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  1. #1
    Join Date
    4th September 2009
    Hamilton, Ontario

    Canted thrust line in OR: BGM-71C TOW

    This is a project I've been contemplating for a while, and I finally said "enough!" and just started building. This thread is about making a semi-scale model of a BGM-71C ITOW.

    Click image for larger version. 

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    I've put this thread in the "Software" section because a major component of it to date has been the model I built in OpenRocket to prove that the idea has merit and to get an idea on stability/flight performance/timing for the model. (I've attached my OR model file in case anyone wants to check it out.) My plan is to launch on a composite Aerotech D13-7 or D24-7 and in-flight start a pair of D12-0's to simulate the "flight motor". My flight motors will be mounted at an angle, which of course is the root of the problem for the OR model: OR doesn't support "canted motors".

    Click image for larger version. 

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    I've built my OR model with motors installed offset to the correct location but pointed straight down knowing that this will give me an overly optimistic altitude and correspondingly long "optimal delay" predictions. I'd like to refine how I've used OR to better predict the max alt and ejection delay - does anyone have any suggestions on how to account for the angled flight motors within OR? The flight motors should be installed with an angle of ~25deg off the long axis of the model, so I'd thought about re-generating a custom D12 thrust curve with each data point reduced by (cos 25), but perhaps there's a more elegant way to get OR to do the work for me?

    BTW - as I'm building, I've also done up a bunch of drawings in AutoCAD to keep my thoughts straight - here's a little bit of what it looks like:

    Click image for larger version. 

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    I'd planned to eject the laundry using motor-ejection from the launch motor. (This is the reason for my interest in optimal ejection timing. I think I'm actually closer to 5 sec optimal delay even though OR suggests more than that.) The flight motors will be fired from an MT-4/onboard battery.

    More to follow!

    Attached Files Attached Files
    Kevin McLeod
    ...With enough thrust, lift is irrelevant!!!

  2. #2
    Join Date
    6th September 2009
    You are on the right track to adjust the motor thrust curve by cos 20. The eng and rse files are just text, and easy to edit. I am sure this is far easier that getting OR to perform an unnatural act.

  3. #3
    Join Date
    14th July 2015
    Randolph, NJ

  4. #4
    Join Date
    4th September 2009
    Hamilton, Ontario
    Thread continued in the MPR section.
    Spoiler alert: The first flight was awesome!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Kevin McLeod
    ...With enough thrust, lift is irrelevant!!!

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