Question about motor mounts

Discussion in 'Beginners & Educational Programs' started by veebirdman, Jan 9, 2020.

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  1. Jan 9, 2020 #1

    veebirdman

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    So when assembling a motor mount with a retainer clip do you put a ring inside the mount in front of the clip? Some people do it, and some I don't see mention it. Also I have these thin plastic rings for motor mounts from the designer special, what do you use them for?
     
  2. Jan 9, 2020 #2

    neil_w

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    A motor hook is pretty much always paired with an engine block in front of it.

    Larger rockets (and some smaller) use different types of motor retention, often but not always some type of screw-on. Those rockets may or may not use an engine block, depending on the type of motors that are planned for use. Composite motors do not need an engine block in front, as a rule.

    The thin plastic rings are typically slipped over the hook, to hold them in place. Scratchbuilders probably use black electrical tape or the like for that purpose more often, but those rings are fine.
     
  3. Jan 9, 2020 #3

    kuririn

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    Yes, I put a thrust ring in front of the engine hook. Reason is the front tab of the hook will concentrate all the thrust of the engine in one spot on the body tube. That can lead to tearing of the motor tube in that spot.The ring will distribute that thrust over the entire interior wall of the motor tube.
    Are the rings you are referring to the thin white mylar rings? Those are typically used as engine hook retainers, in lieu of tape.
    EDIT: Looks like Neil was quicker on the draw than me.
     
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  4. Jan 9, 2020 #4

    prfesser

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    I've wondered about that. If the motor hook was simply taped in place, I could see the potential for a torn MMT. But it seems to me that if the motor hook is installed properly -- bead of glue on the top inch or so, and mylar ring or several wraps of strapping tape or the like, to hold the hook firmly in place -- the force on the forward end of the hook isn't localized at the slot that was cut in the tube, but is distributed over the entire tube (mainly the circumference covered by the mylar ring).

    And the maximum thrust of a model rocket BP motor is about eight pounds; it's not like there's a tremendous force on the forward end of the hook.

    Have there been many incidents of a torn motor tube from not having a thrust ring? If so, how many of those had the motor hook installed properly? (Inquiring minds want to know! :))

    Best -- Terry
     
  5. Jan 9, 2020 #5

    neil_w

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    I wonder how many mounts have been built without one to even have a decent sample size.

    Once you have the hook in there, though, there is literally no reason *not* to have a thrust ring. The hook will already prevent larger motors from going in there. I would not want all the motor's force concentrated on that little tab of metal, even though in many cases it would probably work OK.
     
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  6. Jan 9, 2020 #6

    kuririn

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    Hey prof.,
    First, congratulations on having your book "Experimental Composite Propellant" listed as a new product at Apogee.
    I know some kit instructions have you glue a thrust ring ahead of the engine hook and some don't. I can only speak from my personal experiences.
    I have had the front tab of the engine hook slide under the tube, even with a dab of glue at the front. I also have had some tearing. Not visible, but apparent because of the sliding of the engine hook in the motor tube after a launch. Don't know how common this is but I consider using a 15 or 20 cent thrust ring to prevent this as cheap insurance.
    As for the mylar retainers, installation typically involves sliding the ring onto the hook and wicking thin CA around the ring. I can see where this will bond the ring to the motor tube and hold down the hook. Not sure how well it bonds the hook to the mylar.
    EDIT: Plus, sometimes I like to throw in a D24 instead of an A to surprise my fellow club members.;)
     
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  7. Jan 10, 2020 #7

    ebruce1361

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    Even with kits that do not include a thrust ring, I feel better putting one in anyway like kuririn said as cheap insurance because some of my older rockets from my earlier days have sliding motor hooks. This is one of the reasons I like to hold on to at least one or two used BP motor casings after every launch because I can cut them into thrust rings. I have even drilled out a used 24mm motor and made it into an adapter that holds an 18mm motor perfectly. It's great for gusty days when a D12 might be too much, and I have no C11s on hand.
     
  8. Jan 14, 2020 #8

    BABAR

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    Agree a thrust ring inside the motor tube is cheap insurance. Even if the motor thrust doesn't deform the forward metal tab, repeated exuberant motor insertions over time might.

    I have not used a "dedicated" thrust ring for over a decade. I cut a 1/4" piece of body tube same diameter as the mount, cut a small chunk out of it (just enough so it rolls up inside the motor mount tube), and shove in place with white glue (paper to paper, less likely to "grab", doesn't retract) with an expanded motor casing to appropriate position just forward to where the forward metal tab will be (if I use a clip--- most of mine are friction fit with a piece of external tape over tail of rocket and external part of motor casing.)

    I only fly low power or rarely midpower
    I have yet to have a failure of these self made rings. I HAVE had clip failures, but these were always retention failures where motor ejected out the BACK. I now add an external tape strip for insurance even when I DO use a clip
     
  9. Jan 15, 2020 #9

    Tyler P

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    I always install the rings ahead of the hook. The hook itself is more so there to keep the motor in the rocket on parachute deployment, not so much to keep it from thrusting through the body tube. That's really the job of the engine block/ring. Not that the hook top isn't capable of holding the motor in, but like others have said, the motor tubes are generally a little flimsy and will tear over time and repeated launches if you don't have the block in place.

    On rockets flying on 24mm motors, I've taken to installing the Estes screw-on retainers. After a few launches where the motor was ejected past the retaining hook, I decided I liked the more positive retention the screw-on type afford. Not that I'm particularly worried about being hit by a not-so-heavy cardboard engine tube, but they can potentially start fires if they fall in dry grass or the like. Another benefit of keeping the motor in the rocket is keeping the weight in while the drifting on the chute. The particular rocket in question that I was having issues with ejected motors was quite light, so sending it up on an E and then not having the weight of the motor made it drift way farther.
     
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  10. Feb 2, 2020 #10

    BABAR

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    Actually biggest problem if an engine hook fails and the motor ejects is that it is often accompanied by failure to push the laundry out the front
     
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  11. Feb 2, 2020 #11

    gna

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    Two of the first rockets I built were the Semroc Vega and the Estes Alpha, neither one of which used an engine block ahead of the hook. Somehow it came loose on my Alpha after a few years:



    The Vega also puts a loop of kevlar under the front end of the hook to anchor the shock cord. I copied that design on some of my first scratch builds, and on one it ripped the engine tube a little. I was able to insert an engine block from above on a dowel and glue it in place. I always include an engine block now.
     
    Last edited: Feb 2, 2020
  12. Feb 13, 2020 #12

    Back_at_it

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    I've gone back and fitted every one of my rockets with an engine block. After returning to the hobby last year I ran into an issue with one of my old builds from 25 yrs ago. It was a Custom Rockets Brand flying on a D12-3. On the 2nd flight the engine hook broke and the motor exited the top of the rocket and flew around erratically.

    Upon inspection I found that the small tab at the top of the hook broke off. The rest of the hook was still in place. I pulled it out the bottom and replaced it with an engine block.

    If you currently have rockets without an engine block you can cut about 1/4" piece out of the ring and slide it up into position around the end of the tab.
     
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  13. Feb 20, 2020 at 10:01 PM #13

    MIFFRocket

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    So what I'm gathering from this is that I won't need a block to run 24 or 29mm composite engines? I just got a couple of Mach 1 kits and I noticed that there was no retainer clips or motor blocks. So I'm guessing I don't need a block, but what about retainers? Anyone have any suggestions on retainers??
     
  14. Feb 20, 2020 at 10:44 PM #14

    rockladen

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    Retainers at: aeropack.net
     
  15. Feb 21, 2020 at 12:58 AM #15

    tOD

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    I've built three kits from Mach 1 and used Aeropack retainers on all of them.
     

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