#347 — Pressure Cooker Steam Cannon

15 PSI of superheated steam released through a barrel in a quarter second. Civil War artillery principles meet a Goodwill pressure cooker. Neighbors will call someone.

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🧪 What Is It?

Steam cannons are one of the oldest projectile weapons ever conceived. Legend has it that Archimedes designed one to defend Syracuse in 212 BC — a sealed bronze vessel filled with water, heated over coals until the steam pressure launched a stone ball from a tube with enough force to crack a ship hull. Whether Archimedes actually built one is debated. Whether the physics works is not. MIT built a working replica in 2006 and confirmed: superheated steam released suddenly through a barrel launches projectiles at terrifying velocity.

A stovetop pressure cooker at 15 PSI contains a chamber full of steam at 250°F. That steam wants very badly to expand — it's being held at roughly twice atmospheric pressure by nothing but a lid gasket and a locking mechanism. Replace the jiggler valve with a ball valve connected to a barrel, open the valve quickly, and all that compressed steam expands through the only available exit: the tube. Anything sitting in that tube leaves in a hurry.

The math is straightforward. 15 PSI acting on the cross-section of a 2-inch diameter barrel produces about 47 pounds of force on the projectile. A tennis ball weighs 2 ounces. Force equals mass times acceleration. The ball exits the barrel at roughly 100-200 mph depending on barrel length, seal quality, and how fast you open the valve. The report — the sound of 15 PSI of steam venting through a 2-inch opening in a fraction of a second — is a deep, percussive THOOM that sets off car alarms and causes every dog within a quarter mile to lose its mind.

This is not a toy. This is a functional pneumatic cannon powered by phase-change thermodynamics. It uses no compressed air tanks, no combustion, no chemicals — just water and heat. The same thrift store appliance that makes pot roast is now an artillery piece. The founding engineers at Presto did not see this coming.

🔨 Build Steps

1. Remove the jiggler valve and install the ball valve. Pull the jiggler weight off the lid. The goal is to connect a full-port ball valve to this opening. A full-port valve has an internal bore the same diameter as the pipe — no restriction, no choke point. This matters because you want the steam to release as fast as physically possible. Thread or adapt a brass ball valve into the jiggler port using appropriate reducers and PTFE tape. The valve handle should rotate smoothly and snap from closed to open in a quarter turn.

2. Attach the barrel. Connect 2" ID pipe to the output side of the ball valve using reducing adapters. PVC works for low-pressure applications and is lighter, but steel pipe is stronger and won't shatter if something goes wrong. If using PVC, use Schedule 80 (thick-wall) rated for pressure — standard Schedule 40 drain pipe is not pressure-rated and can fragment into shrapnel. The barrel should be 3-4 feet long. Longer barrels give the steam more time to accelerate the projectile, up to a point — beyond 4 feet, friction losses eat into the velocity gains.

3. Install the pressure gauge. Add an inline tee fitting between the cooker and the ball valve with a 0-30 PSI gauge on the branch. This tells you exactly how much pressure you've built before firing. Operating at the cooker's rated 15 PSI is the baseline. Never exceed the cooker's rated pressure — the safety valve should prevent this, but monitor the gauge anyway.

4. Test for leaks. Fill the cooker 2/3 with water, lock the lid with the ball valve closed, and bring to pressure on the stove. Watch the gauge climb. Check every fitting, adapter, and joint with soapy water — bubbles indicate leaks. Tighten or re-tape as needed. The system must hold full pressure with the valve closed. If it doesn't hold, you'll get a sad wheeze instead of a cannon shot.

5. Load and fire. Take the pressurized cooker outside (use oven mitts — it's 250°F). Set it on a stable surface with the barrel pointed at open ground, away from people, animals, cars, and windows. Drop a tennis ball into the muzzle end of the barrel and push it down to the valve with a dowel rod. Verify the area downrange is clear for at least 200 yards. Stand to the side of the barrel, reach over, and snap the ball valve open in one fast motion. The steam releases with a thunderous THOOM and the projectile launches. Close the valve immediately to conserve remaining steam for a second shot — you'll get 2-3 shots per pressurization depending on barrel volume versus cooker volume.

6. Iterate on barrel and valve. Experiment with barrel length, projectile fit (tighter seal = more velocity but higher friction), and valve opening speed. A solenoid-actuated ball valve connected to a button gives you remote firing capability, which is both safer and more dramatic. A longer barrel with a tighter-fitting sabot around the projectile captures more steam energy. Keep a notebook — this is experimental ballistics and the variables are surprisingly sensitive.

⚠️ Safety Notes

Spicy Level 5 build. This is a projectile weapon powered by a pressure vessel. Treat it with the same respect as a firearm.

• Never point the barrel at anything you're not willing to destroy. A tennis ball at 150 mph causes serious injury. A potato at that speed breaks bones. Treat this as a weapon, not a toy.
• The report is extremely loud — comparable to a shotgun blast. Wear hearing protection. Warn neighbors. Do not fire in residential areas where noise complaints or police response are likely.
• Never look down the barrel while the system is pressurized. If the valve leaks or opens unexpectedly, the projectile exits at lethal velocity with zero warning.
• Never exceed the pressure cooker's rated operating pressure. Overpressurization of any vessel is a bomb — the failure mode is shrapnel traveling at the speed of sound.
• PVC pipe under pressure can shatter into razor-sharp shards instead of deforming like metal. If using PVC, wrap the barrel in several layers of duct tape as a fragmentation sleeve, or use metal pipe instead.
• The cooker and all fittings are at 250°F during operation. Use welding gloves or heavy oven mitts when handling. Steam burns are instant and severe.
• Check local laws. Pneumatic cannons may be regulated as destructive devices in some jurisdictions. Many areas have noise ordinances that this will violate spectacularly.

🔗 See Also

• Pressure Cooker Steam Generator
• Electromagnetic Firework Launcher