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  1. #31
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    Quote Originally Posted by Peter Olivola View Post
    Look at the certification requirements for TRA/NAR/etc. The acceptable range is significant. The certification test only sees a very small number of motors.
    The acceptable tolerance is significant, but I would be very surprised if any manufacturers allow their QC to result in that much variability.


    Steve Shannon

    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  2. #32
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    I've seen that much variability doing testing for altitude events with low and mid power including composites. While not measured, I've also observed it with high power composites. Getting the last percent of accuracy out of a simulation will never overcome motor variability.

    Quote Originally Posted by Steve Shannon View Post
    The acceptable tolerance is significant, but I would be very surprised if any manufacturers allow their QC to result in that much variability.


    Steve Shannon

    Peter Olivola

  3. #33
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    Quote Originally Posted by Peter Olivola View Post
    I've seen that much variability doing testing for altitude events with low and mid power including composites. While not measured, I've also observed it with high power composites. Getting the last percent of accuracy out of a simulation will never overcome motor variability.
    Of course it won't. But beginning with the most accurate model you can construct helps reduce other sources of error which might stack up.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  4. #34
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    Maybe. Or maybe you're attempt will induce an error stack up. It still gets down to being able to recognize the sanity of a simulation. Solid propellants aren't reliable enough in the sizes we fly and the production volumes of our motors to make chasing the last decimal place meaningless.

    Quote Originally Posted by Steve Shannon View Post
    Of course it won't. But beginning with the most accurate model you can construct helps reduce other sources of error which might stack up.


    Steve Shannon
    Peter Olivola

  5. #35
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    Quote Originally Posted by Peter Olivola View Post
    Maybe. Or maybe you're attempt will induce an error stack up. It still gets down to being able to recognize the sanity of a simulation. Solid propellants aren't reliable enough in the sizes we fly and the production volumes of our motors to make chasing the last decimal place meaningless.
    How can a more accurate model possibly add to error?
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  6. #36
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    Other variables in the rocket combined with a delta between the model and the actual motor add up. Not knowing the delta is only one of the problems. Understanding the range of possibilities is more important than you're giving credit in interpreting the model.

    Quote Originally Posted by Steve Shannon View Post
    How can a more accurate model possibly add to error?
    Peter Olivola

  7. #37
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    Quote Originally Posted by Peter Olivola View Post
    Other variables in the rocket combined with a delta between the model and the actual motor add up. Not knowing the delta is only one of the problems. Understanding the range of possibilities is more important than you're giving credit in interpreting the model.
    I agree with all of that, but it's still useful to improve every piece of the model, if only to help understand what else is missing or incorrect.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  8. #38
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    Well, given the certified variability of motors, how do you know you've achieved your goal with any other part of the rocket?

    Quote Originally Posted by Steve Shannon View Post
    I agree with all of that, but it's still useful to improve every piece of the model, if only to help understand what else is missing or incorrect.
    Peter Olivola

  9. #39
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    Quote Originally Posted by Peter Olivola View Post
    Well, given the certified variability of motors, how do you know you've achieved your goal with any other part of the rocket?
    Once the motor burns out, the rocket experiences the deceleration due to drag. The motor burns for just a very short time during the ascent. With the correct electronics the rest of the ascent can provide a measurement that can be compared to the model. With multiple flights the model gets closer.
    Once a person feels the model is good enough (it'll never be perfect and I heartily agree with you that it's pointless to hope for perfection) then the model, and inflight measurements can be used to measure the thrust and impulse of the rocket motor.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  10. #40
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    You must have quite a budget to run so many flights.

    Quote Originally Posted by Steve Shannon View Post
    Once the motor burns out, the rocket experiences the deceleration due to drag. The motor burns for just a very short time during the ascent. With the correct electronics the rest of the ascent can provide a measurement that can be compared to the model. With multiple flights the model gets closer.
    Once a person feels the model is good enough (it'll never be perfect and I heartily agree with you that it's pointless to hope for perfection) then the model, and inflight measurements can be used to measure the thrust and impulse of the rocket motor.


    Steve Shannon
    Peter Olivola

  11. #41
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    Why, do you only fly each rocket once?
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  12. #42
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    Depends on whether I recover it. Getting close enough with simulation is usually enough motivation to move on to the next challenge.

    Quote Originally Posted by Steve Shannon View Post
    Why, do you only fly each rocket once?
    Peter Olivola

  13. #43
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    Quote Originally Posted by Peter Olivola View Post
    Depends on whether I recover it. Getting close enough with simulation is usually enough motivation to move on to the next challenge.
    Agreed!


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  14. #44
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    Quote Originally Posted by Steve Shannon View Post
    Of course it won't. But beginning with the most accurate model you can construct helps reduce other sources of error which might stack up.


    Steve Shannon
    Exactly.

  15. #45
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    Poor choice of word. The acceptable variability in motor performance makes "exact" more of an approximation.

    Quote Originally Posted by TRFfan View Post
    Exactly.

    Peter Olivola

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