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since July 2000

A portable CO₂ rocket-filling system

Thank the paintball people

Thanks to the popularity of the sport of paintball, there are a number of companies that manufacture and sell comparatively inexpensive portable tanks, regulators, and fittings for compressed carbon dioxide or air.

I put together a system for filling water rockets. I describe the system below, to give people an idea what is required to get a portable CO₂-tank system up and running.

Parts list

I bought

• A 5-pound CO₂ tank with valve, cost US$75 at a local welding-supply store. It holds 5 pounds of liquified CO₂—the steel tank obviously weighs more than this. Refills at welding-supply stores cost about US$20. Most places swap your empty cylinder with a filled cylinder of the same size, since the actual refilling takes place at a centralized facility. Refills for 20-pound cylinders cost about the same, so over the long term you save money with a bigger tank even though you pay more initially.
• A 20-ounce paintball CO₂ tank with valve, cost US$30 on ebay.com. I also bought a “fill station” on ebay for about US$25. The fill station is basically a few fittings that allow you to connect a paintball tank (CGA 320 threads) to a standard CO₂ cylinder in order to refill the small tank.
• A “Stabilizer” adjustable pressure regulator, cost US$74 by on-line order from Palmer’s Pursuit Shop. I bought the side-line low-pressure version (up to 300 psi output). For an additional US$26 I also got a set of quick-disconnect fittings, an inline on/off valve, and a brass T-connection. Like a lot of paintball equipment, the regulator connections and other fittings are based on 1/8-inch NPT pipe threads used in the United States.
• A large-face 200-psi pressure gauge with 1/8-inch NPT nipple, cost US$10 on ebay.com. This was installed on the extra port of the pressure regulator.
• Miscellaneous: 1/4-inch polypropylene tubing, compression fittings, and teflon pipe tape purchased at the local hardware store. The tubing is used for a delivery line so I can pressurize rockets from a safe distance.

Was it worth it? Counting shipping charges and taxes, I spent about US$270 on the whole enterprise, not counting the CO₂ itself. This equipment is definitely more expensive than a bicycle pump, but it sure speeds up the launches. I can pressurize a soda-bottle rocket in a few seconds. The 20 ounces (0.57 kg) of liquid CO₂ in the paintball tank expands to about 45 liters of gas at 6.9 bar (100 psi) pressure. This means I can launch up to thirty or so 2L rockets on one fill. Because the tank and regulator are designed for paintball, they are quite portable—I’ve carried them to the park in a backpack. Another thing I like about the system is I can use it in the future to fill high-pressure rockets. Liquid CO₂ can deliver approximately up to 60 bar (870 psi), with the value depending on the temperature.

Pictures

Shown below are the small and big CO₂ tanks. Attached to the big tank is the fill station. I changed it so that the small tank connects to the filling station or to the pressure regulator with a quick-release fitting. The main trick for getting the liquid CO₂ to transfer from the big tank to the small tank is to make sure the small tank is colder than the big tank. You can do this by exhausting gas from the small tank into the atmosphere, which cools the tank down. My own invention is instead to put the small tank in the freezer for 30 minutes before filling it. If the big tank does not contain a siphon tube, then it must be inverted to get the liquid out. My fill station came with instructions and safety information.

Below is a close-up picture of the regulator (the black cylinder-thing). Pressure is adjusted using a hex wrench inserted in the left side. Palmer's Pursuit Shop sells an adjustment knob that permanently attaches to the regulator, if desired. I installed an on-off valve leading to the delivery line. I fill rockets by turning the valve on and off while leaving the regulator at a set pressure.

The cut-off end of a bicycle pump is attached with a hose clamp to the end of the delivery line.

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