The external forces commonly acting on vehicles flying in the earth's atmosphere are thrust, aerodynamic forces, and gravitational attractions. Other forces, such as wind or solar radiation pressure, are small and generally can be neglected for many simple calculations.
The thrust is the force produced by the power plant, such as a propeller or a rocket. It usually acts in the direction of the axis of the power plant, that is, along the propeller shaft axis or the rocket nozzle axis. The thrust force of a rocket with constant mass flow has been expressed by Eq. 2-6 as a function of the effective exhaust velocity c and the propellant flow rate m. In many rockets the mass rate of propellant consumption m is essentially constant, and the starting and stopping transients are usually very short and can be neglected. Therefore, the thrust is
As explained in Chapter 3, for a given propellant the value of the effective exhaust velocity c or specific impulse Is depends on the nozzle area ratio and the altitude. The value of c can increase by a relatively small factor of between 1.2 and 1.6 as altitude is increased.
The drag D is the aerodynamic force in a direction opposite to the flight path due to the resistance of the body to motion in a fluid. The lift L is the aerodynamic force acting in a direction normal to the flight path. They are expressed as functions of the flight speed u, the mass density of the fluid in which the vehicle moves p, and a typical surface area A.
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