Motors for Acme Spitfire

Discussion in 'Low Power Rocketry (LPR)' started by BDB, Aug 10, 2018 at 4:05 AM.

  1. Aug 10, 2018 at 4:05 AM #1

    BDB

    BDB

    BDB

    Absent Minded Professor

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    I’m starting to build an Acme Spitfire with my daughter and the first step is to instal the engine block. I know TRFers often say to not install the engine block so you can fly bigger motors if you so desire. But this seems like the kind of kit where everything is delicately balanced. Flis suggests a C11-3 or D12-3. Has anyone tried anything else?
     
  2. Aug 10, 2018 at 7:45 AM #2

    kuririn

    kuririn

    kuririn

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    If you build it stock, you can fly it with the C-11, D-12, and Aerotech 24/40 case and reloads ( E and F ), as well as most single use composite motors.
    You could fly it with an E motor with the motor hanging out the back, past the engine hook. You would need to tape the engine to prevent it from ejecting, and check for stability, since you are shifting the CG further rearward. Explained with pictures here:

    https://www.apogeerockets.com/education/downloads/Newsletter415.pdf

    If you change the engine hook and motor tube to accommodate the longer E-9 and E-12 black powder motors, you can still fly the aforementioned motors with a 1" orange adapter (included with many Estes kits), or cut a 1" section from a spent 24mm motor casing. You would need a 3.75" long hook, often called an "E" engine hook, and BT-50 tubing for the motor tube at least 3.75" long. And, of course, you would have to check for stability and possibly add more nose weight. If I'm using an engine hook I like to also glue in a thrust ring (engine block) above it so that the engine thrust on the front end of the hook does not put undue stress on the slit in the motor tube where the hook enters.

    Alternatively, you could ditch all engine hooks and install a screw on retainer. No thrust ring needed, however if flying BP motors you need to wrap a thin strip of masking tape to the end to act as an aft thrust block. Composite motors have a rear flange molded into the case.

    The ditching of the thrust ring is a more common practice in 29mm motor mounts, where engine casings have a greater diversity in length. A thrust ring will limit the length of engines that you can fly.

    When comtemplating using non-recommended motors, Open Rocket and RocSim are your friends. They allow you to simulate flying with these motors and what the result will be: speed off the launch rod, maximum apogee, when in the flight profile the ejection happens, success or crash. OR is free.
     
  3. Aug 10, 2018 at 11:51 AM #3

    BDB

    BDB

    BDB

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    Can I sim the Spitfire in Openrocket? I assumed that was beyond it’s capabilities.
     
  4. Aug 10, 2018 at 12:49 PM #4

    kuririn

    kuririn

    kuririn

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  5. Aug 10, 2018 at 2:33 PM #5

    Zeus-cat

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    I would recommend doubling up on the centering rings to give the body tubes something to stick to. There just isn't a lot of "meat" to glue to where the oddly shaped body tubes come together. Alternatively, you could also use small blocks of wood around the rings to give yourself more surface area to glue/epoxy to. That is what I did and it really helped.

    I would also recommend making your fins through the wall if possible. I didn't and have had to glue fins back on many times.
     
  6. Aug 10, 2018 at 2:36 PM #6

    neil_w

    neil_w

    neil_w

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    You could probably get a *very rough* estimate by putting in a straight body tube, and entering overrides for the weight and CG. It still won't be able to account for the extra drag from the crooked body, but you should at least be able to get a decent CP and therefore be able to monitor stability with the different motors.
     
  7. Aug 10, 2018 at 3:20 PM #7

    BDB

    BDB

    BDB

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    I should have thought to check RocketReviews before posting! That's exactly the information I was looking for. I don't have a 24 mm reloadable case, but think I'll build it so it can fly on single use E15 and E30 motors. They are some of my favorites in the LPR/MPR range.

    As for the .rkt file, Openrocket really isn't a fan of the fins over the transition. I'll have to work on adapting this file (or just rely on my trusty ole' mind sim).
    Screen Shot 2018-08-10 at 10.15.15 AM.png
     
  8. Aug 10, 2018 at 9:20 PM #8

    Mushtang

    Mushtang

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    I've built and flown two. I'd recommend doubling the transition as a "super shroud" instead of thru the wall fins. It seems like a much easier way to sturdy up the fins.

    I did this on my Beer Can version of the Spitfire:
    https://www.rocketryforum.com/threa...on-build-thread-finished.133424/#post-1573081


    Hcmbanjo came up with this idea and wrote it up in an article posted in the Apogee Peak Of Flight newsletter in 2013:
    https://www.apogeerockets.com/education/downloads/Newsletter349.pdf
     

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