High Power Shock Cord

Hi everyone,
I'm in the final stages of a scratch built level 1 rocket. It is 5.5" diameter and 50" long. I expect it will weigh somewhere around 8lbs when it is launch ready. My question for the group is around the shock cord. I have 5/8" diameter nylon cord and I was planning on 3x the airframe length (or 15'), although I guess I could go longer. For high power, do I need anything to absorb some of the energy from the charge? I don't think elastic would be strong enough, but perhaps surgical tubing would. Would a deployment bag help in this area? I really appreciate your input.

don.

mad4hws said:

Hi everyone,
I'm in the final stages of a scratch built level 1 rocket. It is 5.5" diameter and 50" long. I expect it will weigh somewhere around 8lbs when it is launch ready. My question for the group is around the shock cord. I have 5/8" diameter nylon cord and I was planning on 3x the airframe length (or 15'), although I guess I could go longer. For high power, do I need anything to absorb some of the energy from the charge? I don't think elastic would be strong enough, but perhaps surgical tubing would. Would a deployment bag help in this area? I really appreciate your input.

don.

Click to expand...

You don't need anything to absorb the shock.

A deployment bag is used with pilot chutes to properly sequence the opening of a parachute; they do not on their own deal with the jerk from the separation reaching the end of the shock cord.

I would go longer on the shock cord 25-30 ft.To absorb some of the shock fold the cord back and forth about 4" long about 3 Ft wrap once with masking tape do another 3 ft section and continue till done . When the parachute opens it will start snaping the tape and help cushion the shock

Thanks guys. this is very helpful.

The deployment will greatly reduce the opening shock of the parachute on the rocket.


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There's a lot that goes into your recovery design, especially if you are doing Dual Deploy. With motor deploy, there is less to consider, but size of the ejection charge is the primary issue. As long as you are not using an overly large charge, you should be fine with the 3x cord length. The problem with longer cord is that is usually used to compensate for too large of ejection charge. If the charge comes late and the rocket is pointed down, then the chute and nose cone are forced below the rocket, the chute may open below the lower section. A rare occurrence would be the lower section fouling the chute and everything dropping fast. Much more common is the chute open and the lower section falls to the end of the shock cord. It puts a huge load on the system when that happens. Worst case is a system failure and the lower section free falls. This is much more common with DD when going drogueless, but can happen with motor deployment when very long shock cords are used.

You mentioned a 5/8 nylon - that should be just fine. 15 foot should be plenty long as long as your careful about the size of your charges. Ground Test!!!!! Don't try to guess what will work and then put it to the test on the field, test it in your backyard before you fly it. Then there won't be any worries come Level 1 flight day. Before long, I'm sure you will get folks here telling you it should be shorter, yet some others telling you it should be longer, and they'll all have what sounds like a sound reason behind the argument either way. Plenty of high power rockets are flown with the same formula your using - just make sure you do some ground testing and you will be just fine.

Golfpro62 said:

You mentioned a 5/8 nylon - that should be just fine. 15 foot should be plenty long as long as your careful about the size of your charges. Ground Test!!!!! Don't try to guess what will work and then put it to the test on the field, test it in your backyard before you fly it. Then there won't be any worries come Level 1 flight day. Before long, I'm sure you will get folks here telling you it should be shorter, yet some others telling you it should be longer, and they'll all have what sounds like a sound reason behind the argument either way. Plenty of high power rockets are flown with the same formula your using - just make sure you do some ground testing and you will be just fine.

Click to expand...

If you ground test, how close is that ground test to what happens in the air using an altimeter and echarges? Is the error factor likely to be: the charge will blow out the NC further at 3000ft? Or not as far at 3000 feet, as compared to ground test.