Recovery shock cord length

[darrena]

hi,

I am finishing off my LOC Tweed B 38mm
This kit splits in half on ejection and is about 77 inches tall.
I have upgraded to kevlar shock cord and a ring bolt in the back of the engine mount but wondered how long to make the cord and where to tie the chute.

Regards
Darren

 

[Salvage-1]

if that were mine, I would go with a 21ft shock cord, and tie a loop 1/3 way down for the chute. Short section of cord goes to the nose section. This means that when its on chute, the bottom half hangs lower than the nose section so they shouldnt smack into each other.

 

[darrena]

21ft?????.....

 

[chadrog]

Since there is virtually no give to the kevlar, any deployment shock is quickly transmitted to the anchor points of the cord. I've seen rockets literally ripped apart due to too short of a cord. If you're using kevlar, there's no such thing as too long.

 

[Threemorewishes]

Daisy chain the harness. It absorbs alot of the opening shock.

 

[SacEsq]

Daisy chain??? Please explain for the ignorant lol

 

[Threemorewishes]

Daisy chain??? Please explain for the ignorant lol

https://www.instructables.com/id/Paracord-Whip/step3/How-to-do-the-daisy-chain/

 

[CPUTommy]

There was an AWESOME video from Apogee.. Tim shows you how to do it and from the video.. very simple.. I will look for the vid..

Tom

Found the video..

https://www.apogeerockets.com/Advanced_Construction_Videos/Rocketry_Video_31

Well worth the time to watch.

 

[Serpico]

There was an AWESOME video from Apogee.. Tim shows you how to do it and from the video.. very simple.. I will look for the vid..

Tom

Found the video..

https://www.apogeerockets.com/Advanced_Construction_Videos/Rocketry_Video_31

Well worth the time to watch.

That's a great tutorial.

 

[darrena]

Thanks all,

I think the cord I have is too think and not long enough for this maybe 3m long, will use it all anyway!

 

[Handeman]

Since there is virtually no give to the kevlar, any deployment shock is quickly transmitted to the anchor points of the cord. I've seen rockets literally ripped apart due to too short of a cord. If you're using kevlar, there's no such thing as too long.

YES there is! It's much worse when using dual deploy, but too much can be as bad as too little. How much shock is on the system when the chute and nose cone is virtually stopped and the heavy fin can hits the end of that LONG shock cord?

 

[Handeman]

if that were mine, I would go with a 21ft shock cord, and tie a loop 1/3 way down for the chute. Short section of cord goes to the nose section. This means that when its on chute, the bottom half hangs lower than the nose section so they shouldnt smack into each other.

If the shock cord doesn't go into a BT on the upper section, it's tied to an eye or u-bolt, tie that about 1/4 - 1/3 the way down the shock cord and attach the chute on the end. That way, the nose cone/upper section isn't swinging around the chute/fin can like a pendulum and there should be less rocking back and forth from the chute.

 

[Salvage-1]

If your rocket is 77" long total (6ft 5"), a 3m (13ft) long shock cord may be a problem.
I like to go with 3:1 on shock cord to length of rocket. Now I am starting Dual Deploy, I am still sticking to that. So EACH shock cord in my 6ft rocket is a minimum of 18ft long. I usually have added a few extra yards just for comfort.

with my 21ft comment for your rocket, I was going on 3 * 6.5ft = 19.5ft then throw in a little extra to allow for knots, loops, etc. = 21ft. That would be approaching the minimum length I would be comfortable with, and I am currently using tubular nylon, not Kevlar as you stated. Tubular nylon has some stretch in it, Kevlar does not. When tubular nylon reaches its normal length and keeps going, it will stretch and soak up a lot of the energy of deployment. If kevlar gets to its end and there is still energy in the deployment, it will be taken up (hopefully) by either the daisychain, taped loops, chute deployment, etc. If the energy does not get soaked up somehow, there will be a failure somewhere. Could be a zipper into the airframe, kevlar is nice for doing that; or one of the attachment points will give. If you have done a good job with the epoxy on the motor mount, and you are using 'open' eye bolts, I am pretty sure that they will 'give' before the motor mount does. Coz the kevlar aint gonna snap.

 

[New Ocean]

Could be a zipper into the airframe, kevlar is nice for doing that;

I learned this the hard way... first zipper ever and it was with a hefty fiberglass airframe and thick (3/8ths kevlar.) The length is 15 feet for a 48 inch rocket. Tapped most of the kevlar into loops with blue tape. It was probably vaporized on ejection and did nothing to prevent the zipper. : (

I mean zipper on fiberglass? I never expected that before.

 

[chadrog]

YES there is! It's much worse when using dual deploy, but too much can be as bad as too little. How much shock is on the system when the chute and nose cone is virtually stopped and the heavy fin can hits the end of that LONG shock cord?

I can't say I've ever had a recovery issue that made me say "Hmm, guess the shock cord was too long." Clearly my mileage has varied.

 

[Handeman]

I can't say I've ever had a recovery issue that made me say "Hmm, guess the shock cord was too long." Clearly my mileage has varied.

I haven't personally had an issue either, but I've seen quite a few, including a L3 where the main opened and the chute and upper section almost stopped in mid air as the lower fin can fell past the main. It tore the 3/8" u-bolt right out of the forward 3/4 aircraft ply centering ring when the lower section hit the end of the 75ft shock cord. The lower section never even slowed down at the end of the shock cord, it just kept on falling. A drogue chute and shorter shock cords would have definitely saved that one. I've seen others where the fin can didn't break off, but you just have to wince when it hits the end of the long shock cord because it's falling so fast by the time it gets to the end.

 

[CPUTommy]

First time (two weeks ago) I launched my first Reloadable motor system in a scratch built rocket. I used an elastic cord that was from another kit rocket.

Rocket went UP beautiful... At appogee.. POOF a nice red cloud.. (line chaulk dust) and we all see the chute..

The chute just hung there and then started to drift.. then got smaller.. smaller ..and smaler..
A farmer found the rocket and brought it to me, The shock cord snapped and the nose cone and chute are probably in england at this point...

Im luck to get my rocket and motor casing back with no damage at all.. well..minus the nose and chute...

3 lessons I learned and will not repeat.

1-Delay time.. Had a bit to much delay . My mistake
2-Not enough lenght in the shock cord My mistake
3-Not thick enough shock cord..

Based on whats left of the cord, and where it broke...I can assume that the mount to the body tube held up, it was the failure of the cord probably due to the lenght as well as the diameter. I need to give the recovery aspect more springiness and play to account for the delay times as well as the ejection charge forces.

Tom..
P.S. Alwalys try to learn...