Module II PDF

# Module II ## Airframe Components ### 1. Fore Body or Nose It is the first and foremost component of a missile that experiences air while travelling through the atmosphere. Several types of nose sections were used in various types of missiles. Some of the types are: - **Conic** This type of for...

# Module II ## Airframe Components ### 1. Fore Body or Nose It is the first and foremost component of a missile that experiences air while travelling through the atmosphere. Several types of nose sections were used in various types of missiles. Some of the types are: - **Conic** This type of forebody is used in missiles intended to fly at supersonic speeds. In a supersonic missile, while travelling in the atmosphere, an oblique shock is formed at the apex of the cone and inclined with respect to the incident upstream flow direction. In the case of the conical nose, there are various noticeable aerodynamic and thermodynamic changes in the flow characteristics of air. - **Ogive** An ogive is similar to a cone except that the plan form is shaped formed by an arc of a circle instead of a straight line. This **blunter nose** provides structural superiority. If the speed of a rocket is less than the speed of sound, the best shape of a nose cone is a rounded curve, and at supersonic speeds, a narrower and sharper point is preferred. - **Hemispheric** This nose is used on some missiles, particularly those that uses infrared (IR) seekers. This type of nose imposes a high drag penalty on the missile. Thus its use indicates the extent to which an aerodynamicist must compromise to achieve an optimum and feasible missile system. ### 2. Main Body or Mid Section The midsection of a rocket in most configurations is in cylindrical shape because it is advantageous due to the: - low drag - ease of manufacturing - load carrying capability The parasite (zero-lift) drag of a cylindrical body is caused by skin friction force only. At a low AOA, a very small amount of normal force (i.e. lift) is developed on the body, this results from the "carryover" from the nose section. ### 3. Aft Body or Boat Tail Section The tapered portion of the aft section of a body is called the boat tail. The purpose of the boat tail is to **decrease the drag** of the body's midsection. Because of the large base area, the midsection has relatively large base pressure and, consequently, high drag values. By boat tailing, the rear base area is reduced, thus reducing base drag. However, the decrease in base drag may be partially nullified by the boat's tail drag. ### 4. Control Surfaces The purpose of putting control surfaces is to provide lift, control, and stability of the missile's flight path. The wings may be movable or fixed. ## Control Surfaces Many use the generic term “fin” to refer to any control or aerodynamic surface on a rocket. Rocket designers, however, are more precise in their naming methodology and generally consider these surfaces to fall into three major categories: canards, wings, and (tail) fins. Control surfaces are lightweight, low in drag, easily made and attached, can have just about any planform as is required, and probably most importantly, provide **good assurance of a dynamically stable rocket**. This is because rocket aerodynamic wings provide a high restoring **lift force** (balances the force due to wind, and the rocket remains stable, with its flight path only slightly altered) at even small angles of attack. This is important to reduce the turning **momentum** of the rocket due to its mass which can lead to an under-damped wobbling rocket that would zig-zag during its ascent, or at worse **dynamic instability** caused by the restoring force being insufficient to overcome turning momentum. ## Aerodynamic Wings Although the wing is made of relatively thin sheet material, such as aluminum, it is beneficial to shape the edges to provide something of an aerofoil shape to reduce pressure and induced drag. The figure shows the plan form of rocket wings of a different type. - **Rectangular** Simple to make, least aerodynamic - **Clipped Delta** Good aerodynamic fin, used on low-drag, high-performance rockets - **Swept** Simple to make, slightly better aerodynamics - **Trapezoidal** Good aerodynamic fin for payload rockets, moves the Center of Pressure forward. - **Tapered Swept** Moves Center of Pressure back, good design for fast moving rockets. - **Elliptical** Best aerodynamic fin, difficult to construct. The planform of the wing (the outline when viewed from above) shown below are the **raked tip fins** providing considerably reduced unwanted shock wave effects. ## Supersonic Wings Subsonic and supersonic wings have a symmetrical cross-sectional base and a small thickness ratio (span over thickness). Supersonic fins have the same shapes at the edges. Fairings reduce the interference drag. The main cross-sectional shapes for supersonic wings are: - **Double Wedge** The air flows over and under a double wedge airfoil **without developing a severe shock wave**. It has the **least drag** for a given thickness ratio but, in specific applications, is inferior because it **lacks strength**. - **Modified Double Wedge** 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. - **Biconvex** The biconvex section provides larger wedge angles at the leading and trailing edges. It causes **considerable drag** (one-third more than a double wedge of the same thickness ratio) but is the **strongest** and most **challenging to manufacture**. The absence of sharp corners affects the flow conditions over the surface, so it has a slight advantage in minimum drag for unit cross-sectional strength.