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 upto 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. Beyond that, is ionosphere or ozonosphere and the last is exosphere. The very high speed fighter aircraft fly upto altitudes of about 30 km, while transport jets fly upto 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 swiveled. 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 maneouvrabilitv. 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. 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 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, maneuverability, 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. Some of the major considerations the latter should have for an optimization of design are enumerated here.
 

• Simplicity in external configuration to reduce development time and cost.

 

• Efficient aerodynamic control surfaces to simplify control and guidance system circuits and to minimize servo power requirements.

 

• Missile range, speed and other performance characteristics that satisfy the mission requirements. Adequacy of the airframe from the standpoint of stability, maneuverability and dynamic responses.

 
 

• Simple, efficient and highly reliable power plant.

 

• Low cost, productability and light weight airframe constructions.

 

• Accuracy of the control and guidance systems to accomplish the desired mission.

 

• Reliability of the complete weapon system as we11 as its individual components.

 

• Efficiency in packaging the various major components to facilitate check-out and replacement.

 

• Degree of complexity in the preparation and delivery of the missile to accomplish its mission.

 
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. 

The body of the missile may be divided into three major sections the forebody or the nose, the Mid-section and the aft or boat-tail section.
 
Nose Section
 
Forebodies 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 sturctural integrity, resistence 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.
 
Conical forebody 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).
 
 
An ogive is similar to a cone except that the planform shape is formed by an arc of a circle instead of a straight line. The ogival shape has several advantages over the conical section.
 
These are:

• Slightly greater volume for a given base and length (eld ratio).

 

• A blunter nose providing structural superiority.

 

• Slightly lower drag.

 
 
The hemispherical forebody type of nose is more widely used particularly in rnissiles 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 ogives are sorne of the other shapes of noses used in present-day missiles.
 
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. 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 suface. 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 = O), some lift can be obtained by using what are called as airfoil sections.
 
The effects of mid-section or afterbody extension on the aerodynamic charcteristics of the conical and ogival nose bodies have been investigatted and it is seen that the effect of afterbody 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.
 
Boat-tail Section
 
Base drag is the drag resulting from the wake or “dead air” region behind the missile. Base drag is less of a problem during powered flight but during free flight it can account for as much as 50% of total drag. Base drag can be reduced by tapering the tail (boat tailing).
 
A boat-tail is the transition section at the tail of a rocket (or other vehicle) that gradually narrows the body down to the motor diameter. It thereby helps reduce base drag.
 
Base drag is a component of aerodynamic drag caused by a partial vacuum in the missile's tail area. The vacuum is the hole created by the rocket's passage through the air. Base drag changes during flight. While the motor is firing, the drag is minimal since the tremendous volume of gas generated by the motor fills this void. The drag takes a sharp jump at burnout when this gas disappears.
 
Base drag can be reduced by the use of a boattail to transition the main body diameter down to the motor diameter which helps direct air into the evacuated area. When properly designed, a boattail can reduce base drag below zero (i.e. actually generate a small amount of forward thrust) by making use of the "pumpkin seed" effect.
 
 
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 generalisations 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 percent 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 nominaI 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.

 

• The nose may be rounded off to lower radii without causing any drag penalty.