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Raw fiberglass tube made by wrapping FTC tubing.

Date:  21st February 2009
Location: Workshop
Conditions: Pleasant
Team Members at Event: GK and PK

This update covers a number of projects we have been working on over the past few weeks. As we gear up for fiberglassing body tubes we have done a few small fiber glass projects as well.

Ring Fin

Our first fiberglass projects was a ring fin. We wrapped 3 layers of fiberglass matting on a 2L bottle, and just used polyester resin on it. It was quite simple to make and hopefully will be a bit tougher than a regular ring fin made from PET plastic. We used glad bake baking paper wrapped on the 2L bottle to make it easy to remove. The ring fin is quite strong and lightweight.

FTC Payload Shells

Since we are rebuilding the FTC payload section, we decided to make the splitting nosecone shells from fiberglass as well. This was mostly because when you cut the FTC in half it partially curls in on itself, and makes it harder to fit to the nosecone and over the FTC. The fiberglass shells are strong and light and are much more dimensionally rigid than the FTC.

The procedure was very similar to the ring fin, but this time we used more resin and also wrapped the whole tube with electrical tape to help the resin penetrate all the way through and give a smoother external finish. This worked out quite well.

We then sanded and painted it.

FTC Nosecone

We tried several different techniques in building a new nosecone for the upgraded FTC payload section. We wanted a more streamlined nosecone compared to the Kinder Surprise half egg we used in the earlier version. I machined up the nosecone mold from a solid piece of plastic and polished it.

Attempt #1 - Heated the base of a small PET bottle and forced it over the nosecone mold and tried to heat shrink it over the top. Heat gun was too close and the base started crystallizing. The nosecone ended up flying quite nicely into the rubbish bin.

Attempt #2 - Heated the base of another small PET bottle more gently this time. This was only a slightly better result than the previous one. Before long he joined his friend in the bin. It just could not be shrunk down enough to the 30mm diameter.

Attempt #3 - Coated the mold with release agent and impregnated fiberglass matting with resin and then started wrapping it on the mold, but as you can imagine a nosecone isn't easy to round off with flat matting. Before long I had a nice mess on my hands with lots of bubbles, folds, and the rounded front was anything but. After another 5 minutes of trying to correct it, I finally pulled the whole mess off the mold and guess where it went?

Successful nosecone attempt

I realised that a stretchy material was needed, and having looked through the kids sock drawers I drew a blank. So I went to the local supermarket and bought a couple of pairs of pantyhose (nylon stockings). Okay I did get a few strange looks at the checkout counter as they were the only things I purchased. Little did they known those stockings would do over 200km/h one day.

Nosecone build procedure

1. We stretched the thumb section of a disposable rubber glove over the mold and tied it off. It was stretched sufficiently so there were no creases anywhere.
2. We then proceeded to stretch 8 layers of pantyhose over the top of that again tying it off at the bottom. Making sure there were no creases. The stocking material stretches well but inevitably ends up being thinner in the very tip section compared to the base. Hence the reason for so many layers.
3. We poured the fiberglass resin all over it and worked it in with a brush and gloves.
4. After the resin was fully cured we cut the nosecone off around the base with sharp knife and pulled it off.
5. Once it fully cured we gave it a light sanding. At this stage the nosecone was still a little more flexible then regular glass, this was probably due to the nylon and the lower strength polyester resin. So we coated the outside with 5 minute epoxy.
6. After the epoxy had cured we sanded it back to a smoother finish.
7. The last few bumps and divots we filled in with spray on putty and again sanded it back to a smooth finish.
8. We then gave the whole nosecone a couple of coats of gloss spray paint.
9. We trimmed the bottom portion with a Dremmel tool.

The nosecone turned out quite well, but will use proper epoxy glue next time instead of the polyester resin.

Pressure Testing

We have pressure tested a 2L spliced pair of bottles that only used Sikaflex as the glue. The splice held up to 130 psi before giving way. There was no damage done to the bottle or the sleeve. The glue simply let go. Based on those results we would not recommend using it for splices that need to hold more than 100psi.

Polaron VIb & Polaron VII

We've started rebuilding the Polaron VI rocket after the crash last year. The setup is going to be almost identical, and some of the parts are being reused. We are building a second Polaron VII that will have an extra spliced pair, and will have a slightly different deployment mechanism. Everything else will be a standard size so it will be interchangeable for spare parts with Polaron VI.

Due to the last crash of Polaron VI we are changing a couple of things to try to prevent that from happening again. We have drilled out the nozzle so that it is now 9.45mm diameter instead of the 8.9mm it had previously. This gives the nozzle a 14% larger cross sectional area. This was done in order to empty the water a little faster reducing the amount of weight near the tail, helping to improve stability. We have also created a new fin set that almost doubles the fin surface area. The fin set build procedure has been added to the construction section.

For Polaron VII we also redrilled our old aluminium CD nozzle and converted it back to a straight through nozzle with a hole of 10.5 mm. This gives a 44% larger cross sectional area compared to the original Polaron VI nozzle. This was done in order to generate more thrust, and still allow us to use the standard 9mm Gardena quick release. This was possible because we use a thinner o-ring and hence the o-ring groove can be shallower allowing for the greater internal diameter.

We have also been splicing 2L bottles like there is no tomorrow. These will be used on the Polaron rockets, their boosters and the new Acceleron Rocket. Each spliced pair requires 5 bottles.

Miscellaneous

Following are small snippets of other things we have been working on:

• We added lever arm extensions to our medium launcher to enable us to launch the FTC rocket at higher pressures. During the last launch day, the release mechanism was having a hard time releasing the rocket at higher pressures.
• The fins for the FTC rocket have now been replaced with smaller ones that weigh only half of the previous ones. That saved about 10% weight of the overall rocket. 
• I've been spending a bit of time soldering up the flight computers as time allows. It's taking about 30 minutes to make each.