HiFlyer XL Shock Cord Alternative

[vulpex_rocketry]

I was looking for a shock cord alternative as the mid power engine and heavier rocket make me uneasy with the Estes glued paper method. I was working on a bracket design that allows for easy shock cord replacement and think I have a good design but wanted to see other's thoughts as I am still very new to the rocketry world.

Any thoughts on this design? I mostly saw designs that attach to the motor mount in some way. This is suspended in a lower but similar section as the paper mount. it is chamfered on the bottom to prevent any turbulence from the ejection charge or hang up of wading, etc. It has two channels for use if no breaks or potential for a centered parachute line/dual parachute lines.

 

[SolarYellow]

I have done something very similar with the motor block in a BT-20 MD rocket and 100-lb Kevlar. Works great.

I would not place it in the path of the wadding. If the wadding has a tight enough fit in the tube to do its job, it will hang up on that ring. The ring needs to be below the wadding. You might even put a bar across the middle to support the wadding, acting as a "laundry shelf."

 

[Scott_650]

Looks viable, though attaching a Kevlar “leader” to the engine mount centering rings is fairly straightforward, you can even use a thin tube as a pass through (the typical source is the hollow plastic “stick” from a higher end cotton swab) with the attachment at the aft end of the mount so the Kevlar can be replaced as needed. Your printed ring looks workable as well and with a bit of dexterity the Kevlar could be replaced.

The trifold mount is really pretty good considering it’s simplicity - I built an Estes PSII Ventris completely per the instructions and the cardstock trifold mount has held up just fine. But it can cause the recovery gear to hang up inside the airframe.

 

[bjphoenix]

The instructions for older LOC kits did something somewhat similar to the Estes tri-fold but using epoxy. You could do something like that.

I like the loop around the motor mount with holes through the centering rings. It's easy to do and it's easy to replace if needed.

I switched to thin kevlar a few years ago and did some experiments. The paper tri-fold method will zipper if you load it enough. I found a method using cloth that works for me and is much stronger. The way I fold the cloth it doesn't zipper until you put a lot of load on it.

This thread has the results of my studies plus discussion of other methods.
https://www.rocketryforum.com/threads/shock-cord-mount-studies.167751/

 

[Ronz Rocketz]

The reason why I stay away from the tri-fold is that any slight restriction could possibly block the chute deployment in the BT60 given the right circumstances. I've had better success with tying the Kevlar to the inside MM. The only time this has failed me is when I used dog barf and the ejection burned through the Kevlar.

Some like to attach the elastic cord through the aft so the elastic is against the forward opening. I prefer to loop the Kevlar outside the tube and attach a swivel hook with heat shrink to prevent zippering.

Another alternative is to use a Qualman baffle with an eyelet hook to attach the cord. No wadding needed. Be sure to have 4" of gap between the ejection and baffle or the motor might pop out. I've also had the baffle almost pull out so lots of glue.

 

[n3tjm]

I have found the most reliable method of shock cord attachment is the tri-fold method. Easy to do, easy to replace if needed (which I find is rare), and does not pose all the issues that kelvar cord has (tangling, burning at the mm tube, less likely to have zippers.

 

[86mustang408w]

Kevlar to the top CR.