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Date:  31st October 2009 (8am - 1:00 pm)
Location: Doonside, NSW, Australia
Conditions:  Warm 23C. Light winds, overcast
Team Members at Event: GK and John K.

This week we revisited the experiment carried out on Day 80 while looking back at the bottom part of the rocket during flight. We changed a couple of things in the experiment to make it easier to see what happens in the lowest bottle during flight:

• We painted the fins a neutral grey to help prevent the camera seeing too much white making everything else dark.
• We only used coloured water this time instead of foam to help provide a better contrast.

Since one of the fins was going to take up quite a bit of the video frame, we decided to attach a set of 8 tell-tails (in two rows) to see what the airflow is like around the bottom bottle. We weren't looking for any particular result with these, it was more or less out of curiosity.

Flight day report

• We arrived at the launch site about 8am. Setup was fairly quick and straight forward with no issues.
• The rocket was pressurised to 130 psi and launched. The flight was straight and stable. The parachute deployed around apogee, but it was quickly obvious that the parachute didn't want to play ball. The parachute became tangled and acted more like a streamer than a parachute. With such a heavy nose, the rocket impacted quite hard into the ground.
  
  The slow motion video showed how much the boom arms bent during the crash, but sprung back again without problems.
  
  The nosecone was only slightly damaged, and the top two bottles on the rocket buckled. All important components survived well without problems.
• The video from on board the rocket was also good.
• I always bring along spare nosecones and bottles, so it literally took 5 minutes to unscrew the buckled bottles from the rocket, and screw in the new ones. I also configured the new nosecone's flight computer to the correct flight profile settings, and the rocket was ready to go again. It's great having this modular capability in the rockets for events such as this. The only difference was that the new nosecone did not have the extra ballast, so I extended the parachute deploy delay a little.
• The second flight was similar to the first, but this time the parachute opened well and the rocket landed without incident. Good video of the lowest bottle was obtained.
• The third flight was again launched at 130 psi as the previous one, and good onboard video was acquired again. The parachute opened a little late than we would have liked.

We only flew the three flights on the day, as we spent a bit of time video taping pyro rockets on the day.

What we learned

Both of the experiment improvements worked well, and the detail of the water was a lot clearer than the last time around.

It was quite obvious from the footage that even under acceleration the water still gets pushed up the sides of the bottle during the air pulse. Some of the water can be seen draining from the rocket while under parachute.

The retained water may only translate to a couple of percent reduced performance, but it is something to consider when designing high performance rockets with Robinson couplings. People optimize the weight of their recovery systems and 20mL of water saved is 20 grams of payload.

The tell-tails showed the airflow quite well along the fins, although I had expected to see the ones closest to the rocket show more of a curvature as the air comes back together near the bottom of the rocket.

It was also interesting to see exactly when the rocket actually started falling backwards after apogee as the tell-tails could be seen lifting. Something that is normally difficult to judge from onboard video of the ground, since the closest reference is at least a couple of hundred feet away.

Splice Experiments ... continued

Since starting the splice testing a couple weeks ago, we've tried the Epoxy and PL splice as well as the Sikaflex and PL splice. Both used the #5 splice arrangement.

Epoxy

After letting the epoxy splice sit for a week we pressurised the spliced-pair and at around 120 psi a small leak developed. We kept increasing the pressure until the splice failed at ~150psi.

It was the PET bottle that failed rather than the splice. It tore itself fairly cleanly from around the edge of the sleeve. The epoxy glue though separated cleanly from the bottle that flew off. This means that the epoxy did not do as good a job of holding onto the PET, even though the 1cm x 1cm tests showed promising results.

The reinforcing shells worked well and the bottles did not show any signs of stress in the neck area.

Sikaflex 11FC

When both the Sikaflex and PL cured we pressure tested the spliced-pair again to destruction. This time the results were a lot more promising. The splice remained sealed during the entire test.

The final burst pressure was 190 psi (13 bar). In the slow motion video you can see the reason for the failure. The glass strapping tape broke over one of the splits in the reinforcing jacket, which resulted in the jacket no longer reinforcing the bottle, and the bottle exploded. Normally these bottles burst around 165psi. It tore itself away from the other bottle in a similar manner as the Epoxy test, but this time you could see traces of glue on both bottles which suggests the Sikaflex was holding well to the PET all the way to the end.

The region around the edge of the splice undergoes quite a bit of stretching, which appears to have helped the epoxy fail, but the Sikaflex being so flexible continued to seal despite the stretching.

The bottle has a cross-sectional area of 9503 mm squared (14.73 square inches) which means that at 190 psi the splice was holding the equivalent of 1,269 kg (2798 pounds) stopping the two bottles from flying away from each other. That's the equivalent of hanging a small family car from the end of the splice!

A little bit of air entered the splice because all the water ran out in the air hose so when the bottle let go there was quite a loud boom. Luckily the neighbours now know that this sort of thing happens on the other side of their fence from time to time and don't bother asking about it anymore.

Before the test I attached a skewer stick to the end of a piece of blue-tack and stuck it on the bottle right next to the sleeve. This was intended to help show the bottle expanding more outside the sleeve than close to the sleeve. In the end it was difficult to tell if there had been significant distortion because there was a wrap of glass tape right next to the sleeve. You can see the distortion more clearly on the other side because the reinforcing tape is further from the sleeve.

Next we will be making up a number of these spliced pairs and test them to 10% over the intended pressure and build a rocket out of them.

Flight Details