Lets say we only have the z axis to deal with( 1 dimention)
key variables on Rigidbody
Gravity = off
Drag = 5
Mass = 20
Force = 6600
assume everything else is set to the default settings
what is the formula to work out the velocity?
if possible what would the formula look like with gravity On (not required but curious)
Rigidbody.velocity 
Seriously though, thinking back 15 years, velocity is the integral of acceleration. I dont recommend performing integration in your code.
(a=m/s^2, v=m/s)
Here is a sheet of formulae that will help you, particularly
velocity, v = v0 + at
v0=initial velocity
a=accel
t=time
Ofc F = Ma or a = F/M
Velocity = Initial velocity + acceleration * time
http://www.ajdesigner.com/constantacceleration/cavelocity.php
