Wednesday, May 20, 2009

Enhanced Motor Retention for Semroc Brighton

The Semroc SLS Brighton is a a very nice versatile kit. It has a motor mount system that can take a wide variety of single motor and cluster configurations. You can fly it on a cluster of four C motors (18mm), a cluster of three D-E motors (24mm), or single 29mm motor for flights up through H size motors. You can also fligh on a single 24mm motor using and included 24mm-29mm adaptor for the single 29mm motor mount.

The only major drawback to the stock design is the small engine hook (One typically used with 18mm motors) that is used to keep the motor mounts from ejecting. You have an overall very strong design of though-the-wall basswood fins and very strong tubes that when glued with epoxy make for a robust rocket that can be flown on motors up to high powered range. But then you haveonly the tiny little hook to hold in the removable motor mount. In the picture above you can see the small hook on the left side of th picture.

To compensate for this, I used Aves Fix-It epoxy putty to mount two threaded rods between the sustainer tube and the internal mount for holding the interchangeable motor mounts. I used one 3" 6/32 threaded rod cut in half. That made each rod 1.5" in length.

Shown above are the 3/16" One Hole Midget Cable Straps and wing nuts I used for the retainers. I was going to use these wing nuts as a "tool-less" design for ease out in the field. However, I will replace those with regular nuts to fly to cut down on the drag of the "wings" on the nuts. As you can see from the picture above, I also used the Fix-It epoxy putty for the fillets on the launch lugs and fins. I plan on fiberglassing the fins and lower body tube for added strength for high powered flights.

Hope this helps out some of you building this model. I had read several stories of high powereed motors being ejected and lost because of relying on the supplied hook. I would actually just have left it off to save on weight, if I had known I was going to add this modified retainer system.

2 comments:

Chris Andros said...

Hey, so I experienced the exact worry you have addressed here. I was making my first flight with my new Brighton, which they have since upgraded to having 2 retention clips, and on the ejection charge of the F50-4 motor,the entire motor came out. I had even taped the hooks, as I had heard this was a possible solution. Despite this, the hooks bent, and then ripped through the tape.

I was wondering exactly how you fit quarter inch threaded rod in between the body tube and the motor mount adapter. Looking at my Brighton, it looks like I have less than an eighth of an inch of space. Did you flex the body tube, mill down the threads, or did you have more space than me?

Thanks,
Chris

Black Belt Rocketeer said...

Hi Chris,
I actually looked at it again. I think the pictures in the post should show enough of what I did. Not sure I could really show more now that it is together.
But the main thing is that they are 3/16" diameter rods, not 1/4".
They just barely fit between the two walls, so I'm sure a 1/4" diameter rod would bend the weaker of the two tubes. It was a tight fit, but worked for me and I am able to change mounts fine.
If the 3/16" rods do not fit yours, it makes me wonder if they are using thicker tubes now? Mine are decent strength so I would be surprised unless their supplies changed.
And I highly recommend the Aves epoxy putties for jobs like this. Worked great.
I haven't had any issues since this mod.
Good luck and maybe post back as to how yours turns out. I am curious if anything besides the added hook of their changed. I always recommend this rocket to people and I am partial to it because I used it for my Level 1 cert.