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Date:  13th September 2009 (8am - 12:00 pm)
Location: Doonside, NSW, Australia
Conditions:  Warm in the morning turning hot during the day 30C. Strong wind: 20-25km/h gusting to 35 km/h
Team Members at Event: GK, Paul K and John K.

We had a good day flying this weekend at Doonside. It was the first Sunday launch for NSWRA this year and the turnout was reasonable. Due to the strong wind conditions not a lot of rockets went up. Dad being in Europe over the last few weeks we didn't get a chance to launch the Acceleron V rocket. It takes two cars to get out there and both of us to set it up and launch it. We'll try to launch it again at the next opportunity.

A Different Perspective

One of the things we've wanted to investigate over the last few months is how water/foam behaves in the lowest bottle in flight. Static tests are limited by the fact that the water/foam inside the rocket is not accelerated. Other than air pressure inside the rocket, acceleration causes the water to be forced towards the nozzle even more. An early observation showed that when using Robinson couplings water was pushed up the inside walls of the rocket as the air pulse started.

Although it is a small amount, the weight of the water is not being efficiently utilised for maximum thrust. The main question was: Does the acceleration of the rocket cause this extra water to drain properly or does it still move up the sides during flight.

To try to answer this question we modified one of our rockets by mounting a long boom to the side of the rocket with a camera at the end so that the camera could look back at the lowest bottle and nozzle spray. We used our new MD-80 camera for that.

Boom

The boom was made from long strips of Corriflute material. The boom extended the same distance on both sides of the rocket. The actual boom was made up of 4 strips of Corriflute material joined together in pairs by gluing bamboo skewers inside each channel with PL premium. This gave the strips extra strength where they were attached to the rocket.

A counter weight was made for the other side of the boom that had the same weight and drag profile as the camera housing. The housing was also made from strips of corriflute material to hold the camera and protect it during landing.

Two tensioning ropes were attached to both the boom ends and around one of the bottle necks further up the rocket body. The tensioning mechanism was a tiny bent aluminium plate like used on tents.

Extra Ballast

Because of all this extra weight near the tail of the rocket, the Cg moved back about 200mm. Although the boom would act partially as a fin, it would only do so in one dimension, so there was potential that the rocket could be unstable along the line of the boom. So we added another 140grams of ballast to the very nose of the rocket to move the Cg back to where it should have been.

We weren't concerned with any altitude penalty on this flight. The rocket with everything came in at just over 1Kg.

Fins

The fins were also specially made for this experiment. We glued the fins to clear section of bottle first. There were no fillets on the fins to allow more of the bottle to be seen. The fin skirt was then slipped over the neck of the bottle and attached with clear tape near the top of the main bottle. 

Launcher mods

Because of the long booms we could not use our regular medium launcher. We slightly modified the booster launcher to hold the 2m guide rail closer to the release head. We then glued guide rail buttons to the rocket. The 2m guide rail was necessary to make sure the rocket could build up enough speed for the fins to keep the rocket flying straight.

Misc

We also used thermal covers over the black Tornado couplings on this rocket in the form of a roll of white paper cut out from ordinary A4 printer paper.

The simulator predicted that the optimal water fill would yield an acceleration of around 2.1G for water only. This would have meant closer to 1.5G for foam. We reduced the optimal water amount to 1.4L which raised the acceleration to over 3.2G.

Flight day report

• We arrived at the launch site early as there were strong wind predictions. Since early mornings are usually the calmest we wanted to get the launch in early. Doonside has changed considerably since our last visit. Most of the construction dirt has been removed and all the tall grass has been replaced by new short growth after a large grass fire that went through there a few weeks ago. This makes locating rockets very easy now.
• Setup was quite straight forward with Paul helping out quite a bit during the preparation. He's becoming a quite a good rocketeer. :)
• We pressurised the rocket to 130psi and launched. The rocket went up straight without any problems and pitched over into the strong breeze. The parachute came out well after apogee and the rocket landed well without issues. The only casualty was that the ping pong nose popped inside as the rocket landed on the road. The parachute was tied a little too far back which made the rocket come down nose first. The boom also gave the tail of the rocket extra lift on the way down.
• The camera recorded good video of the flight, but I did forget to add food colouring to the water to give a better contrast. The other issue was that the fins were too white which affected the camera's auto iris when they were facing the sun. This made the background darker. For the next flight these should be painted a dark colour.
• We didn't fly a second flight because by the time the data was downloaded the wind had picked up and was well over our launch criteria. (20km/h)
• We flew Paul's small Pod 1 water rocket a couple of times to fill in the time. The rocket flies well even in higher wind conditions. Both were good flights and both landed well.

What we learned

• It was easy to see that the water/foam was pushed up the sides even during flight. This is good in some respects in that ground static tests can be used for evaluating solutions for reducing the effect.
• The long booms remained quite stable during flight, allowing this camera position to be used for other experiments like looking up towards the nose of the rocket.
• The weight of the camera and the counterweight on the ends of these long booms also likely helped with rocket spin control. (Kind of like a ballet dancer spreading her arms out to slow down her spin.) This approach could be used for taking stable video while looking out the side of the rocket.

Flight computer progress

I've spent quite a bit of time over the recent weeks working on V1.7 and V2.0 of the flight computers.

V1.7

The prototype for V1.7 is up and running including the code. As components are starting to trickle in from different vendors, I've been working on the PCB layout checking the physical clearances. The current design has the PCB about half the size of V1.6. I am also using 3.7g servos for this design, and a small lightweight 6V battery. The aim is to have the entire package weigh around 20 grams, including the servo, FC, G-switch and battery. The FC can still be used with larger servos but then it also needs bigger batteries.

V1.7 is aimed at simple operation and primarily for parachute deployment, but it can be used for staging as well.

V2.0

V2.0 is still in the circuit design stage with some of the external modules still being designed. I'm going to need to get some input from the pyro guys about designing the igniter side of the circuit so that it is as safe as possible and reduces the chances of false triggers.

V2.0 is being designed with both water and pyro rockets in mind that carry more complex payloads that need a certain amount of control.

I've also started developing a simulator application for V2.0 that allows configuration for it to be tested before programming the actual computer.

Flight Details