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Tips for external recovery cords?

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brewster_rockit

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Lately I've been launching lots of minimum-diameter (BT-5, BT-20) models and noticing that the shock cords are burning through (especially when using sewing elastic vs rubber shock cords.) My chutes and streamers are so tightly packed that they don't deploy well either. I've been thinking about switching to external shock cords to create a bit more room for the streamer inside the model, reduce the chance of burning through the shock cord, and lower the model in a horizontal fashion to minimize damage.

There's not a lot of advice online for external recovery cords, besides epoxying the kevlar portion to a fin root and letting it hang unglued starting at the burnout CG of the model. I'm thinking the kevlar portion should end before reaching the rocket's separation point, and tie onto a piece of thin tubular sewing elastic. The sewing elastic would slide through a hole in the nose cone shoulder, and be long enough to stuff into the body tube without having to stretch. Are there any rules of thumb for designing an external recovery cord besides what I've surmised?
 

Micromeister

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Lately I've been launching lots of minimum-diameter (BT-5, BT-20) models and noticing that the shock cords are burning through (especially when using sewing elastic vs rubber shock cords.) My chutes and streamers are so tightly packed that they don't deploy well either. I've been thinking about switching to external shock cords to create a bit more room for the streamer inside the model, reduce the chance of burning through the shock cord, and lower the model in a horizontal fashion to minimize damage.

There's not a lot of advice online for external recovery cords, besides epoxying the kevlar portion to a fin root and letting it hang unglued starting at the burnout CG of the model. I'm thinking the kevlar portion should end before reaching the rocket's separation point, and tie onto a piece of thin tubular sewing elastic. The sewing elastic would slide through a hole in the nose cone shoulder, and be long enough to stuff into the body tube without having to stretch. Are there any rules of thumb for designing an external recovery cord besides what I've surmised?


Competition Model rocket flyers have been using external shocklines for a very long time. Generally we do a lariat loop around the motor casing, run the kevlar shock line up the external side of the model and into the body tube with either a slit in the balsa nose cone or file a channel in the styrene shoulder, the Chute, shroudes and remaining shockline (NO Elastic) has loosely packed on top of the ejection styrofoam plug with or without a 1/4 sheet of wp wadding. The only trick to this no elastic set-up is to have a long enough shock line for the NC to slow down before it hits the end of the kevlar line:) Over time we've found the 30 to 36" of 50-70lb kevlar is generally enough. I'll some times go with 48" of 50lb kevlar in SuperRoc models from the forward end of the body tube to ensure I've given plenty of shockline slack.

if you intend to epoxy the kevlar into the Fin/body joint it is advisable to drill a small hole in the aft end of the fin and thread the kevlar though this hole. A small overhand knot can be used but I've found over time the knot is not necessary. instead tack the end of the kevlar to the opposite side of the fin/body joint the fillet both sides with a small epoxy fillet.

I've used the same set up on much larger models using 1000lb kevlar shockline teathers with nylon Hemi chutes on several larger 3X Upscales. On These larger mass models the Kevlar enteres the forward end of the body tube where it is joined to a section of OVAL elastic or 1/4" elastic attached to the nose cone with a buttlerfly knot about a third of the way between NC and Body where the chute is attached with a HD snap swivel. Works like a champ.
below are a couple photos that may be of help.

As for the Ole "Plastic Wad" problem. Talc baby powder is your friend. Talc both sides of your chutes before folding. It is also a very good idea to "season" your plastic or mylar chutes by crumpling them into your hand several times to loosen the fabric a bit. I generally grab a shroud line corner, crumpling the chute into my palm, relax and flatten the canopy then repeat all the way around the chute twice. Then baby powder the heck out of the new chute and about every other launch there after.

640a-sm_TaperPaper 18-13mm PD&SD_03-16-00.jpg


641a-sm_TaperPaper 24-13mm PD&SD_03-16-00.jpg


617-sm_25A-B 105mm PD & SD_06-20-98.jpg


216-a-Mk4_MM .125A-PDa,b,c,f, Mk2,3 & 4 models_02-18-13.JPG


149d3_4D Ultra Orb-Trans_flt c&d Liftoffs pg_95&97.jpg


Crumpling-e_4 pic Page_05-15-04.jpg
 
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Woody's Workshop

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BT-5's are hard to get anything into.
The ones I have either glued the nose cone in and did a motor eject with tumble recovery...
or did away with the streamer and just used a really long kevlar string to the nose cone for tumble recovery.
Unless your landing in gravel, concrete or blacktop; there's not enough weight to damage anything.
BT-20's in the olden days use to cut couple slits 1/4" above one another in the body tube close to the nose cone, slip the shock cord from outside to inside to outside again, knot and glue.
This did away with folding tab mount and only 1/4" shock exposed inside the body tube, and you could put masking tape over it, or glue, or what ever you wanted to.
Kept the chute from hanging up on the mount too.
Look through some of the old kits PDF's "HERE" to get a visual of what I mean.
 
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