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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Domed or "Guppied" Nose Cones

Members post instructions to construct water rockets, launchers, deployment systems, staging mechanisms, splicing bottles, and payloads.
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Domed or "Guppied" Nose Cones

Post by SaskAlex » Sat Jun 08, 2013 3:21 pm

Most experienced water rocketeers probably already know most of this, but I was asked some questions about it so I thought I'd put together a quick how to and some background info.

People have been using heat and pressure to make the bottoms of bottles more round or elliptical for a long time. Most people call this "guppying". The result is a "guppied" bottle. This technique is mostly used to create more aerodynamic nose cones, but I actually use it to improve the shape of the pressure vessel (some "pentaclaw" bottles always fail at the base, and guppying can increase burst pressure). I think Clifford Heath was the first to guppy bottles; see his page here- http://polyplex.org/rockets/guppy.html. Or just do a google search for "guppied nose cone". Warning- some people (like Clifford Heath) perform this at fairly high pressures. I never have. The heat can easily melt a hole and cause a bottle to burst. I've melted holes at 20 psi, and the bottles have not burst. The air just shoots out the small hole. This is all at your own risk, so think about what you are doing.

My technique is fairly simple- somehow secure a bike valve in the end of a bottle cap (this is a very useful and basic water rocket tool, I won't describe it here) and pump up a bare bottle to about 20 psi (no water at all, just air). I then set a heat gun on the work bench and have it pointing outward. Hold the bottle so that the base is about 2" from the end of the heat gun (with the axis of the bottle at about a 45° angle from the direction of air flow) and spin the bottle constantly. I actually insert the valve stem on the bottle cap into the chuck of a cordless drill. Hold the drill with one hand, and cup the bottle around the middle with a rag in the other hand. The rag is just used to reduce friction. The drill helps me spin the bottle at a faster, uniform rate. It's not very difficult, just grab a few bottles and experiment with it.

My bottle ends don't usually turn out perfectly symmetric, but some people claim to get very good results after practice and careful application of heat. One source I read (can't remember where) said that it helped a lot to use a fine pencil tip on the heat gun. I don't really care about getting a perfect shape, because I do this to my pressure vessels instead of my nose cones. The bottles I've used it on the most are SmartWater bottles (1L and 1.5L). They have a flat base which bulges out under pressure, and I'm quite certain they would crack and fail at a fairly low pressure without any modifications (but I've never actually tested this). They can certainly handle the 20 psi I put in them during the guppying process, though. Here are two 1L SmartWater bottles that I have guppied, compared to an unmodified bottle. Note that they are all lined up at the bottle necks.
DSCF2783 (640x363).jpg
Also note that you can use heat and pressure to reshape other parts of the bottle (such as the ridge around the bottle near the top of a small pepsi bottle) but it's more challenging. Gary Ensmenger describes how he uses an iron to reshape bottles here- http://www.h2orocket.com/topic/iron/iron.html. If you have any links to other good articles, feel free to post them here.

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Re: Domed or "Guppied" Nose Cones

Post by bandsons » Sat Jun 08, 2013 4:01 pm

Thank you! This could really help with wrapping the bottles.

I've noticed that the Pepsi 2 liter bottles we use change shape on the bottom somewhere around 100psi. Maybe it would be good to start from this shape? I will read up on your links.

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Re: Domed or "Guppied" Nose Cones

Post by U.S. Water Rockets1 » Sat Jun 08, 2013 10:27 pm

You might find that putting your bottle in the chuck of a small electric drill will allow you to spin it in the hot air stream for more even heating. That would help achieve a more uniform shape.

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