3D Printed Bottle Rocket Launcher (OLD BETA VERSION)

The newest version (v1.4) from September 2023 can be found here: https://www.instructables.com/Simple-3D-Printed-Water-Rocket-Launcher/

An updated version (v1.0) of this design was released on 01 October 2021 https://www.instructables.com/3D-Printed-Water-Rocket-Launcher/

This Instructable will be preserved to illustrate the iterative design process over the years.

Further details are available at https://www.marimolabs.com/water-rocket-launcher

This is a soda bottle rocket launcher built from 3D printed parts and PVC tubes. It's potentially useful for kids, teachers, and beginner water rocketeers (like myself). There are many water rocket launchers available. This particular launcher draws on the design principles below:

- Ease of part sourcing -> Common parts at hardware stores in the US and easily found online

- Ease of construction -> Few specialized tools or techniques needed

- Robust design -> Strategic reinforcements developed through repeated rounds of destructive testing and failure-mode analysis

- Ease of transport, setup, and use -> Portable, quick setup.

- For novice water rocket builders -> Uses "launch tube" design for success with even plain bottles and does not require special nozzles

- Modular -> Clear purposes for each part, adaptable to re-design

VERSION

Models and instructions are updated to version beta 3. Images are from an earlier version which used a 10-24 screw (current version uses larger and stronger 1/4-20 screw). At some point I will update the images.

DISCLAIMER

Things you should know by now: No warranty, expressed or implied, is made regarding the safety, performance, or suitability of this design. Pressurized systems are inherently dangerous. Do not attempt without appropriate qualifications, protective equipment, and tools. Fumes can be dangerous. Work in well-ventilated areas and follow all instructions on tools and materials. 3D-printed components can be of variable strength and stability, and structural defects can be invisible. It is incumbent on you to inspect the 3D models and assess whether they meet the need of your application. Do not rely on the strength of 3D-printed components for safety. No liability of any kind is assumed by the creator of this design. Use at your own risk.

Supplies

Try your local small hardware store (e.g. ACE Hardware affiliate) to get single parts. Bring this list. They can help you find each of these items.

3D MODELS

Files on Thingiverse

PLUMBING

- PVC pipe, 1/2" nominal, Schedule 40, 3 ft (This is the most commonly found, standard 1/2" PVC pipe). $2

- PVC elbow, 1/2" nominal spigot ("slip fitting") to 1/2" female pipe thread ("FPT" or "female NPT"). $1

- Bushing, 1/2" MPT (male pipe thread) to 1/4" FPT. Typically found in brass, rarely in PVC. $1-5

- Schrader valve (common bike valve) to 1/4" MPT fitting. $1-5. Less common at hardware stores. May need to buy online or try an automotive store. Alternately, there are tutorials online about repurposing bike inner tubes and using a PVC end cap to accomplish the same purposes as the elbow, bushing, and valve combination here.

- O-ring. 7/8" OD, 3/32" thick, 11/16" ID (Ace #64812-K). $0.45. See image.

FASTENERS

- Machine screw, 1/4-20 thread, 2.5" long, Phillips head or hex head, stainless steel preferred. $1

- Lock nut with nylon inlay, 1/4-20 thread, stainless steel preferred, $0.40

- (optional) For DIY ground peg: 3x Machine Screw, 10-24 thread, 4" long, 3x 10-24 wide washer, 3x 10-24 locking nut, any metal

MISC TOOLS

- Method for cutting PVC pipe. Saw or pipe cutter.

- Ruler, permanent marker for measuring and cutting pipe

- PVC primer/cement. E.g. Oatey Handy Pack Purple Primer and PVC Cement. $8

- Disposable gloves (for PVC pipe assembly)

- Philips head screwdriver or adjustable wrench

- Slip-joint pliers (for tightening nuts and disassembling tight dry-fit parts)

- PTFE ("Teflon") pipe thread tape. $1

MISC ITEMS

- Cord for launch pin (at least 24")

- 3x Standard rubber bands

- Metal tent pegs, 9" long (or use DIY ground pegs above)

- Work surface for PVC cement

Step 1: Launch Mechanism Primer

This launch mechanism is based on the Clark Cable-Tie Release Mechanism designed by Ian Clark, which has been one of the standard ways to launch water bottle rockets.

Briefly, the Clips hold the bottle onto the Tube Adapter and Base. The Collar holds the Clips in place and the Launch Pin holds the Collar in place. Rubber bands provide the down-force to pull down the collar.

Another way to understand the mechanism is by following the forces. The pressurized bottle wants to move upward (axially). The Clips, via their slanted bottle interface, translate that force into an outward (radial) direction. The Collar contains those forces until it is time for release. When the Launch Pin is removed, the rubber bands pull the collar down, releasing the clips, which releases the bottle.

(Note that these images are from an earlier version of the launcher so eagle-eyed readers will note that some parts look slightly different)

Step 2: Structural Primer

Safety and success require understanding how pressure is contained in the launcher, and where the weak points are. Pictured are the pressure-holding assembly and a close-up of the Tube Adapter. Mouse-over for details.

3D-printed parts are inherently poor at holding pressure due to tiny holes between fused filaments. There are numerous work-arounds on the internet but I found them to be inconsistent. This launcher addresses this problem by:
1. Using PVC pipe wherever possible to contain pressure.
2. Using PVC primer and cement to seal small holes in the Tube Adapter where #1 is not possible.*

Additionally, 3D-printed parts are weakest against shear and distraction (pulling-apart) forces. These forces are intense at the Tube Adapter, especially when loading heavy bottles. This launcher transfers the stress onto a stainless steel machine screw by a similar mechanism as pre-stressed reinforced concrete. While the launcher would work without the screw, it would not be durable.

*Please note that #2 is certainly a hack, but appears effective and reproducible in assembly. Using new PVC cement (low viscosity, not yet turning jelly-like) will help wick into small holes. Ideas for improvement are appreciated. A slow leak is not necessarily a bad thing, and could be considered a safety feature for gradual pressure-relief in case of launch failure.

Also, please note that PVC primer and cement do not actually solvent-weld PLA, the most common 3D printer filament. This assembly works by solvent-melting the PVC, which then wicks into the tiny nooks and crannies of the PLA, forming a sufficiently-strong connection. I have pressure-tested the launcher up to 140psi without catastrophic failure. Suggestions with data on adhesives that properly bond PVC and PLA would be appreciated. Plastiweld Plastruct has been suggested as a possibility. Please let me know if you test it.

Step 3: Tube/Fitting Prep

See Intro/Supplies Page for a Bill of Materials.

PVC PIPE (1/2") PREP
- Cut 3x 8" lengths of pipe for the legs (longer if desired)
--- Edge finish for the legs is not important. Good to do these first to get your technique down
- Cut 1x 4.5" length of pipe for the upper launch tube
--- Tube ends should be flat across and cleaned up so there is no flaring or burr
- Cut 1x 3.5" length of pipe for the lower launch tube
--- Tube ends should be flat and cleaned

BRASS FITTING PREP
- Apply PTFE tape thread to brass male threads. Three times around is usually sufficient

(Optional) ASSEMBLE GROUND PEGS
- As an alternative to finding tent pegs, you can make your own using machine screws, washers, and locking nuts.
- See the pictures for one way to do this

Step 4: Assemble Tube Adapter and Dry Fit Launch Tube

ASSEMBLE TUBE ADAPTER
- Place the machine screw through the center of the Tube Adapter
- Place lock nut (with nylon inlay)
- Screw on the nut very snugly with the pliers/wrench
- Check the O-ring fits on the little lip

DRY FIT THE LAUNCH TUBE
- Make sure all tubes fit together correctly and smoothly as per the image. This is the time to make sure all the tube ends are flat and fit well together.
- If everything fits, disassemble the dry PVC pipes (leave the screw/lock nut in the Tube Adapter) and set aside the O-ring and bushing+Schrader valve

Step 5: Build Launch Tube and Legs

Set up your workspace. Open a window, cover your work surface, and grab a couple paper towels. Read the instructions for the PVC primer and cement. Loosen the caps of both the PVC primer and cement.

If it's your first time working with PVC primer and cement, do the legs first. Those joints are not as critical.

BUILD LAUNCH TUBE

Make sure the O-ring is off the Tube Adapter but that the screw and nut are installed.

Start with the lower launch tube (3.5" length).
- Prime the Tube Adapter and the inside of the lower launch tube. Also put a little on the outer edge of the lower launch tube as per the picture.
- While the primer is still wet, apply PVC cement to the Tube Adapter and the lower launch tube
- Squeeze them together and give them a quarter-twist, and hold for 30 seconds
- Wipe off excess cement

Add the upper launch tube (4.5" length, ends flat and smooth).
- Note that we will cover the Tube Adapter's sides (not the end, where the screw is) liberally with primer and cement. This will seal the small holes in the 3D print and is important for airtightness.
- Prime the sides of the Tube Adapter LIBERALLY and to the inside of the upper launch tube
- While the primer is still wet, apply PVC cement to the outside of the Tube Adapter LIBERALLY and to the upper launch tube
- Squeeze them together and hold for 30 seconds, but DON'T twist. Twisting risks loosening the lower launch tube. The upper launch tube/Tube Adapter interface is not subjected to strong forces.
- WIPE OFF EXCESS, ESPECIALLY THE GROOVE WHERE THE O-RING WILL SIT. Fold a piece of paper towel to get into the crevice.
- Look inside the tube to make sure the air/water channels are open. A couple clogged segments are not an issue. If there are more than that, consider giving the long tube a puff of air to open up segments.
- Set the assembly aside to cure for at least 30 minutes. We will return to do the elbow. In the meantime, we can...

BUILD LEGS

Do one at a time.

- Prime outside of Foot and inside of leg (8" length)
- Squeeze Foot and leg together for 30 seconds

Step 6: Assemble Launch Tube to Base and Elbow

After the launch tube has cured for 30 minutes, set up for assembling to the base and attaching the elbow. Note that after the elbow assembly, there is no easy way to disassemble the launch tube from the base.

ASSEMBLE LAUNCH TUBE TO BASE AND ELBOW
- Place the lower launch tube through the center of the base. The cone surface of the Tube Adapter should mate with the cone surface of the top of the Base.
- Join the elbow to the lower launch tube and hold for 30 seconds. This joint will compress together as the PVC solvent-welds together.
- Wipe up the excess primer and cement
- Set the assembly aside to cure for 30 minutes
- We are done with PVC pipe joining. You can clean up your workstation.

Step 7: Install O-ring and Valve Fittings

INSTALL O-RING
- After the cement is no longer sticky, fit the O-ring over the end of the upper launch tube and gently roll it into place in its groove
- It will be slightly stretched after going around the launch tube. It will return to its normal size
- Gently smooth the ring into its groove
- You may apply a small amount of olive oil to the ring to make bottle fitting easier

INSTALL VALVE FITTINGS
- Take your Schrader valve and bushing, which should have PTFE tape on them already, and screw them together. Snug it down with pliers/wrench
- Screw this into the PVC pipe a vigorous hand-tightening, and give it another 3/4 turn with pliers/wrench. Do not overtighten this, as it can cause the PVC to crack. You can always tighten more if you have a leak, but if you crack the elbow, you'll have to start over.

Step 8: Assemble Retention Mechanism and Launch Pin

ASSEMBLE CLIPS AND COLLAR
- Place a rubber band around the top of the Base, overlying the six notches that will hold the Clips
- Note the orientation of the Clips. The skinny end goes down.
- Place the six Clips in their notches, using the rubber band to temporarily hold them in place
- Slide the Collar down over the upper launch tube and over the tops of the Clips
- Remove the rubber band
- Pull the Collar down past the Clips

ASSEMBLE LEGS
- Slide each leg pipe into the Base

INSTALL RUBBER BANDS
- Take a rubber band and thread it under the leg support as shown in the picture
- Loop both ends of the rubber band up to the corresponding side bracket of the Collar
- Repeat twice for the other legs

ASSEMBLE AND PLACE LAUNCH PIN
- Tie a knot in one end of your launch cord and thread the cord through the hole in the Launch Pin
- Note that one of the Base's leg supports has a "PIN =>" notation. This is the direction from which you should insert the Launch Pin for maximum stability when pulling it on launch
- Lift the Collar with one hand and place the Launch Pin in the recommended direction. You will need to hold down the Base while you do so. An assistant or your knee gently placed on a PVC pipe leg will do the trick.

Step 9: Test and Launch!

Allow at least 24 hours for the PVC cement to cure before applying air pressure to the assembly.

TIPS
- If you have trouble getting the bottle over the O-ring, make sure the O-ring is smoothly seated in its groove. Apply a small amount of olive oil to the o-ring. When you put the bottle on, give it a slight twist at the O-ring to help it slide on.
- Stake the launcher in the ground or put weights on the legs, especially for young rocketeers who may very enthusiastically pull the launch cord, and perhaps at a suboptimal angle. You don't want the launcher to fall over.
- If the Collar does not come down as expected, you probably need stronger rubber bands.
- This launcher does not have a built-in launch-abort system. Consider integrating one in case of failure to launch. Alternately, consider any slow leaks of pressure through 3D printed parts as a gradual depressurization safety system.
- Wear safety glasses when near a pressurized rocket

FEEDBACK
- Please share your experiences with all aspects of this design, especially failures and challenges. I would also be interested in experiences with use in educational settings. My goal here is to make water rocketry more accessible to all.

SPECIAL THANKS
- I am new to water rockets. I learned a lot from, and was particularly inspired by, two groups linked below. Visit their websites for great tutorials and amazing videos. Thanks for sharing your knowledge and passion with the world!
- US Water Rockets
- Air Command Rockets