Altitude = 15 km, Vz= 88.7 m/s (using Cd = 0.8) Vz = 124.5 m/s using spherical drag modelAltitude = 10 km, Vz= 58.6 m/s (using Cd = 0.8) Vz = 89.3 m/s using spherical drag modelAltitude = 5 km, Vz= 43.6 m/s (using Cd = 0.8) Vz = 70.0 m/s using spherical drag model

@aero, please check your calcs, because with a simple formula ( http://en.wikipedia.org/wiki/Terminal_velocity )sea level terminal velocity for Falcon 9 should be 200 m/s.(mass 20000 kg, air density 1.2 kg/m3, area 10.5 m2, Cd 0.8 ).

Aren't the propellant tanks filled with helium during recovery? That should give them about 400 kg of buoyancy if so.

You could try to do the evaluation backwards, assuming Elon Musks statement is correct. He said the final burn will consume less than 5% of the empty stage weight. So calculate the burn for 1t of fuel and see what terminal speed can be killed with that burn. Then you can calculate the drag required to achieve that terminal speed. Should be interesting.

Quote from: guckyfan on 12/23/2013 06:54 PMYou could try to do the evaluation backwards, assuming Elon Musks statement is correct. He said the final burn will consume less than 5% of the empty stage weight. So calculate the burn for 1t of fuel and see what terminal speed can be killed with that burn. Then you can calculate the drag required to achieve that terminal speed. Should be interesting.Matches our figures, as a first approximation.5% of empty stage mass is about 1000kg.At sea level, full throttle, the single merlin 1d impart 3.1g to empty stage + 1 ton.It would burn through that 1 ton of fuel in 4.25 seconds, delivering total delta of about 129m/s.

Quote from: aero on 12/23/2013 05:19 PM Aren't the propellant tanks filled with helium during recovery? That should give them about 400 kg of buoyancy if so.Their content is irrelevant, as the first stage is a volume-constrained rigid object.Its buoyancy would be exactly that of the air mass displaced, which is about 440m3. At sea level, that is about 525kg. At anything over 10km altitude, a very very good approximation of zero.

“(Also worth noting,) you don’t need wings to steer aerodynamically, you just need some lift over drag numbers and lift vector.”