This project started as a simple request to make a previously designed paper rocket launcher for STEM events. The original design had many flaws, and hopefully I've mitigated most of them to make the launchers safe, easy to use, cost effective, and easily transportable. I don't know if the version is actually version 5, but it's something around there. Every time a rocket launcher gets broken, I have modified the design, and many people have offered input to make the launchers better.

The students build the rockets out of paper and tape, so they can choose many designs and draw on their rocket if they'd like. Once the rocket is done, they take their rocket to a launcher, and pump up the air with a bicycle pump. The launch is controlled by a standard electronic irrigation sprinkler valve so that the student can stand far away from the launcher.

These are great for STEM events, or for home use. There are a few Boy Scout troops that have built the launchers, but my work also takes them to schools in the area. Many lessons can be applied to using the launchers. From the aerodynamics of the rocket, simple trigonometry for height competitions, or more advanced math to calculate the rocket trajectory, or just having fun letting kids watch their rockets soar, the possibilities are endless.

My version 2.0 launcher Instructable is here:

https://www.instructables.com/Air-Powered-Rocket-Launcher/

The improvements include:

Easier to find parts
Cheaper overall cost
3D printed parts can be printed on a typical print bed
Easier to transport due to stackable design
Compact design makes it easier to carry
Everything is attached to the frame to again make transport easier and makes it harder to lose components
Includes a blow-off valve for additional safety
New TPU printed part to keep from breaking

If you think of any improvements or modifications, please let me know. I'd love to make these launchers better and safer so that more people can use them and learn about STEM!

With that out of the way, lets get into the build!

Supplies

Tools and workshop materials

Tape measure
Drill bit set (7/16 inch and 1/2 inch bits)
1/4 in NPT tap
Soldering iron
Solder
Heat shrink tubing or electrical tape
A way to cut the PVC pipes
Miter saw is easiest
Hack saw with miter box
PVC pipe cutter
3D Printer
Hot glue gun and glue sticks

Lowes Parts:

Frame
1-1/2-IN X 10-FT SCH40 PIPE pressure PVC
Lowes 23830
4x 1-1/2-in 90 Degree Schedule 40 PVC Elbow
Lowes 23909
2x 1-1/2-IN X 1-IN SXT TEE
Lowes 317755
1-IN X 2-IN SCH80 NIPPLE (2 inches long)
Lowes 57071
1-IN CLOSE SCH80 NIPPLE (1.5 inches long)
Lowes 56860
1-IN SCH40 CAP threaded
Lowes 23887
Launch Valve
1in Rainbird sprinkler valve (Orbit can be used, but I’ve had a lot of luck with Rainbird)
Lowes 23026
Launch Tube – Designed for a ½ inch launch tube, but tests have shown that a ¾ inch launch tube launches the rockets further.
1-IN SCH40 ELBOW – threaded street elbow
Lowes 126824
1-IN x 3/4-IN BUSHING threaded
Lowes 51348
3/4-IN X 1/2-IN ADPT
Lowes 22698
1/2-IN X 10-FT SCH40 PIPE
Lowes 23966
Miscellaneous
OATEY 8-OZ HANDIPAK PVC primer and cement
Lowes 150887
PTFE TAPE 1/2-IN X 43-FT
Lowes 456832
Spray paint to protect PVC frame from the sun
Lowes 3209531
220-grit sand paper to prepare the PVC for gluing
Lowes 140381

Amazon

3D printed pieces
OVERTURE PETG Filament 1.75mm
OVERTURE TPU Filament 1.75mm Flexible TPU Roll
Launch Trigger Box
9V Battery Clip Connector
24-AWG 2 Conductor Wire
RCA 100' 24-Gauge Speaker Wire (can be used instead, but gets tangled much easier)
2-pin Wire Connectors
2x Momentary Metal Push Button
Heat Shrink Tubing
Air Input
Right Angle Valve Stem
Blow-off Brass ASME Safety Valve 60 PSI
Brass nut
Bike Pump (or an air compressor)

Pipe Nipples

These are by far the hardest part to find at a reasonable price or locally. I luckily have 4 Lowe's stores relatively close to me, but I have still had to go to each to get enough of the schedule 80 nipples to build the launchers. If you have a problem finding them at Lowes, here are some other options:

Lasco part numbers:
210015 - 1" x 1-1/2" SCH 80 Nipple
210020 - 1" x 2" SCH 80 Nipple

I also found them at a decent price on American Copper and Brass online.

Air Powered Rocket Launcher v5 Parts List.pdf
Download

Step 1: Why Use This Design?

You can see the original T design I started with. The T shape was based on a really old design that had a ball valve, but I wanted to improve the experience using a sprinkler valve. The initial deployment was a great success, and everyone loved them. After I loaned them out to another work group and they destroyed them by carelessly tossing them in the back of a truck, I decided I needed to rethink the design.

I scrambled to make a couple new U shaped launchers for an airshow. I did this to remove the strain on the sprinkler valve since this was the main broken part from the T shaped launchers. The other most commonly broken part was the valve stem for the air input. With the T design, I couldn't replace the valve stem.

Now with a rectangular pressure chamber, all of the components face inward. This makes it much more difficult to accidentally break the parts off. Should the sprinkler valve or air input be broken off, they can be easily replaced. It's very unlikely that the frame itself will be broken.

To make sure that the sprinkler valve is well supported, I created a 3D printed bracket that braces the outflow of the valve to the sides of the pressure chamber. Since there was already one bracket, I created another to go on the input side and added ledges so that the launchers can easily stack on each other. These brackets also hold the launch trigger and the launch tube. That means that everything the launcher needs (except for the bike pump) can be easily transported. I've learned that if the launchers are easy to transport, the users are much nicer to them.

The addition of the safety valve makes the design even safer by setting a hard limit for the max pressure.

For the launch trigger box, I really liked having a safety toggle and momentary button, but no one would turn the toggle off after a launch, and the toggle switch makes stacking the launchers harder (also the switch is relatively expensive). Now the launch boxes have two momentary buttons instead, which makes the triggers safer. I also replaced the standard low gauge speaker wire with a two conductor wire with rubber-ish insulation. This wire tangles less, and I'll see how it holds up.

Lastly, since I loan them out to many groups I've found that the easier the launchers are to transport, the less often they are broken. This design footprint comes out to 18"x11" which fits pretty well inside some rolling toolboxes. The launchers stack on each other so the group leaders can easily transport the launchers to a location and then pack them up safely.

Step 2: Print Parts

The printing will take some time, so start it first. A normal Ender 3 size bed can be used for all of the parts. The main brace for the sprinkler valve is structural, so I highly recommend to use PETG with a 50% infill. The brace on the air input side can be printed in TPU since it is more resilient, and the students are typically pretty hard on the launchers (although we've had far more launchers broken by staff during transport than students at events). The launch trigger boxes take quite the beating as well from kids dropping/throwing them so they can run after their rockets, so TPU can be used for the box too, but I use 100% infill for them typically.

Notice the "mouse ears" on the pieces. These help prevent the pieces from peeling up while printing. They easily pry off by hand, and then can be trimmed flush with a utility knife.

Prints settings required:

15% infill PETG
1.5_in_rocket_launcher_brace_side_4_spacer v3.stl
2x rocket_launch_trigger_holder_short v4.stl
rocket_launch_controller_two_paint_mask v3.stl (only required if you want to paint the launch trigger box)
100% infill PETG (can be printed together, but the quality was much better for me when printed individually)
rocket_launch_controller_two_button_bottom v5.stl
rocket_launch_controller_two_button_top v5.stl
rocket_launcher_safety_valve_nut_spacer v2.stl
50% infill, 4 outer perimeter walls, PETG
1.5_in_rocket_launcher_brace_side_3 v12.stl
15% infill TPU (can be printed in PETG instead)
1.5_in_rocket_launcher_brace_side_4 v22.stl

Step 3: Cut and Prep PVC Parts

Using a miter saw is the easiest way to complete this step, but a hacksaw or PVC pipe cutter can be used as well. The largest PVC pipe cutter that I could easily find could cut up to 1.5 inch pipe, which is part of the reason I now use 1.5inch pipe instead of 2inch.

With the 1.5inch pipe, cut:

4x 2.25inch pieces - these will connect the elbows to the tees
2x 12.5inch pieces - these will be the main frame tubes

With the 1/2inch pipe, cut:

12 inch piece - this will be the launch tube.

After the cuts are made, prepare the pipes for glue by lightly sanding the ends so that they go from a shiny service, to being matte and lightly scuffed.

Also sand the insides of the sockets of the elbows and the tees. The last socket that needs sanded is the 3/4inch to 1/2inch adapter that the launch tube will be glued into.

Step 4: Make the Launch Tube

We start with the launch tube since it will be used to help align the main frame in the next step.

Glue the launch 1/2 inch tube to the 1/2inch slip to 3/4inch threaded adapter.
Using the PTFE tape, attach the 1inch threaded to 3/4inch threaded bushing. The bushing will not need to come off, so get this very tight so that it doesn't fall off and get lost.

Now with this done, the launch tube can be screwed into a 1inch female threaded pipe.

Step 5: Glue PVC Frame

Glue the four short 1.5inch pipes to the smooth sockets of both PVC tees.
First use the purple primer on both the socket and the pipe to clean the pieces.
While the primer is still wet, apply cement to both the socket and pipe.
Insert the pipe into the socket, turning the pipe and socket as you push it in. The twist helps spread the cement and prevent air leaks.
Use a paper towel to wipe off excess glue.
Use these gluing steps for all of the next joins.
Glue a 90 degree elbow to one of the launch frame tubes.
Make sure you have a flat surface. This will be used to get the next elbow to be level with the other.
Glue a second 90 degree elbow to the frame tube. Press the open elbow sockets onto a flat surface to make sure they're both level with each other.
Repeat steps 2-4 for the other side of the frame.
Screw the launch tube into one of the tees. Just hand tighten the tube.
Glue the tee onto one of the 90 degree elbows and make the launch tube parallel to the frame tube. It's very important that the frame is aligned properly.
Move the launch tube to the other tee.
Glue the tee with the launch tube onto the same frame tube that has the other tee already glued to it. Again use the launch tube to align the tee with the rest of the frame. The launch tube should be pointed directly at the other tee opening.
Prepare both the sockets of the 90 degree elbows that will be glued to the short pipes coming off of the tees. You will have to move fast once glue is on the pipes.
Glue the short pipes onto the 90 degree elbows. I press the frame on the ground and then use my body weight to help push the ends together.

Step 6: Paint the Frame

This is optional, but plain PVC is not UV resistant. The primary goal of painting the frame is to look good, but it also helps protect the frame. With paint applied, it's much easier to see if the frame has been damaged or chipped.

Use masking tape to tape the inside threads of the tees. If paint gets on the threads it could cause air leaks.

If you'd like to paint the 90 degree threaded street elbow as well, make sure to tape off the threads first.

Step 7: Drill and Tap for Blow-Off Valve

The safety valve is optional, but highly recommended. PVC does not fail gracefully! And it is generally not a good pressure vessel. Since these rockets don't need high pressure, we've never had an issue, but know that without a safety valve you will need to use these launchers with extra caution.

Drill a 7/16 inch hole into the middle of the two inch SCH80 nipple.
Using a 1/4 inch NPT tap, tap the hole. I bought my NPT set at Harbor Freight which was by far the cheapest place I could find.
My safety valve came with a hard thread sealant, but I removed most of this using a 1/4 inch die. I was worried that it was too hard for the PVC.
Put PTFE tape on the safety valve threads.
Screw the safety valve into the nipple as far as you can without stripping the PVC threads.
Put the 3D printed nut spacer on the inside of the nipple.
Place a small dab of thread locker on the safety valve threads.
Using the brass retaining nut, screw it onto the safety valve from inside the nipple. This is just an extra safety feature.

You can see how the safety valve fits into the 3D printed brace piece. Another reason for printing this piece using TPU is so that if there was a blowout, the TPU would flex and absorb some of the force.

Step 8: Drill and Set Valve Stem

If you want to be able to stack the launchers, you need to use a right angle valve stem.

Drill a hole in the top of the 1 inch female threaded cap that is the right size for the valve stem you are using.
Most valve stems have a rubber o-ring so the hole can be slightly bigger than the valve stem.
You can use a small drill bit at first to get the hole centered.
After drilling the hole, a reamer and sand paper can be used to clean up the hole. The hole needs to be clean to get a good air seal.
I used a right angle valve stem for some launchers so that it's easier to attach an air chuck.
Some valve stems use a retaining nut. If yours has one, then tighten this down.

The point of using the cap to hold the valve stem is so that it is easy to replace or repair. I also have some caps that have quick connects that are used when connecting the launchers to a launch control station that fires multiple rockets.

Step 9: Prepare the Sprinkler Valve

Adding the connector makes it easy to swap out the launch trigger if the batteries go bad.

Pick the male side of the 2-pin connector. When delivering power, the male side should always receive the power since it has exposed pins.
Strip back the wires on both the valve solenoid and the connector pigtail.
Put the heat shrink tubing over both individual wires, and one that will be big enough to cover both finished wires.
Use a heat gun to shrink the tubing since it won't singe the tubing, but a lighter can be used as well.
The flow control valve is not needed, so unscrewed the valve, put CA glue on the threads, and then screwed it back down tightly. After the super glue has dried, use a hacksaw to cut off the valve top.
This step helps the launchers stack, so it isn't strictly needed if you don't plan to stack multiple launchers.
I have had this valve come loose on a few launchers and people have thought the launcher is broken, so it could be worth while to glue this shut anyway if you are letting other people use the launcher.

Step 10: Install Sprinkler Valve

Take the 1-1/2 inch long nipple and wrap PTFE tape clockwise around one side of the threads.
Hand tighten the nipple into one of the tees with the PTFE tape facing into the tee.
Wrap PTFE tape around the exposed pipe threads on the nipple sticking out of the tee.
Take the solenoid off of the sprinkler valve. Otherwise the solenoid will hit the frame tubes.
Screw the input side (indicated by a flow arrow pointing into the valve) onto the freshly wrapped nipple. This will help tighten the nipple into the tee as well.
Once the sprinkler valve is securely on the tee, reinstall the solenoid.
Take the 3D printed (PETG part) and slide it onto the frame so that it fits against the sprinkler valve.
The 90 degree street elbow will hold the 3D printed part on. Wrap the elbow male threads with PTFE tape (clockwise).
Screw the street elbow into the sprinkler valve.
Now take a trigger holder and slide it into the brace.
Using a couple small screws, secure the launch holder into the brace.

Step 11: Install Safety Valve and Valve Stem in Frame

Take the nipple with the safety valve and wrap PTFE tape clockwise around one side of the threads.
Screw that side into the open frame tee.
Orient the safety valve so that it will fit in the 3D printed brace piece (this piece should be printed with TPU if possible).
Wrap PTFE tape clockwise around the nipple threads. This is important to do before putting the 3D printed pieces on to make sure that you fully wrap all the threads!
Place the 3D printed spacer on the nipple.
Slide the brace over the nipple and make sure the safety valve fits in the slot.
Screw the air input cap onto the nipple. You may need to use channel locks or something similar to help tighten the cap.
The cap will hold the "brace" piece on. This piece isn't really a brace, but it helps hold the launch trigger, the launch tube, and allows the launchers to stack.
Attach the launch trigger holder on the 3D printed brace using a few small screws like in the previous step.

Step 12: Assemble Launch Box

The solenoid for the sprinkler valve operates on 24v AC, but three 9v batteries in series will give us 27v DC power. I haven't had any issues using the 9v batteries, but some users have complained that they go dead quickly. I've used the same batteries for multiple all day events. There are opportunities to improve this by quite a bit, but I was going for the cheapest and simplest design possible.

To improve the lifespan of the solenoid, don't apply power for very long. All that is needed for launching is a quick press to release the pressure. Having two momentary buttons help minimize the power on time, and act as a safety so the user has to intentionally launch the rocket. Usually people let go of the buttons as soon as the rocket launches.

Wire everything in series. The batteries are in series to add up, and the buttons are in series so that both have to be pressed at the same time to complete the circuit and launch the rocket.

If you want to paint the front, I recommend using the 3D printed paint mask. I tried spray paint, but didn't get great results since the force from the spray blead onto the front of the launch box. Instead, use a paint brush and the paint mask and things will look much better. See the picture for more details.

Trim off the "mouse ears" from the 3D printed shells. Use a utility knife to smooth them out.
Optionally paint the front to make the letters more readable.
Strip back the three 9v battery connectors.
Wire the three in series (red 1 to black 2, red 2 to black 3).
Solder the two connections and cover with heat shrink tubing.
Solder the female 2-pin connector to ~10 feet of speaker wire. Do the same steps that were done to solder the male connector to the sprinkler valve.
Place the two buttons into the launch trigger top shell.
Put the wire through the hole in the shell.
Leave enough space to reach both buttons, and then strip back the outer insulation.
Tie a knot into the insulated part of the wire right before the stripped off part. This will keep the wire from being pulled back through the hole.
Connect the positive wire to one of the buttons. Connect the negative to the other button.
Connect one of the 9v battery connector wires to one of the open button terminals.
Connect the other side of the 9v connectors to the other button terminal.
Hot glue everything! It's a great strain relief. Make sure to leave room to close the shells.

The standard speaker wire gets tangled pretty easily, and doesn't seem very robust, which is why I switched to the fully insulated two conductor wire. You can also see how much slimmer the launch boxes are now without the bulky toggle switch. The kids really like flipping the toggle switch, but since everyone just leaves the toggle switch on after a launch, I think having two momentary buttons that require both hands and in sort of an awkward position, the new design should be safer. Additionally, the toggle switch makes stacking the launchers harder since it hits the sprinkler valve solenoid, and the toggle switch was more expensive than the momentary switches.

Step 13: Build a Rocket

It's easiest to complete the rocket while it is still on the PVC tube. The tube will help hold the rocket body's shape.

Print out the templates to make it easier to make the fins and the nose cone. If you'd rather make a 3D printed rocket, print it with TPU and in vase mode.

Take a spare 1/2 inch tube, and wrap a piece of paper around it long ways.
Make sure the paper isn't too tight around the PVC tube. You want it to be slightly loose so that it will easily slide off the tube.
Secure the paper tube with tape.
Use markers to customize your rocket!
While the paper is still on the PVC pipe, tape over one end of the paper tube to seal that end.
This is the most important step! Without the tape seal the nose cone will blow off.
Cut some fins out of index cards or card stock and tape them to the sides of the rocket.
To make the nose cone, cut a "PACK-MAN" shape out of paper (a circle with a wedge cut out of it).
Tape the cone to the body.

Step 14: LAUNCH!

Always use caution when using the launchers! Safety glasses should be worn at all times!

We usually have tables set up for the students to make their rockets away from the launchers. We use cones to mark off the area near the launchers, but another user mentioned using a rope staked to the ground to make sure students don't cross into the area.

The main point, please be careful!

Take the launcher out to a large open space with a bike pump and a rocket.
Take the launch tube out of its holder and screw it into the sprinkler valve output.
Unwrap the speaker wire from the launch trigger and plug it into the sprinkler valve. Place the launch trigger box away from the launcher.
Attach the bike pump to the valve stem on the launcher.
Wear safety goggles any time you are using the pressurized launchers.
Place the rocket on the launch tube BEFORE pumping the launcher up with air.
Pump the launcher up to 30 PSI. I've found this to be the sweet spot that is easy enough for most children to pump to and still launches the rockets very high. Yes, pumping up to a higher pressure is possible, but it isn't really necessary.
Have everyone back away from the launcher.
When the area is clear, press both launch buttons at the same time to launch!

Step 15: Transportation

If you're only building one launcher, this step probably won't apply to you. For larger groups that require multiple launchers for events, this step is very important! The easier the launchers are to transport, the longer they will last and the more they will get used. As I mentioned in the first step, a team took my original design to Oshkosh and most of the launchers were destroyed when transporting them back because they were tossed in the bed of a truck. Save yourself from that sadness by planning ahead and making transport a part of the design! Every part of this design was made to stack to make transporting and setup easy.

The Rigid toolboxes that I found fit up to four launchers in the bottom, and the top toolbox can hold all of the supplies. I am still working on designing a holder on the top to secure the bike pumps, but now one person can easily move everything required using one hand. Since the boxes break down (the wheels and handle even pop off!) it is easy to put the boxes in any sized car.

I bought the toolboxes because they were on sale at Home Depot, and the deal was too good to pass up. But I later saw very negative reviews because the handle isn't very strong. I hopefully proactively solved this by using a 3D printed reinforcement that I slid into the handle. Plus I'm not putting heavy tools in the boxes, so the handle should be okay for this use. I included the .stl for the handle if you use these same boxes. I had to swap filament in the middle of the print but the part is very solid. The caulking helped the piece slide in and should help hold it in place, although it was a pretty tight fit, so I'm not worried about it falling out.

rigid_rolling_toolbox_handle_reinforcement v2.stl
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Air Powered Rocket Launchers V5.0