Water bottle rocket using a hand pump and plastic pipe to launch the rocket. (Credit goes to 2nd Kirkham for the idea and information) http://e-voice.org.uk/secondkirkhamscoutgroup/resources/
Pipe - 1000 long, 103 inside diameter, 110 outside diameter
Wooden spine - 1210 long x 95 wide x 44 thick (giving a 210 free section to push the bottle into the pipe)
Tape pipe to wooden spine using 54 wide Duck Tape at 3 locations shown
Legs - 1000 (15mm copper pipe or anything suitable). Flatten end of tube, drill hole and screw into wooden spine 260 from the top (highest point). Add rubber feet (e.g. walking stick rubbers) if wish.
Vertical hand pump - foot pumps have been found to be unsuitable because they have to be secured rigidly to a baseboard and still buckle in use. The vertical pumps are easy and stable to use. Lidl occasionally sells the vertical pump quite cheaply.
Adaptors - a cork through which a plastic lilo inflator (slim cone shape) has been inserted with its wide end attached to a foot pump.
Instructions for Use
Take a 2 litre lemonade bottle. Fill it 25% (only) with water. (Too full and it will balloon along the ground)
Into its open mouth jam the cork adaptor as tightly as possible
Push the bottle up the pipe that has been securely aimed somewhere safe
Pump away and it self launches at about 20-25psi - flies about 60-80ft high then tumbles slowly and safely down where we normally have Beavers and Cubs waiting to catch it. It takes an adult about 4 or 5 big pumps, Beavers about 10 short pumps to launch.
Points to note
It's safe as long as no one is allowed to peer down the drainpipe.
It's fast to relaunch - 2 to 3 goes a minute.
You don't need many bottles.
Some smaller bottles have the same size neck and fly well
Corks are not so easy to get . Build up a supply. Plastic corks tend to be too narrow. Rubber bungs of the right size are rare. Corks of cork are good though some may leak slightly - only about 10% seem to fit.
We used bicycle valves for a long time as adaptors - got through a lot. One day we ran out of them and a Scout used the lilo adaptor in desperation. It is far better, the air goes straight through into the bottle and the adaptor lasts and lasts and lasts. However, lilo adaptors are not easy to get hold of - they are included in packets of spare valves and also usually are included with a new foot pump.
The kids get their feet wet which everyone likes.
The ground will get soggy
Cost : pump with adaptor ~£4-50. + cork (free) + bottle (Free) . Just add drainpipe ~£5 or less ??, In fact, the wooden spine and legs are optional extras - say, around £10 - 15 total + paint. Best £10 - 15 we have ever spent. We have 2.