Rocketman Parachutes posted decent rates not matching reality

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Mason T

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When I flew my my L1 flight I used a standard Rocketman parachute I sized it according to the chart on the rocketman website. My rocket had a rapid decent. faster than expected. After talking to people at the field it seemed like common knowledge that to have to use a larger parachute than the charts indicate. I didn't have electronics on my L1 so I do not know the exact rate of decent.

Come to my L2 flight. I have an Eggtimer proton to record my decent rate. I flew a cardboard madcow DX3 with a burnt out weight of 6lb 3oz. My eggtimer shows 18 - 20 fps decent rate under main using a 7 foot standard parachute.

According to the website the decent velocity table says:
17 ft/sec 11lbs
20 ft/sec 15lbs

That is close to 100% error when I am getting this decent velocity at 6lbs.

Please don't take this as me hating on the Rocketman Parachutes. I absolutely love them and will continue to use them in future. I just want to know if there is a good way to determine the correct size as I continue to build larger rockets.

Do you have any tips or rules of thumb to get the right size?
 
Buy a different vendors chute. For your 6lb weight there are 5' options that would work better than the 7' you have.
What I've learned is that there is no consistency on how a parachute is measured. Rocketman measures in what I would call up and over which makes them sound much bigger than they actually are. While there are others that measure across. You really can not compare parachutes from different vendors based on number of feet. which is quite unfortunate.
 
What I've learned is that there is no consistency on how a parachute is measured. Rocketman measures in what I would call up and over which makes them sound much bigger than they actually are. While there are others that measure across. You really can not compare parachutes from different vendors based on number of feet. which is quite unfortunate.

Correct - the legs on an RM chute are included in the measurements so they dont have as much fabric as other chutes. If you want to get your existing RM chute to perform better (note: not to the levels they claim) you can add additional fabric gussets between the legs to fill them out.

IMG_20201213_124524.jpg

With the added gussets they don't jelly fish as much either.
 
I loathe the fact that there is zero consistency between parachute makers' soecifications. A 7' parachute seems HUGE for a rocket just over 6 lbs dry. I fly a 4" rocket that is 7 lbs dry and use a 54" Spherachute. Measured average descent velocity (via Eggtimer Quantums) is between 22 and 26 ft/s at 75' ASL.
 
The only manufacturer that has descent ratings I trust is Fruity Chutes. Their chutes cost about 50% more than Rocketman for the equivalent chute, but you get what you pay for.

Pretty much all my parachutes 10 feet and under are Fruity Chutes and they are very consistent with the manufacturer's ratings. They also have the best descent rate calculator on the web (IMO).

For rockets about 80lbs to somewhere around 120lbs, I use Cert-3 chutes. I have lots of flights on Cert-3 chutes and am comfortable with their performance in that weight range.

For rockets above 120lbs, I do use Rocketman chutes because they are the only ones that make chutes that big at a reasonable price. My largest chute from them is a 50 foot standard which they rate to a 550 pound rocket, but I only use it in rockets up to 450lbs. I recommend taking Rocketman's estimations at about 70-80% of the descent rate they claim and you will be in the ballpark.

Above 450 pounds, I recommend getting an Army surplus cargo chute. An Army G-12 is a 64ft chute that can handle up to about 2000 pounds.
 
Correct - the legs on an RM chute are included in the measurements so they dont have as much fabric as other chutes. If you want to get your existing RM chute to perform better (note: not to the levels they claim) you can add additional fabric gussets between the legs to fill them out.

View attachment 576874

With the added gussets they don't jelly fish as much either.
That's a great idea. Thanks for the idea!
 
The only manufacturer that has descent ratings I trust is Fruity Chutes. Their chutes cost about 50% more than Rocketman for the equivalent chute, but you get what you pay for.

Pretty much all my parachutes 10 feet and under are Fruity Chutes and they are very consistent with the manufacturer's ratings. They also have the best descent rate calculator on the web (IMO).

For rockets about 80lbs to somewhere around 120lbs, I use Cert-3 chutes. I have lots of flights on Cert-3 chutes and am comfortable with their performance in that weight range.

For rockets above 120lbs, I do use Rocketman chutes because they are the only ones that make chutes that big at a reasonable price. My largest chute from them is a 50 foot standard which they rate to a 550 pound rocket, but I only use it in rockets up to 450lbs. I recommend taking Rocketman's estimations at about 70-80% of the descent rate they claim and you will be in the ballpark.

Above 450 pounds, I recommend getting an Army surplus cargo chute. An Army G-12 is a 64ft chute that can handle up to about 2000 pounds.
This is great info thank you very much!
 
When I flew my my L1 flight I used a standard Rocketman parachute I sized it according to the chart on the rocketman website. My rocket had a rapid decent. faster than expected. After talking to people at the field it seemed like common knowledge that to have to use a larger parachute than the charts indicate. I didn't have electronics on my L1 so I do not know the exact rate of decent.

Come to my L2 flight. I have an Eggtimer proton to record my decent rate. I flew a cardboard madcow DX3 with a burnt out weight of 6lb 3oz. My eggtimer shows 18 - 20 fps decent rate under main using a 7 foot standard parachute.

According to the website the decent velocity table says:
17 ft/sec 11lbs
20 ft/sec 15lbs

That is close to 100% error when I am getting this decent velocity at 6lbs.

Please don't take this as me hating on the Rocketman Parachutes. I absolutely love them and will continue to use them in future. I just want to know if there is a good way to determine the correct size as I continue to build larger rockets.

Do you have any tips or rules of thumb to get the right size?
Are they the standard chute or the 2.2CD?
 
I am using the standard.
The Standard chute is listed with a drag coefficient of 0.97. This doesn't even match the descent tables provided on their website. Using the 17 ft/sec part of their table (assuming MSL as the landing altitude), the average drag coefficient for all of the standard chutes is 0.8359. There is quite a spread though; 0.74-0.90. In practice however, the drag coefficient that tends to be more accurate is in the 0.65 plus or minus 0.10 range.

I would use a drag coefficient of 0.65 when you plan your next flight and purchase accordingly. That said, I just used your data and came up with a Cd as low as 0.3389, which would be a 9ft chute. That's a little big IMO. There may have been other factors contributing to the faster than expected descent rate, to include poor data. If the rocket was swinging it will certainly come in faster. If it was swinging, lengthen the shock cord a little to reduce the rocket's affect on the chute oscillating.

I hope that helps in some way, but without repeating the flight a few times and having more data, I'm not entirely sure why the descent rate was so high.

The drag coefficient calculation was taken from Knacke's Parachute design manual, and is Cd=W/(P*V^2*((D/2)^2*Pi)/2)
Cd=drag coefficient
W=weight
P=air pressure in slugs
D=diameter of chute
 
Do you have any tips or rules of thumb to get the right size?

Here's the best parachute size calculator I've found:

https://www.translatorscafe.com/unit-converter/en-US/calculator/parachute-size/
parachute descent calc 01.JPG parachute descent calc 02.JPG

Unlike the FruityChutes calculator, this one lets you specify the shape of the parachute—flatsheet (circle, hexagon, octagon or square), or dome (hemispherical)—as well as the size of the spill hole.

Start by specifying the desired descent rate, enter the Cd ( .75 for flatsheet, 1.50 for hemispherical), mass of the rocket with spent motor, spill hole percentage (of canopy diameter).

It also wants air density at your elevation. I get that info here:

https://aerotoolbox.com/atmcalc/
Set the first popup menu to Altitude + Air Temp.

parachute descent calc 03.JPG

If you want to make your own parachute using that info, get Richard Nakka's gore generator, available here:

https://github.com/Case-Rocket-Team/Parachute-Gore-DXF-Generator/releases/tag/v1.1.0
 
What I've learned is that there is no consistency on how a parachute is measured. Rocketman measures in what I would call up and over which makes them sound much bigger than they actually are. While there are others that measure across. You really can not compare parachutes from different vendors based on number of feet. which is quite unfortunate.

Correct, and any mention of Cd is meaningless without also providing the reference area used in the denominator. Actually, the product CdA would be a better measure of the parachute performance and eliminates ambiguity.

Do the vendors do drop tests on their products and measure descent rate or just assume Cd, A and do a simple terminal velocity calculation? Who knows. The Rocketman data is all over the place. Unfortunately, you don't know for sure until you fly it on your rocket.
 
"The only manufacturer that has descent ratings I trust is Fruity Chutes."


Spherachutes has been spot on for me. They are my go to chutes. Top flight descent chart has been accurate for me also. Those are the 2 vendors I highly suggest. Take this as a learning experience.
 
"The only manufacturer that has descent ratings I trust is Fruity Chutes."


Spherachutes has been spot on for me. They are my go to chutes. Top flight descent chart has been accurate for me also. Those are the 2 vendors I highly suggest. Take this as a learning experience.
I love sphereachutes
 
Do the vendors do drop tests on their products and measure descent rate or just assume Cd, A and do a simple terminal velocity calculation? Who knows. The Rocketman data is all over the place. Unfortunately, you don't know for sure until you fly it on your rocket.
Most vendors do conduct some test to determine their drag coefficients or they rely on legacy test data. To the best of my knowledge, the following is the summation of testing for the various vendors.

Fruity Chutes- they have done a tremendous amount of testing on their own and have used user data
Top Flight- the flat chute is a simple legacy design where there is plenty of data for all descent ranges
Front Range Rocket Recovery- Steve flies his designs quite a bit to ensure that he is not misrepresenting the drag coefficient
Spherachute-third parties have independently verified the performance of Julie's chutes and she has gone with the conservative drag coefficients; Spherachute also uses a hemispheric chute that NASA tested a lot in the past so there is plenty of legacy data
SkyAngle- I don't think they test their chutes and only rely on user data
Rocketman Standard- this is the Deist Safety dragster parachute and other than user data, I don't think this has been tested except for high speed (which does not translate to slow speed data). I don't know about their other chutes
Apogee-Tim has very stringent test standards for every product he sells, including his chutes

Testing cost money and is usually passed along to the customer in the final price. I look at it as spending the same amount of money, I could save on the chute and damage my rocket or I can spend a little more to protect something I spent hundreds on and hours of work.
 
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