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  1. #1
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    Rocketman Parachute Drag Coefficients

    I like the rocketman parachutes for their simplicity, 4 shroud lines and easy packing. However I am getting faster decent rates than their decent rate charts indicate. In Open Rocket I need a drag coefficient of .5 to match the rates I am seeing. Anyone else have similar experiences?

    TRA #16452
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  2. #2
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    I don't have an answer as to drag coefficients but I have also observed much higher descent rates than what Rocketman publishes. My most recent launch came in at almost 10 fps faster than the published rate!

    -Dave
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    Level 2 - Madcow Adventurer 3

  3. #3
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    Are you talking about the descent rates in OR, or actually observed descent rates from altimeters?

    If the later, please provide example including weight of rocket, which chute, descent rate, and method used to determine descent rate.

    Something does not add up as a flat sheet parachute has a higher Cd of about 0.7.


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  4. #4
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    Quote Originally Posted by markkoelsch View Post
    Are you talking about the descent rates in OR, or actually observed descent rates from altimeters?

    If the later, please provide example including weight of rocket, which chute, descent rate, and method used to determine descent rate.

    Something does not add up as a flat sheet parachute has a higher Cd of about 0.7.


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    Madcow DX3 2.6" FG rocket weight 48.9oz before motor. Used a Rocketman 4ft with a decent rate of 21ft/sec determined by a Alt3.
    TRA #16452
    L1 Wildman Drago

  5. #5
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    So, with motor let’s say 3.5 pounds. What altitude is your launch site at? If I recall the predicted rates are t sea level. The predicted rates are just predictions- they can vary a lot with altitude, air pressure, and possibly wind.

    For what it is worth, 21 feet/sec is not that bad.


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    Mark Koelsch
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  6. #6
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    Quote Originally Posted by markkoelsch View Post
    So, with motor let’s say 3.5 pounds. What altitude is your launch site at? If I recall the predicted rates are t sea level. The predicted rates are just predictions- they can vary a lot with altitude, air pressure, and possibly wind.

    A four foot Rocketman is measured over the canopy- this means the opening is likely less in the sim.

    For what it is worth, 21 feet/sec is not that bad.


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    Mark Koelsch
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  7. #7
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    The launch was at MWP 737ft alt. OR is giving decent rate 5-6 ft/sec slower.
    TRA #16452
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  8. #8
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    Altitude a bit higher than sea level will increase the descent rate. If there was wind it will generally effect descent rates a bit.

    Could also be the model/part in OR was not setup correctly.


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    Mark Koelsch
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  9. #9
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    Quote Originally Posted by dhkaiser View Post
    I like the rocketman parachutes for their simplicity, 4 shroud lines and easy packing. However I am getting faster decent rates than their decent rate charts indicate. In Open Rocket I need a drag coefficient of .5 to match the rates I am seeing. Anyone else have similar experiences?
    Drag coefficient without the reference area is rather ambiguous. Rocketman chutes are odd-shaped, and they do not indicate the reference area used. If you want to match the stated parameters inside OR, then make sure product of CDA is the same.

    I took the table from RM website and plotted the CDA. I then assumed A = d^2, and computed CD as well. Calculations assume constant gravity and density at sea level standard day. Your actual launch day conditions will affect the results. Not to mention, the drag of all the rocket stuff hanging under the chute comes into play.

    This is what RM says the performance to be under their assumptions, and may not be the actual performance you see in real life.

    Click image for larger version. 

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  10. #10
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    So, let me phrase it this way- when you look at the parachute in OR how is it setup? What is the diameter? Is it set for the correct number of shroud lines and length? A R4 does not have a diameter of four feet- the 4 is the distance over the top of the chute. So the “diameter” is a fair amount smaller than four feet.


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    Mark Koelsch
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  11. #11
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    Quote Originally Posted by markkoelsch View Post
    So, let me phrase it this way- when you look at the parachute in OR how is it setup?... Is it set for the correct number of shroud lines and length?
    I don't think OR is smart enough to adjust CD or A based on shroud lines. They are probably only used for design purposes and maybe mass calculations.

    Nonetheless, we are saying the same thing in the previous 2 posts - Reference area matters when trying to simulate or extract CD.

  12. #12
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    I had the correct number of lines and length, and the chute was set for 4ft. Just adjusted the alt above sea level and to my surprise that made a big difference. Now OR is showing a decent rate within 2-3 feet of the observed decent rather than 6ft.
    TRA #16452
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  13. #13
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    Quote Originally Posted by Buckeye View Post
    I don't think OR is smart enough to adjust CD or A based on shroud lines. They are probably only used for design purposes and maybe mass calculations.

    Nonetheless, we are saying the same thing in the previous 2 posts - Reference area matters when trying to simulate or extract CD.
    That is exactly what I was getting at. You answered more directly than I did- was trying to lead the discussion, so to speak.


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    Mark Koelsch
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  14. #14
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    Drag coefficients are harder to find even for engineer students and professional research dorks. I'm talking about a wind tunnel test or weeks of CFD for one silly number. But you need that number to be accurate for a precise drag force calculation. Companies should release that info. Took a hypersonic flow expert three weeks to get one on a SEDS rocket for a college team by CFD. Longest part was all the meshing of cad models I designed. Normally it's real nice when you have a damn table of values and you don't have to ask a professional graduate level ultra dork grey hair type person for hey can you find a drag coefficient of this object no one has a value for at these conditions? Literally. There's ways to get drag coefficients and you guys and gals won't like how tedious it is. Call up manufacturer and ask for technical support then a drag coefficient of the product itself. Sales should redirect to an engineer, lol. Beats having to brute force dork it.

  15. #15
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    Shoot rocket weight go up, come down. Soon napkin say good. Take meds, relax breathe.

  16. #16
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    Quote Originally Posted by Andrew_ASC View Post
    Drag coefficients are harder to find even for engineer students and professional research dorks. I'm talking about a wind tunnel test or weeks of CFD for one silly number. But you need that number to be accurate for a precise drag force calculation. Companies should release that info. Took a hypersonic flow expert three weeks to get one on a SEDS rocket for a college team by CFD. Longest part was all the meshing of cad models I designed. Normally it's real nice when you have a damn table of values and you don't have to ask a professional graduate level ultra dork grey hair type person for hey can you find a drag coefficient of this object no one has a value for at these conditions? Literally. There's ways to get drag coefficients and you guys and gals won't like how tedious it is. Call up manufacturer and ask for technical support then a drag coefficient of the product itself. Sales should redirect to an engineer, lol. Beats having to brute force dork it.
    Andrew, sure that method would work if you hav CFD and knowledge to use it. So would a wind tunnel and a bunch of testing. Drop tests as well. All take time and effort, and other than potentially the drop test, are beyond the means of essentially all hobbyist and the manufacturers of chutes within the hobby.

    Throw in a crosswind and a lot of this carefully predicted/ tested stuff goes out the window.


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    Mark Koelsch
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  17. #17
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    Quote Originally Posted by Andrew_ASC View Post
    Drag coefficients are harder to find even for engineer students and professional research dorks. I'm talking about a wind tunnel test or weeks of CFD for one silly number. But you need that number to be accurate for a precise drag force calculation. Companies should release that info. Took a hypersonic flow expert three weeks to get one on a SEDS rocket for a college team by CFD. Longest part was all the meshing of cad models I designed. Normally it's real nice when you have a damn table of values and you don't have to ask a professional graduate level ultra dork grey hair type person for hey can you find a drag coefficient of this object no one has a value for at these conditions? Literally. There's ways to get drag coefficients and you guys and gals won't like how tedious it is. Call up manufacturer and ask for technical support then a drag coefficient of the product itself. Sales should redirect to an engineer, lol. Beats having to brute force dork it.
    As somebody else noted in one of your other tangents, you are answering questions that nobody is asking.

    We are talking about parachutes that fall at 15-60 ft/s. Your hypersonic CFD SEDS rocket is irrelevant. Again.

    Yes, you do have a "damn table of values." Try to keep up. That is the very topic of this thread. Nearly all the parachute makers provide some kind of descent charts that are good enough. I used the Rocketman charts to compute CdA and Cd in post #9. Gee, took me 5 minutes, not three weeks of CFD computations.

    Yes, us guys and gals do know how tedious aerodynamics analysis can be. However, we also realize that at some point you need to shoot the engineer (in your case, the college student) and move on with the project.


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