Estes V2 Motor Mount with an Alternative Shock Cord Anchor

 

I'll get back to the  Super Big Bertha in the next post. For now, here's more progress on the Estes V2.

The forward centering ring has a slit for the rubber shock cord to pass through. You're meant to push one end of the shock cord through and knot the other end to secure it to the rocket. There's no tri-fold "tea bag" mount included in this kit.

The advantage of Estes rubber shock cords is that the cord is much less likely to damage the end of the body tube if your parachute ejection doesn't happen right at apogee. This can be because you have the wrong delay for your flight, or the delay grain has a flaw, causing it to burn too long or short. Or, if you have to launch at an angle due to wind, sometimes the rocket doesn't slow down as much at apogee.

If the chute opens when the rocket is coasting, it can pull the cord against the opening of the body tube. This can pull hard on the shock cord, and a stiff Kevlar cord can sometimes damage your rocket when this happens. It can be minor damage, like a small nick in the tube, or a long rip, called a zipper.

This rocket has a very long Kevlar shock cord. Even with all textbook flights, it has had some damage.

While I do like a shock cord with some elasticity, I rarely use the ones supplied with the kit. I usually make a long shock cord made of two parts - a length of Kevlar thread to secure it to the rocket, and a very long piece of sewing elastic.

With a short rubber cord, sometimes the nose cone can snap back and hit the rocket, damaging the paint. A long cord gets the NC and chute away from the rocket and helps reduce the risk of damage.

Some rocketeers just use very long Kevlar thread for shock cords, and I've done so as well, but Kevlar is inflexible, and I've had some damage to the ends of body tubes even with very long shock cords. I like having a loop of Kevlar which ends just inside the body tube.

The Kevlar comes from emmakites.com, a kite supply website, and is meant to be for flying large kites. Their prices are good, and shipping is fast and free, and they have some very heavy duty stuff suitable for larger rockets. They even list some of their product now as "for rocketry." It seems they've caught on that we like their Kevlar.

For the V-2, I mounted it right to the motor mount.

The first step was to punch a hole through the forward centering ring. I cut a length of Kevlar longer than I would need, and tied a loop in one end.

I passed the free end through the loop, forming a kind of lasso, which I wrapped around the motor tube just behind the forward centering ring. The free end was threaded through the hole in the centering ring and the lasso was secured in place with a fillet of wood glue.

Once the glue was dry, I slid the motor mount into the rocket at the correct depth to see how long the Kevlar lead should be. I then removed the motor mount and tied a simple loop. 

After a couple of tries, I had the perfect length.

Having access to the Kevlar loop means I can attach a long piece of heavy sewing elastic, and I can replace it when the elastic gets brittle.

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