Minimum diameter motor retention

[Iamtannorv]

Hi all, I was looking at picking up one of wildman's minimum diameter blackhawk kits, specifically the 38mm, and wanted to ask for some tips/suggestions on motor retention. This is my first minimum diameter rocket, and second rocket overall, so I'm a little unsure of how to go about the process. I was looking into some of the motor mounts that are epoxied into the BT and screw into the fwd closure, however, if possible, I would like some type of motor retention that allows me to utilize motor eject as a backup, and it also seems like that would possibly limit the size of motors I would be able to fly. Another option I was looking into were the slimline retainers from GiantLeapRocketry and simply mounting one of those to the bottom of the BT like a standard motor retainer. Seeing as it is a minimum diameter kit, I would like to get good performance out of it, but I don't mind sacrificing a little performance for easy of use/construction. Any and all advice and tips are welcome!

 

[tim cubbedge]

I used the GLR slimline. I wasn’t worried about performance. Only issue I had it is slightly larger the the Black Hawk tubing so I had to shim it to fit

 

[David Schwantz]

Building a 29mm as we speak. Can still use motor eject with an internal retainer. I drill holes in the body to allow ejection gases through. But I will be using friction fit on this one along with aluminum tape around the thrust ring and BT joint.

 

[Jmhepworth]

I use aluminum tape.

 

[Iamtannorv]

Building a 29mm as we speak. Can still use motor eject with an internal retainer. I drill holes in the body to allow ejection gases through. But I will be using friction fit on this one along with aluminum tape around the thrust ring and BT joint.

I use aluminum tape.

When using the aluminum tape, do you just wrap it around the motor case and build up a layer until you have a snug fit? Does that work consistently with no issues?

 

[Iamtannorv]

I used the GLR slimline. I wasn’t worried about performance. Only issue I had it is slightly larger the the Black Hawk tubing so I had to shim it to fit

Did you just use normal wood shims to get a snug fit and then fill the gaps with epoxy or did you epoxy the shims to the BT and then the retainer to the shims?

 

[Buckeye]

I like to keep it simple for min diameter kits like the Wildman BH38. No extra hardware. Just tape and knots!

This for motor retention. I was afraid to do simple friction fit at first, but it works. The aluminum wrap around the outside is the key. Don't skip this part.

https://www.rocketryforum.com/threads/mach2.156841/post-1949748
This for shockcord anchor, if you use traditional Aerotech forward closures. Seriously. Simple knots work fine.

https://www.rocketryforum.com/threads/attach-harness-to-forward-closure-why-knot.159622/

 

[tim cubbedge]

Did you just use normal wood shims to get a snug fit and then fill the gaps with epoxy or did you epoxy the shims to the BT and then the retainer to the shims?

I used JB Weld and then shimmed it with toothpicks while setting.

 

[manixFan]

The key to friction fitting is to build up the thickness in several layers. I cut the masking tape so the leading edge forms a point - makes it easier to slide into the motor mount. The aft end is cut so it's wide to provide thickness. Put the longer strips down first, then shorter ones in between so the tape gets progressively thicker. You also want to wrap the tape about a 1/2 turn or so around the body tube, like a screw thread, and twist the motor as you push it in. It should require a fair amount of effort the last inch or so, (more for longer motors). If you pull straight back on the motor, you should not feel any movement. It may require that you push the motor out the front using a dowel when done properly, or wrapping masking tape around the closure and using wide mouth pliers to help twist it out.

I've been flying MD rockets for years and have used the above method without issue on up to 54mm motors. You do need to double check the fit if the motor/rocket have sat for a long time, or if the temperature conditions change.

Good luck,


Tony

 

[boomtube-mk2]

https://wildmanrocketry.com/collections/minimum-diameter

The one issue with these is that they cannot be used with motor-ejection.

 

[Titan II]

You can also use a set screw to eliminate the screw head. This is a Blackhawk 38 using a CTI motor.

 

[David Schwantz]

I use yellow masking tape on the inside and the aluminum on the outside around lower part of body tube and the motor thrust ring.

 

[teepot]

How are you guys attaching a shock cord in MD rockets? I'm doing a 29mm and then a 38mm. I was thinking of using a centering ring above the motor with the shock cord pinched between the BT and ring.

 

[Titan II]

How are you guys attaching a shock cord in MD rockets? I'm doing a 29mm and then a 38mm. I was thinking of using a centering ring above the motor with the shock cord pinched between the BT and ring.

https://www.rocketryforum.com/threads/38mm-minimum-diameter.59465/#post-613531

 

[Banzai88]

How are you guys attaching a shock cord in MD rockets? I'm doing a 29mm and then a 38mm. I was thinking of using a centering ring above the motor with the shock cord pinched between the BT and ring.

https://aeropack.net/webstore/products.asp?groupid=10

 

[teepot]

https://aeropack.net/webstore/products.asp?groupid=10

Thanks

 

[Banzai88]

Thanks

I actually secure mine in with screws so that I can remove it to replace the harness. Above it I glue in and additional 1 inch piece of coupler so that it always has a shoulder to index against as well.

 

[cwbullet]

I use aluminum tape.

Cheap, low tech, and it works.

 

[bjphoenix]

I think I've read about people embedding a piece of threaded rod in the fin fillet then using a clip over the end of the motor bolted onto this threaded rod.

 

[Jmhepworth]

When using the aluminum tape, do you just wrap it around the motor case and build up a layer until you have a snug fit? Does that work consistently with no issues?

On my minimum diameter rockets, I leave a half inch of body tube below the fins and wrap a single wrap of aluminum tape so it covers both the body tube and the motor.

 

[Iamtannorv]

On my minimum diameter rockets, I leave a half inch of body tube below the fins and wrap a single wrap of aluminum tape so it covers both the body tube and the motor.

Do you use any tape on the motor case itself or do you simply stick the motor in and tape the closure to the BT

 

[cautery]

Hmmmm... aluminum tape for motor retention? I don't think I could bring myself to it....

 

[David Schwantz]

I have used this on 54 mm before.

 

[Jmhepworth]

Do you use any tape on the motor case itself or do you simply stick the motor in and tape the closure to the BT

Nothing on the case. Just a wrap at the aft end of the rocket, securing both the body tube and the motor’s aft closure. It’s secure and it doesn’t take up space in the airframe.

 

[Jmhepworth]

Hmmmm... aluminum tape for motor retention? I don't think I could bring myself to it....

I didn’t either. CJ talked me into it. It works fine.