upgrading to kevlar

When I upgrade a shock cord to kevlar on a mid power rocket, what is typical for kevlar length compared to elastic? One to one, shorten, lengthen?

Thanks!

Lengthen. A lot. Kevlar has 0 stretch. Period.

blackbrandt said:

Lengthen. A lot. Kevlar has 0 stretch. Period.

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Right now I am looking at a 29mm motor in a 2 inch body, 12oz total and it came with an 8 foot shock cord.

Time for 12 feet? 16?

Mine are a little longer. You can use masking tape to absorb some shock. Do not trust the plastic nose cones' loops like on loc cones.

Kevlar with reefing. Reefing is taking several loops of shock cord and taping them together with electrical tape. The loops are 4-6 inches each (depending on rocket size) and three to 6 loops per bundle, typically, and are z-folded before placing at least two wraps of tape around the middle. This is your shock absorber. The Kevlar will not stretch. The shock after separation will be dissipated as the reefs are ripped apart. Without the reefs, all the shock will be on your eye bolts. I've straightened out un-forged eye bolts on even 54mm diameter fiberglass rockets by forgetting to reef.

Thanks, but still looking for a length recommendation. How much kevlar should I use for my 2 inch wide 12oz rocket, possibly up to H motors?

I would use 8 -12 feet with a foam ball to prevent zippers.

Net 16....add another 1-2ft. depending on what type knots you use.

A loop & knot can use easily a foot @.

Super long is super cool. Kinda depends of what kind of mid power rocket it is, flying conditions and the motor and delay you are using. I have seen some cool and bad dudes crocheting their kevlar to fit in with their big foam balls:y:

cwbullet said:

I would use 8 -12 feet with a foam ball to prevent zippers.

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How does the foam ball work? I am imagining it attached to the cord and able to slide to nearly the end of the airframe split, but not all the way?

A few things to consider - elastic SC's are typically long - the reason being, that stretchy thing. If you separate with enough force that that band is fully stretched, you could have the NC/Payload bay snap back and damage the rocket - therefore, they are long.

Also, the motor choice comes into play, but not in motor class, but the size of the ejection charge. If, for example, you are flying 29-40/120 reloads, you can adjust the amount of BP in the charge (the reload kits come with .7gms of BP, IIRC). If you don't have much space to pressurize, you can use far less than .7gms, and the ejection is far less "energenic" (or in reality, not "overly energenic"). You really just need to separate the parts, and not blast the NC to the next county!

watermelonman said:

How does the foam ball work? I am imagining it attached to the cord and able to slide to nearly the end of the airframe split, but not all the way?

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I've used rubber balls and tennis balls. You poke a hole in the ball and threat the tether/SC thru it, and when prepping, line up the ball so it is near the top of the BT when the cord is extended. That's a good way to avoid zippers. The ball keeps the kevlar from cutting into the open end of the BT

And do you have a problem with those types of ejection charges?

RecoRocket said:

I've used rubber balls and tennis balls. You poke a hole in the ball and threat the tether/SC thru it, and when prepping, line up the ball so it is near the top of the BT when the cord is extended. That's a good way to avoid zippers. The ball keeps the kevlar from cutting into the open end of the BT

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Do you somehow attach so it is at a fixed position along the cord, when the cord is fully extended?

watermelonman said:

Do you somehow attach so it is at a fixed position along the cord, when the cord is fully extended?

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Friction has always worked for me. But you can tie a knot on the rear end of the SC to prevent the ball from sliding south. And remember, you don't want the ball to burn from the ejection charge.

blackbrandt said:

And do you have a problem with those types of ejection charges?

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This is rocket science, not fireworks..