Guppy Water Rocket

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Elliptical nose cones

I figured out how to stretch the bottom of a 2 litre bottle into a perfect nosecone shape (newer method, easier and safer than previous)!

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.

Guppy Rocket

My high-flying Guppy rockets (named for their shape) are made by using a bottle with a stretched elliptical nose, and then shrinking the body of the bottle by capping tightly and rotating over a BBQ. The heat must be very carefully applied, but because the bottle is capped as soon as it shrinks, internal pressure supports the shape and prevents it from collapsing entirely. For a Guppy, don't shrink any part of the nose cone, so that the nose cone is the widest part of the bottle. This results in a very low drag shape. I have built accurate altitude measuring equipment now (trundle wheel and inclinometer), and although the first Guppy has flown, the launcher was leaking and I think I only got it to 50-70 PSI. The height was measured at 90 metres (300 feet), so when I can get a Guppy up to 120 PSI (or more with filament tape reinforcing) I believe that 200 metres (650 feet) may be feasible from a single stage.

Super-Guppy Rocket

In October 98, I launched my first Super-guppy. This rocket (prototype shown here) is a full tear-drop shape with a tail that is tapered right down to the mouth, and flies higher than anything else I've launched. Although it is about the diameter of a 1.25 litre bottle, it's actually a shrunken 2 litre. It's harder to get them to stay symmetrical like this, so future ones will just use smaller bottles.

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!

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