water rockets
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Hello children, welcome to the wonderful world of water rockets. Items you will need include:-

Some air and water for fuel.

A compressor - I use one of those cheapy 12v car tyre inflator things, but will soon be trying a 240v fridge compressor.

Some sort of lightweight, high pressure, composite cylinder thing - a 2litre drinks bottle should be fine. (maybe with a big compressor you could try something bigger - like a 9litre water  fire extinguisher ?).

Some self adhesive tape and round nosed scissors ??


The 12v tyre inflator is a good choice for a basic launch system as it is small and portable, the only down side is the time it takes to get the rocket up to launch pressure. My initial plan was to find a cork that was a tight fit in the top of the 2l bottle, then drill a hole through it and glue in a valve from an old tyre. Due to an ongoing 'no cork' situation I had to resort to a bodge. I cut the old valve stem from a car tyre, wrapped it in gaffer tape, slid it into a 2" long bit of garden hose, then added more gaffer tape until it was a tight fit in the neck of the bottle.

That's it really, very basic. You have no control over when the rocket actually fires, but that's half the fun :-) 

Start by 1/4 filling the bottle with water, then stick the valve bung thing in, find some safeish way to keep the way to keep the rocket up the right way, start the compressor and run away a bit incase it all explodes.

A trolley jack makes a good mobile launcher, just tape a couple of bits of stiff wire to the rocket to make loops to go round the vertical stick thing.

If you can't be bothered to make proper tail fins, then tape a couple of short bits of rope to the back of the rocket like kite tails. These will provide enough drag to stop the rocket from tumbling in flight.


Rocket on launcher outside the bunker at minimission.co.uk HQ in Dorset after about 2 mins on the compressor.

Not long after.

Pressure at launch was around 35psi.


After a couple of successful launches of the small boring  rocket it was time for something bigger. A 18litre water bottle from the office water cooler !!! Purely by chance the bung valve for the 2litre bottle fitted perfectly into the hole in the blue cap on the 18litre which made things easier.

The main concern with the BIG bottle was that if it kind of exploded then there could be quite a lot of 100mph sharp plastic bits to dodge. The bottle has a handle in the side, with lots of tight radius mouldings that look great for producing stress risers.


During the first pressure test, we underestimated (completely ignored) how long it would take to get to 18 litres to 35 psi using the tiny 12v tyre inflator!!!. The middle of the driveway may not have been the best location, but luckily no body needed to get past.

After about 5 mins the gap between the handle and body had gone from 25mm to about 10mm.

Then after about 10 mins the cap blew off.

Explosive decompression causes the surface of the water to 'boil' producing a nice cloud!

Much to our suprise the bung had not come out of the bottle cap, the whole cap had come off of the bottle !! Instead of a 20mm exhaust port, we would now be using a 50mm one. Not too sure of the implications to acceleration of a rapid discharge wide bore vs a slower discharge small bore though......


As we had no idea of the amount of thrust being produced it was decided that the for the first launch the rocket should be tied to the trolley jack, to stop it sailing off into the neighbours greenhouses etc.

The bottle is getting close to launch pressure and is starting to leak a bit of water.

Then without warning the thing takes off!

0.2 seconds after launch

0.4 seconds after launch

The 2 ton capacity trolley jack being lifted up to 45' shows how much power the rocket has. You wouldn't want to be in front of one as it went off !!!


A second tethered launch was set up, but one of the test team (thanks Ade!) undid the knot in the rope !

AVI of free launch 1.4Mb

Note the all AVI files have been compressed  with DivX codec  divx_3.11alpha.zip 

For more info check http://be.divx.st/download


On the pad...


T + 0.1

T + 0.2

T + 0.3

T + 0.4

T + 0.5

The last pic shows the last bit of water coming from the rocket.

Maybe using a smaller exhaust port would mean thrust for longer, but the weight of the rocket would be higher... Tricky one !


More Rockets soon !!

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All .avi files compressed using the DivX codec. Download v3.11a here 701kb