Making a Water Rocket from FTC (Fluorescent Tube Cover)
Water rockets made from Fluorescent Tube Covers (FTC)—have an advantage over rockets made from bottles in one important respect: they are smaller in diameter, and so have much less induced drag as they move through the air. The higher drag of “fatter” rockets (2 liter bottle rockets for example) can be compensated for by using smaller nozzles—smaller nozzle = less thrust (but spread over a longer duration) = lower velocity = less drag = higher flights. However, all else being equal, the rocket with less drag will be able to fly higher . . . soooo, the FTC rocket.
FTC Rocket (brightly painted to aid in recovery)
FTC is made from polycarbonate plastic (as used in shatterproof safety goggles) and is intended as an “unbreakable” protection from shattering glass of fluorescent bulbs/tubes in settings such as restaurants—who wants bits of crunchy glass in their food? FTC comes in several diameters that correspond to the bulb diameters, which are measured in eighths of inches as expressed in “T” numbers: T8 (8/8 or 1 inch), T12(12/8 or 1.5 inches) are the two most common sizes, available in 4 foot or 8 foot lengths.
The photos and text below document the building of a rocket from T12 4-foot-long FTC (bought at a local Lowes home center for about US $2.00).
T12 4-foot-long FTC
PL Premium (polyurethane) Construction Adhesive
Several 500 ml PET plastic soda bottles (here Coke and Sprite)
1 foot or so length of 1/2 inch PVC plastic pipe (here gray electrical conduit)
1 1/2 inch to 3/4 inch copper reducing coupling (also known as a “bell”)
Materials for fins (here, three fins made from plastic "credit" cards bent and joined to a split skirt of FTC with double-sided foam tape)
PLastic FTC end caps (come with FTC)
Plastic Easter egg ( 1/2 for nose cone)
1 inch Schedule 40 PVC plastic coupling or end cap (for aid in cutting FTC)
X-acto or utility knife
Hacksaw (or here bandsaw)
Sink or bucket of cold water
Making the Upper Parts of the Rocket:
Using the Schedule 40 PVC plastic coupling or end cap as a guide/support, use an x-acto/utility knife to cut a length of FTC for use as a payload/altimeter chamber/nose cone. Cut another section—a length appropriate for containing a parachute to add below that.
Squirt a thin bead of PL Premium around the top outer circumference of one of the FTC caps and insert it into the Easter egg half. Dry/friction fit the other FTC cap into the bottom of the nose cone length of FTC.
The longer (parachute chamber) length of FTC will be pressed down over the bulkhead after completing the steps below. Warming the bottom of it with a heat gun may help it slide on easier.
In this design the nosecone pulls the parachute out because it is wrapped tightly with the shroud lines. . . .
pull it into a point from the center, fold it into
a 2" length starting with the center/tip and progressing to the end using a zig-zag folding process. Finally, wrap the shroud lines very tightly around the outside. This drops in and out of the lower FTC chamber.
The piece of rope attaches the nosecone to the rocket via a paper clip on the nose end and the rubber band looped around the bottle neck (bulkhead). This allows plenty of room for the nose to fall and pull the parachute out easily (shroud lines are, like the rope, attached to the paper clip).
The real "secret" is the loose (you may need to wrap some tape around if it's too loose) but precise fit between the FTC end cap and the FTC.
FTC Rocket Upper Parts
Making the Rocket Nozzle and Upper Bulkhead:
The first step is to fashion a “jig” for heat-shrinking soda bottles (the nozzles/bulkheads) to fit snugly over the FTC. Using a pipe cutter, cut about 1/4 inch off the end of the 1 1/2 inch to 3/4 inch copper reducing coupling. This step is not “absolutely” necessary, but it will allow the inner shoulder of the bottle to seat against the outer shoulder of the copper coupling for better alignment.
Next, using a hacksaw or bandsaw, cut the endmost inch or so off the necks of several soda bottles—these will be used for bushings/stops to help align and hold things together while heat-shrinking.
Use an x-acto/utility knife to cut the bottoms off of a couple of soda bottles—leave them long, as they will be trimmed after heat-shrinking. One will be used as part of the jig, and the other will be used as the actual FTC nozzle/bulkhead.
Slide the copper coupling onto the length of 1/2 inch PVC plastic pipe. Slide one of the short soda bottle necks (bushings/stops) onto the PVC and inside the coupling until snug. Slide one of the soda bottle tops on the other end, and over the copper coupling. Finally, slide another of the bushings/stops onto the PVC until snug against the soda bottle mouth. Sight down the length and make sure everything looks properly aligned.
Prop up your heat gun at a comfortable height and turn it on. Holding the jig assembly with both hands, bring it close to and above the end of the heat gun. Spin it rather quickly with your fingers with the heat directed first against the cut end (bottom) of the soda bottle, and then spiraling toward the neck as it shrinks. This may take a few tries before you get the hang of it—it is important to try to work quickly, as the PET plastic doesn’t always shrink evenly/uniformly, and shrinks better on the initial heating than on subsequent passes over the same area. Don’t worry if the cut end refuses to shrink tight and ends up looking like a flower, since most of it will be cut off anyway. When your done heat-shrinking, dip the jig assembly into a bucket of water or run it under water in a sink to cool everything down.
Use an x-acto/utility knife to carefully cut the shrunken bottle near the end of the copper coupling. Make several passes pressing moderately hard rather than trying to cut through all at once—the shrunken PET plastic is thicker and very tough!
Pull the nozzle form you have just made off of the copper coupling and PVC pipe and slide it on the other end, leaving a 1/4 to 1/2 inch gap between the shoulder of the copper coupling and the cut end of the nozzle form. The copper coupling is exactly the same diameter as the outside of the FTC, so this first nozzle form will add just enough thickness to allow the next heat-shrunken nozzle to slide over the outside of the FTC. By not covering the copper coupling all the way up to the shoulder, there will be a section that is smaller in diameter, and will act as a stop when you slide the finished nozzle onto the FTC.
Completed Nozzle Jig
If needed, slide another of the bushings/stops onto the PVC to secure the first (now upside down) nozzle form, then slide another of the soda bottle tops on the other end, and over the copper coupling. Finally, slide another of the bushings/stops onto the PVC until snug against the soda bottle mouth. Again, sight down the length and make sure everything looks properly aligned.
(left) Bottle mounted for heat-shrinking (right) Starting to shrink
Heat-shrink the finished nozzle form as above, and carefully cut it off after cooling. Squirt a thin bead of PL Premium around the bottom inner circumference of the nozzle and push it over the FTC , pressing it in until it seats evenly against the end of the FTC.
Repeat the above steps to fashion and attach an upper bulkhead to the rocket. Screw on a bottle cap extra tight to seal the pressure chamber.
Please note that the nozzle being heat shrunk (left) is clear, while the finished/mounted one (right) is green
The fins are simply fashioned by bending plastic "credit" cards and attaching them with double-sided foam tape to a longitudinally split length of FTC as a skirt. This allows you to mount the fin assembly with tape so that it is easy to remove/replace in order to try other fin designs.
Plastic Credit Card Fins—FTC skirt is just barely visible
I will be using my sport cap release with this rocket—documented on the homepage of this site.