TARC frustration: advice needed

I'll second the idea that for TARC, the rocket design itself is secondary to good recordkeeping, good simulations, and a good test program. Teams score well with all kinds of different designs.

boatgeek said:

I'll second the idea that for TARC, the rocket design itself is secondary to good recordkeeping, good simulations, and a good test program. Teams score well with all kinds of different designs.

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I’ll third that. An imperfectly built rocket that accomplishes the goals of the competition is much better than a rocket with perfect paint and fillets that doesn’t.

Hi everyone, my TARC team had our first launch today and we did pretty, well but we have a few quick questions:
Our whole rocket is made out of BT-70 tubes, so we use a coupler with a bulkhead inside to separate the bottom section from the upper(payload bay). Because of this, two problems have appeared.

1. We attached the shock chord to the top section using the method in this video:
However, during the launch, the shock chord was ripped off of the coupler on both the attachment points causing the top and bottom sections of the rocket to come down separately. The parachute was attached to the shock chord, so top section came down in freefall. Is there a better way to attach the shock chord to the top section so it won't come off next time?

2. Also, after we recovered both parts of the rocket, we noticed that the nose cone came off during the flight. We don't think it came off when it landed though because neither the nose cone nor the top body tube were really damaged in the fall and the nose cone wasn't found near the tube. We suspect that, because we used the method in the above link to separate the top and bottom sections of the tube, we left a few holes when gluing the bulkhead to the coupler, so the pressure from the ejection charge made it to the top of the rocket and ejected the nose cone as well. Could this be the reason why the nose cone fell off or are there any other guesses. And if so, how can we stop this from happening next time?

Thanks for all the help and Merry Christmas!!!

Pictures are always extremely helpful when trying to diagnose problems.

firpisto34 said:

1. We attached the shock chord to the top section using the method in this video:
However, during the launch, the shock chord was ripped off of the coupler on both the attachment points causing the top and bottom sections of the rocket to come down separately. The parachute was attached to the shock chord, so top section came down in freefall. Is there a better way to attach the shock chord to the top section so it won't come off next time?

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How long was the shock cord, and what was it made of? Without referring me to the video, can you describe exactly how you made your attachments? Can you show a picture of where the shock cord came detached? There are many different ways a shock cord could become detached, no way to know what happened in your case without more info.

2. Also, after we recovered both parts of the rocket, we noticed that the nose cone came off during the flight. We don't think it came off when it landed though because neither the nose cone nor the top body tube were really damaged in the fall and the nose cone wasn't found near the tube. We suspect that, because we used the method in the above link to separate the top and bottom sections of the tube, we left a few holes when gluing the bulkhead to the coupler, so the pressure from the ejection charge made it to the top of the rocket and ejected the nose cone as well. Could this be the reason why the nose cone fell off or are there any other guesses. And if so, how can we stop this from happening next time?

The bulkhead should be completely sealed, and should not allow any ejection gasses to get into the payload section. If it's not sealed right now, then seal it.

There are other reasons why the nose cone could come off. Assuming it was a friction fit, how tight was it before the flight?

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neil_w said:

Pictures are always extremely helpful when trying to diagnose problems.


How long was the shock cord, and what was it made of? Without referring me to the video, can you describe exactly how you made your attachments? Can you show a picture of where the shock cord came detached? There are many different ways a shock cord could become detached, no way to know what happened in your case without more info.

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I unfortunately can't send pictures because we are currently on break and I don't have access to the rocket but ill do my best to describe what happened. While building, I made sure we made the attachments EXACTLY as the video showed. We first cut a piece of paper the same size as he has in the video and used wood glue to attach the shock chord horizontally across it; we did this twice for the two attachment points. Next, we put a gentle curve onto the two papers and glued them down with wood glue to the sides of the coupler, making sure they were approximately the same distance from end of the coupler(so equal pressure would be put on both the sides). We then glued a wooden disk (bulkhead) to the end of the coupler. As an extra measure, after all the glue was dry, we put a little more wood glue on the two papers, and after that was dry, we added a bit of epoxy as well.

The shock chord became detached on both sides of the coupler. On each side, the shock chord came out of the paper and the paper came off the coupler. So, the glue holding the shock chord to the paper and the paper to the coupler both "failed."

I hope thats enough information, tell me if you need any more

So after flight you had a shock cord with paper (and epoxy) broken free from the tube? Was it the force of the ejection charge? Was the fitment too tight and the speed of the ejection caused it to break free? What motor was used? There are plenty enough people here to help you figure this out.

firpisto34 said:

The shock chord became detached on both sides of the coupler. On each side, the shock chord came out of the paper and the paper came off the coupler. So, the glue holding the shock chord to the paper and the paper to the coupler both "failed."

I hope thats enough information, tell me if you need any more

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How long was the shock cord and what was it made of?

neil_w said:

How long was the shock cord and what was it made of?

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it was about 2.5 times the length of the rocket and it is made of kevlar.

To me that sounds pretty short for a non-elastic shock cord, but since I normally use elastic shock cords I don't have specific advice to give here. Hopefully someone else can chime in.

I think minimum 3x length of rocket is standard length, anything less is likely to be too much force. Bigger and heavier rockets even more.

If it helps my thought is this: When we put kevlar in paper the length becomes even more important since kevlar doesn't stretch much and sort of cuts like a wire when really thin in light small rockets. Reattach in a new spot-in future builds maybe consider securing the kevlar at the motor mount (unless minimum diameter). Also, I fold over my kevlar and flat nylon shock cords neatly and bundle them in sections with small dental rubber bands (not really tight) looping back and forth. This means more force is required to extend it all, slowing the force down a little more. Get the shock cord length right and reattach and try adding neat folds with rubber bands, it will help you pack your rocket neatly too-really easy to work with if you need to take everything out again.

Interesting video, but there's some issues. The method of attachment ( anchor points) would be good for a light payload but nothing like what you're doing. There are a couple of problems with this method. The biggest one is that Kevlar will cut through paper and cardboard when a sudden force is applied to it. Notice that the in the video, the "strength" test used a gradual build up of force to show how strong the anchor point(s) are . During deployment the force applied to your anchor points was sudden, a jolt as opposed to a pull. When force is applied like this Kevlar line tends to cut through weaker material like a knife. Paper and cardboard don't fair well when this happens.. The second thing is that Kevlar attachment points should only use epoxy. Kevlar will soak up epoxy but not wood glue. Wood glue just sets on top of it instead of bonding with the Kevlar. This results is a weak joint.

So, how to fix the problem? You can still use the method in the video with two minor modifications. First , use epoxy and be generous with it. Second, go to any auto parts store and go to the body repair aisle. You are looking for fiberglass cloth. Unfortunately , you'll end up buying more than you need, but this is the easiest way to get fiberglass cloth.

Cut a strip of FG cloth about 1/2" wide and 1-1/4" long. This will be your anchor point. Tie several simple knots at the end of the Kevlar cord. THe goal is to produce one big knotted ball instead of a line of small separate knots. --O--------- not --o--o--o------ . Leave about one inch of Kevlar above the big knot and unravel it so that it looks frayed. Mix up some epoxy and wet the FG cloth and the frayed end of the Kevlar. Lay the Kevlar in place making sure to leave room for the FG cloth strip. Press the strip in place with the big knot on the outside edge >>>O FG-------- -> to parachute. THe Big Knot should always be on the opposite side of where your parachute is going. the Knot acts as a anti-pull through stop.

If you are still using a plywood bulkhead (Fixed in place by glue fillets on both sides of the bulkhead) you can simplify making an anchor even more. Take a length of Kevlar about 18" long. Feed both ends through the holes in your bulkhead and tie them together using a square knot. Put a drop of epoxy on the knot to make sure it doesn't come undone. It would be preferable if the knot where inside the payload section instead of on the parachute side. You shout now have a loop of Kevlar. Cut some of the afore mentioned fiberglass cloth in the shape of your bulkhead. It should be a bit smaller ( decrease the diameter by 1/4") than the wooden bulkhead. Wet the cloth with epoxy and press in to place over the Kevlar making a FG - Kevlar- Plywood "sandwich". Tie a loop on the parachute side like in the video and you're finished.

I would add a 2 to 3 foot length of 1/4" wide elastic to the system. With one end tied to the Kevlar loop and the other to the parachute. Just make sure you have enough wadding between the parachute/elastic band and the ejection charge. The elastic band will absorb a lot of the deployment forces and this will result in less strain on your Kevlar anchor regardless of the method used. Your egg will also be happier that you used elastic.


Good Luck!