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  1. #1
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    Open Rocket altitude question

    Running sims on my Uber Der Red Max (86" tall, 32.8 pounds, 7.5 inch diameter) the sim for a K2050ST runs about 920' up. Actual data was about 200' short. However, with a L1000W, the sim shows 2180' and actual data was within about 30 feet of that.

    Both flights were really straight up, so weathercocking wasn't the issue with the K2050.

    Anyone have any ideas? Was the K2050 I used just underpowered? Was the base drag higher than the sim suggests and the L1000 was overpowered? If I add a massless cone to the aft end to simulate base drag so that the K2050 matches, the L1000 sims at about 400 feet below actual.


  2. #2
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    Sounds like you might be within the variance of the motor cert. Anyactual flight within 10% of sim is great.

    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  3. #3
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    I have similar results myself. One motor I miss by 400 feet and the next I'll be within 10 feet. Same rocket, different rocket, doesn't matter. I'm all over the place. Which is why I rely mostly on electronic deploy and usually use the sims for stability and estimated speed off rail.

    Maybe the motor files are incorrect. Curious to know myself.
    Nothing is foolproof to a sufficiently talented fool.
    TRA #16513
    Level 1: Danger Close ---AT H123W to 1240'--- 29 OCT 2016
    Level 2: Binder Design Tyrannosaur ---AT J315R to 2148'--- 30 SEP 2017

  4. #4
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    Motor certs require that they perform within X percent of the designed performance, so if its + or - 5% there is your altitude variance just by that, add the motor delay variance and it can shorten the altitude by a bit more.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  5. #5
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    Quote Originally Posted by rharshberger View Post
    Sounds like you might be within the variance of the motor cert. Anyactual flight within 10% of sim is great.
    If the L1000 sim is right and the K2050 is off, that's about 22% variance. If the K2050 is right and I adjust the drag to match that sim, the L1000 is off about 23%. If I was within 10%, I'd just chock that up to margin of error. About 25% though... First time I've been that far off.

    Just for kicks, I checked the motor files for both the K2050 and L1000. They both match ATs published data.

    And I was using altimeters and no motor eject.
    Last edited by Tonimus; 25th May 2018 at 03:03 AM.

  6. #6
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    Another thing that can affect altitude is wind, my L3 rocket simmed to 8200' on a M1297 and actual was 7800' iirc due to 10-15mph winds and weathercocking.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  7. #7
    Join Date
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    Open Rocket altitude question

    As I made the motor files I can tell you that they are accurate, and created from the certification data.

    That said, you can have variation in motor performance that meets or exceeds your variation. This is allowed under NFPA 1125.

    Have you modified your sims to account for wind speed, launch rail angle, temperature, and humidity? Until you do that it is easy to blame motor files.

    Also, both Rocksim and Open Rocket tend to underestimate base drag.

    So, throw this all together and if you are within 10% you are doing well. Conversely, if you are off of that consistently you might need to revisit the accuracy of your sim versus reality.
    Mark Koelsch
    Tripoli 6155 L3
    Owner of http://www.rocketryfiles.com/
    Editor of http://www.thrustcurve.org/
    Member of the Tripoli Motor Test Committee, and keeper of the motor file

  8. #8
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    Quote Originally Posted by markkoelsch View Post
    As I made the motor files I can tell you that they are accurate, and created from the certification data.

    That said, you can have variation in motor performance that meets or exceeds your variation. This is allowed under NFPA 1125.

    Have you modified your sims to account for wind speed, launch rail angle, temperature, and humidity? Until you do that it is easy to blame motor files.
    Mark, sorry if it came across as a issue with the motor files, it was individual motor variance I was referencing.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  9. #9
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    Mark, I was sure the motor files were correct. Just wanted to head off those questions.

    Rich, there was not any discernible weathercocking with the K2050 flight. Here are the flights.




  10. #10
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    Well the only things left that are easily identified are humidity and temperature.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  11. #11
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    Adjusted for temperature and barometric pressure, with base drag cone added, the K2050 sim is about 15% off and the L1000 sim is off about 5%.

    Just because, I added 10% in thrust (in the form of a G127 in an outboard) and the K2050 sim gained 14% in altitude.

    I think this may be a case of stacking tolerances. I could've had an under performing motor and an over performing sim. Didn't think they cumulative effect would be that high with a 35 pound rocket, but the numbers suggest otherwise. Looks like I need more data.

  12. #12
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    That's an excuse to fly more...😀
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  13. #13
    Join Date
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    Post your sim files and altimeter data. The altimeter is a "measurement", and not necessarily "actual."

    Like it or not, the actual motor not burning like the thrust curve is the biggest variable in sim/flight agreement, especially for heavy rockets only going 1000-2000 ft. The OP said the flights were straight up, so that rules out wind and rod angle. Humidity amounts to peanuts in simulations. Launch site altitude, temperature, and pressure should be checked.

  14. #14
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    I'll get the sim file posted in the morning. Not sure if I have the altimeters here or if they're at the shop. Either way, I'll get the data eventually.

  15. #15
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    The K2050 results in higher velocity at lower altitudes which greatly increases the effect of drag both in real life and simulations.
    I would try overriding the drag coefficient for the rocket to get the simulation for the K2050 closer to the actual flight. Then look at what that does to the simulation for the larger motor.
    Steve Shannon
    L3CC, TAP,
    President, Tripoli Rocketry Association

  16. #16
    Join Date
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    Quote Originally Posted by Tonimus View Post
    Adjusted for temperature and barometric pressure,
    Quote Originally Posted by Buckeye View Post
    temperature, and pressure should be checked.
    Replying to my own post...

    Baro altimeters don't adjust for temperature nor pressure, so no need to adjust beyond standard day conditions in the sim, if correlation is the goal.

  17. #17
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    Sorry it took me so long. Here's the sim file and the raw computer data.
    L3 UDRM.ork
    Flight data.txt


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