Guppy Water Rocket
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Simply fit an old car tyre valve assembly into a bottle cap (10mm drill, and be careful to get a round hole), and pump a bottle up hard (more than 70 PSI). Light the barbecue (or stove - electric preferred), and get it good and hot. Wave your hand over it to find where the heat is distributed - BBQ's in particular often have hotter and cooler spots.
Hold the bottle by the tyre valve, with the bottom hanging about 20-30cm (8-12") above the heat, and spin it in your fingers. Don't let the bottle get within 10cm (4") of a naked flame, and don't let it touch any hot metal parts, or it will instantly get a hole and let the air out. In 20 or 30 seconds the bottom of the bottle will become hemispherical, and with longer in the heat will become elliptical. If it becomes assymetrical, throw it out and get another - you won't be able to straighten it.
Watch carefully and don't heat too fast - the plastic holds heat and will continue stretching after you remove it from the heat. If it stretches too far it will presumably explode. You might consider having cold water available to cool the plastic as soon as it has stretched far enough.
You can either cut off the nice shape and use it as a nose cone, or just use the whole bottle as a rocket. The plastic inside the base of a bottle is laminated due to the way they're made, and won't survive many crash-landings especially if it is also pressurised, but they're easy to make, so just replace them when needed!
I have received reports of mixed success in using hot air from a hair dryer for this - if you try it and it doesn't work, you can make the air hotter by restricting the air intake of the dryer with your hand (obviously you can make the dryer overheat this way too, but most dryers have an over-temperature cutout for this reason).
Please don't use my old method of using hot water for this. It is unsafe, and folk have landed in hospital with severe burns. After one accident I thought it was unlucky, but we've had a second report so please don't do it.
I know I'm going to get a heap of questions about how I do this, and apart
from the comments above I don't think words will help, I need to put up
pictures. Anyhow I'll try. I blow the nose cone, then with the cap on
and no pressure inside, rotate above the BBQ until the top half of the
bottle has shrunk. Keep heating, and any initial assymetry will become
less as it takes up the shape of least stress. Let the shrinkage extend
until it's close to the bottle mouth, then cool. You have a bottle with
a reduced top half, with basically parallel sides. The reduced section
is noticably necked-down from the lower half (perhaps 30%). Now pressurise,
and heat just the middle section (not the bottle neck) until it expands
again. If you got it right, you now have a Super-guppy pressure vessel!