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Date:  15th April 2010
Location: Workshop
Conditions:  Warm 24C.
Team Members at Event: PK and GK

Fiberglassing

Two weeks ago we took our first plunge into fiberglassing pressure vessels for our rockets. We know that eventually in order to get peak performance we will need to be using more expensive composite materials, but at least this way we can limit the costs while learning the necessary techniques. We have previously made some fiberglass components such as nosecones and fins but have not tried reinforcing bottles.

We are using West Systems 105 Epoxy and 206 hardener as the resin. For the first tests we used 200gsm E-glass wrapped on a spliced-pairs of bottles. The spliced-pair acts as the internal air bladder. We only spliced these with the Sikaflex 11FC to give an air tight seal, but we are relying on the fiberglass to prevent the two ends from flying apart.

After wrapping the bottles we set them up on an improvised rotisserie and placed them next to a series of light bulbs to cure the epoxy faster. We measured the temperature carefully so that it was around 30 C at the surface of the bottles to prevent the bottles from shrinking.

We did not try any peel ply, or heat shrink tape on the outside for these tests. We will most likely try that in the future to remove the excess epoxy and give a smoother finish. Vacuum bagging is also something we may consider though that may be a little tricky.

Tests

WARNING: If you are going to attempt to reproduce any of these results, PLEASE pay attention to safety due to the higher pressures involved. Always conduct these tests behind a safety barrier and use eye and ear protection. Make sure your test equipment is rated for the pressures used.

We tested 4 spliced-pairs of bottles. One pair used a single wrap of glass on a 90mm body (there was about 1 inch of overlap), two used a double wrap on a 90mm body, and the last used a double wrap on a 110mm body. The 90mm spliced-pairs hold 2L each and the 110mm spliced-pair holds 3.15L.

We hydrostatically tested these inside an old scuba tank with the bottom cut off to contain the noise. We used our high pressure panel for the tests.

The results:

These results are preliminary and will need to be repeated a number of times to confirm their accuracy.

• The single glass wrap on a 90mm spliced-pair was a little disappointing at 220psi because it means the actual launch pressure would be closer to 170-180psi.
• The 300psi+ pressure for the double wrapped bottles was a very good result. We don't know how much more it can hold, but the launch pressure would be at least 250psi. Our high pressure panel only goes to 300psi so we could not test to higher pressures. The bottles did not show any sign of stress when inspected after the test.
• The 300psi test told us that the regular bottle caps we use can also hold at least that much pressure.
• The Sikaflex 11FC alone was able to provide an air tight seal without needing the PL premium sleeve to help hold the splice together.
• Each double wrapped bottle costs about $4 in glass and glue materials, which is quite reasonable. 
• From the 4.8L of epoxy and hardener we can make about 100 spliced-pairs of 1.25L bottles, and about 65 of the 2L bottles. 
• The double wrapped 110mm spliced pair had a 300+ psi burst pressure. This was a better result than we had expected.

Simulating performance

Because the reinforced spliced-pairs are heavier we ran a number of simulations to see at what point the glass reinforcing is going to give us better performance over the lighter but weaker spliced bottles.

We weighed all of the non-pressure vessel related components from one of our typical rockets (Axion) which are flown regardless of the material the pressure vessels are made of. The total weight of these components was 380 grams and included, parachute deployment mechanism, nosecone, parachute, camera, tornado tubes, nozzle, fairings and fins.

We ran two sets of sims on two different rockets. One rocket was made of 3 x 90mm spliced pairs giving a total capacity of 6L with a 9mm nozzle. The other rocket was constructed from 3 x 110mm spliced-pairs giving a total capacity of 9.3L with a 15mm nozzle. We did not use a launch tube for either simulation.

We used Clifford's simulator to calculate the expected altitude in 10psi increments. In all cases the coefficient of drag was the same, and we always let the simulator pick the optimal amount of water for each pressure. In the graphs below we compare the 4 different 90mm spliced-pairs and their expected behaviour. The second graph represents the rockets made out of the different 110mm spliced-pairs. The solid lines represent the range of safe launch pressures, and the dotted line represents pressures up to the burst pressure. The end of the dotted line represents the burst pressure.

Graph 1 - 90mm rocket performance comparison using different reinforcement techniques. 

Graph 2 - 110mm rocket performance comparison using different reinforcement techniques.

Conclusions

In Graph 1 the PL sleeve line represents the splicing technique we have been using for most of our rockets. From the graph you can see that the single glass wrap only gives modest altitude gains and requires about 20psi more to break even. However, the double glass wrap gives significantly higher altitudes and takes an extra 30psi over the PL sleeve to break even.

The actual altitudes are only theoretical, so real world results will differ.

Similar results were obtained for the 110mm spliced-pairs. I look forward to launching a 4 x spliced-pair with a long launch tube. :)

Tornado Tubes

We hydrostatically pressure tested the tornado tubes to 170psi (before an non-reinforced bottle connected to it failed). We'll have a go at building a rocket using these in upcoming test flights. We think that they should be able to hold higher pressures than the 170psi. We'll put these on the high pressure test panel to see if they can hold up to the 300psi mark.

In the last week we have tested the small 70mA LiPo batteries and voltage regulators with our current flight computers. They seem to be working well and are nice and compact. We will test fly these on upcoming flights to see how well they perform during the whole launch day.

We also received the 350mAh LiPo batteries this week and so set about replacing the batteries inside the FCO's cameras. The power issues have been a major reason why we have stopped using them recently. These batteries are higher capacity and bigger than the original batteries and so need to be housed externally. Due to their higher capacity they take about 3-4 hours to fully charge from the USB port. Having waited a full 24 hours after the charge, I did some recording tests today. This was a test to allow me to charge the cameras the day before a launch.

I recorded a couple of long movies until the card was full. After 43 minutes (when I actually checked) the camera was still on and was showing that the card was full. It looks like the new batteries are working well.

One of these will be placed back in the Acceleron V booster. With the new battery the camera now weighs 44 grams compared to the 39 grams previously.

For charging the little 70mA batteries I bought a couple of the SparkFun LiPo chargers. The charger can only charge a single cell at a time, but having two means we can be charging two batteries simultaneously. This is useful when we need to recharge several battery packs the night before launch.

Loc/Precision Weasel

Last month I bought a pyro rocket kit from Suburban Rocketry that can handle bigger motors than the ones we have used until now in Paul's rockets (up to C impulse motors). This new rocket can take 29mm motors up to G impulse. Being a kit, construction was quite simple, although very different to water rockets of course. I've posted some build photos on the left. We will most likely fly it on a E15 first with a 24mm adapter to see how it flies and then go up from there. It has a separate payload bay big enough to carry altimeters or cameras or both, so I'll look into making use of it in the future. So far I've painted it white, but still need to decide if I am going to put any design on it ... and probably should give it a name.

And no, we are not migrating to pyro rockets, just expanding our knowledge of the rocketry hobby. :)