Shock cord-shock absorber cord

little confused on shock cord use.To me a "Shock cord" is a shock absorber. h: Elastic. Now they have kevlyr cords that have no absorbtion to them. I see this as a reason for so many "zippers" I have a Q "Superbird" that has a kevlyr cord on BT for chute and a small elastic cord on NC for NC chute.Why not just elastic on both? Its hard for me to post what Im trying to get at.Anywhoo- if anyone can catch my dirft on this,could you please inlighten me on use of Shock cord And shock absorber cord.:confused2::confused2: Scotty Dog

Not sure I can answer your question about your particular model but can maybe add a little light on the theory behind the lack of elastic.

Until ejection our models are traveling (coasting) at a pretty high rate of speed. and the airframe is in or should be in fairly aerodynamic shape...Pointy end forward
At ejection we "briefly" accelerate the nosecone and laundry while the momentium of our coasting model decelerates rapidly with the in rush of air into the forward end and the trubulance caused by all that ejecting material, misaligned parts etc which slows the body pretty quickly while the NC continues away but is also now slowing by air friction. The trick is to have enough slack in the Shockline to allow both bodies to continue to seperate while moving forward until they either stop completely before falling out of the sky or the recovery system stops their forward motion at deployment. If we have enought line to allow this to happen before the NC hits the end of the length of attached line NO real Shock is felt. I'm sure you've seen some of the what looks like a mile long shock line coming out of some of the bigger models. this allows air friction to slow the NC down before its weight is thurst against the slow moving attachment to the main body.
If you have a nice long Kevlar shockline you should only need a small elastic loop between NC and Chute to absorb the shock of the Chute opening againt the attachment to the NC.
Hope that helps a little.

In larger rockets, I tend to call the non elastic cord a 'recovery harness'.

Scotty Dog said:

little confused on shock cord use.To me a "Shock cord" is a shock absorber. h: Elastic. Now they have kevlyr cords that have no absorbtion to them. I see this as a reason for so many "zippers" I have a Q "Superbird" that has a kevlyr cord on BT for chute and a small elastic cord on NC for NC chute.Why not just elastic on both? Its hard for me to post what Im trying to get at.Anywhoo- if anyone can catch my dirft on this,could you please inlighten me on use of Shock cord And shock absorber cord.:confused2::confused2: Scotty Dog

Click to expand...

I agree. Elastic has a damping effect. It slows the relative motion between the rocket bits without the jerk. Unfortunately, it stores that energy, and then imparts it back into the system thus bringing the bits crashing back together.

Instead, you want something to absorb the shock without storing the energy. It needs to dissipate it rather than store it.

I prefer to use kevlar because it's very durable, especially compared to elastic. It takes the heat well. To avoid the jerk, I form loops in the cord and apply tape wraps. When the chute opens and tries to jerk on the rocket, the tape gets torn thus dissipating the energy without storing it.



But the kevlar cord is still prone to zippering the airframe. So I add some ribbon at the opening to guard against that. In the pic below, you can see some kevlar fabric I got from Quickburst. I folded it double and use it as a flame proof ribbon to spread any forces and avoid cutting the airframe.



By using a combination of cord-ribbon-cord, I minimize the volume taken up in the ariframe, and also minimize the cost (versus all kevlar ribbon).

HTH.

Doug

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In smaller rockets (C or lower), you can use either elastic or kevlar cord, or a combination of the two. If I need a bit of "cushion" to the shock cord, I will create a loop in the kevlar and tie a bit of elastic in parallel to it, with a some slack in the kevlar cord. This creates a "shock absorber" as the kevlar stretches the elastic as far as the slack in the kevlar allows. If, however, the elastic should burn through, you still have the kevlar as a backup since it runs from the rocket to the nose cone.

As far as zippering, I wrap a flap of masking tape around the kevlar just as it exits the body tube. That way, when the kevlar goes tight, the masking tape helps protect the top of the BT from zippering. Cheap and effective.

Beware having too short an elastic shock cord - that is what causes the notorious "Estes Dent" in the top of the BT as the nose cone snaps back against it after ejection. A long enough shock cord or a combo of elastic and kevlar will help prevent it though.

I get shock cord now.Thanks to yous guys. I understand the violence of an ejection with the two objects now. To be honest, I thought most set-ups were overkill. :confused2: The part about the stored energy I never figured in either. And there is different ways to softing the blow. Geeezzzzzeeee I keep getting schooled and I may become a Rocket Scientist Scotty Dog