By Marcus Wilson 05/07/2011 2

So, last Thursday and Friday, we had a great demonstration of some rockets, thanks to Steve Chrystall. Some of our visiting school students had made water rockets, and these were launched across our sports field at lunch time.  There were some pretty impressive entries (both in terms of distance travelled and blowing-up on the launch pad). The furthest student rocket travelled a staggering 130 metres, though Steve himself demonstrated one that  from memory reached around 180 metres.

We also had a demonstration of some pyrotechnic rockets – two on Thursday, two on Friday.  Of those, we had two and a half successful firings – the ‘half’ being a rocket travelled a couple of metres into the air before exploding early.

Rockets are extremely simple in concept. They are a demonstration of the conservation of momentum. The idea is that the rocket throws out exhaust backwards at high speed – this exhaust carries momentum – and as a result the rocket gains a momentum in the opposite direction. Overall, the momentum starts at zero (stationary rocket and fuel) and ends at zero (fast rocket and fast fexhaust, in opposite directions). As fuel you can use anything that can be expelled at high velocity. Commercial rockets use exhaust gases that have been created as a result of a chemical reaction (e.g. hydrogen with oxygen). You can make a tiny chemical rocket using vinegar and backing soda in an old 35mm film cannister (it releases carbon dioxide) but in the case of water rockets we can use water with compressed air to achieve the same thing.

The ‘design’ is really straightforward. Take a 1 litre soft-drink bottle, drink the contents, then place a bit of water inside and put a bung in the end, in which a valve (like on a bicycle or car tyre) has been inserted.  Then pump up the air inside to about 80 psi (about 50 kPa) – high enough for a good effect but not too high to blow open the bottle. (You’ll need to hold the bung in place while you do this.) Finally you let the bung fly out. The air/water mix comes out the back carrying momentum, and the rocket accelerates in the opposite direction.

You’ll need to experiment a bit with getting the right amount of air and water in the rocket, and putting a bit of weight on the nose so that it flies straight, but getting that right is part of the fun. Probably the most difficult bit is constructing a launch mechanism that holds the bung in place while you pump up the pressure but then releases the bung at your command.  

See if you can get one travelling over a hundred metres.


2 Responses to “Rockets”

  • I was on the measuring crew on Friday. Steve’s rocket was closer to 200m. It may have even been over 200m as it landed outside the boundary of the “safe” zone!

    My students at Hillcrest are keen to try and combine the best elements of each of the rockets seen on the day and hopefully break the 200m barrier. It seems that aerodynamics might play much more of a role than trying to maximise the impulse on take off. Any idea what the terminal velocity of an ideally shaped and weighted water bottle rocket is?

    A challenge like this should be a part of the lectures every year!

  • Not sure of the terminal velocity of the rocket – how about getting your students to think of ways of measuring it? Now, what challenge can we think up for next time?

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