Posted: July 18, 2011 in design - launchpad
*I took this picture just now in the middle of a redesign of the plumbing, so the central tube is much shorter than it should be!*
Here you can see the wooden support, complete with decorations, for holding the launch tube. It also has the bottle locking mechanism (the 4 x meccano strips and two tent pegs).
The tent pegs slip over the lip just above the thread on a soft drink bottle and prevent the bottle taking off until you tug on the string (on the left, in its cunning cable tidy) and pull the tent pegs out. the two pegs are held together with a bit of coat-hanger wire on the left hand end and the string is attached to this to ensure that both pegs come out together and the rocket launches smoothly.
The launcher is really basic, made from one old shelf. I cut the two ends off and screwed them in to make the uprights. Two pipe clips hold the tubing in place, and crucially allow you to take it out to fit to your rocket. Fitting is done off the launcher so that you hold the rocket upside down, insert the tubing and make the seal, then carefully turn it right way up so that the water doesn’t go into the tubing but stays in the bottle.
The two holes in the board at the right for are putting two more tent pegs through to anchor the launcher to the ground, so that it doesn’t move when you tug on the launch string.
That is essentially it. All in all it took two days-ish to make, but that’s only because the bloody glue takes so long to dry.
*the yellow circle bit on the left is an off-cut from the new design and should be in the bin.
I started with designing the launchpad and release mechanism.
The launcher needs to do the following things:
- Hold the rocket in place
- Allow presurising of the rocket reliably
- Provide a quick release launch mechanism
I spend a good few days reading various websites and looking at numerous designs. One of my goals for the design was to be as cheap as possible, so I settled on a system based on some PVC pipe, an old shelf that I’d recently take down and a couple of extras from B&Q. Total cost for the launcher is probably about £10.
- 21.5mm PVC overflow pipe
- 2 x 22mm PVC push-fit end caps
- 1 21.5mm PVC pipe t-connector
- PVC Pipe Weld glue
- Evostik Serious Glue
- Schrader valve from a bike innertube
- PTFE tape
- 2 x 21.5mm PVC pipe clips
- 4 lengths of Mecanno
- a bunch of screws
- 4 tent pegs
- a bit of coathanger wire
- various bits of coloured insulating tape and spray paint for decoration
Using a hack saw I cut a lengh of pipe that was the length of the inside of a 2-litre bottle + the height of the upright on the launchpad. I wraped some PTFE tape around one end and used the Pipe Weld glue to secure it into the centre of the t-piece. On each end of the t-piece I did the same fitting process with two short lengths (about 4 inches long), and left it all to cure for 24 hours.
Next day I drilled a hole in one of the end caps the same diameter as the Schrader valve. I managed to blag a burst bike innertube from the service desk in Halfords, and set about cutting the valve and the small sourounding circle of rubber from the tube. Using the Serious Glue I fitted this from the inside of the end cap before welding both end caps onto the arms of the t-piece. Once this is set the plumbing for the launcher is complete. The final part of the tubing is to wrap some PTFE tape around the section of the upright pipe where the neck of the bottle will sit to make for a better quality seal.