OpenRocket - Low Altitude and error

Discussion in 'Rocketry Electronics and Software' started by Pyropetepete, Sep 17, 2019.

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  1. Sep 17, 2019 #1

    Pyropetepete

    Pyropetepete

    Pyropetepete

    Well-Known Member

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    HI,

    I've redesigned a L1 rocket I once flew to 2818ft. The redesign is using lighter materials, shorter length but the simulation on same motor says 275ft for apogee.

    Not sure what I am doing wrong, perhaps have one wrong.

    The build will be

    PML 4'' nose cone
    PML 4'' to 3'' tail cone
    Loc 38mm motor mount 14' long
    Loc 4'' body tube 34'' long

    Hopefully attached is the OR file. Anyone able to help please.

    Thank you
     

    Attached Files:

  2. Sep 17, 2019 #2

    neil_w

    neil_w

    neil_w

    When in doubt, add more rocket TRF Supporter

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    Extremely low altitude estimates usually means the rocket is unstable. What does stability margin show for your chosen motor?
     
  3. Sep 17, 2019 #3

    kuririn

    kuririn

    kuririn

    BARGeezer TRF Supporter

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    I looked at your sim. Your CG is behind the CP, margin of stability is -.568 caliber on the I540 motor.
    When I add 10 oz. of nose weight I get caliber of stability close to one, and an apogee of 3665 ft. on same motor.
     
    SammyD likes this.
  4. Sep 17, 2019 #4

    Pyropetepete

    Pyropetepete

    Pyropetepete

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    I just tried that and it worked, 3884ft it now says. Still not sure how to know it's on the right caliber for stability.
     
  5. Sep 17, 2019 #5

    neil_w

    neil_w

    neil_w

    When in doubt, add more rocket TRF Supporter

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    As a very general rule of thumb, you want stability margin to be 1-2 calibers (higher for very long skinny rockets.) This is reported in the upper right corner of the rocket display, along with CP and CG. What you care about is stability margin when the motor is selected (stability with no motors doesn't tell you anything meaningful).

    Stability margin = (CP - CG) / max diameter of rocket

    Therefore CP must be greater than (i.e., behind) CG to maintain positive stability.
     
  6. Sep 17, 2019 #6

    Pyropetepete

    Pyropetepete

    Pyropetepete

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    Neil thank you, been about 6 years from doing this so racking my brain a little.

    Having to rethink as just ran the sim with it being 54mm and a K1440WT. 1244mph, not sure paper will work
     

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