aerodynamic force, and gravitation. Other external forces such as centrifugal force, Coriolis force, and solar radiation pressure are generally insignificant due to the relatively short time of powered flight and small size of spacecraft, and may generally be neglected in simplified performance calculations. Propulsion The thrust of a rocket engine, in the general case of operation in an atmosphere, is approximated by: {| border="0" cellpadding="2" |- |align=right|where: | |- !align=right| |align=left|= exhaust gas mass flow |- !align=right| |align=left|= effective exhaust velocity (sometimes otherwise denoted as c in publications) |- !align=right| |align=left|= effective jet velocity when Pamb = Pe |- !align=right| |align=left|= flow area at nozzle exit plane (or the plane where the jet leaves the nozzle if separated flow) |- !align=right| |align=left|= static pressure at nozzle exit plane |- !align=right| |align=left|= ambient (or atmospheric) pressure |} The effective exhaust velocity of the rocket propellant is proportional to the vacuum specific impulse and affected by the atmospheric pressure: where: has units of seconds is the gravitational acceleration at the surface of the Earth The specific impulse relates the delta-v capacity to the quantity of propellant consumed according to the Tsiolkovsky rocket equation: where: is the initial total mass, including propellant, in kg (or lb) is the final total mass in kg (or lb) is the effective exhaust velocity in m/s (or ft/s) is the delta-v in m/s (or ft/s) Aerodynamic force Aerodynamic forces, present near a body with significant atmosphere such as Earth, Mars or Venus, are analyzed as: lift, defined as the force component perpendicular to the direction of flight (not necessarily upward to balance gravity, as for an airplane); and drag, the component parallel to, and in the opposite direction of flight. Lift and drag are modeled as the products of a coefficient times dynamic pressure acting on a reference area: where: CL is roughly linear with α, the angle of attack between the vehicle axis and the direction of flight (up to a limiting value), and is 0 at α = 0 for an axisymmetric body; CD varies with α2; CL and CD vary with Reynolds number and Mach number; q, the dynamic pressure, is equal to 1/2 ρv2, where ρ is atmospheric density, modeled for Earth as a function of altitude in the International Standard Atmosphere (using an assumed temperature distribution, hydrostatic pressure variation, and the ideal gas law); and Aref is a characteristic area of the