Thoughts on Shock Chord Length

[lcorinth]

I know the question has come up many times "how long should the shock chord be?" And I know the general answer is about (or at least) 3-5 times the length of the rocket, because a longer shock chord will allow the parts to slow down as they separate, lessening the chance of damaging the rocket. I've even heard some people say "as much as you can fit into the rocket."

And I'm sure there's no such thing as a shock chord that's too long. But I'm building a rocket at the moment and about to cut some shock chord, and had a few thoughts, and wanted to ask what you guys think.

Let's say you've got two rockets, both BT-60 (1.637 inch diameter). One has an 18-inch long airframe, and the other has a 36-inch long airframe. Both are meant to fly on D black powder motors.

With a - for simplicity, let's say 3X length - shock chord, the first rocket would need a chord 54 inches long, whereas the second would need one 108 inches long.

But both ejection charges will have roughly the same force. And the nose cone and body tube are only going to fly sideways so far before gravity takes over and they're just going downward. Wouldn't there be some kind of law of diminishing returns to adding length to a shock chord? In other words, if 54 inches long is plenty for the 18-inch rocket, wouldn't it be the same for the 36-inch rocket? Or, at least, wouldn't it make sense to make a 3X chord for the 18-incher, but a 2X (72 inches) chord for the 36-incher? Make the shock chord longer than on the smaller rocket, but not that much longer?

I could see how things might change for high power or dual deployment, where a larger charge would be used for a longer rocket, but I'm not sure it seems that necessary for a LPR rocket. And if you go with a 5X length chord, then for the longer rocket - 15 feet - seems like it might be overkill.

Any thoughts on this subject?

Afterthought: Do people actually mean to go with the longer (say, 5X) for shorter rockets and shorter (3X) for longer rockets with the same power motors?

I'm just curious. I've got plenty of shock chord. I just want to hear people's thoughts and experiences on the matter.

 

[rocket_troy]

Put it this way, the longer the shock cord, the less chance of ripping your airframe. This is because (primarily) there’s a certain amount of “give” in your SC or “specific give” and what I mean by that is the amount of give per unit length of cord (be it meters/feet/whatever). When I say “give” I mean the amount of semi elastic stretch that will absorb the kinetic energy of the separating masses without returning it to the masses like an elastic cord would do. Different common SC materials have different degrees of “specific give” so say tubular nylon provided you with 4 units of “specific give” per meter and Kevlar provided you with 1 unit of “specific give” per meter, then you would need a Kevlar SC that’s 4 times longer than the tubular nylon to provide you with the same amount of shock absorption.
This ignores the deceleration of the separating parts or additional shock suppressors that can be included.

Sure the weight and volume requirements of extra SC needs to be accounted for so there are limits.

As for the rules of thumbs – x3 times the body length or x5… they’re just rough guides and should always be taken with a wary eye on suitability to airframes that might substantially fall outside what’s considered standard or separation energies that might be more aggressive than standard.

Troy

 

[dford]

Where are you attaching the shock cord. The airfram eats that length right away.
Low power. I'd assume not that big of a deal. I've put 12' into an 18 inch total length rocket. I think the actual airframe was something like 10" on my own design mid power.

 

[lcorinth]

Where are you attaching the shock cord. The airfram eats that length right away.
Low power. I'd assume not that big of a deal. I've put 12' into an 18 inch total length rocket. I think the actual airframe was something like 10" on my own design mid power.

In this case, near the top, but I always ignore what's inside the rocket anyway. For instance, on my Leviathan, I anchored the shock chord at the forward centering ring, but I measured the length starting where it came out of the airframe at the top.

 

[Zauskycop]

Okay, I speak from being somewhat of a noob myself, but I always go with 3x length of rocket. Pretty easy math actually...you mount the parachute 1/3 of the way from the NC, or AV bay...whatever is separating, and that leaves 2/3 of the length to keep the two parts away from each other. I can see an issue if you are breaking a rocket in the middle and the upper length is still fairly long...that may call for more shock cord? Hopefully I will learn something here also as to when to use a longer cord!

 

[caveduck]

Length of shock cord translates to how much deceleration of the two parts can take place after a (variably!) energetic separation. The shorter the cord, the more velocity when "dog reaches end of leash". The worst zipper damage situation is on a late ejection when deployment takes place at high velocity and the body gets abruptly yanked 180 degrees when the chute snaps open. I tend to think more in terms of the mass of the rocket and possible ejection velocity rather than its length...minimum 3-5 ft for LPR, 6-12 ft for MPR, 18-30+ ft for HPR. In the LPR case you can trade some length for elastic material. Personally I don't use short full length elastic/rubber shock cords anymore because of nose section snap-back. hcmbanjo has some really great ways of rigging shock lines in his blog posts.