Model Forum / General / Rockets / June 2006

I'm prety new to this myself but have a couple of thoughts for you:

Natral fibres may be beter and could be soaked in a bicarbonate of soda so
make them self extinguishing.

I've used a shot length of picture wire - just enough to have a loop coming
out of the top of the tube to which other shock cord can be attched.

Good luck,

Halam

So... use it.  Also, look up the tips and techniques for securing the
kevlar inside the rocket.  I've been looping around one of the BTs
behind a centering ring, either at a transition or around the engine
mount.  Also, I've learned to use a loop of kevlar, with the knot
buried in the epoxy (i.e. nothing to come untied during flight).

For heavier loads, I learned to do a four-strand braid.  Start by
twisting an inch or so of the *centers* of two strands together.  Loop
that twist over something (it becomes your loop), then braid the rest
of it.  Now, feed the non-loop end through a hole in a centering ring,
split it into two sets of two strands, and tie the two sets together.
Now, run each set around opposite sides of the BT and tie them
together on the other side.  This is fairly secure on its own, but
smear epoxy over the whole thing (behind the CR) to lock it in place.

Make sure the strands are long enough that the loop just sticks out
the front of the rocket, so you can tie your shock cord onto it.

I use kevlar string where the shock cord mount is down near the ejection
charge. Then I tie a loop in the kevlar string up just below the plane of
the  body tube where the nose cone sits.  To this loop I tie my shock cord
be it nylon or elastic.  This keeps the shock cord away from the ejection
gasses and the kevlar string below the exit plane of the body tube.  I keep
the kevlar string below the exit plane of the BT to keep it from being
pulled through the side of the BT (Zipper effect).  Since the kevlar string
is very thin it does not take much to pull it through the side of the tube.
However the shock cord is wide and thicker so it won't unzip the tube.  By
tying the shock cord to the kevlar just below the tube end I get the best of
both worlds.  Kevlar does not burn and the wider shock cord does not easily
cut through the BT.  On bigger heavier rockets I actually braid the kevlar
string to make it 5 times stronger.  Then I do the same thing.  Never had a
failure in the kevlar nor a tube that got zippered.

Be sure to pull the shock cord and kevlar string out and tight while putting
in the wadding to keep them above the wading and protected as much as
possible.

I don't have a lot of experience in the High Power arena.  So failure may
happen to me some time or another.  I also had an experienced fellow tell me
not to use elastic for HP rockets.  He said,"The only thing those are good
for is to hold up your underwear".  I followed his recommendation and
replaced my elasic shock cord with a 30 foot long tubular nylon shock cord.
It worked great!  Thanks James.

KT

for higher powered models I would install an eye or U bolt in the last
centering ring, then the thick (1/2 inch) kevlar shock cord is tied into a
loop (or some people sell pre sewn loop... which is more convienent) and
attach it to a climbing hook or those chain link with a screw on it (forgot
whats it called) and attach one end to the eye hook or U bolt. Then the
other end I would attach nosecone and recovery material, and flameproof
cloth. Everything is attached with a bolt of some kind so the recovery
material can be easily and quickly disassembled. That way you can have one
recovery system for several rocket (assuming they need to use the same kind
of chute....) and not have to get super expensive ones for every one of
them.

Many thanks for the replies guys - lot of useful info. I'd go for the kevlar
but I've not seen it in any local hobby shops (Western Australia). Most
hobby shops have little more than half a dozen kit rockets and a handful of
estes motors.

I might use a short length of light gauge multistrand stainless trace wire
and go with some of DJD's and Halams ideas to prevent cutting u the side of
the tube.
cheers guys.

Hey, I just bought a box of 24 assorted Estes engines for $95 australian -
say $4 each. Hows this compare with US/UK prices.
rob