Are motor retainers required, or are built-on retainers suitable?

[nickrulercreator]

Building a large level-2 rocket, and I want to know if I need to install a retainer for the 54mm engine. My rocket also has 3, 38mm tubes around the 54mm tube, and if I am to use a retainer for the 54mm motor, then I couldn't use the 38mm tubes since the retainer overlaps them.
The reason I can't use tape is because the tubes are flush with the bottom centering ring (they don't stick out), so there's no surface to tape the motors too.

Thanks for any help, images attached.

 

[chris m]

Bolt washer and wing nut for all I will post pictures soon for you

 

[Steve Shannon]

Building a large level-2 rocket, and I want to know if I need to install a retainer for the 54mm engine. My rocket also has 3, 38mm tubes around the 54mm tube, and if I am to use a retainer for the 54mm motor, then I couldn't use the 38mm tubes since the retainer overlaps them.
The reason I can't use tape is because the tubes are flush with the bottom centering ring (they don't stick out), so there's no surface to tape the motors too.

Thanks for any help, images attached.

View attachment 309430View attachment 309431

You can still use tape on the cases of the motors to create friction. That method is usable for every size of rocket motor. Wrap painters tape around the motor case in two or three places, with at least one near the forward end of the case. It should be tight going in.

An even better way to retain the motors in your instance is to use mirror clips or Kaplow clips (some googling required).

In the future you might want to include the retention in the rocket's design before building.

Steve Shannon

 

[chris m]

same as poster above

 

[rstaff3]

Is that ring bonded or dry fit? My preference on that sized ring would be to install T-nuts from the inside, spaced so a washer will overlap the rim of the motors. I'd use three. If it's already bonded, maybe try threaded inserts to do the same thing.

Tape has been proven to work, but I choose more positive methods whenever possible.

 

[pyrobob]

I've used variations of all suggested methods for 54's and below and they work just fine.

 

[rstaff3]

I've used variations of all suggested methods for 54's and below and they work just fine.

Yup, seen the concept once and it is easy peasy to figure something out.

 

[KennB]

You've got some good options listed above.
Check, too, with the club you'll be flying with; the field in Maine that is home of MMMSC is a sod farm. We allow friction fit for BP motors as the paper tubes will not be a big problem for the mowing/harvesting equipment. However, positive retention is required for the plastic and metal cased composites as we don't want the rockets to spit those casings at ejection.

 

[manixFan]

Have you tried to fit the 54mm and 38mm with the rear closures installed? It looks like it might be a pretty tight fit between the closure rings depending on what motors you are using. There does not look like a lot of space between the central mount and the surrounding ones. Before going too far with a method other than friction fit I would test fit all 4 motors.

That said I have tried all the methods mentioned - clips, washers, and friction fit. All work great but I use friction fit the most. I have friction fit 3" motors in min diameter rockets with no issue. I use a spiral wraps of blue tape near the bottom of the motor with a couple of extra wraps right at the base. The spiral wrap is narrower at the front so it's easier to get the motor started in the mount and then it gets thicker as you get near the base. You can also slightly twist the motor along the direction of the spirals to help it go in. It should be very snug the last inch or two. In all my years I've never kicked out a motor case using friction fit.

Good luck, should be an interesting flight.


Tony

 

[blackjack2564]

Sheetrock screws and large fender washers. 3 sets, slightly offset. Each washer/screw will catch the 38 case & 54 case.

Cheap easy & functional works great.



Shown above with a 6in motor & 3 54 motors.....1/4lag bolt & fender washers.
I use same style on my 6in rocket with 54 center and 3 outer 38's. Plain ole sheet rock screw and fewer washers