Introduction: 2 Liter Bottle Launcher
After over 20 years of service and thousands of rocket launches, it is time to retire my old rocket launcher and make a new one. This device is used in my classroom as part of my rocketry unit. We study rockets and then build and launch pop bottle rockets as a culminating activity. It is run using a portable compressor and air hose connected to the launcher and operated using a ball valve.
This will be a total rebuild using new parts and incorporating a few upgrades that should make it last until my retirement.
Just as a note, you will find a lot of launchers that use a bicycle pump or other ways of creating pressure but I feel an air compressor is the best solution. The main reason is consistency. My students are trying to design the best rocket possible and without a regulated air supply you cannot get reliable data.
Compressed air is dangerous and I don't allow students to launch their rockets. I use between 80 and 100 psi. When I began, I tested some bottles and they burst at 200 psi so I feel I have a good safety margin. I have only ever had one rocket "blowup" on me and that was a result of a cracked end on a used bottle.
Step 1: Materials
- 3 pcs @ 9” x 12” of ¾” plywood
- 1 pc @ 11” x 7 3/8” of ¾” plywood
- 1 pc @ 4 ½” x 4 ½” of ¾” plywood
- 2 pcs @ 2 ½” x 11” of ¾” pine
- 2 pcs @ 2 ½” x 8 7/8” of ¾” pine
- 2 pcs @ 1” x 12” of ¼” aluminum bar
- 1 pc @ 18” x ½” PVC pipe (4 ½” and 11 ½”)
- 1 pc ½” PVC 90 deg. Elbow
- 1 ea. 1/2 in. Lead Free Brass Threaded FPT x FPT Ball Valve
- 1 ea. 1/2 in. x 1/4 in. Galvanized Malleable Iron MPT x FPT Hex Bushing
- 1 ea. ¼ in. MPT air fitting
- 2 ea 1in. EMT Galvanized Strap 2-Hole
- 1/4 in. -20 tpi hex bolts, fender washers and nylon lock nuts
- 3 ea. 2 inch
- 2 each 1 ½ inch
- 2 ea 2” x 3/8” springs
- See step 9 for updated parts and pictures.
- 1 ea 2 1/2" long 1/2" steel pipe
1 ea. 1/2 in. PVC Sch. 40 Female MPT x S Adapter
Step 2: Construction
In this step we will build the base and the adjustable platform. Once you have cut your pieces to the sizes listed on the materials list, it is time to assemble the pieces. Locate the centers of the 7 1/2" x 11" and the 4 1/2" x 4 1/2" piece and drill a 1 1/8" and 7/8" hole respectively as shown in the picture. Use a piece of pipe and elbow to hold the two pieces aligned so that the holes are concentric and then glue and screw the pieces together. Next glue and screw the side pieces to the base and the adjustable platform. Do not add the top pieces to the platform at this time. You may choose to add them if you want but I left them off so that I could easily paint all of the surfaces.
One of the upgrades, I have added, is the use of solid pieces of wood for the narrow pieces on the platform. On the old one, I had to replace one of the pieces because it had delaminated.
Step 3: Priming
I first primed all of the pieces with an oil based primer. I was able to coat all sides of all the pieces by either hammering nails into the wood to act as legs or I used a piece of wood with drywall screws as a standoff. I have a stack of these around my shop that I use for various projects.
Another of the upgrades I wanted to give my launcher is a good paint job. If you look at the old one, many of the pieces are starting to delaminate. Even though it is nearly impossible to prevent fir plywood from checking and splitting a good paint job goes a long way in making it last as long as possible.
Step 4: Painting
After the primer was dry, I painted the launcher with exterior latex paint. I also screwed the top pieces onto the platform and just used the wet paint as a glue. The 2 1/2" drywall screws will be strong enough to hold it together.
I got the paint half off at our local hardware store because it was a color that they mixed up for a customer that didn't match. If you don't care about the color, most paint stores have cans of mixed paint that they will sell at a discount.
Step 5: Assemble Launch Tube and Valve
Next, I assemble the parts to the launch tube. This is fairly straight forward. Use PVC cement to assemble the plastic pipe and use Teflon tape on all of the screw fittings.
Update See step 9 for updated assembly.
Step 6: Attach Launch Tube to Platform
Once the launch tube is dry, attach it to the bottom of the platform. I used 1" EMT straps which almost perfectly fit the outside diameter of the 1/2" PVC pipe fittings. I had to add a few layers of black electrical tape to get a tight fit. I also added a little bit of masking tape to the 1/2" elbow where it goes into the bottom of the platform. the 1 1/8" hole is just a little bigger than the fitting. I didn't want any play in the launch tube because it might affect the launch of the rockets.
Step 7: Connect the Launch Platform to the Base
To connect the launch platform to the base, lay out holes as shown in the picture above. The top hole is located for a 45 deg. angle. My old launcher was designed so that it could be set for 0, 15, 30 and 45 deg. angles but I found that I only ever used 0 and 45. The hole on the opposite side is measured the same as the lower left hole. Drill the holes with a 17/64" drill bit in the base first, then set the launch platform in the base and use 1-1/4" spacers to hold the platform at the right height. Drill through the base and platform at every hole. Attach the platform to the base with two 2" 1/4"-20 tpi bolts and fender washers and secure with nylon lock nuts. Do not over tighten. The platform should be free to pivot. Angle the platform to a 45 deg angle and check your hole alignment. Mine was off slightly and I had to run the drill though to clean out the hole. Insert a 2" 1/4"-20 tpi bolt and fender washers and secure with nylon lock nut.
Step 8: Clamping Mechanism
Now that the platform and base are assembled, it is time to add the clamp. To launch a rocket, you slip a bottle on to the 1/2" PVC pipe; squeeze the two aluminum bars together so that they capture the flange on the pop bottle; open the ball valve and then let go of the 2 bars. Two springs pull back on the bars and the rocket flies into the air.
First drill 2 17/64" holes 1/2" from the end of each bar. Drill 2 matching holes, 2" apart, in bottom cross piece of the launch platform as shown in the picture.
Next attach the 2 springs to the aluminum bars. They should be attached even with the top of the top cross piece. I used some #10 x 3/4" self taping screws but I pre-drilled the aluminum. I then cut them flush with the back side and sanded it flat as shown in the pictures.
Use two 1 1/2" 1/4-20 tpi bolts and fender washers under the bars and on bottom side of the cross piece. Then secure with 1/4" nylon lock nut. Do not over tighten. The bars need to swing freely.
To attach the other end of the spring, I removed the bottom screw from the cross piece and used it to attach the spring
To finish the clamp, you need to add two stops for the bars. I had a scrap piece of 1/2" plastic to use for the stop. It could be made of almost anything or you could most likely get by without a stop. I cut out 2 pieces using an 1 1/8" hole saw and then fastened them using the 2 top screws of the cross bar like I did for the springs.
Step 9: Update! PVC Breaks After a Few Days of Launching.
It turns out that going 100% PVC was a bad idea. The 1/2" PVC to 1/2" male pipe adapter was the weakest link. It was a quick fix however. I bought a 1/2" PVC to 1/2" female pipe adapter and a 2 1/2" long piece of 1/2" pipe and made a quick fix. It works great now.
Here is the finished launcher with the old one so that you can compare the two. The last improvement I added was rounding the ends of the aluminum bars. You can see on the old one I had wrapped tape around the ends because my hands would get sore from the square corners.
Watch the video of how to launch a rocket.