# Parachute drag coefficient on openrocket

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#### Hakan Demir

##### Member
Hey folks! l am a student freshman level at university. l am planning to build HPR and have a problem. l want to use Dacron polyester material for my parachute. l am not sure whether Cd is correct on openrocket software or not. As Nasa resources, ıt would be 1.75. But as openrocket software , it would be 0.8.

how can l find correct Cd for my rocket?

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#### Steve Shannon

##### Well-Known Member
The Cd of the parachute is dependent on the shape of the parachute and to some extent the porosity of the fabric. The actual type of material though has no effect on Cd. A hemispherical dome of one non porous material will have exactly the same Cd as a hemispherical dome of another non porous material.
The best way to find your true Cd is to drop test or fly the rocket and see what the terminal velocity is of the falling rocket under chute and then solve for Cd. It’s a dimensionless value that is typically found empirically. Until then using 0.8 is probably close enough.

Edit: well, the simple truth is, never trust things like this to memory. My original comment about reasonable Cd values above was completely wrong! I edited it.

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#### Hakan Demir

##### Member
The Cd of the parachute is dependent on the shape of the parachute and to some extent the porosity of the fabric. The actual type of material though has no effect on Cd. A hemispherical dome of one non porous material will have exactly the same Cd as a hemispherical dome of another non porous material.
1.75 is much closer to correct than 0.8. No reasonable chute would have a Cd that low. The best way to find your true Cd is to drop test or fly the rocket and see what the terminal velocity is of the falling rocket under chute and then solve for Cd. It’s a dimensionless value that is typically found empirically. Until then using 1.75 is probably close enough.

Thank you for your response Steve

Probably, our chute shape would be hexagonal.

Hakan Demir

#### gtg738w

##### FlightSketch - flightsketch.com
Parachutes will have a range of values depending on design. Small flat parasheets for low power are actually pretty close to 0.8. If you have a large hemispherical chute it will be closer to the NASA number.

The shape of a flat sheet when inflated isn’t as effective and you could argue the reference area drops when it pulls into shape.

Testing is the only way to know for sure, but at 95cm, you can probably pick a number in the middle and not be too far off.

#### Hakan Demir

##### Member
Parachutes will have a range of values depending on design. Small flat parasheets for low power are actually pretty close to 0.8. If you have a large hemispherical chute it will be closer to the NASA number.

The shape of a flat sheet when inflated isn’t as effective and you could argue the reference area drops when it pulls into shape.

Testing is the only way to know for sure, but at 95cm, you can probably pick a number in the middle and not be too far off.

Could you give me advice about the shape of the parachute? Which one gives us the closest results? l just know hexagonal or round shape.

#### gtg738w

##### FlightSketch - flightsketch.com
If it is a single piece of fabric, that would be a flat sheet. The actual shape, round, hexagon, octagon doesn’t change things much.

You can also cut many tapered pieces of fabric, “gores”, and sew them into a shape much closer to a hemisphere. This is like the NASA chutes or an umbrella. Much better drag but more complicated. There are other shapes like cruciform or annular as well.

#### Alan15578

##### Well-Known Member
Hey folks! l am a student freshman level at university. l am planning to build HPR and have a problem. l want to use Dacron polyester material for my parachute. l am not sure whether Cd is correct on openrocket software or not. As Nasa resources, ıt would be 1.75. But as openrocket software , it would be 0.8.

how can l find correct Cd for my rocket?

Throw it out the window! Well, that may be a little too crude, but seriously, a little testing will determine you drag coefficient much more accurately and more easily than any other approach.

Alan

#### dhbarr

##### Amateur Professional
Flat sheets are, in fact, pretty close to 0.8 IIRC.

#### SpaceManMat

##### Space Nut

https://forum.ausrocketry.com/viewtopic.php?f=6&t=5312&hilit=Purple+parrot&start=75#p66208

Happy Heyoka said:
lots of kinds of chutes, obviously... but for descent rate (vs gliding for example) Cd is king; then opening force, stability etc.

lots of publicly available stuff on military air drop tech worth reading... here's a table from "Parachute Recovery Systems
Design Manual"

#### ibanerjee20

##### Member
Hey folks! l am a student freshman level at university. l am planning to build HPR and have a problem. l want to use Dacron polyester material for my parachute. l am not sure whether Cd is correct on openrocket software or not. As Nasa resources, ıt would be 1.75. But as openrocket software , it would be 0.8.

how can l find correct Cd for my rocket?

Try running some simulations with both the input parameters. There are a few platforms which can help you design your parachute and then simulate the designs in your desired operating conditions. This way, you'll be able to make a fair judgement.

"A picture is worth a thousand words"
"A simulation is worth a thousand pictures!"

Best wishes
Ishan Banerjee
Zeus Numerix

#### Andrew_ASC

##### UTC SEDS 2017 3rd/ SEDS 2018 1st
The only time I accurately solved for a drag coefficient for new object involved a CFD at university for senior design rocketry project. Specially for an airfoiled multistage rocket at M2.4 which was later flight tested. You can also experimentally find Cd in a wind tunnel if the tunnel offers a velocity high enough to match the precise real world value. Ask some grad students or a professor in fluid dynamics for help.

Practically speaking... When I design HPR rockets if it bugs me I’ll buy a fruity chute then use the manufacturer posted Cd in open rocket. When it doesn’t bug me I use the default open rocket value and a top flight thin mill. Both will work in practical sense. Have deployed top flight thin mill chutes at 324 mph in university competitions. My two cents it’s worth it to buy a chute. It’s flight proven. Here’s the oddball thing, a top flight chute will pack tighter than a fruity chute and it’s cheaper which is important because chutes in this hobby get damaged by powder spray sometimes. Apogee rockets also has neat chute design info.

Drag coefficient varies with geometry, angle of attack, and velocity. It’s just not something easily solved for. Either a computer throws **** loads of differential equations at it or you simply real world test it in a wind tunnel. For hobby use the OR default value won’t break your rocket imo.

#### Andrew_ASC

##### UTC SEDS 2017 3rd/ SEDS 2018 1st
Richard Nakka also has good chute design info, specifically excel sheets for sizing and cutting gores of elliptical chutes.

#### SammyD

##### Well-Known Member
This thread caught my eye as I've wondered why parachute makers (FruityChutes) don't put a drag coefficient in their descriptions. Telling me that a 108" Iris Ultra brings 50lbs down at 20fps really doesn't help me much when the rocket weighs 25 lbs. I'd like to add the Cd to my OR files to better be able to estimate ground hit velocity. For a chute that can cost in the hundreds of dollars, that would be a nice bit of information that I'd think they'd have available to them.

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#### timbucktoo

##### Well-Known Member
Staff member
Global Mod
This thread caught my eye as I've wondered why parachute makers (FruityChutes) don't put a drag coefficient in their descriptions. Telling me that a 108" Iris Ultra brings 50lbs down at 20fps really doesn't help me much when the rocket weighs 25 lbs. I'd like to add the Cd to my OR files to better be able to estimate ground hit velocity. For a chute that can cost in the hundreds of dollars, that would be a nice bit of information that I'm would think they'd have available to them........
If you go to the Fruity Chute descent rate calculator page, you will see that they list the Cd for all their chutes; however, I do not see an Iris Ultra in 108" size.
https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

#### SammyD

##### Well-Known Member
I
If you go to the Fruity Chute descent rate calculator page, you will see that they list the Cd for all their chutes; however, I do not see an Iris Ultra in 108" size.
https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

I was going from memory - sorry on the 108". I think I used 108" in the sim for a 96" Iris Ultra Standard chute because I couldn't get a Cd that seemed correct. Since those chutes seem to have substantially higher drag (thus slower decent), it's only Cd that I can really input to make up for the difference OR increasing the size of the chute.

The rocket weighs 25# without a motor, flew it this past weekend with a 96" Iris Ultra Standard, and it floated quite gently to the ground after the main deployed. I've clearly got the right chute for this rocket, but I have other Iris Ultras in larger and smaller sizes that I'd just like to be able to use without uncertainty...

#### Buckeye

##### Well-Known Member
This thread caught my eye as I've wondered why parachute makers (FruityChutes) don't put a drag coefficient in their descriptions. Telling me that a 108" Iris Ultra brings 50lbs down at 20fps really doesn't help me much when the rocket weighs 25 lbs. I'd like to add the Cd to my OR files to better be able to estimate ground hit velocity. For a chute that can cost in the hundreds of dollars, that would be a nice bit of information that I'd think they'd have available to them.
Use the equation in post #1 to compute CD from the information given.

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