Shock Cords

9/16" or greater but rubber band it so it deploys with force.

I masking tape

Please enlighten further...

~ Duane Phillips.

On a long shock cord, you Z fold the cord and wrap the ends with either rubber bands or tape. The tape breaks very easy but it keeps it from getting tangled as well as eats up some of the shock.

Geoff

Masking tape is usually more effective if it is stronger. But if you use many layered rubber bands and use a new one on each new fold on ALL folds it makes the strength progressively stronger and provides the "thrust curve shape" most desireable.

Mere tech Jerry

Forgive the tech post!

I do the same and use masking tape. Works like a charm. (Unless your coming in at terminal velocity.) ;-)

-- Drake "Doc" Damerau

NEPRA President NAR Section 614 NAR 79986 L3 Remove "My Shorts" to reply

So is 1" tubular fine for 7.5" ?

I have plenty of 9/16" but it seems so small for 5.5 and 7.5 rockets.

Or is that just because of the larger is better hype I see at all the Vendor tables at the HPR meets selling 1, 1.5 and 2" tubular and kevlar.

I also hear that while kevlar won't burn up easy by the cannon charges (yes, our ejection charges are really antique canons , made from paper tubing) but that it can break easier then tubular.

So far i've used 25 feet 3/4 for apogee and 18 feet 1 inch for main in my

5.5 inch rockets flying on Is, Js and Ks

/ArtU

If all I had was 9/16" I would use it and if I was concerned I would double it up but make the lengths slightly unequal. So if one broke the other one would have a fresh stretch action before itself failing. Presumably the first one absorbed alot of energy. Due to "jerk force" I would use a fail strength about 10x the mass of the rocket. Sounds excessive but it is not.

I think REI.com has strength ratings or if not this is certainly a good one for the strength of materials site (never rely on evil Jerry for free test sample stuff because you could never trust the results then even though they were performedby a certified government lab).

You can protect nylon strap from BP by putting heat shrink tubing near the base which prevents first order damage.

Jerry

Part tech, part [POL]. Pick your poison.

HEY!!! I resemble that! My data is MORE accurate that what is published by any shock cord manufacturer or vendor.

I think Jerry is trying to push my buttons. ;-)

-- Drake "Doc" Damerau

NEPRA President NAR Section 614 NAR 79986 L3 Remove "My Shorts" to reply

But if you use

Hmmmm.....

Velllly Intelestink.

Never tried it that way.

-- Eric Benner TRA # 8975 L2 NAR # 79398

Art, It's nice to use 2in for the first few feet of harness where it attaches in the airframe so you have a two inch strap pressing against the airframe opening, then transition to something smaller. This helps to avoid zippers.

I also like to z-fold the cord and tape it together.

If you make the cord too long the sections can accelerate to higher than the deployment forces before being stopped by the chute, so be carefull.

Sometimes I like to make a harness a few feet long out of Kevlar that attaches to the bulk plate and the harness attaches to this, this can handle higher heat. But if you use 2in TN it will be fine. Just sew loops in the end and wrap with duct tape to protect the thread.

It's working and you ROCK.

You should think about rocket mass more than airframe diameter for shock cord sizing. The shock cord has to dissipate the force of the two rocket parts separating from each other by way of ejection charge. Therefore, the mass of the rocket pieces on each end of the cord and the force (size) of the ejection charge will determine the force (momentum) that must be absorbed / dissipated by the shock cord under normal deployment. Using longer shock cords will allow the pieces to slow down due to friction with the air before the cord goes taught. rubber bands or tape as others have mentioned also help.

"blow it out or blow it up" may help gaurantee 'chute ejection, but it increases the force on your shock cord, so be carefull.

There are also other ways to deploy chutes to minimize the forces on your shock cords.

I have a 7.5" X 8.5 ft rocket that weighs about 30lbs on the pad. It has all

9/16" tubular nylon shock cords. BUT... This rocket deploys at apogee in the following manner. The charge blows the nosecone off which pulls a 36" Skyangle chute into the airstream. As that chute fills, it pulls a deployment bag housing the main (Skyangle Cert3 Large) out of the rocket. As the nosecone and rocket body separate, the main shock cord is pulled taught and then the shroud lines for the main are pulled out of the loops sewn on the outside of the deployment bag. All of this dissipates energy from the ejection charge. Finally, the bag is pulled off of the main. The main then fills and brings the rocket down. The nosecone descends independently on the 36" chute. That looks like a lot when written out in words, but it is really very simple and I have absolutely no fear of overstressing the 9/16" tubular nylon.

-- Eric Benner TRA # 8975 L2 NAR # 79398

I guess since I use a "car polishing towel" wraped in duck tape for my anti-zipper ball this help avoid that problem as well ?

thanks for the reasons below, they reminded my why their was larger sizes , yes.

/ArtU

OK , here is what I want to do. the 7.5 : rocket is like a bruiser exp with a 4" motor mount. it will travel to about 7-9k so dual deploy is needed.

All the laundry will be in the payload bay.

The primary apogee charge cannon will blow the nose cone off and the nose will pull a sky angle drogue chute out held to the payload bay with a ring link held to an ARRD, say 25 feet of 3/4" cord. The secondary apogee charge cannon will seperate the booster from the payload bay. say 35 feet of 1" cord.

If the primary charges fails to fire, the seconday charge will end up seperating the nose from the payload (no shear pins) in the standard inertia mode that causes so many main deploy at the top situations. If the seconday charge fails, then the seperation of the booster and payload will take place with the drogue becomes taught.

at 800 feet, then the main match fires the arrd, and at 700 feet the secondary match fires the arrd weather it needed to or not.

That realeases the ring on the arrd, and lets the droge pull the main and it's deployment bag out and do all that stuff.

I've got to be missing something, so let me know if I did.

ohh, the harnesses have the "car polishing towel" wrapped in ducktape anti-zipper ball on them.

anyway, my 5.5 rockets just do the drougeless two step and so far have been fine, crossing streams, no that's fingers.

/ArtU

That sounds OK.

Personally, I wouldn't feel comfortable relying on inertia for a backup plan. With my luck, the parts will stick at the wrong time.

That sounds OK. I have a project coming up which will require an ARRD or some similar device. I would like to use another as a backup instead of just a backup match in one ARRD.

Drogueless dual deploy is my most common high power rocket recovery method. Works great and usually brings everything close to the pad.

Eric B.

the ARRD works in reverse, so if two are used, and only one fires, you don't have deployment ? I can't see how two of them does anything good ?

Oh, ok... sorry, I was zipping through the messages and I mis-read it...

Thanks.

~ Duane Phillips.

Thanks, lernt something new today.

~ Duane Phillips.