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Date:  22nd May 2011
Location: Workshop, NSW, Australia
Conditions: pleasant, temp ~20C
Team Members at Event: Paul K, John K, GK

Aim

Since a bottle stretches when pressurised, how much energy is stored in the bottle itself? How much of this energy is returned during the thrust phase?

Experiment Setup

Background

In order to measure the stored energy in the bottle walls we decided to completely fill the bottle with water, pressurise it and measure the thrust produced when it is released. Because water is essentially incompressible at these temperatures and pressures all the energy that produces thrust should come from bottle contraction.

In order to get an accurate thrust measurement we needed to eliminate the effects of gravity because the bottle would get lighter as it expelled water. We suspended a 2L bottle horizontally so that it was free to swing along it's axis. We also attached the neck of another bottle to the base of the test bottle so it could be attached to a load cell. Another loop of rope was used to stop the bottle from moving upwards during the thrust phase.

Since the thrust phase was going to be fairly short we used a small 5mm nozzle to stretch the thrust curve out in time as we only sample the load cell at 240Hz. The total impulse is still roughly the same as if we had used a larger nozzle.

Experiment Setup

Even the smallest bubbles in the bottle can give significant errors in the measurements because air is compressible. In order to prevent air entering the bottle during pressurisation we filled the air hose completely with water to fill the void as the bottle expands. We also submerged the entire bottle in a tub of water and fitted the nozzle,  release head and the air hose full of water so that everything was completely free of bubbles. We then placed the bottle on the test stand and suspended the hose vertically so that potentially any remaining bubbles would float to the top of the hose.

From previous experiments we know that a standard 2L bottle increases its volume by 157mL or 7.5% when pressurised to 120psi.

Results

Three horizontal experiments were carried out to measure the thrust produced by bottle stretching alone. One vertical experiment was also carried out  to compare how much thrust a typical 2L bottle produces at the same pressure and same nozzle using the same measuring equipment.

The bottle capacity was 2096mL and was pressurised to 120psi.

	First horizontal test setup. Firing horizontally eliminates the effects of gravity on the thrust reading as the bottle loose water.
	Attaching the nozzle and release head to the bottle while completely submerged under water to eliminate bubbles.
	The bottle is suspended at one end by a string, while the other end is attached to the load cell.
	Test #4 was carried out vertically. The support brace around the bottle keeps it aligned but allows it to move freely up and down.
	Here it is connected to the air hose and release mechanism. A non return valve is built into the release head. The bottle was filled with 800mL of water.

The following graph shows the recorded thrust curves for all three horizontal tests. All 3 thrust curves have a similar shape and timing.

Graph 1 - 3 Horizontal tests

The following graph shows the 3 horizontal thrust curves with the vertical test thrust curve using air and 800mL of water.

Graph 2 - Comparison of normal thrust curve and the bottle stretching thrust curves.

The following graph superimposes the Simulation results from Clifford Heath's simulator thrust curve confirming that the measured results are in close agreement with simulation results.

Graph 3 - Theoretical thrust curve superimposed on the measured results.

The data from the load cell was then further processed in an Excel spreadsheet. The data was first filtered with a 5 point moving average before the total impulse was calculated. The vertical experiment data also had water loss compensation added for the water phase part of the thrust curve.

The following table lists the total impulse for each test run.

Conclusion

This experiment showed what the upper bound is for the thrust produced by stretching alone. It is only an upper bound as in a real rocket the energy is returned over the entire boost period which includes the air pulse. In an ideal rocket with 1/3 the volume filled with water the energy returned during the water phase will span a pressure drop of ~40psi. (Going from 120psi to 80psi during the water phase).  The rest of the energy is returned during the air pulse as the pressure drops from 80psi to atmospheric. But the impulse normally produced during the air pulse is approximately 1/3 of the total impulse.

Taking this into consideration the net thrust produced during a real launch from bottle stretching is likely to be around 1/2 of the thrust measured in this experiment.