Why Missiles and Rockets are Blunt from Top?
Everyone knows that air planes, missiles and rockets are designed to minimise the air friction by creating an “Aerodynamic shape”. We all were taught in school that they have a streamlined design that allows air to pass easily. But what makes the design more streamlined? A Pointed top like in most fighter jets or a Blunt top like in missiles or Rockets.
Fighter Jets and Rockets fly at a very high speed. Most of the jets and all the rockets exceeds the speed of sound ,i.e., Mach>1. This type of speed is known as supersonic speed. For rockets, this speed even crosses 5 times the speed of sound, i.e., Mach>5. This is known as Hypersonic speed. The speed of the aircraft plays an important role in defining if the nose should be pointed or blunt.
For any aircraft design, the main aim is to reduce the drag. There are three different types of drag which acts on any body – Skin drag, Form drag and Wave drag.
For Subsonic Flow – Mach<1
Commercial jets and some fighter jets fall in the category of subsonic flow. For subsonic regime, the first two types of drag plays an important role, viz., Skin drag and form drag. If we consider our flight as a pure cylindrical object with a nose, keeping the length and cross sectional area same, following values of Drag Coefficient Cd is achieved for different shapes of nose.
Coefficients indicating the forebody pressure drag of a series of cylindrical body – Fluid Dynamic Drag by Hoerner
This means that all other things being equal, the blunt nose has lesser drag compared to sharp ones in subsonic flow. In particular, the blunt nose has lesser surface area compared to the sharp, long ones and thus lesser skin friction drag.
Thus in Subsonic flows, Blunt nose is more aerodynamic than sharp nose
For Supersonic Flow – Mach>1
When it comes to supersonic flow, wave drag becomes the main concern. In aeronautics, wave drag is a component of the aerodynamic drag on aircraft wings and fuselage, propeller blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects, and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to the Critical Mach number.†
In this case a sharp nose reduces wave drag because in a sharp section, the cross sectional area varies gradually, which helps in prevention of local acceleration (which can result in local sonic conditions).
Thus in supersonic flows, sharp nose is preferable over a blunt nose.
Hypesonic flow – Mach>5
You will find that aviation design is full of compromise. for Mach>5 wave drag still dominates over other two types of drag, but it dominates so much that it will produce an attached shock which will heat the tip to something close to stagnation temperature of the flow.This will put the nose material to an upper limit of aerodynamic heating.
Though a blunt nose creates a more drag in hypersonic flow, but it is capable of distributing the heat load uniformly and thus sustain the harsh conditions of aerodynamic drag at such a high speed.
Thus in hypersonic flow, blunt nose wins because slow and steady wins the race.
†Definition of Wave drag from : https://en.wikipedia.org/wiki/Wave_drag