Rocksim altitude accuracy

Discussion in 'Rocketry Electronics and Software' started by rewilfert, Sep 6, 2019.

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  1. Sep 6, 2019 #1

    rewilfert

    rewilfert

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    I'm modeling a rocket in Rocksim. The design is a simple 4" rocket about 5' long with 3 fins. It will fly on I and J motors.

    To those that have modeled a rocket and then flow with an altimeter for data. Does Rocksim do a fairly good job of estimating the flight altitude? Or do you find it over or under estimates? Obviously, real world conditions will have an effect, but I'm curious how accurate it typically is.
     
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  2. Sep 6, 2019 #2

    GaryT

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    RS does pretty good but the easiest way to do it is build the rocket, add everything! that'll be in the rocket except the motor. No need to weight every little piece as its all going to add up to the same weight anyway. Now weight the rocket and add that total weight to your RS file, let RS determine the rockets CD. After you've flown the rocket adjust the CD in RS to net you exactly what you got in altitude. After a few flights and tweaking the CD it'll be pretty dam close in predicting altitudes. My Sims are usually +/- 125'
     
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  3. Sep 6, 2019 #3

    GaryT

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    This was a duplicate deleted
     
  4. Sep 6, 2019 #4

    mbeels

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    I get very close results to my simulations if I set the surface finish to "matte", or "unfinished". I find that if I leave everything on the default setting of "polished", I get very optimistic sims. That may say more about my finishing skills than Rocksim. It also does a decent job of including the effects of winds (and weather cocking), but that adds a lot of variability. Usually there is at least some wind.
     
  5. Sep 8, 2019 #5

    rewilfert

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    Thanks for the feedback. I have certain performance goals I am trying to achieve with a fixed weight and 2 specific motors. The altitude simulation comes up on the low end of where I’d like to be with the smaller of the two motors. Unfortunately, I can’t fly it first to see how accurate the simulation is. I’m going to have to change my approach to the problem to make it work the way I want.
     
  6. Sep 9, 2019 #6

    ihbarddx

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    Just want to point out that one can fly the same configuration N times and get N different altitudes. There is inherent variation that simulations aren't going to predict. Cd is another source of inaccuracy. Even if simulations are very good at computing 0-Angle-Of-Attack Cd (and in my experience they are not), AOA varies during flight, and depends on things like turbulence. Not only can AOA variations affect drag, they affect trajectory, which affects altitude.

    Then there are the motors. BP varies in Isp, and the manufacturer compensates by varying propellant mass. The result is not equivalent, but it's reasonably close if propellant mass fraction is low and if burning time isn't affected too much. AP composites are more consistent, but Aerotech changes motor design from time to time, affecting total impulse. You have to make sure you're using the right thrust curve. Naturally, ignition anomalies affect thrust curves. Launch venues from high elevations affect thrust curves (enhancing Isp), as does ambient temperature. All motors have inherent variation.

    With a new configuration, I'm personally happy with +/-20% accuracy.
     
  7. Sep 10, 2019 #7

    jmmome

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    You'll probably find that RockSim will over-estimate your altitude. And as previous posts mentioned, you'll get different actual altitudes with the same rocket and the same motor 10 times out of 10 launches. For me, RockSim is more about confirming the stability of a design before you actually fly it.

    Sounds like you should go with the larger of the two motors, if the smaller motor sims out on the low end of what you want.

    Mike Momenee
    TRA #12430 L3
     
  8. Sep 10, 2019 #8

    AllDigital

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    I agree with the feedback above. Rocksim does a great job at the simulation, but usually comes out on the high end, especially if defaults are left in place. In addition to capturing a perfect build (weight, polish, nose type, etc.), make sure you account for all the specifics of the launch, including baseline altitude, temp, wind, etc. If you are going for a specific altitude for a competition then the only way to get accuracy is to fly a lot of flights... use the data from previous flights to inform your adjustments for the next flight.
     
  9. Sep 10, 2019 #9

    rcktnut

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    .............and determining the closest/ best delay to use.
     
  10. Sep 10, 2019 #10

    Crayok

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    Hi All

    There is going to be many factors that need to be considered in this question and most errors are not the sim but the numbers put in;

    First rocksim has the ability to choose finish for each of the parts, most people seem to choose the polished or maybe gloss finish. I believe that is used to help determine Cd. Play with those parameters for the exterior surfaces and use the calculate Cd option to see if that gets you closer. Use the Cd analysis function (rocket tab/Cd Analysis ), to show you some of the complexity that RS considers.

    Second, under the prepare for launch function and launch conditions. Make sure you have these values correct Altitude, humidity temp and pressure. Setting I use for the Rock Lake site in Alberta are 3500 feet, 20% humidity, 25.999" in Hg for pressure. Using the actual numbers for flight conditions will be best. These numbers are used to determine air density which is in the Cd formula, one of the reasons aircraft on long haul fly high, less air density therefore less drag. Note that pressure is station pressure not the adjusted sea level pressure used by weather folks. As you gain altitude air pressure is less but to keep things consistent for consumers of weather info they adjust the pressure numbers for comparison to other areas. STP (standard temp and pressure) formula also is interesting in that high humidity conditions, air density is actually less than desert conditions. Temp also plays a big role. So much more to explore here.

    Third, is the motor actually performing as stated in the RS file? Remember that the motor burn characteristic is based on a few test motors on a test stand, maybe several years ago. has the manufacturing process changed? formula slightly adjusted? does the age of the motor change the thrust profile? With the Cd you calculated you can do some calculations with previous flight acell data from the flight computer to see if the motor file in RS is accurate or not.

    Forth, wind arcing will reduce altitude. Arcing will have a significant loss of altitude. again if you have acell data that will tell you the probable straight up flight but it will be a barometric data that is correct and generally, ok always lower, than the acell altitude.

    Fifth and one not really considered is rail drag, I can't tell you how much effect this has but it is there, flight observations is probably best to determine if this is big or not.

    Sixth is the altimeter and its accuracy. just the barometric types can have issues due to vent hole placement and even accuracy when temperature changes. as mentioned before accell types can have their own issue too, low sample rates being the biggest error maker, and as mentioned arcing will show higher altitudes than reality

    Hope this helps
    David
     
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  11. Sep 11, 2019 #11

    JohnCoker

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    In addition to all the caveats Crayok posted, don't forget that motors can have quite a variation from firing to firing.
    www.thrustcurve.org/certification.shtml

    But I agree that simulators are generally optimistic in terms of altitude.
     
  12. Sep 11, 2019 #12

    OverTheTop

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    Rocksim is generally optimistic for altitude, unless you start dialing in surface finishes and drag coefficients etc, like was mentioned earlier in the thread. Mostly I find the estimates reasonable, but I did have one flight that underestimated the altitude by about 1500'. I ended up with 21500', which was well above the simulations. Luckily it was well within the 120k' waiver ;).
     
  13. Sep 11, 2019 #13

    DAllen

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    I find the altitude inaccuracies that happen with RockSim and OpenRocket usually stem from not getting all the settings right. You have to go in and adjust all the weather conditions correct, fin cross section, also change the default finish to rough or unfinished or something like that. The CoD for polished seems rather unrealistic for the majority of rockets we build.

    Usually, what I do with a rocket is put it all in the sim as accurately as possible to make sure the CP/CG is correct and then go fly it. Get the altimeter altitude and compare to the sims. From there I'll adjust the CoD from there for all future flights and that usually works out pretty well.

    -Dave
     
  14. Sep 11, 2019 #14

    jderimig

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    The flaw in many of these accuracy discussions is assuming that your barometric altimeter altitude is accurate. Figure that error +/- 5% based on SAM error alone. If you fly in warm weather your altimeter will report a lower alt than real .
     
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  15. Sep 11, 2019 #15

    cerving

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    I think OR's value is primarily for determining stability, comparison of different motors, and getting a ballpark apogee figure (i.e. "8 or 10 thousand feet"). Like DAllen said, the accuracy is largely dependent on how much work you put into it... it agreed pretty closely with my L3 flight because I spent a lot of time with it and put in all the parameters (temperature, launch site altitude, CoD derived from test flights, etc.), but most of the time I'll just leave it mostly at the defaults knowing that it may be optimistic by as much as 20%.
     
  16. Sep 12, 2019 at 2:09 AM #16

    rewilfert

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    Thanks for all the feedback. Based on the max altitude attained in the simulation with the smaller motor, I decided to stop the path I was going down until I can fly the rocket on a larger motor. Then I at least have baseline flight data to work from. It may be possible to use the smaller motor, but I don't want to go that route right out of the gate.
     

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