Empty falling speed is the terminal velocity at sea level on earth of the rocket typically expressed in meters per second. The empty falling speed increases as the mass of the rocket increases and as the engine nozzle increases in cross sectional area, for instance when it is optimized for vacuum operation. A model rocket will have an empty falling speed of around forty meters per second and a Saturn V class rocket will have an empty falling speed of around three hundred meters per second.

The higher the empty falling speed the lower the aerodynamic drag per unit mass at a given airspeed. A high empty falling speed is good when traveling through an atmosphere. A low empty falling speed is good when reentering and especially when landing on rocket thrust, because a braking rocket has to provide a velocity change equal to the empty falling speed. Ideally, a rocket would have a low empty falling speed and somehow start off above most of the atmosphere. Given empty mass, exit area and mass the empty falling speed can be calculated.

area = pow( ( mass / 13000 ), 0.6666 ) + exit area
drag = 0.4 * 1.24 * area
empty falling speed = sqrt( empty mass * 9.8 m / s^2 / drag )
 
 

This is used in pumped rocket and rocket cost.
 
 

Feedback   Free Electronic Nation Home    Rocket   GNU Free Documentation License