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Shock Cord Mount Repair Solution
#1
Hi Eveyone,

When I build new rockets I always use a motor mount-attached Kevlar shock cord, however I have some old rockets that I built or were built by other people that have the conventional rubber or elastic cords glued into a teabag mount. When (not if) these cords break, they always break at the mount, where they receive the most heat from ejection. The only repair possible is to remove as much of the old teabag as possible and glue in a new cord and mount.

My repair solution is a hybrid: I make a "shopping bag" with a Kevlar loop to replace the old teabag mount, and then loop the new shock cord to that. So far, the cords are lasting longer.


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Living life dangerously...launching C's on a B field.
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#2
(10-03-2015, 01:05 AM)Joe Shockcord Wrote: ...My repair solution is a hybrid: I make a "shopping bag" with a Kevlar loop to replace the old teabag mount, and then loop the new shock cord to that. So far, the cords are lasting longer.


Good looking idea.  I'm rebuilding the 'launch probe' of my TIE Fighter and will likely do this.
Phill Ash
Secretary - SouthEastern Virginia Rocketry Association (SEVRA)
2015 Tot Impulse: 1398.8 Ns (L2, 45 Flights, 57 Motors)
2016 Tot Impulse: 190.3 Ns (L2, 14 Flights, 16 Motors)




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#3
Yeah, I think I'll try that too. Thanks!
Member of MDRA, NAR and NARHAMS;
Level-2 certified but mostly fly G and under;
Volunteer compiler of manufacturer's news for ROCKETS Magazine.
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#4
Hmm, it doesn't look to me as though the elastic would be much further from the heat this way. So if they're lasting longer, I'm curious as to why. You're not using glue on the elastic. That might be part of it; glue could make the elastic brittle.
Rich Holmes
Camillus, NY
Secretary / newsletter editor
Syracuse Rocket Club

http://richsrockets.wordpress.com
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#5
(10-03-2015, 01:50 PM)Rich Holmes Wrote: Hmm, it doesn't look to me as though the elastic would be much further from the heat this way. So if they're lasting longer, I'm curious as to why. You're not using glue on the elastic. That might be part of it; glue could make the elastic brittle.

Well, like I said, this is a repair technique that I use on rockets that were built with a teabag. Glue on the elastic could be an issue, and if so, the Kevlar loop allows you to eliminate any chance of that. I agree that the elastic is only moved a little bit farther away from the heat and therefore the improvement is marginal (the elastic will break again eventually), but it is just enough—in my experience so far—to slow down that process. Also, replacement of the elastic is easier and quicker with the loop than gluing in another teabag.
Living life dangerously...launching C's on a B field.
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#6
(10-03-2015, 01:05 AM)Joe Shockcord Wrote: Hi Eveyone,

When I build new rockets I always use a motor mount-attached Kevlar shock cord, however I have some old rockets that I built or were built by other people that have the conventional rubber or elastic cords glued into a teabag mount. When (not if) these cords break, they always break at the mount, where they receive the most heat from ejection. The only repair possible is to remove as much of the old teabag as possible and glue in a new cord and mount.

My repair solution is a hybrid: I make a "shopping bag" with a Kevlar loop to replace the old teabag mount, and then loop the new shock cord to that. So far, the cords are lasting longer.

I suppose that is why they call you "Joe Shockcord" ...   Smile 

I do the same thing.  I've found that the old "teabag" mounts will usually peel up OK.  On all new model rocket builds, I go down to the motor mount so nothing is glued on the inside of the body tube to possibly cause a hangup of the streamer or chute. I throw all Estes shock cords away, and just use elastic purchased at the fabric store.  Its the only time I am willing to go into JoAnn's with my wife, without throwing a temper tantrum ....
Lee
NAR 55948, L2
In Build: Mars Snooper
Next: upscale Sky Hook (Panavia kitbash)
In Finishing:  Estes Honest John, Red Nova, Super Cobra
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#7
I've mounted a kevlar lead in epoxy clay, and worked that onto the top CR, or, even, along the inside of the airframe where the tri-fold kind of mounts would go. It has worked well... as another option.


Later!

--Coop
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#8
I just received FOUR low power rockets from an aging Rocketeer who is cleaning out his fleet at the request of his wife. As a result, I have a Big Bertha (original) and three other units that have high quality enamel paint jobs. I was told they will need a little TLC and began to inspect them. One, has no motor retaining clip.... so I supposed that it will require a wrap or two of tape for a friction fit motor.

But the reason why I post, is the condition of one of the shock cords. Contrary to an earlier allegation that they ALWAYS break at the tea-bag attachment, this one appears to have a clean break about 6 inches away from the blown plastic nosecone...and the rest of the shock cord appears to be an elastic band (similar to Jo-Anne fabric band) that is just a little dried out, and about two feet long.

How would you approach repair/replacement of this shock cord?
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#9
(10-19-2015, 05:19 PM)Kirk G Wrote: How would you approach repair/replacement of this shock cord?

If there's enough of the original shock cord (and it seems to be in good condition) you could just tie another onto it to lengthen it.
Living life dangerously...launching C's on a B field.
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#10
(10-19-2015, 05:19 PM)Kirk G Wrote: How would you approach repair/replacement of this shock cord?

Kirk,

I would peel out the old mount, and would not trust that shock cord.  Start with something like a knife or really small screwdriver to get under an edge of the old mount and work the old mount free.  If a bit of the tube peels up, just make sure it gets glued down when you glue in the shock cord mount.

For smaller diameter tubes like BT-50, I've cut a piece of notecard/card stock, curled it to lay against the inside of the tube, and glued in a Kevlar loop while gluing into the body tube, for attaching your favorite type of shock cord.  These mounts lay much flatter against the tube than the "teabag" style, so there is less chance of hang up.
Lee
NAR 55948, L2
In Build: Mars Snooper
Next: upscale Sky Hook (Panavia kitbash)
In Finishing:  Estes Honest John, Red Nova, Super Cobra
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