Bottle Neck Caps For Fluorescent Tube Covers

    A technique for making ends for water rockets using fluorescent tube covers and 500 ml Dr Pepper bottles.

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    Fluorescent tube covers are long plastic (polycarbonate) tubes sold in hardware and lighting stores.  They are designed to slip over Fluorescent light bulbs to prevent a broken bulb from showering anyone below it with broken glass.  Readily available in the USA, they might be difficult to find in countries with fewer or different safety regulations.  Needless to say, being extremely tough, about 42 mm (1 3/8 in) in diameter and coming in lengths over 2 meters (8 ft), it was only a matter of time before a water rocketeer noticed this material.

    There are reportedly some good fits to FTC using commercial model rocket nosecones.  This will work fine if you don't need to put anything on top of your rocket, but it will get expensive unless you are using water recovery.*  The following describes a method I use based on the conical shape of 500 ml Dr Pepper bottle neck.

*It is possible to trim one of these rockets so that it descends horizontally at a non-dangerous speed, but I suspect this will ultimately cost you altitude.

    Make a mold for plaster of paris by drilling a carefully centered  1 3/8 inch hole in the bottom of a Dr Pepper bottle using a spur drill (A spade drill bit with spikes at the corners of the cutting edge.)  Smear Vaseline inside the bottle neck as a release agent,  cap the bottle and insert a square cut piece of FTC that is long enough to stick out the bottom of the bottle.  Pour plaster into the FTC and let it harden.  If you are smarter than I am you might think about how you are going to hold this assembly while the plaster sets before you mix the plaster.  When it is hard, you can twist and pull the form out of the bottle, but leave the FTC on the plaster.  Clean off any plaster that might have leaked out around the FTC inside the bottle.   I made a line 3 cm from the shoulder on the FTC to guide my scissors when I trim bottlenecks before shrinking them.
Picture of the Plaster bottleneck formCut off the neck of a Dr. Pepper bottle at the shoulder, slip it onto the plaster form, trim it to the line, and shrink it with a heat gun.  You will find this makes a snug fit with FTC in a minute or so.
    The contact surfaces inside the neck and outside the FTC are roughened with coarse sand paper, and I glue them together with Household Goop because it is hard to find the superior but uglier PL Premium where I live.  Using Goop, it is easy to level the glue squeezed out with a moistened finger.  Use rubber gloves if you handle PL Premium at all.

    Using this technique, it is easy to put two nozzles on an FTC.  Thinking about it another way, it is also easy to put on a nozzle and a threaded fitting.  I glue (Goop again) two bottle caps together, back to back, and put a Pop rivet through them.  Use a washer to get the rivet tight.  This makes an air-tight female-female adapter.  Then anything I can make based on the male threaded neck of a bottle, for example, payload chambers or recovery systems, securely screws onto the front of my rocket.  I make a fairing from another bottle or short piece of FTC to cover the linkage.  These rockets are modular, and it is easy to switch and swap parts.

7/25/00  Gordon B. McDonough
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