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HPR Shock Cord Setup

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ActingLikeAKid

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Building a scratch-built rocket from BlueTube. Total weight is going to come in at just under 1 kg. Length is ~130cm. 38mm mount. I'm planning on (unless maybe I try dual deploy in the future, and I'm building it so that's a possibility) doing a Jolly Logic Chute Release. I was thinking about shock cord setups, and here's what I'm thinking:

The break in the rocket (Is there a term for this?) is about halfway up, so the "nose cone" section is almost as long as the bottom. To avoid things banging together, I'm thinking of having about 6 feet of shock cord, then the chute, then another 2 feet of shock cord to the upper body section. Obviously when it's free falling before the CR pops, it's just going to tumble and do its thing. But once the CR opens, I don't want the two halves of the rocket banging into each other, hence the uneven cords. In my head, as it's descending, this would look like an old grandfather clock with one weight down and one weight up (if that makes sense) ...

Further details: A long loop of 1/4" tubular Kevlar with both ends "capsuled" along the mmt and the top of the loop just below the top of the lower body tube. A 6 foot section of 1/2" tubular nylon is attached to the Kevlar at one end and a quick link with the chute at the other. Also attached to the quick link is a 2 foot section of nylon that goes to the nose cone.

Any comments/thoughts/suggestions on this?
 

Rex R

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you'll want the shock cord to the upper section longer than the shroud lines on the chute. if it were me, I would go with about 15' between sections.
Rex
 

Handeman

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I would recommend the Kevlar loop extend about 3 inches above the BT. Then sew or whip the Kevlar about 2 inches from the end to form a loop. I've been doing this and it works very well. Tends to keep the Kevlar loop centered in the BT, at least in one direction. It also makes it easier to attach a swivel or quick link and the extra length keeps the metal parts from direct contact with the top edge of the BT.

I would also recommend a swivel between the Kevlar and the nylon shock cord. When DD rockets drop, it's the fin can that spins a lot. I had one spin so bad before I added the swivel that it twisted the shock cord so bad it wrapped the drogue chute up in the twisted shock cord. Once the main opens everything tends to hang straight below the chute and spinning stops. I assume a motor eject with a chute release would do the same. I'll be finding out this weekend at BattlePark. I just got my new JL CR.
 

Bat-mite

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Rule of thumb is 4-to-5 times the rocket length, so I'd go with 16' - 20' of harness.
 

ActingLikeAKid

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Rule of thumb is 4-to-5 times the rocket length, so I'd go with 16' - 20' of harness.
Even with just motor ejection? I guess I could get a 10-15 foot piece to go from the kevlar to the chute, then add the 6 feet I have from chute to nose...
 

Bat-mite

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Yeah, because if the cord is too short and the NC snaps back and hits the booster, you can still get damage, tangling, etc.
 

NateLowrie

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Even with just motor ejection? I guess I could get a 10-15 foot piece to go from the kevlar to the chute, then add the 6 feet I have from chute to nose...
I agree with John's assessment. At least 15' with 20'-25' being preferable. The only reason to go less is if you are build a high performance bird.
 

Zeus-cat

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First thing I thought when I read your post was that you need more shock cord. Seems everyone else agrees.
 

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