OpenRocket: Someone Please Explain Engines...

Discussion in 'Rocketry Electronics and Software' started by Solomoriah, Oct 10, 2015.

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  1. Oct 10, 2015 #1

    Solomoriah

    Solomoriah

    Solomoriah

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    I've noticed in recent versions of OR that, on the "Select a rocket motor" dialog, there are sometimes engines that have multiple thrust curves. For instance, the Estes B4 engines have one that is red, and one that is blue.

    So where is it documented which engines have which curve??? That is, looking at a B4-4 engine from my stockpile, how do I know if it's the red one or the blue one?
     
  2. Oct 10, 2015 #2

    Rex R

    Rex R

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    well since the red curve uses data from the NAR db published in 2000 (and matches the Estes catalog info) I would use the red curve for the B4, no idea where the blue curve came from.
    Rex
     
  3. Oct 10, 2015 #3

    Solomoriah

    Solomoriah

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    I was hoping to hear where those curves came from. You've given me half the answer, anyway. Surely it's documented somewhere?
     
  4. Oct 11, 2015 #4

    wcravens

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    We'll have to look into the documentation situation. But here's a quick overview of how it works.

    We currently download all RASP (.eng) and RockSim Engine format (.rse) files from ThrustCurve (http://www.thrustcurve.org/) and create our motor database when we build OpenRocket. You can add your own files to the database by following these instructions: http://wiki.openrocket.info/FAQ#How_do_I_add_motors_to_the_database.3F. I have had to do this at least once because I had a motor with a delay that wasn't otherwise available.

    There are times when there is more than one datafile for a given engine. In these cases we provide all thrust curves to you and you can pick from them in the user interface.

    We recently opened a new development issue for the OR project about how we use data from ThrustCurves. You can follow along and provide your input at https://github.com/openrocket/openrocket/issues/246

    Wes
     
  5. Oct 11, 2015 #5

    Buckeye

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    You can never know for sure how your stockpile of motors will burn and how well they follow a published thrust curve. (The official NAR/Tripoli tests give some mean and standard deviation statistics). This is the biggest source of error in correlating sims to real flights.

    Thrustcurve is crowd-sourced. From the webpage:

    "motor data files can be contributed by anyone who has them. The data is no longer as "authoritative," but at least whatever is floating around can be available in one place."

    You can debate the pros and cons of this approach.

    As an aside, I was told by the the RASAero guys a few years ago that they investigate duplicate motor files and leave only the "best" one in the RASAero database.
     
  6. Oct 11, 2015 #6

    markkoelsch

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    As the editor of Thrustcurve I can explain a little. We take motor files from anyone. That being said, there are a small group of us that have done the vast bulk in the last 5-6 years. I can tell you that almost every motor I have created has come directly from the cert documents.
     
  7. Oct 11, 2015 #7

    Solomoriah

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    So I guess it's a user interface problem. There should be something more than color to distinguish these. It'd be nice to know where the curve is coming from in each case.
     
  8. Oct 12, 2015 #8

    wcravens

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  9. Oct 12, 2015 #9

    Solomoriah

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    The bug report is somewhat tangential to my issue, and honestly I'm not sure how to fix it. Probably because I have NO IDEA what "red" and "blue" means... so I don't know how to describe how it OUGHT to work.
     
  10. Oct 12, 2015 #10

    wcravens

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    Ok, That's fair. But just so you know, the ticket isn't really a bug report. It's a discussion around how we can improve the import, management and usage of the ThrustCurve data. I could try and add your thoughts myself but by you adding your comments it helps us to know where suggestions are coming from, i.e. the users, and it would help you track the progress of the discussion and ultimate solution.

    Wes
     
  11. Oct 12, 2015 #11

    kruland

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    red and blue are simply colors so you can see the different curves. When you select an individual curve (using the "select thrust curve" drop down on the top left), the metadata from the file is displayed in the "Show Details" tab. Some files have good information, some do not.

    The fact is the standards on motor variation are pretty weak and only regulate total impulse, burn time, and ejection delay. The actual curve itself has no standards what so ever. In general, it doesn't really matter which curve you select (provided that it isn't crazy wrong, then it should be reported to thrustcurve.org) because the motor you pull from your box will only be close anyway.

    Kevin
     
  12. Oct 12, 2015 #12

    Solomoriah

    Solomoriah

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    Okay, so you have to look at a different tab to figure out which curve you have. That's useful information. Lemme go try it...

    ... ah, so. I see how it works. The red curves for the B4 and C6 engines have their provenance given:

    Estes B4 RASP.ENG file made from NAR published data
    File produced October 3, 2000

    But the blue curves do not indicate their source. Would be nice if they did.

    I'm particularly surprised at the difference between the B4 curves... one is obviously stronger than the other.
     

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