Surgical Tubing for use as a shock cord...?

[Ralph M Bohm]

Hi all,

I am thinking of using surgical tubing like the kind in the attached image for a shock cord. Would this be advisable for a vehicle approx mass of between 1.5 and 2 kilpgrams? I would tie off to a length of kevlar.

 

[prfesser]

Most high-power rocketnutz shy away from elastic shock cord. It doesn't stand up to ejection charges as well as kevlar, or even nylon. And a heavy nose cone or payload section snapping back on the airframe can do a fair bit of damage. Tubular nylon or kevlar is pretty standard.

 

[cwbullet]

I would not use it. Elastic materials are better left in underwear.

 

[Funkworks]

If it's anything like a slingshot elastic cord, it's heavier than kevlar, bulkier, weaker, and flammable.

 

[cerving]

Chances are pretty good that, assuming the tubing survived the ejection charge, your nose cone would come smashing back into the body. Not a good choice for a shock cord... but great for launching R/C gliders.

 

[Ralph M Bohm]

Good morning and thanks for the replies. What do you all use to lessen the sudden tensile shock of deployment?

 

[Kelly]

Good morning and thanks for the replies. What do you all use to lessen the sudden tensile shock of deployment?

Some combination of
(1) a length of nylon, which has a bit more stretch than Kevlar,
(2) a long enough cord, so that the components have a chance to slow down due to drag (many recommend 3-5X the length of the rocket),
(3) ground testing with your deployment charges to ensure that the components are not separating with so much force that they put a strain on the cord.

 

[Funkworks]

A right amount of BP. If the shock is too big, there's too much BP. The blast should be big enough to overcome friction between body tubes, but not so much as to overly stress the cord's attachement points. I enjoyed doing ground tests to see this up close (with safety glasses). With kevlar attached at both ends to an epoxied eye bolts, I can't imaging the tensile shock to be a problem. And as Kelly, says, the longer the cord, the less the shock.

 

[tfish]

Good morning and thanks for the replies. What do you all use to lessen the sudden tensile shock of deployment?

I use break points...bundles of the shock cord taped together with electrical tape....these bundles can be made with various layers of tape an either one, two or three per bundle depending on the size/weight of the rocket. experiment...take some shock cord make up some bundles...pull on the ends of the shock cord and see for yourself how much energy it takes to release the cord from the bundles..




a short video of a 40+ pound 6" rocket using 1" tubular nylon with break points


ideally you will have some break points not "used" after your flights..

Tony

 

[MClark]

Surgical tubing and bungee cord was used early in HPR but was found lacking in strength and spring back can cause problems.
Do what Tony says

 

[Ralph M Bohm]

Thanks!
Great ideas for sure. And yes, I am most certainly scheduling in ground tests before any launches.

 

[ZEDL1]

I agree with Mark that I would not use it for that purpose, today.
That said, I knew Frank Kosdon quite well, and he liked it for this purpose. I believe that Mark will remember...

This thread brings back memories of Frank. I own a very old, early-nineties I think, Dangerous Dave Gawlick Mad Dog 40/40 4" 8' tall all-handmade glass kit that was owned by Paul Robinson. Frank used the surgical tubing for the "shock cord", and gave or sold it to Paul.

Two lost people that we older ones miss...