Good Idea ??

Building a Callisto 38mm. Didn't like the idea of tying knotts to the cone or the piston. Used rivets with washers. Look O.K. ??

Sorry about the quality but you shold get the idea.

Looks "clean".....

...but the Boy Scout in me tells me that a properly tied knot will not fail before the cord itself will. It seems to me that that putting rivets through the cord may in fact *weaken* it - the rivet going through the cord making a possible failure point.

Not that it won't work - I'm just sayin.

Based on my experience, I would say the eyehole on the nc will fail before anything else. Recently I've started drilling two holes in the base of the nc and threading kevlar thread through the holes (multiple times and then twist them), and then tie a knot in your recovery harness around the kevlar thread loop. It should be very sturdy.

Josh

That will probably be OK, but on those PML nosecones I drill two holes in the base, thread the shock cord through the holes, tie a not, and then put epoxy over the knot. It works quite nicely. However, I think that your way will probably be good, but I have never done it.

It's probably fine for the nose cone, but on the piston (where it has to support the shock of the chute slowing the entire rocket) a knot really would be better. I tested the grommets (basically like rivets) on the strap BSD includes with their kits and looked like it had less than 50% of the strength of the cord (based on what was ripped out and what did not break). BSD says the strap is OK in their kits, and I'm sure it is, but I didn't use it because I wanted to use Kevlar for heat-resistance.

Should be fine on a kit like this - NC of a Callisto is not that heavy, so there won't be a huge amount of stress on it.

A significant contributor to the stress on the eye loop will be the amount of black powder in the deployment charge. Particularly with pistons, people over-do the charges.

In a 4" PML Amraam, I ripped that eye loop in your photo by putting 2-grams of BP in the deployment charge. Since then, even when I get aggressive with BP, I always use a Giant Leap Kevlar Bungee cord on the upper shock cord (6' long) to absorb some of that deployment shock....

I'd say it would be fine for a small nosecone like this. What I usually do is epoxy an eyebolt into the bottom of the NC, and use that to tie the knot around, since it is much stronger than the weak tab thing.

2 thoughts I'll throw into the mix.

1) You rivet is more (or less) of an issue based upon how long the recovery harness. On a short run of harness I'd say that rivet could be an issue.

2) A knot is stronger than that rivet...big time. A knot is stronger due to the amount of surface area it has to put friction on itself. A knot used in climbing such as the munters hitch (figure 8) is extremely reliable...(this is the knot that generally joins a climbing rope to my climbing harness).

As many have pointed out...probably won't be a big deal due to the light weights you are dealing with. And it's cool to see some new ideas getting tossed around. That's what this place is for.

Good Luck!

Westy

I've used rivets on rockets (not on the shock cord) before and I really don't like them. Knots are better, but using both is even better. Tie off the knot and then rivet the loose end of the shock cord to the shock cord like you did. That will never come undone.

I have to agree with Forecheck on this. Putting holes into the shockcord may not be the best option. As also previously mentioned, the platic eyelet will fail long before the shockcord.

However for future reference, sewing the shockcord and then applying a small bit of epoxy over the sewn threads has always worked very well for me. I have literally snapped the shockcod before the sewn loop failed. FWIW................

Knots weaken the cord significantly. I am no expert on this but I have read this over and over and over on climbing sites and knot sites. The average is a 50% reduction in breaking strength.

Now that I have said that - I have no idea what reduction you get with Rivets. Sewn Loops are most likely the strongest method of attachment based on what I have read - assuming that you do not use glue. When you add glue the info out there seems to suggest that this also reduces the overall strength.

If needed I can post some site links to this type of info but I'm sure a quick google search will reveal many sites with this info.

NOTE: Since many people "overbuild" the recovery harness there is a significant margin of safety involved that allows for reductions in strength of 50% without risk. When you have a 1500lb shock cord strength in a 3" 3lb-5lb rocket you can lose 50% to 750lbs without any concern.

Uncle Vanya...interesting info. Perhaps it's wrong of me to compare climbing lines (that are somewhat elastic in nature) to tubular nylon webbing or tubular kevlar webbing which do not posess the same traits.

I do know I trust my life at the end of a climbing line with a properly tied munters hitch. I would NEVER climb with a line that was rivetted, glued or even sewn to create a loop that I could clip into. But perhaps I'm comparing apples to oranges (different applications).

Additionally I fall into that overbuilding category you are describing. For my rockets I use:

- Tubular Kevlar (longest lengths that will fit safely in the rocket)...the ends are sewn shut with kevlar thread (Giant Leap Rocketry).

- the hardware and quicklinks are probably more robust than need be

- nomex heat shield to protect chute

- nomex / kevlar sheaths on the lower part of recovery harness to further protect the integrity of the line.

I guess I just like to take every opportunity to be sure no failures occur. (this is common in rocketry from what I can tell).

It would be cool to see video of lab equipment pulling the nose cone from an airframe until failure occurred. My vote is in that 9/10 times the hardware pulling out of the nosecone would be your first failure point. It would not matter much on whether you had tied knots or sewn loops at the end of your webbing I'd guess....but never tested.....anyonle want to volunteer their rocket at the next launch for a tug-o-war ?

Westy

To do esentially the same thing on all of my non sewn loops... My favorite material's to do it threw are kevlar and climbing spec TN. The kevlar is for abvious reasons, it is very hard to shear. The climbing spec TN just seems like a tighter weave but I have used this technque with regular TN. My 15 pound nike smokes 1/2" CS TN harness's use this method with a single machine screw, the trick is to use an al (not sure how to spell it) to get the screw through a hole in the TN, without cutting to much material. So strength is not comprimised very much. Also sometimes I fold it over, then fold it over into the loop so the screw goes threw 3 layers of TN, just for a bit more secure hold. Oh and the nike with this setup has flown MANY times without any damage to the area... (knocking on wood... wouldnt want to jinx the rocket).