Those pesky shock cords.

[Back_at_it]

I don't build anything smaller than BT20 anymore but even those are a bit of a PITA to get a shock cord into. I've taken to using Kevlar line in all of the BT20 and BT50 stuff. For BT20 using 13mm mini motors, I simply tie a length of Kevlar line around the upper motor mount ring and run it up the body tube.

For BT20 rockets that are flying on 18mm motors I'll add a second thrust ring with the Kevlar tied to it just ahead of the motors thrust ring so there are two of them. One about 3 inches up the tube (Further up if there is room) with the Kevlar tied to it and then the normal thrust ring that keeps the motor in place.

A good example of this is the mini mean machine which uses BT20. I ditched the stock 13mm motor mount for a thrust ring to run 18mm motors. Since I had the length to do so, I decided that I didn't want the Kevlar right at the ejection point so I added a second thrust right just above the body tube coupler with the Kevlar line attached to that.

When building BT50 rockets I will do the same thing but I will add a length of 1/8 elastic to the end of the Kevlar to act as an actual shock cord. In every instance where Kevlar is used. It's a good idea to run a little thin CA around the inside of the body tube and allow that to dry to help aviod zippering.

 

[Retrospace101]

Usually with BT-50 or Bt-50< size tubes you can try putting a long kevlar cord on the MMT between the Centering Ring and the Motor tube ( Make sure you have the END ( 1/2"-1") of it attached to the MMT, the rest can be exposed), then you can use the rest of the cord for your recovery, but if you wanna take it one step further you could attach a small Kevlar cord instead and loop it, then secure the loop with a heat sensitive sleeve, electrical tape, epoxy, or something and tie another shock cord to the loop, but I recommend this step to BT-80 and BT-80<.

 

[Knuckledragger]

Would it be possible to notch both centering rings on the far outside edge, loop a strand of kevlar around the engine mount on the outside, run the kevlar up through the notches and in to the body tube, put a piece of elastic on the far end and connect the recovery device to that? The wadding would give the elastic some protection. That way if the kevlar is looking questionable, you remove the old and drop a new one in. PTFE tubing from the RC hobby stores is $2 for 24" and has some heat resistance too; if theres room enough to run that up the inside to the bottom of the NC and thread the kevlar through it.

 

[ebruce1361]

@Knuckledragger That's a pretty good idea! Lining up a fresh piece of kevlar cord through both centering rings might be tricky, but could be done with a thin wire or some other kind of rigid guide. An added bonus I see to that setup is the holes/notches in the centering rings would double as vent holes to allow for pressure equalization in the airframe. The main concern with that setup is to make sure the kevlar cord is securely attached to the motor mount and doesn't come untied or slip off. A piece of masking tape and a drop of glue on the knot would probably be sufficient to prevent those problems, but some people might not like it due to the aesthetic effect on the tail, but that's not a big deal for me. The only situation where I can see this not being a feasible solution is with rockets that have a boattail that is flush with the motor tube.

 

[hcmbanjo]

I came up with a replaceable Kevlar line engine mount in 2013.
Here's the article from the Apogee Peak Of Flight Newsletter:
BT-50, 55 and 60 mounts:
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
BT-20 and ST-7 mounts:
https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

My Odd'l Rockets product line produces a HDEM (Heavy Duty Engine Mount) kit
with the replaceable Kevlar feature, available from the major vendors.

 

[mbeels]

My Odd'l Rockets product line produces a HDEM (Heavy Duty Engine Mount) kit
with the replaceable Kevlar feature, available from the major vendors.

That's a nice mount.

 

[ebruce1361]

@hcmbanjo
I like the addition of the plastic swab stick into the mount. That is considerably better than using a wire or some other item to guide the shock cord through the notches in both centering rings like I was originally thinking. I'll likely incorporate your method into my next scratch build!