The parachute is an example of an object falling at terminal velocity.

The terminal velocity of a rocket under chute is described by the equation:

V = square root [(2 x W) / (Cd x rho x A)] where V is the terminal velocity in ft. per second, W is the recovery weight of the rocket in pounds, Cd is the drag coefficient of the parachute, rho is the atmospheric density in slugs per cubic foot, and A is the area of the chute in sq. ft.

Drag coefficients range from 0.65 to 1.25 depending on parachute design. Many calculators simply use 1which is not a bad approximation for descent rates in the 15-20 fps range. The drag coefficient drops as the descent rate increases and you migh want to use 0.7 for a drogue in the 60-80 fps range.

At 70 °F and 14.696 psia, dry air has a density, rho = 0.002328 slugs/ft3.

The atmospheric density drops by 5% for every 1000' increase in altitude.

A simple calculator is here.

http://www.digitaldutch.com/atmoscalc/
A pictorial description of terminal velocity is found here and there is a simple applet at the bottom of the page that does the calculation for you.

http://www.grc.nasa.gov/WWW/K-12/airplane/termv.html This Java calculator is accurate and allows for altitude compensation.

Using the calculator with a Cd = 0.9 and an 8' chute you get ~15 fps at sea level and 16 fps at 5000' as the descent rate of for a 12 pound rocket.

Bob