shock cord

[bachsta]

what are som good ways to mount the shock cord onta a mid power rocket?

 

[rocketsonly]

What kind? Kevlar? Nylon? Elastic?

 

[JeffS]

I like the LOC method. There's a thread with a good photo here.

I seem to have the best luck by moving the shock cord and mount as far away from the motor as possible.

Jeff

 

[wwattles]

There are several methods. You can go with the tried-and-not-so-true Estes-style mount (not recommended). You can go with kevlar cord attached to the MMT, or the fwd centering ring. You can get some kevlar elastic and have both a spring effect along with your flameproof qualities. You can attach an eyebolt or U-bolt to your forward centering ring and hook onto that. Tubular nylon is popular for shock cords themselves, and tubular kevlar has a few devotees as well. Some people will even epoxy the cord directly to the centering ring, although it's not highly recommended, either, as it actually creates a very weak point in the cord right at the edge of the epoxy.

There are other design considerations as well, such as the presence of baffles or a piston ejection system, which change your connection configurations around. You can even go with a "zipperless" design.

Personally, I'd go with a U-bolt on the forward centering ring with a kevlar elastic cord clipped onto it (so it can be replaced if necessary), if your rocket is wide enough for you to be able to access it after it's built. Otherwise, when you put in the MMT, have a length of kevlar already attached to it so that it's just enough to reach a little past the forward end of the rocket. That way, it's permanently attached to the MMT, and thus the heaviest part of the aft section, and can be easily attached to a chute and the remainder of the shock cord for the nose cone.

WW

 

[bachsta]

thanks, its an loc rocket and i was just wondering if there method was actually any good. according to you guys it is so ill just go with that.

 

[sandman]

I had abandoned the Estes trifold paper method of shock cord mount for the Kevlar tied to the motor mount.

I have been flying model rockets for 40 years and honestly I have never had a trifold mount break in flight.

They are easy enough to test before launch by pulling hard and easier to replace.

I have had two of the Kevlar tied to the motor mounts fail. They appeared to have been burned but I could not think of a way to replace them...so i used the Estes trifold.

This is just my obsevation and not a critisism...but i'm beginning to wonder if the Kevlar meathod is sound.

Maybe it's the brand of Kevlar.

For may trifolds I don't use regular paper. I have been using heavier card stock.

 

[DynaSoar]

Originally posted by sandman
I had abandoned the Estes trifold paper method of shock cord mount for the Kevlar tied to the motor mount.

I have been flying model rockets for 40 years and honestly I have never had a trifold mount break in flight.

They are easy enough to test before launch by pulling hard and easier to replace.

I have had two of the Kevlar tied to the motor mounts fail. They appeared to have been burned but I could not think of a way to replace them...so i used the Estes trifold.

This is just my obsevation and not a critisism...but i'm beginning to wonder if the Kevlar meathod is sound.

Maybe it's the brand of Kevlar.

For may trifolds I don't use regular paper. I have been using heavier card stock.

I've had two burn through also, both because the kevlar ran too close to the ejection gasses. Kevlar is heat resistant, not heat proof.

One one I used the LOC method to fix it, on the other an Estes trifold.

 

[Gus]

Funny, I just had my first kevlar failure. Initially I thought it was a burn-through but it broke right at the point where it leaves the body tube, and looks more frayed than burned.

I've never had a trifold mount pull out of the rocket, but I HAVE had about a bazillion burn-throughs of elastic shock cords, which is why I've switched to kevlar.

 

[JStarStar]

Originally posted by Gus
Funny, I just had my first kevlar failure. Initially I thought it was a burn-through but it broke right at the point where it leaves the body tube, and looks more frayed than burned.

I've never had a trifold mount pull out of the rocket, but I HAVE had about a bazillion burn-throughs of elastic shock cords, which is why I've switched to kevlar.

You probably would get a little abrasion/fraying on the kevlar where it rubs against the end of the tube. Of course, this usually results in a zippered tube, but if the kevlar even just got nicked a bit, that might be enough to let it break.

Since I've really seriously started building kits again, this is the one thing that hacks me off, the 4-inch long strips of elastic Estes is using for shock cords. Come on, is elastic that expensive that it would kill you to put another foot in? (It isn't - I got like 12 yards at the craft store, for a buck.)

As far as the shockcord anchor question, this has been a bone of contention since the 1960s when you had the double-slit method vs. the revolutionary new (at the time) trifold method.

I don't like the trifold on smaller tubes... a BT-50 or smaller - those folded paper anchors clog up the tube too much in my opinion.

In the Handbook of Model Rocketry, G. Harry Stine had a method which I think works pretty well:

Cut a piece of medium-thickness cardstock about 1" square.

Cut two small rectangular slots in the cardstock, then thread your shock cord through both of the slots.

Pre-curve the cardstock anchor to approximately match the interior of the body tube, then coat the underside of the anchor with glue or epoxy, then slide into the body tube at least 1" down the tube (to allow nose cone insertion).

After the anchor sets, coat with another thin coating of glue or epoxy over the top of the anchor.

This attaches the shock cord pretty securely, yet does not obstruct the interior of the tube as much as a trifold anchor would.

As always, be careful to wipe or sand off any excess glue that spills on the inside of the tube (try to avoid doing it in the first place, too. )

I think GHS came up with this method in about 1965, it looks as good as most anything else I've seen.