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Date:  22nd April 2011 1:30pm - 2:45pm
Location: Denzil Joyce Oval, NSW, Australia
Conditions:  Swirling wind 10km/h with gusts to 15km/h. Temp ~25C
Team Members at Event: PK, Paul K, John K, Jordan K and GK

Quick Launcher

This week dad built a great launcher to replace the damaged medium launcher. We call it quick as it vastly reduces the setup and pack-up times. This is also a rail launcher as opposed to a tower launcher as before. This requires the rockets to use rail buttons, but those are relatively easy to make and attach.

The launcher uses interchangeable release heads so we can continue to fly our rockets with or without launch tubes and different sized nozzles. The release mechanism is attached to a sliding platform that allows it to be adjusted for rockets with different diameters. Two wing nuts lock it in place. This is much easier to adjust compared to the several minutes it took with the previous launcher.

Having no ring braces also allows us to use fins of any dimensions without the risk of them hitting the ring brace.

The launcher comes in two parts. The base consists of a 1.5m vertical strut with a pair of extendible stainless steel legs. The legs fold out and are then pined to the ground with tent pegs. Because the vertical strut only has a small footprint it can be tilted in various directions allowing us to point the rocker in the direction we need. The strut is also pinned to the ground to prevent it lifting during launch. The launcher can now be easily adjusted for distance launches as well, something not possible with the previous launcher.

The second part is the 2m guide rail itself with the attached release mechanism. This allows us to leave the base permanently attached to the ground while allowing us to load the rocket onto the guide rail and locking it into the release head without spilling any water. The whole rocket and guide rail then simply locks into the base and is secured with a single pin. The whole launcher is sturdy enough to launch the 3m G2 rocket.

At the end of the day the launcher neatly folds up into the two long thin parts. Detailed photos are shown on the left.

Update: See Day 133 for explanation of how the Quick Launcher has been improved.

Rail Buttons

We made a number of rail buttons on the lathe and attached them a couple of different ways to the rocket. Where the button needs to be attached to the side of the pressure vessel, it is first screwed to an aluminium backing plate which is then taped to the rocket. This allows us to reuse the button between rockets. The other attachment method is through a fairing wall with a backing plate to stop the fairing from warping. Because we screw the different sections together, the buttons would not normally line up, so we attach the rail button to the pressure chamber first and then rotate the fairing until the buttons line up.

Thermal Pressure Test

As we investigate the reason for the G2 pressure chamber failure, this week we performed a pressure test with a temperature probe inside the rocket to measure the the rise in temperature during rocket pressurisation. We wanted to see if pressurising the rocket quickly will increase the air temperature enough to affect the PET plastic lining in the pressure chamber.

We used a 90mm spliced pair of bottles wrapped in a couple of layers of 200gsm glass. We drilled a hole in a bottle cap and epoxied a thermocouple into it. The whole experiment was placed inside a thick aluminium pipe to stop any shrapnel should the bottle fail.

We filled the 2.1L bottle at our regular fill rate slowly increasing the pressure to 160psi. This increased the temperature from 18C to 36C.

We let the air out and let the air temperature equalize back to ambient of 18C.

With the pressure regulator left set at 160psi we opened the tank valve and let the bottle fill much quicker. The temperature quickly rose to 47C.

This experiment was carried out in the evening, and so there was no additional heat supplied into the system from the sun. The air hose was in the shade and hence at ambient temperature.

Conclusion

This test showed that it may well be possible to come close to the 65C mark that PET plastic starts becoming weak. On the launch day the ambient temperature was about 10 degrees C higher and the rocket had sat in the sun for almost an hour. We also filled the rocket to 250psi rather than 160psi which would have raised the temperature further. The black air hose was also in the sun.

Though we are not conclusively saying this was a root cause of the G2 failure, it may have been a factor. We are going to fill the next rocket a lot slower to allow it to cool as the pressure builds. We will also measure the temperatures in the spliced test bottle at the range to compare how hot things really get.

Launch Day Report

We took the new launcher down to the local park to see how it worked in a real launch day situation. It was very easy to set up. What's nice is that you can carry the entire launcher in one hand making it easier to get to the launch site. We launched a small rocket 3 times at 100psi since the park is small and there was a breeze blowing.

The first launch was less than ideal, as the parachute tangled on itself (one shroud line caught on the knot) and resulted in the rocket coming down pretty hard nose first. The nosecone was a little bent as was the top bottle, but not badly enough so we just popped it back out and used a bit of tape to repair it. We flew the rocket a couple more times after that with good deploys. The rocket also used the Servo Timer II prototype again as we continue it's flight testing.

It also gave us the opportunity to fly our new HD video camera on the rocket. (see below for details). The camera recorded good video on the two flights that we had it attached to the rocket.

We're very happy with how the launcher performed, and we will bring it with us to Doonside for the G2 launch.

HD in-flight Video

We bought the #11 camera from this http://myworld.ebay.com/hxelepro360/ eBay seller as recommended on the RC forum here: http://www.rcgroups.com/forums/showthread.php?t=1362692&pp=100#post16951767

The camera cost $39.95 including delivery, though it does not come with the micro SD card. It arrived in about 8 days. As it ships the camera has the time stamp turned on which is quite big, but thankfully on the above forum are instructions how to update the firmware to remove the timestamp. It is only a matter of copying the correct file to the SD card and turning on the camera. The camera then loads the new firmware. The process is simple and quick and only took about 1 minute. Here is how: http://www.rcgroups.com/forums/showpost.php?p=16952818&postcount=12 . It is also easy to reverse if you decide you want the time stamp back again.

Both the video and audio are quite good from this camera with really no evidence of dropped frames. Footage from the camera is included in the highlights video:

Day 104 - Highlights

Polaron G2b - Repairs

The G2 repairs are going well, with the backup parachute payload bay and the nosecone fiberglass body now repaired. We really only have to assemble the rocket and attach the new rail buttons. With the new launcher we are looking good for trying another launch again at the next opportunity. I am still waiting for the MD80 clone camera and SD cards to arrive, but with the Easter break these may take a couple more days.

Flight Details