Pressurized systems are dangerous and water rockets are no exception. Failure modes tend to be explosive. Small defects tend to propagate to catastrophic failure. I’ve done my best to mitigate safety hazards, but I am not an expert and hold no formal qualifications in plumbing or hydropneumatic engineering.
As with the rest of this blog, please read this section as one interested person’s experiences and thoughts on a complex matter. Please do not take these thoughts as guarantees or warranties. Tinkering is an intrinsically hazardous activity, as are all activities of discovery. There’s a lot we don’t know and can’t predict.
Per the disclaimer on every page: Know what you are doing and take appropriate safety precautions. Make your own assessment about whether this project is appropriate for what you are trying to do, and use at your own risk. Do not attempt without appropriate experience, protective equipment, and tools. Most water rocket guides recommend not pressurizing past 60psi/4 bar. Always secure the launcher to the ground.
This blog uses US customary units of pound-per-square-inch, or psi. Quick unit conversion approximations: 1 atm = 1 bar = 15 psi = 100 kPa. Apologies to my readers around the world, and yes, your units do make a lot more sense.
For a sense of scale:
air pressure at sea level = 1 atm = 15psi = 1 bar. Remember all pressures below are RELATIVE to this 1 atm
water pressure at depth of 10 feet/3m = 1 atm = 15 psi (in addition to atmospheric air pressure)
mountain bike tire pressure = 30psi = 2 bar
car (sedan) tire pressure = 35psi
soda bottle pressure at room temperature = 60psi = 4 bar
road bike tire pressure = 90psi = 6 bar
challenging for an adult to pump a standing bike pump = 120psi = 8 bar
blow-molding machine (method of making soda bottles) pressure = 150psi but with metal mold for shaping/pressure = 10 bar
scuba tank = 3000psi = 200 bar
Here is a non-exhaustive list of ways the launcher can fail, and various mitigation strategies.
Mechanism: plastic (non-elastic) deformation and bursting of bottle
Increased risk with: Damage to the bottle; larger bottle (larger diameter); bottle reuse; use of bottles for noncarbonated beverages (tend to be thinner-walled)
Failure experienced: Not in my testing, with smaller 1/2 and 1L bottles
Design precautions: Launch Pin can be pulled from far away using a long cord
Operating precautions: Recommended keep-out zone around a pressurized rocket. Only use bottles designed for carbonated beverages. Retire plastic bottles if damaged or after a certain number of uses.
Mechanism: Launcher falling over, most likely during pulling the Launch cord, launching rocket sideways, causing injury or property damage
Increased risk with: Not fixing launcher to the ground. Not fully seating Legs into Base sockets.
Design precautions: Integrated Pegs for anchoring to the ground
Operating precautions: Launcher must not be used if not properly anchored to the ground
Mechanism: Collar or clamps jam, such that when the Launch Pin is pulled, the pressurized bottle does not launch.
Increased risk with: Sand/grit in mechanisms; varying printer tolerances
Failure experienced: Yes. This was a frequent issue with early versions of the launcher, but should be resolved as of v1.44. Please report failures. In Beta versions, this was due to high O-ring friction. In later versions, this was due to finding the right balance between secure clamping and smooth unclamping.
Failure mode: Rocket not released when expected; concern for unexpected launch subsequently.
Warning signs of failure experienced: High friction with moving the collar. Momentary delay of Collar falling when Launch Pin is pulled.
Design precautions: Removal of hose extension valve center allows for launch abort without manipulating the bottle on the launcher.
Operating precautions: Never lean directly over the launcher. A stick can be used to gently nudge down the Collar if it sticks.
Mechanism: Clamp or Clamp interfaces (to the bottle or Base) failure due to high tension forces
Increased risk with: Heat-weakening of the 3D print or other processes that may lead to deformation of the plastic; poor 3D print strength
Failure experienced: No, with testing to 100psi.
Failure mode: Uncontrolled release of rocket
Warning signs of failure experienced: None
Design precautions: Redundancy in retention (6 Clamps); Base design keeping lower Clamp interface in compression. Aligning rails on Clamps and Collar.
Operating precautions: Avoid high temperatures (closed cars, prolonged time under direct sunlight). Never lean directly over the launcher.
Mechanism: Bottle not clamped or improperly clamped
Increased risk with: Unfamiliarity with the launcher. Use of bottles with flanges of less than 33mm diameter. Note: to my knowledge, all carbonated soda bottles in common production are 33mm. There are possible exceptions in Europe (GME 30.37 and 30.40 specifications allow for smaller diameters) but I have not yet heard of them being found in stores. PLEASE NOTIFY ME if you come across such bottles.
Failure experienced: Not in my testing, but one report of early releases due to particular bottle geometry; this vulnerability was patched in v1.43.
Failure mode: Unanticipated launch when pumping pressure overcomes O-ring friction and rocket weight.
Warning signs of failure experienced: Greater than expected wiggle of the rocket on the launcher due to not being clamped
Design precautions: Geometry makes it challenging to load the bottle in a way where it does not clamp when the Collar is raised. Launch Pin seats securely.
Operating precautions: Never lean directly over the launcher. Use silicone lubricant on the O-ring to reduce friction so rockets pop off at low pressures if not clamped.
Mechanism: Launch cord knot comes undone when cord is pulled
Increased risk with: Slippery cords increase chances of knot coming apart
Failure experienced: Yes, with slippery poly braided cord
Failure mode: Launch Pin remains in place or partially pulled and rocket does not launch (or launches unexpectedly when Launch Pin falls out)
Warning signs of failure experienced: Loosening knot on Launch Cord
Design precautions: Use of higher-friction cord
Operating precautions: Never lean directly over the launcher. If this occurs, launch abort via bottle decompression is recommended, rather than pulling the Pin manually.
Mechanism: Failure of bond between Valve Retainer and lower launch tube pipe due to high internal pressure
Increased risk with: Poor bonding during assembly
Failure experienced: No. Tested to 140psi
Failure mode: Theoretically ejection of the Valve Retainer and tire valve from the bottom of the Type A Core
Warning signs of failure experienced: None
Design precautions: Tire valve holds pressure. Increased pressure improves seal of tire valve, limiting the amount of ejective force experienced by Valve Retainer. Tire valve puts compressive force on the Valve Holder.
Operating precautions: Cores should be mounted in the Base and the launcher should be anchored to the ground. A failure would therefore eject the Valve Retainer and tire valve (attached to the pump) directly into the ground. High pressure air/water would be directed into the ground.
Mechanism: Failure of bond between Tube Core and either the lower or upper PVC pipe pipe bond
Increased risk with: Poor bonding during assembly. Wear and tear
Failure experienced: Not in operating conditions, though there was one instance where an upper Launch Tube (SN008) detached from the Core Center under non-operating stresses (was twisting it to seat a PVC elbow for pressure testing). Not noted with pressure testing to 140psi.
Warning signs of failure experienced: Yes. See above.
Design precautions: A break of the bond to the upper pipe is well-tolerated and may not even be noted in operating conditions. The upper tube does not actually hold pressure. On launch, the predominant force on the upper tube will be pressing it into the Tube Core. A break of the bond to the lower pipe will result in ejection of the lower pipe out the bottom of Base.
Operating precautions: Same as above
Mechanism: Shearing between layers of the Tube Core from tensile forces resulting in misalignment of the bottle causing bottle retention failure
Increased risk with: Problems with the stainless steel bolt, e.g. not using lock nut
Failure experienced: No. Tested to 140psi.
Warning signs of failure experienced: In an early alpha/beta build, a (non-lock) nut backed off and the Tube Core split
Design precautions: Strong 1/4″-20 stainless steel bolt reinforces the Tube Core. Avoidance of tensile forces. Encasement of Tube Core by Base and Clamps. Precautions for Bottle Retention Failure.
Operating precautions: Basic precautions
Mechanism: breakdown of hose extension crimp connections, causing hose to pop off and decompress rocket rapidly.
Increased risk with: damage to hose extension; lightweight hose extensions; severe bends to extension.
Failure experienced: Not operationally, but noticed lightweight hose extensions coming apart on assembly, especially if they are pulled on to seat the tire valve stem in the printed Core.
Warning signs of failure experienced: See above.
Design precautions: Recommendation to use heavy-duty hose extensions and only to pull on the metal parts. Hose extension failure will either direct pressure straight into the ground (if fails at upper crimp) or toward pump. Snorkel ensures that air is vented rather than water, and narrow internal passageways in Core B slow speed of discharge.
Operating precautions: Handle hose extensions gently.
Have a large margin of safety with pressurized systems. While this system is aggressively tested, no guarantees of safety or suitability can be made. Throughly test your builds and have safety protocols in place.
Please contact me if you do have a failure so I can troubleshoot and improve the design.
This concludes the subsections. Return to Design Notes.
Job Hazard Analysis by the Montgomery County Public Schools (Maryland) for the AquaPod commercial water rocket launcher