A couple of quick questions...

First: How long should a shock cord be for a 70" long scratch-built rocket? I've got a kevlar string attached inside the body tube and it sticks out about 6". How long of a shock cord should I attach to that?

Second: Is there a quick table showing how big a parachute should be based on the weight of a rocket?

Thanks!

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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Another question/request:

I have a BT-80 that I want to put 5 fins on. Does anyone have a fin wrap template for this tube and that many fins? If so, could I get a copy please?

Thanks.

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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go to www.rocketreviews.com; in the tool bar on left is a "calculator" to tell u the size a chute should b based on weight of rocket. Good Luck!

Once u have figured out size of parachute, shock cord is about 3x's diameter. There is probably a calculator on www.rocketreviews.com too. Good Luck!

Here is where you will find the tools..There is a calculator for chute size and for fin tmplate(Template Widget)..

__________________
Jon
SAM #0396
BAR 02/07
NAR 86940
KF4GUL
Aim high, fly straight!

Thanks, all, those links should do the job.

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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Shock cord length 3x the diameter of the parachute? I always make mine 3x the length of the rocket.

I also make the Kevlar portion just long enough to reach the inside top of the tube. Having it extend beyond the top of the tube usually won't cause any problems, though, but just in case, it is a good idea to attach some padding to it right where it would contact the lip of the tube. One method that I have used is to glue a small rectangle of cardstock onto the Kevlar; you can do this by punching 2 or 3 small holes in a piece of cardstock that has been doubled over and then threading the cord through the holes in a manner similar to the way that the cord was threaded through Centuri's shock cord mounts. Then use white glue to bond it at the right place on the Kevlar, and optionally, cover the rectangle afterward with a couple layers of masking tape. Then seal all edges with a thin layer of white glue. As the glue is curing, flex the cardstock back and forth slightly from time to time to keep it from becoming totally rigid. The idea is that if the cord is ever pulled hard against the side of the tube, the padding will spread out the force and keep the Kevlar from slicing through the upper lip of the tube (zippering it).

MarkII

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SAM # 0011

Ahh, so THAT'S how zippering happens! I'm cloning an Orion and attached a long length of Kevlar because I didn't want the nose cone re-bounding and messing anything up. Now I know how to protect it. Great tip!

47 feet long should be just about plenty for a 70" rocket.
That will be more than enuff to ensure against the dreaded snapp-bakk.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, TURMOIL, FIASCOS, and HAVOC !

Mark II

Do you have a pic of your technique for the kelvar attachment thingy?

Daniel