How much water to use in a water rocket?

Since this is a question we get asked all the time, today we performed the experiment that many students perform at school to see what the optimal amount of water is for a water rocket. Here is some background information on why we use water. A question that also often comes up is how high would a rocket fly with air only, and so we looked at that as well.

We used our Axion III rocket for this experiment because it has a smaller volume compared to the larger Axion rocket which meant we would use less air from the scuba tank. The rocket parameters were:

	Parameter	Value
	Capacity	3.35L
	Dry Weight	415g
	Diameter	90mm
	Nozzle	9mm
	Launch Pressure	110psi

Because the capacity of this rocket is 3.35L we chose the following water amounts to test with: 3%,15%,30%,45% and 60%. Here are the results including simulator predicted altitudes from two different simulators.

Results

Sim A was Clifford Heath's simulator and Sim B was Dean Wheeler's Simulator.

 *Due to the weight of the 2L flight and the small nozzle the rocket had very low acceleration and weathercocked significantly and therefore didn't reach as high as predicted. With the simulator predicting optimal water amount of 1.1L for this rocket we see that there are two nearly equal altitudes either side of this peak. Although the 45% percentage fill gave a slightly higher altitude we must remember that the error in our measurements is in the order of 10%. See day 159 for details of how the 10% is arrived at.

Rolling out safety tape the easy way

Mixing food colouring

Ready to launch

Setting up

Documenting experiment

Visitors checking out what all the fuss is about

Back to munching on grass

Conclusion

As can be seen from both the observed and simulated results the optimal amount is about a 32% full of water and the altitude drops off significantly when you use too little water. With air only the rocket flew to only 37% of the altitude when compared to the optimum.

Video

Other flights

A big thank you goes to John Beans from Jolly Logic who generously donated an AltimeterThree for our experiments. Obviously we were very keen to try it out on our rockets. We installed the AltimeterThree App on Paul's iPod so we could download the data. I like the fact that it not only records altitude but also acceleration. Something we don't usually record.

We flew the Axion rocket with foam for this test to see how well it would detect launch (because the foam flights tend to have a slower lift-off) and then to see what the recorded flight profile was like. The altimeter was very easy to set up and get going. We only flew it on one of two flights, because the wind was blowing straight towards the tree-line and we didn't want the altimeter becoming a Christmas ornament for the holidays. The second flight was similar to the first, it flew nice and straight all the way up.

Here is a plot of the altitude and acceleration:

All the flights flew and recovered well on the day, so we were happy.

Axion flying with the AltimeterThree

Flight Details