Formula for Calculating Descent When Cd is Known?

Bought a Rocketman Kevlar flameproof 3 foot chute some time ago, which I planned to use as the drogue for my 22 pound "V-2 Stretched". Now that I know that the recently posted Cd (after I purchased it) from his site is 1.85 for the chute, RockSim gives a crazy-low descent rate of 37 fps under drogue.

What is the formula for calculating descent rates when the Cd is known? The online calculators I've found don't allow you to enter the Cd; only the shape of the chute.

I'd like to double-check this 37 fps result from RockSim before I use the chute as the drogue, and then have to walk half a mile to recover the rocket- lol.

Mike Momenee
TRA#12430 L3

D = sqrt((8*m*g)/(p*r*Cd*v^2))

(Use appropriate units)
D=diameter (meters)
m=mass (kg)
g=gravity (9.81 m/s)
p=pi (3.14159)
r=density of air (1.22 kg/m^3)
Cd= drag coefficient
v= fall velocity (m/s)

Ironically Bamachutes provides exactly the calculator you are seeking: https://www.bamachutes.com/calculators.html

Thank you fellas!! I never knew this Bamachutes site existed. I got the same 37fps result, so I guess I'll be selling this Kevlar chute- no other need for it. It would have been a LONG walk......................
Mike Momenee
TRA#12430 L3

jmmome said:

Thank you fellas!! I never knew this Bamachutes site existed. I got the same 37fps result, so I guess I'll be selling this Kevlar chute- no other need for it. It would have been a LONG walk......................
Mike Momenee
TRA#12430 L3

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Hello Mike, the Cd on that parachute is .97 Cd, the coefficient of drag is very complicated on Parachutes as there’s a million different ways to measure them. But the Cd that you need to use is .97, also the decent rates are listed on our website.

Buddy Michaelson said:

Hello Mike, the Cd on that parachute is .97 Cd, the coefficient of drag is very complicated on Parachutes as there’s a million different ways to measure them. But the Cd that you need to use is .97, also the decent rates are listed on our website.

Click to expand...

Buddy,
Your website says the Rocketman Kevlar chutes have a Cd of 1.85 in all sizes. As do the standard Rocketman. Is this wrong?
Looks like using the calculator with Cd .97 gets the numbers closer to your listed descent rates than does using Cd 1.85.
This will explain why my rockets seem to come in hot when the sims say it should be a lot slower.

To the OP, my bad. I gave you the 1.85 Cd from Rocketman's site.

MikeyDSlagle said:

Buddy,
Your website says the Rocketman Kevlar chutes have a Cd of 1.85 in all sizes. As do the standard Rocketman. Is this wrong?
Looks like using the calculator with Cd .97 gets the numbers closer to your listed descent rates than does using Cd 1.85.
This will explain why my rockets seem to come in hot when the sims say it should be a lot slower.

To the OP, my bad. I gave you the 1.85 Cd from Rocketman's site.

Click to expand...

That 1.85 Cd is measured by the surface area of the top of the parachute only, it’s conokicated going from one parachute manufacturer to the next. I will make sure to fix that on my website and explain it better. All of our Parabolic Chutes are cut the same form our standards, Pro x, Mach and Kevlar Chutes. Thank you so much Mikey for using Rocketman Products.

Buddy Michaelson said:

That 1.85 Cd is measured by the surface area of the top of the parachute only, it’s conokicated going from one parachute manufacturer to the next. I will make sure to fix that on my website and explain it better. All of our Parabolic Chutes are cut the same form our standards, Pro x, Mach and Kevlar Chutes. Thank you so much Mikey for using Rocketman Products.

Click to expand...

It'd be better if you didn't just post the drag coefficient, but also posted the reference area used to determine the drag coefficient. That'd remove all the ambiguity.

Thank you to all for your input. .97 Cd makes more sense, given the shape of the chute.

Buddy, the chute is beautifully constructed!

Mike Momenee
TRA#12430 L3

jmmome said:

Thank you to all for your input. .97 Cd makes more sense, given the shape of the chute.

Buddy, the chute is beautifully constructed!

Mike Momenee
TRA#12430 L3

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Thank you Mike I really appreciate that! Let me know if you ever have any questions about Parachutes.

jmmome said:

I'd like to double-check this 37 fps result from RockSim before I use the chute as the drogue, and then have to walk half a mile to recover the rocket- lol.

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37 FPS seems too fast for a main 'chute (and too slow for a drogue). I guess it depends what surface the rocket will land on, but I wouldn't come down that fast on the desert playa. I shoot for a tiny drogue and oversized main, intending to drop at the end at around 15 FPS. If landing on grass or crops, a higher speed might well be OK.

Buddy Michaelson said:

That 1.85 Cd is measured by the surface area of the top of the parachute only, it’s conokicated going from one parachute manufacturer to the next. I will make sure to fix that on my website and explain it better. All of our Parabolic Chutes are cut the same form our standards, Pro x, Mach and Kevlar Chutes. Thank you so much Mikey for using Rocketman Products.

Click to expand...

I love the Rocketman chutes. Less lines to fool with. Will be better prepared now that I know the correct Cd.

Yup- 37fps is "my bad". I'll pass it along unused to a good home via the "Yard Sale" forum. I have my trusty ten year old 19" nomex drogue (mother-in-law made!!), and I'll use it for my project instead.

Mike Momenee
TRA$12430 L3