how to prevent busting off fins

[spacecowboy]

I'm sure this is an aged question, but this rookie flier needs some help.

I use the 2X "rule" for length of shock cord. Almost all my "splitter" rox do the same thing, the top end rips a fin off the lower end.

The Estes Polaris does it every time. My Estes Guardian ain't been lit up yet (it's basically the same as Polaris), and it's my clubs' official roc.

If I had to guess, attaching the chute to the cone vs attaching it to the shock cord is my error

spacecowboy

 

[rbeckey]

Always attach the chute at or near the nose cone (or upper section) if possible. Make certain that if the shock cord is folded at the point of chute attachment, the upper section doesn't touch the lower.
My Honest John (Estes K27) splits below the paper shroud. The chute is attached with a snap swivel to a short kevlar leader that is permanently mounted to the NC on a screw eye with the shock cord. At ejection, the rocket splits and the NC pulls the chute out. The lower section ends up at the far end of the shock cord and the two pieces never have a chance to hit each other.

 

[Mark_Newton]

Personally, I think 2X body length is too short. My rule is 4X, if there is enough room to pack it. You'll never make the shock cord too long! If you can't pack that much length, use Kevlar cord, because it does not stretch, and hence, does not snap back. Your broken fins are like due to the nose cone snapping back and hitting the fins.

Another trick some people are doing is to mount the parachute close to the rocket body instead of the nose cone. The idea here is that the parachute doesn't cause the nose to abruptly stop and run into the body when it opens. Also, the nose hits the ground before the body, and reduces the weight (and descent rate) of the rocket just before it touches down. Again, this technique works better with long stretches of Kevlar--get that nose cone away from the body.

 

[Mike]

Originally posted by Mark_Newton
Another trick some people are doing is to mount the parachute close to the rocket body instead of the nose cone.

I'm inclined to disagree with this, I prefer to mount the parachute closest to the nose. You make a good point about the slightly reduced descent rate right at the end of the flight but the nose cone (especially if balsa) is pretty light so might not make too much difference.

The body tends to be the heaviest section of the two so surley it would make more sense to have the longest part of the shockcord on the body side of the parachute. You need to have a sufficient amount of shock cord to absorb the shock of a parachute opened at some speed so mounting the parachute close to the body doesn't really let this happen and leaves you with a long piece of elastic for the nose cone which I don't think it needs.

To me it makes more sense to mount the parachute towards the nose, a light nose cone doesn't need much, if any, shockcord so mounting the parachute close to the body means an unnecessary amount of elastic.

Anyway just my two cents and if your method works then use it.

 

[Micromeister]

I must agree with mark, I almost always go way more then 2x on the shock cord. If your concerned about room, go half kevlar line and half elastic. a 1" piece of masking tape centered on the kevlar just inside the body will prevent zippering.

If you use a long enough kevlar line you don't need the elastic at all. 8 or 10 feet of 70 or 100lb kevlar doesn't take up nearly as much room at even 1/8" elastic. The only down side is it takes awhile to repack.

The note about the chute nearer the body then the nose cone
really does work, preventing the body from passing the nose as the elastic retracts.

 

[jflis]

You could also take a que from competition and design your shock cord (kevlar) such that it attaches to the outside of the body at or near the CG (body only, with expended motor(s) in place (eg: recovery mode CG)).

This will have the effect of bringing the model down with the body parallel with the ground. This will distribute the landing impact over a larger area reducing the shock to the fin (as compaired to a single fin tip being the first thing to strike the ground and absorbing all of that impact).

I use this method for the Cougar 440 and Cougar 660 models (for competition reasons) and a similar attachment on the Nomad (near the CG).

just my 2cents

jim

 

[spacecowboy]

I think I agree with jflis and micromister here.
First, who can argue with somebody that owns their own rocket company ?

Kevlar is the way to go. I've been using 1/4" elastic since about my third roc, and I have enough reco failures and subsequent catos, usually 13mm tubes, but sometimes even 18mm, to prove this idea aint working.

I should have taken my clue from Quest that uses the kevlar/elastic combo (No offense, Jim, I was going to get that complete 2002 set of yours,,,,until I got my layoff notice. Now, I just get to play every once in a while)

spacecowboy

 

[jflis]

Originally posted by spacecowboy
I think I agree with jflis and micromister here.
First, who can argue with somebody that owns their own rocket company ?

spacecowboy

PLEASE don't shy away from arguing with me cuz I own a rocket company

I'm the first to admit I have a lot to learn and I learn a lot of it right here

but i'm also quick to share what I *do* know. And if I had to describe my company in one sentence, it's a company with products that teach you about rocketry.

 

[sandman]

I use a fairly long shock cord. I buy them at a local sewing store (unless Joanne's has a sale) and they are cheap. I use a pretty heavy duty elastic cord too.

I also make a loop about 1/3 or 1/2 the distance (depending on my mood) back from the nose cone and attach the parachute there.

I have tried the Kevlar and I think it works...can't tell yet. I worry about zippering.

I have very few parachute failures. Maybe I'm lucky or just carefull, I can't tell.

sandman

 

[spacecowboy]

Jim, and everybody else

I'm just a simple country boy, playing with the birds. It's hard to argue with a pro.

I think we're all learning. In my case, the scratchers I build are "best guess" efforts, I don't own RocketSim or any of the other CAD s/w.

I think the joy is in the gig when a bird flies. If it doesn't fly, after getting all the "hacked off" out of your system, there's more joy there trying to figure out what went wrong.

Before this thread goes nuts, I'm shutting up. All the input was very useful.
SpaceCowboy

 

[Mike]

Originally posted by Micromister
The note about the chute nearer the body then the nose cone
really does work, preventing the body from passing the nose as the elastic retracts.

OK then, I'll try it next time I launch.