements are also met. In the missiles aerodynamic surfaces called wings, fins, and control surfaces and body called fuselage (with suitable nose shape conical or ogival followed by cylindrical) are designed to provide the necessary lateral manoeuvrability. This is achieved by deflecting control surfaces through actuation mechanism and thereby altering the balance of forces and generating turning moments. This happens at a very rapid rate.
In cruise missiles wings are provided to generate lift force while the missile flies in horizontal level mode. Most of the aerodynamics is studied by mathematical analysis of flow and then further validated by tests on scaled-down models in wind tunnel where forces are measured and correlations generated. An experimental data bank is generated for subsequent designers.
Aerodynamic considerations and structural design factors are intimately related to the propulsion and guidance aspects. Hence the external aerodynamic configuration of the missile is also of primary emphasis. Many aeronautical engineers who were earlier airplane designers transformed themselves from 'airplane to missile' aerodynamicists due to the similar principles involved and the rapid growth in varieties and classes of missiles used in modern warfare. The external missile shape and design is finalized keeping in view the needs of other subsystems and performance criteria. Thus mechanical and electric missile system engineers take equally
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important part in the overall missile design. This calls for a need to have a good insight and appreciation on the part of these personnel for the overall missile design.
Aerodynamic characteristics of various external components and their configuration aid their selection towards an optimum missile performance with respect to its lift and drag characteristics, aerodynamic stability, manoeuvrability, etc. Comprehensive and accurate data to enable a missile technologist to zero-in on a particular configuration is not readily available since much of the essential data is classified. Moreover, the requirement of stupendous quality of data desirable and sufficient for a fairly efficient design is a deterring factor too. However, an important asset the missile engineer: must have in discharging any R&D assignment is a sound understanding and knowledge of the fundamental principles involved in all the subsystems. The fundamentals of many technically specialized areas-aerodynamics, thermodynamics (mainly heat transfer), kinematics, propulsion, structural design-are a necessity though it makes the task of the aeronautical design engineer rather complex.
Fig 4-1. Sections of Missiles
The body of the missile may be divided into three major sections - the fore body or the nose, the mid-section and the aft or boat-tail section.
4.1 Nose Section
Fore body’s may have many varieties of shapes, most common of which are conical, ogival, power series or hemispherical. These shapes are used primarily on the missiles of supersonic speeds and are generally selected on the basis of combined aerodynamic, guidance and structural considerations. A hemispherical nose has very high drag from the aerodynamic drag or performance standpoint, but it is excellent from the standpoint of structural integrity, resistance to aerodynamic heating and amenability to certain types of guidance like infrared guidance. Since the pressure or wave drag may be several times that due to friction at supersonic speeds, careful selection of the nose shape needs attention to assure satisfactory performance of the overall system.
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Fig 4-2. Flow past a Wedge and a Cone
Conical Fore body has given way to other types because of relative disadvantages but the conical one is the basis for the study of aerodynamic characteristics due to its simplicity. Briefly some of the flow characteristics about which an aero engineer will have to be very familiar are the formation of a shock wave, the shock angle, streamlines or flow direction and air properties between the shock wave and surface of the body. The supersonic flow over a cone has characteristics which are similar in appearance as that of a conical one but are markedly different in nature from those corresponding to two-dimensional flow (i.e., flow over a wedge). The similarity in appearance is that an oblique shock wave is formed at the tip of the wedge and apex of the cone (Fig. 4-2).
An ogive is similar to a cone except that the plan form shape is formed by an arc of a circle instead of a straight line as is evident from Fig 4-3. The ogival shape has several advantages over the conical section. These are:
Slightly greater volume for a given base and length (c/d ratio). A blunter nose providing structural superiority.
Slightly lower drag.
Fig 4-3. Conical and Ogival nose cone shapes
The hemispherical fore body type of nose is more widely used particularly in missiles which use infrared (IR) seekers as their homing head. 'The ease of manufacture of this shape is one of the major reasons and advantages for its use in spite of its extremely high drag penalty on the missile. This is a measure of the extent to which an aerodynamic engineer must compromise to achieve an optimum arid feasible missile system. Many modified ogive are some of the other
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shapes of noses used in present-day missiles. In some missiles the shape of the nose section may be defined by the 'power series' of the Van Karman type, named after its originator. Other such elaborate expressions derive purely from radar consideration i.e., to minimize refraction and radar beam distortion. The 'parabolic' type and the 'power series' mentioned above are also derived from aerodynamic considerations.
4.2 Mid – Section
The mid-section in most missile configurations is cylindrical in shape. This shape is advantageous from the standpoint of drag, ease of manufacturing and load carrying capability. From the earlier chapters it is known that the total reaction of the missile at any instant has two components, the lift (components at right angle to the direction of airflow) and drag (those parallel to the direction of airflow). These may be positive or negative. It becomes desirable to have a greater lift than the drag and this can be done by using a curved surface. Angle of attack is the direction of the reaction force with respect to the free stream direction. Even at zero angle of attack, called as the zero-lift drag (x = 0), some lift can be obtained by using what are called as airfoil sections.
The effects of mid-section or after body extension on the aerodynamic characteristics of the conical and ogival nose bodies have been investigated and it is seen that the effect of after body extension is to increase the lift coefficient and move the centre of pressure toward aft end as a result of body carry over and viscous cross-flow effects.
4.3 Boat-tail Section
Boat tail is the tapered portion of the aft section of a body. The purpose of the boat-tail is to decrease the drag of a body which has a 'squared off base. By 'boat-tailing' the rear portion of the body, the base area is reduced and thus a decrease in base drag is realized. However, the decrease in base drag may be partially nullified by the boat tail-drag.
In a nutshell, regarding the aerodynamic characteristics of the complete body the following generalizations may be made:
The drag of the body at supersonic speeds depends primarily on the nose shape and the amount of boat tailing.
Base drag is greatly affected by the presence of the jet.
The majority of the body lift is on the nose section of the body with a small down load on the boat-tail.
The resultant centre of pressure for a conventional body varies between 15 and 20 per cent of the body length at low angles of attack. At higher angles of attack the centre of pressure of the complete body can move forward or aft of the nominal centre of pressure location depending on the amount of boat-tailing.
For moderate boat tailing, say seven degree, the centre of pressure tends to shift rearward with angle of attack.
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CHAPTER -05
Stability and Lift in Modern Missiles
So far we have discussed the principles of producing lift by the use of cambered (curved) wings. Cambered wings are still used on conventional aircraft, but are not used on most present day guided missiles. Most operational missiles use streamlined fins to provide stability and some lift.
In some missiles lift is achieved entirely by the thrust of the main propulsion system. The Navy's Polaris missile, for example, has no fins.
The flight of an arrow is an example of the stability provided by fixed fins. The feathered fins on an arrow provide streamlined airflow over surfaces which ensure accurate flight. Since supersonic missile fins are not cambered, a slightly different lift principle is involved than with the conventional wing. At subsonic speeds a positive angle of attack will result in impact pressure on the lower fin surface which will produce lift just as with the conventional wing. At supersonic speeds, the formation of expansion waves and oblique shock waves also contributes to lift. Figure 3-5B shows the upper surface of a supersonic fin. Due to the fin shape, the air is speeded up through a series of expansion waves. This results in a low pressure area above the fin. Figure 3-5C shows the fin cross section. Beneath the fin, the force of the airstream (dynamic pressure) and the formation of oblique shock waves result in a high pressure area. The differences in pressure above and below the fin produce lift.
5.1 Control Surfaces, External
Movable fins, called control surfaces, provide a means for controlling missile flight attitude. A control surface provides control by presenting an obstacle to airflow. This causes a force (due to impact pressure) to be exerted on the surface. The magnitude of the force depends on the angle between the control surface and the direction and speed of airflow. In figure 5-1, we are looking at one effect of control surface movement.
Fixed (stabilization) fins and movable control surfaces may be located in several ways on the missile. In figure 5-2A and B, we see two possible combinations. In the first, control is achieved by movement of control surfaces located at the missile's center of gravity. In the second, ~ control is achieved by movement of tail surfaces; other combinations are possible, and are used in some operational missiles. The important thing for you to remember is that control is attained due to impact pressure exerted on the control surfaces, and that fixed fins contribute primarily to stability. If you have ever watched an airplane being landed and noticed how the pilot tilted certain portions of the wings, folded other parts, and extended other portions, you have some conception of how movement of missile wings, fins, tails, or other control surfaces can be used to change the course of the missile.
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Fig 5-1. Control surface deflection A. Control surface in neutral position; B. Control surface titled; C. Impact pressure of air causes missile roll.
5.2 Fin Designs
Supersonic fins are symmetrical in thickness cross section and have a small thickness ratio - the ratio of the maximum thickness to the chord length. The double wedge, shown in figure 5-2, has the least drag for a given thickness ratio, but in certain applications is inferior because it lacks strength. As you saw in figure 3-5C, the air flows over and under a double wedge airfoil without developing a severe shock wave.
The modified double wedge has a relatively low drag (although its drag is usually higher than a double wedge of the same thickness ratio) and is stronger than the double wedge. The biconvex, shown in figure 5-2, has about one-third more drag than a double wedge of the same thickness ratio. It is the strongest of the three but is difficult to manufacture.
The plan form of the fins - the outline when viewed from above – is usually either of the delta modified delta (raked tip) or rectangular types shown in figure 5-4A. These shapes considerably reduce unwanted shock wave effects.
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Figure 5-4B shows some of the movable parts of control surfaces that help to offset unstabilizing forces and to keep the missile on course. The ailerons on the wings help in roll control. When one aileron is raised, the other is lowered; the wing with the raised aileron moves down and the other moves up. Elevators attached to the horizontal tail fins are raised or lowered together and help in pitch control. The rudders on the vertical tail fins may be used for tail control. If one rudder is moved to the right, the tail moves to the left and the missile yaws to the right. In most missiles in present use, the movements of the various parts are caused by the hydraulic system in the missile, which receives instructions electrically as to what moves are necessary to keep the missile on its course.
The Bull pup missile uses a form of tab (fig. 5-4B) as a method of control. Some of the newer missiles combine two devices, such as an elevator and an aileron, into an elevon, which allows control of both pitch and yaw by a single control mechanism.
Fig 5-3. Arrangement of exterior control surfaces on missiles; A. Movable surface at the centre of Gravity; B. Movable tail surfaces.
Fig 5-4. External control devices on missiles; A. Plan forms of airfoils; B. Movable parts of fixed airfoils.
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5.3 Arrangement of Fins
Fins are mounted on the airframe in several arrangements, some ofwhich are shown in figure 5-5. The Cruciform is the most popular tail arrangement. It is used in surface-to-air missiles. Both the Inline and Inter-Digital cruciformarrangements are widely used, especially forsupersonic missiles.
5.4 Control Surfaces, Internal
We mentioned earlier that some missiles do not have external control surfaces such as wings, fins,and tails. They may have small control surfacesthat must be supplemented by other means ofcontrol. Two types have been used as auxiliarycontrols-exhaust vanes and jet control.
Exhaust vanes are control surfaces mounteddirectly in the exhaust path of a jet or rocket engine (fig. 5-6B). When the exhaust vanes are moved they deflect the exhaust, resulting in a change inthe direction of thrust so as to keep the missile pointed in the desired direction. Even when themissile has just begun to move after launching, the exhaust velocity is very high. One disadvantage isthat the tremendous heat of the exhaust makes the life of the vanes very short. The German V-2 missile used exhaust vanes of carbon; these lasted, on the average, about 60 seconds before they were burned up completely. Various forms of vanes are used and may be called jet vanes, jetevators, orjetevons. Some are fixed and some are movable.
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Movable jets are another method of controlling the flight attitude of a missile. One method is tomount the engine itself in gimbals (fig. 5-6 C), andturn the whole engine to deflect the exhaust stream. This system requires that the engine be fed with flexible fuel lines, and the control system that turns the engine must be very powerful. Also, it cannot control roll of the missile. To get control of all axes, two gimbal- mounted jets may be positioned in the missile. Both jets must be free to move inany direction, and must be able to respond tosignals from any of the three control channels (pitch, roll, and yaw). Another system uses four movable jets mounted in the aft end of the missile. Two jetscontrol yaw, two more control pitch, and all four together control roll. Sometimes a fifth jet is fixed in the middle, in the space between the movable jets.
Fixed steering jets (fig. 5-6 A) are placed around (inside) the missile so as to give directional controlby exerting a force in one direction or another. Heatshields are necessary to protect the main body of the missile against the heat of exhaust from the jets.The use of these auxiliary jets makes it possible to eliminate all outside control surfaces. Missiles andspacecraft that reach into the higher, rareatmospheres, where external control surfaces are oflittle use, must depend on internal control means.
Fig 5-6. Jet control of Flight Attitude; A. Fixed jets; B. Exhaust vanes; C. Gimbaled engines.
5.5 Missile Airframes
The airframe of a guided missile serves the same purpose as the airframe of the conventional aircraft: it carries the necessary components and controls to ensure proper flight. But, since the guided missile is essentially a one-shot weapon, the body structure can be simpler in structure than that of a conventional aircraft. Missile bodies are designed so that inner components are readily available for testing, removal, and repair. The major components are mounted to form independent units. Adequate room is provided to permit slack in electrical cables and harnesses so that inner sections can be removed easily during maintenance.
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The configuration of a guided missile is the principal factor controlling the drag and lift forces that act on it as it passes through the atmosphere. Supersonic missiles must be designed for minimum drag. Both lift and drag are directly proportional to the square of missile speed. The shape of the nose, the body, and external control surfaces (if used) must be scientifically designed to obtain a maximum of lift and a minimum of drag.
Another effect that must be considered in missile design is that of heat. Heat results not only from friction as the missile passes through the atmosphere, but also from the temperature rise caused by the ram effects as the air is compressed by the speeding missile. A significant part of the development effort for long-range ballistic missiles has been devoted to development of nose cones capable of withstanding extreme temperatures.
Most missile bodies are slender cylindrical structures similar to those shown in figure 5-3. Several types of nose sections are used. If the missile is intended for supersonic speeds, the forward section may have a pointed arch profile in which the sides taper in lines called ogive curves (fig. 5-7A). Missiles which fly at lesser speeds may have blunt noses as shown in fig5-7B. Rounded noses which house radar equipment may look like the one in figure 5-7C. Figure 5-7D shows an air-breathing missile nose which includes the duct for the ramjet propulsion system. The nose design of our ICBM has been modified several times, each one made to meet the need of the missile design with the least drag effect.
Fig 5-7. Missile noses.
Most modern missiles are made up of several sections. Each section is a cylindrical shell machined from metal tubing rather than a built up structure with internal bracing. Each shell contains one of the essential units or components of the missile, such as the propulsion system, the electronic control equipment, the warhead, or the fuze assembly.
Sectionalized construction has the advantage of strength with simplicity, and also provides ease in replacement and repair of the components since the shells are removable as separate units. The sections are joined by various types of connections designed for simplicity of operation. Access ports are sometimes provided in shells, through which adjustments can be made prior to launching.
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CHAPTER -06
Some Future Configurations
As a result of future trends in tactical missiles, a number of new concepts are being advanced to fill the needs. These configurations include waveriders, noncircular bodies, air breathing engines, etc. We will examine a number of these future configurations. Consider first the waverider which is now receiving much attention. Figure 6-1 illustrates a waverider at its design point. It is the type of waverider known as a caret wing. Its upper surface consists of two triangular planes joined at hinge line.
Fig 6-1. Caret wing Waverider
At the design condition, the hinge line is parallel to the free stream direction, and no pressure exists on the upper surfaces. In the chord wise direction, the airfoil sections are all wedge sections of a uniform wedge angle. The flow under the wing is all parallel flow at oblique shock pressure. The region between the bottom of the waverider and a plane containing the apex and wing tips is at uniform pressure. We thus know its lift/drag ratio from oblique shock theory. Waveriders have higher L/D ratios at hypersonic speeds than the usual cruciform missiles by about a factor of 2.Waveriders are seriously considered for designs of hypersonic aircraft by Kucheman and his associates in England about 25 years ago. It is only recently that waveriders have been given serious attention for hypersonic tactical missiles.
A large variety of waverider configurations is possible. The use of waveriders as missiles presents a series of aerodynamic problems such as adding a propulsion system, controls, and a radome to the basic waverider, hopefully without seriously degrading (L/D) max, It is clear that considering the large number of waverider configurations and the above aerodynamic problems, a large and fruitful opportunity exists for research and development in this field.
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With the introduction of air breathing missiles to achieve range, it becomes possible to use noncircular bodies which do not roll continuously. Opens up the design space and allows increased performance and increased stability and control. While rocket powered cruciform missiles usually use skid-to-turn maneuverability, air breathers utilize bank-to-turn maneuvering. Possible advantages claimed in the use of noncircular bodies include higher lift, better storage, improved carriage, better separation, and improved stability and control.
A nonplanar missile on a noncircular body represents an interesting new design possibility which can have different stability and control than the usual cruciform missile. A few examples with noncircular bodies are now presented. A circular body can develop rolling moments by skin friction but they are of small magnitude. It thus has zero effective dihedral. A noncircular body under sideslip can have rolling moment and side force as a result of pressure forces, yielding finite values. Elliptical bodies have good effective Dihedral while the circular body has neutral stability. Directional stability, but the elliptical body is less unstable than the circular body and will thus require a smaller vertical fin.
The stability and control characteristic of monoplanes with elliptical bodies generally provides a good balance between longitudinal and lateral-directional stabilities. Too low a profile, looking normal to the body in the horizontal plane, reduces the directional stability. It also causes unporting of controls at high Deflections with an attendant loss of control.
Hunt and his coworkers have studied hypersonic missile airframes capable of housing a scramjet engine. The studies have shown that engine/airframe integration to be a significant problem for this class of missiles. Also engines can have a significant effect on the missile's stability and control. Spearman analyzes the aerodynamics of some unconventional missiles and considers their applicability to certain missions. The classes of missiles considered are:
(1) delta-wing bodies
(2) ring-parasol wing bodies, and
(3) Monoplanar missile with circular/elliptical body
A monoplanar wing in connection with an elliptical body is a good candidate for a maneuvering missile such as required in air defense or air combat missions. Its high L/D makes it a good candidate for longer range air-to-surface missions.
In this section we have considered four general categories of missile types: (a) waveriders,
(b) flat-top monoplanar missiles,
(c) Missiles designed for scramjet propulsion, and (d) Missiles suitable for particular missions.
Although a number of stability and control problems have been mentioned in connection with these missile types, a great amount of research and development will be required in the future.
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CHAPTER -07
Concluding Remarks
In this paper a wide range of subjects in missile aerodynamics has been covered from a general point of view. Certain subjects such as aerodynamic heating, drag, radar cross-section, and real gas effects have been neglected because of classification restrictions on this particular subject. But at the same time there is a big race among the leading space research organizations to develop and successful use a SSTO (Single stage to Orbit). In which NASA has been a step ahead with development of Pegasus winged missile. Hope fully we can get to see some radical advances in field of Missile aerodynamics by the end of this decade.
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CHAPTER -08
Bibliography
Bertin, John J; Smith, Michael, L. Aerodynamics for Engineers; Prentice Hall, Inc. 1979. Chin, S. S. Missile Configuration Design. Mc Graw-Hill Book Co Ltd. 1961.
Clancy, L. J. Aerodynamics. Halsted Press. 1975.
Kennedy, Col. William, V. Intelligence War. Salamander Books Ltd, 1983. Kermode, A. C. Mechanics of Flight. Himalaya Books, 1972.
Sutton, C. P.; Ross, D. M. Rocket Propulsion Elements: An Introduction to the Engineering of Rockets Wiley Interscience. 1976.
"Missile Aerodynamics," AGARD-CP-336, Restricted NATO, presented at the Fluid Dynamics Panel Symposium, Sept. 20-22, 1982, Trondheim, Norway.
