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Date:  7th August 2011
Location: Workshop, Australia
Conditions:  Mild, rainy
Team Members at Event: PK and GK

Polaron G2 - Phase 2

This week we have been focusing on the phase 2 development of this rocket. Phase 2 is basically the addition of boosters to the main stage, and switching the main stage to the Jet Foaming configuration. This also involves upgrading the cluster launcher to handle the greater loads.

Main Stage - Jet Foaming static tests

This weekend we ran a number of static tests on the main stage to measure the amount of thrust generated and the thrust duration.

We removed the fins and the nosecone from the G2 rocket and added a reinforced spliced pair of bottles to the bottom of the stack. This added another 3L or so to the overall capacity. We also used the same Jet Foaming spacer as we trialled on the Axion G2 rocket. For these tests we just used a standard 9mm Gardena nozzle on the main stage. This nozzle would normally be too small for a rocket this size, however, with the higher pressure and the boost in velocity on launch, it is fairly close to where we want to be. We want a nice long burn from this rocket.

Because of the length of the rocket we had to suspend our normal test stand from the balcony.

On Saturday we ran a couple of tests with 4L of water + foam at 205psi. The tests went well except when we sat down to review the thrust curves we realized we didn't quite have the load cell amplifier turned on (*Doh!*)and so we captured a little bit of noise. The tests at least allowed us to measure the thrust duration.

So on Sunday we repeated the experiment and re-ran the two tests at 200psi with the load cell amplifier turned on this time. Although the rocket will use higher pressure for actual flights we decided to use a lower pressure to reduce the risk of an explosion in our back yard.

We captured good data from the Sunday tests.

Here are the two raw thrust curves from test #3 and #4:

The thrust curve is interesting because after about 1.2 seconds you can see the thrust increase as the pressure drops which is a little strange but when we subtract the weight of water as a function of time we see the thrust curves are more like this:

Note the rate of water loss in only an estimate since we can't accurately measure it. The increase in thrust becomes less dramatic, although it's still present.

The dip in the thrust curve around 1.2 seconds is the result of water running out in the bottom bottle. The 8mm nozzle inside the jet foaming spacer limits the rate at which water can enter the lower bottle, but the 9mm nozzle being bigger can empty the lower bottle faster.

The raw thrust curve is actually the one we are interested in because that is the effective thrust causing the rocket to accelerate upwards since it already incorporates the weight of the water.

Here is a short video of the static tests:

From the tests it looks like we should get a nice long foam trail! :) The interesting thrust curve makes me think it may be possible to customize a thrust curve shape to a certain extent with a series of baffles in the pressure chamber with different sized holes and varying amounts of water/foam above each baffle.

Other Development

Boosters

• The lower spliced-quad of each booster is now reinforced, but still needs to be pressure tested.
• We are using the Acceleron V nozzles for each booster since they are already done. These are 15.6mm in diameter.
• We made up three new booster parachutes, each is 700mm in diameter and weighs 27grams.
• We are in the process of adding the deployment mechanism to each booster but they will be based on this technique like we used last time.

Launcher Upgrades

We need to strengthen the base of the cluster launcher as at 250psi each booster is going to be exerting about 320N on each nozzle seat the equivalent of about 32Kg. The release head also needs to be reinforced as it will have about 1000N trying to rip it out of the launcher. There will be a lot of stress on the Gardena nozzle and it will need to be made of aluminium again.

The stability of the entire launcher needs to be improved because the rocket is very tall, and could easily tip over as it is.

A new narrower and longer guide rail will need to be fitted.

One of the things we need to investigate is how much we need to strengthen the neck and throat of the lowest bottle to handle the static force while on the pad with all three boosters trying to rip it out of the launcher.