Sun synchronous orbit alt = 700 km
Mass of satellite = 800 kg
Assume Cd = 2
Thruster Isp = 80 s
Propellant = N2 (Molecular Weight = 28, etc)
Propellant temperature = 280 K
Length of satellite = 4 m
Width (diameter) of satellite = 1 m
F = (1/2*m)*Cd*A*density*V^2 but how would you calculate the mass of propellant needed.
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The drag force is
Cd = 2 (given),
and we address the values of A, rho, and V below.
The momentum "eaten up" by the drag over lifetime T is
P_lost = F*T = (Cd/2)*rho*A*V^2 *T.
The momentum added by emitting the propellant over the lifetime is
P_added = Isp * g * M,
Isp = 80 s
is the given thruster specific impulse (e.g. see definitions at http://en.wikipedia.org/wiki/Specific_impulse),
g = 9.8 m/s^2
is the free fall ...
This solution calculates the force on a satellite due to drag and the mass of the propellant needed to overcome the drag force.