Drop-down Nozzle

Launching a water rocket with a full bore nozzle, the full open neck of the bottle fit over a half inch size piece of PVC pipe, gives a very powerful blast for lift-off but it doesn’t last very long. People have wanted a way to reduce the nozzle size as pressure drops after lift-off to make the propulsion last longer and give the rocket more altitude.

If you research “T-nozzle” on water rocket web pages you will find several examples. They either seemed difficult such as needing to cement parts while they were inside the bottle, they required parts that I did not have in my shop or the bottle had to be the type that connects to another bottle so that you had that second opening to work through. I came up with a version that I had not seen anywhere else that uses a plain 2 liter bottle and can be removed if necessary without destroying it or the bottle.

My first launcher used a garden hose washer held against the edge of the neck of the bottle as the pressure seal. As I was looking at the hose washer’s fit against the bottom of the bottle I noticed how it would fit inside the tapered part of the bottle neck. Then I worked out a way to get it on a nozzle after it was inside the bottle.

This nozzle reduces the 22 mm. bottle neck down to a 12 mm. jet of water and air for prolonged propulsion.

Revision 1
As test launches were being made at higher pressures, 60 P.S.I. and above, the impact of the nozzle against the inside of the bottle eventually knocked off the hose washer and the part holding it on. It obviously needed to be made heavier and stronger. The hose washer was replaced with a black rubber grommet that is twice as thick.

The end piece will no longer be just forced on. It will be pinned in place by plastic tube running perpendicularly through it, and the inside end of the nozzle tube will also be flared to help keep the end piece on.

The original instructions have been altered to describe this Revision 1.

Parts you will need

(1) 4 inch long piece of 1/2 inch PEX plastic pipe, which is 5/8 inch O.D.
(1) 1 1/4 inch long piece of 1/2 inch PVC pipe ( fits inside bottle neck )
(1) 3/4 inch long piece of 1/2 inch PVC pipe ( end piece )
(1) rubber grommet 5/8 inch I.D., 1 1/8 inch O.D., 5/16 inch thick
(1) 1/4 inch O.D. (or smaller) rigid plastic tube like those used in spray bottles
1/2 inch PVC pipe for your launch tube

Tools you will need

a tubing cutter
a tubing flaring tool (optional)
a drill bit slightly smaller than the plastic tube (for a tight fit)
sand paper and if possible a sanding sponge
a small plastic mallet to drive parts together
a 5/8 inch spade bit
a round or half round file
a 1/2 inch wooden dowel to use with sandpaper
a metal file or
a belt or disc sander is better and saves time
(3) pieces of thin fishing line at least a foot long ( braided, not mono-filament )
(3) strong rubber bands that can wrap around the bottle neck
(1) piece of coat hanger wire with 1/4 inch at end bent into an ‘L’ shape

Assembly

1.With a tubing cutter cut 4 inches of PEX, 1 1/4 inch of PVC and 3/4 inch of PVC.

2. Use the pointy blade on the tubing cutter to slightly bevel the inside of the PVC pieces at each end. Rotate  each end on sand paper or a sanding sponge to smooth it.

3. Sand or file any raised edges created by the tubing cutter on the ends of the PVC until they will just fit inside a bottle neck.

4.You need to taper the grommet end of the assembly. Mark a ring around the smaller piece of PVC about 5/16 inch from the end that will touch the grommet. You don’t want to sand or file off this ring. Slip this piece of PVC, ring end first, temporarily over some PEX pipe. It need not be all the way on to the PEX, just snug. You need to sand or file the end of the assembly like a pencil without the lead. The pointed end should be shaped down to 5/8 inch in diameter, same as the PEX pipe. The taper should go smoothly up to the ring that you have drawn. If you have a power sander go slowly and carefully rotate the assembly to shape it. When finished carefully remove the tapered piece from the PEX.

5.If you have a tubing flaring tool, use it to flare one end of the PEX pipe. You only need to enlarge it some so parts can’t slip over that end so easily. Don’t force it enough to crack it.

6.Start the short piece of PVC pipe, tapered end first, over the non-flared end of the PEX pipe. Tap the PEX gently through with the plastic mallet if you can’t force it by hand. Stop when the tapered edge meets the flared end, don’t go past it. Drilling a 5/8 inch hole through a board will hold the PEX and allow it to pass through as you hammer.

7.Now you will need to drill four holes through these two parts to lock them together. Use a drill bit slightly smaller than your small plastic tubing so that it will be a tight fit. Test the fit in a piece of scrap PVC first. You will want to drill through the PVC piece at the straight area between the tapered part and the edge that touches the grommet. Leave a narrow solid ring next to the grommet. Try to drill straight through toward the center. If possible use a drill press and a scrap of wood with a groove in it to hold it steady. Repeat drilling three more holes 90 degrees apart around the piece.

8.Push the piece of tubing into a drilled hole. Push it in until it is barely through inside. Cut it off flush with the outside surface. Repeat in the other drilled holes.There should not be anything sticking out to keep it from entering a bottle when you test it, so sand or file the tubing flush if necessary. The tubing not only should keep the parts together, the holes should help drain out the last of the water in the bottle. If the pieces of tubing seem to fit loosely, they will be blown out and lost at launch time.

9.Slip the rubber grommet over the PEX pipe and slide it up against the end piece just installed.

10.Force the PEX pipe into the larger PVC piece until the rubber grommet is touching, but not crushed between, the two PVC pieces. Remove the grommet from the pipe by stretching it over the short end and save it for later.

11. Test fit the assembly in the bottle neck and sand it by hand if it doesn’t fit smoothly.

Altering the launch tube

My launcher was designed to have interchangeable launch tubes for different rockets. Whether you need to create a new launch tube or alter the one you have, you will need to ream out at least the top two inches so that the new nozzle will fit loosely inside. Your launch tube will also need to be about two inches shorter than the original to fit inside the bottle with the new nozzle on top. Cut your launch tube to the right length first.

I used a 5/8 inch spade bit, a wood working drill bit, to begin reaming out the 1/2 inch PVC launch tube. Clamp the spade bit upright in a vise, sticking out as far as you intend to drill, and start the tube over its point. Turn the tube slowly clockwise by hand and let the bit cut out the inside. When the bit has cut through far enough, reverse turning and lift off the tube. Repeat this until the bit doesn’t cut out any more of the PVC.

Check the fit of the nozzle in the launch tube. It will probably not fit yet. If by rare chance you have an 11/16 inch spade bit repeat the reaming process. If you have a round or semi-round file that fits inside the tube use that to remove more of the PVC. If not, the rest will need to be sanded out. Wrap sand paper around a 1/2 inch wooden dowel and start twisting it inside the tube. Check the fit and keep sanding until the nozzle fits easily all the way into the tube. As a last step I used some automotive finish rubbing compound. It is a paste with some fine abrasives in it used to buff out scratches and blemishes in car paint. This helped to smooth the sand paper scratches inside the tube and on the nozzle. Apply compound to a damp piece of cloth over a wooden dowel and polish the inside of the tube until the nozzle can drop freely into the tube. Wash off the remaining compound.

Installation – Building a ship in a bottle

1. Set the bottle that you want to use in an upright position. Take each of the three pieces of fishing line and pass an end through the grommet until you reach the middle of the line. Take the grommet and rest it on the neck of the bottle. Position each doubled line one third of the way around the grommet with the six ends hanging down the outside of the bottle.

2. Take the rubber band and stretch it around the neck of the bottle below the flange to trap the lines in place.

3. Bend and push the grommet down into the bottle with a finger while trying to keep the lines somewhat tight and the grommet level if possible. Level off the grommet suspended by the lines just below the neck of the bottle.

4. Add another heavy rubber band over the lines below the flange. You can add another rubber band around the bottle threads as well and hold onto the lines tight with your hand.

5. Insert the assembly’s pointed end into the bottle trying to get it to start through the center of the grommet. A little dish washing soap on the end may help it slip through.

6. Once you have the grommet pulled up far enough on the tapered end so that it doesn’t fall off you should reach the coat hanger wire past the extended nozzle and hook on to the grommet with its bent end. Work the grommet up until it is almost into its groove on the assembly.

7. Carefully loosen one end of each of the three lines and pull them out from between the grommet and the assembly.

8. Use the wire tool to pull the grommet completely into its groove and straighten it out. Remove the wire tool from the bottle and pull on the nozzle to test how the grommet seats itself inside the tapered bottle neck.

9. If you ever need to remove the nozzle from the bottle get a long thin stick, such as a popsicle or craft stick, and while holding the nozzle end, use the stick to carefully push the grommet back over the tapered end so everything can fall out again.

How to use the nozzle

You will need to let the nozzle drop down into the neck of the bottle. Slip the nozzle into its special launch tube and push the launch tube all the way into the bottle. Turn the bottle upright and fill the water through the tube and nozzle. Now turn it over on its side to drain water out of the tube and then quickly insert the tube into the launcher. This method worked for my first launcher because the launch tube was easily removed and installed. If yours doesn’t remove or you don’t want to remove it every time, try holding the nozzle in the neck of the bottle while you fill the water through it then quickly set the nozzle into the launch tube and push the bottle down.

nozzle_in_bottle

nozzle_in_neck

The original version using a hose washer couldn’t take higher pressures.

drop_down_nozzle_V2

Drop Down Nozzle V2

Revision 1 using a heavy rubber grommet and pieces of tubing to rivet the end piece in place. Holes in the side of the end piece also allow less water to remain in the bottle.

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