Open Rocket Atmospheric Model ???

Discussion in 'Rocketry Electronics and Software' started by Buckeye, Jan 1, 2020.

Help Support The Rocketry Forum by donating:

  1. Jan 1, 2020 #1

    Buckeye

    Buckeye

    Buckeye

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    2,223
    Likes Received:
    163
    Can anybody explain how Open Rocket handles the user entries for temperature, pressure, and ground altitude? I am seeing some very odd behavior. First, I ran some tests by varying these values:
    Capture12.PNG
    Sims 1 to 6 are at 0 ground altitude (sea level), and all seem to make sense. Sim 1 uses the check box for International Standard Atmosphere. In Sim 2 , I unchecked the box and manually input the standard sea level values of 15 C and 101325 Pa. The results are the same, as they should be. Check.

    Sims 7 to 12 repeat the temp and pressure variation, but at 1600 m (1 mile) elevation. These results with the ISA model look good, but the rest are wonky. How can the flights at 1600 m now be even less altitude than their sea level counterparts???

    I exported some results for a closer look at the air density. Oddly, of all the variables written by OR, density is not one of them???? Anyway, in Sim 7, T and p on the ground are calculated more or less correctly using the ISA model at 1600m:
    Capture11.PNG
    However, Sim 8 values for T and p on the ground make no sense when manually inputing the standard values of 15 C and 101325 Pa:
    Capture10.PNG
    There seems to be some weird interaction of elevation and the atmospheric model in OR. I think something is wrong, here. This can be a big problem, especially for high altitude simulations.

    Any OR programmers out there who can explain?
     
  2. Jan 2, 2020 #2

    SpaceManMat

    SpaceManMat

    SpaceManMat

    Space Nut

    Joined:
    Dec 20, 2013
    Messages:
    635
    Likes Received:
    40
    Its fairly trivial to get OR to report Air Density.

    Download modified version of OR here:
    https://drive.google.com/open?id=1mWNN3E4C7I9NtPOgUD3KTlaVho76yS_l
    Extract to your desktop and run

    Note I've created a short cut that is required for running OR on my Win 10 machine, not sure how much of an issue you have running OR on your machine.
     
  3. Jan 3, 2020 #3

    Buckeye

    Buckeye

    Buckeye

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    2,223
    Likes Received:
    163
    Thanks for the link. One can also get OR's density from the output of drag or ideal gas law.

    Anyway, T and P vs. elevation looks seriously messed up in OR. If you are trying to be accurate with your sims based on your launch location and weather conditions, you may be doing more harm than good.
     
    Last edited: Jan 4, 2020
  4. Jan 3, 2020 #4

    SpaceManMat

    SpaceManMat

    SpaceManMat

    Space Nut

    Joined:
    Dec 20, 2013
    Messages:
    635
    Likes Received:
    40
    Can you please post some screen shots showing what’s going on and what you expect? As well as your sim?
     
  5. Jan 4, 2020 #5

    Buckeye

    Buckeye

    Buckeye

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    2,223
    Likes Received:
    163
    Yeah, sorry, ork file attached. The name of each simulation is what I changed in the settings for my study. The screenshots are in Post #1. I highlighted in red the results that make sense, and results that do not.

    The detailed spreadsheet data comes from OR's export function.

    FYI, I am doing a similar test with RockSim, and T and P vs. elevation looks much better to me.
     

    Attached Files:

Share This Page