Maker Shed sells $70 kits for these, but why bother when you can get everything you need from the local hardware store? The primary ingredient is a sprinkler system solenoid valve which can be had for $12.
The only real trick is power for the solenoid. The valve is rated for 24V AC. The article's author used a 7.2V DC remote control car battery. The Maker Shed kit uses two 9V batteries in series. I personally think the 9V battery is the worst as to cost per mAh and I would be loath to put one in a project, let alone two, so I came up with several very unique power supply solutions.
My first solution was to use a 18V battery from a electric power tool. This is economical only in fact that I could borrow one. The problem of making an electrical connection without altering the battery was solved by soldering two rare-earth magnetic balls to the leads. This had the added benefit of easily pulling free when an over-excited 3-year old ran off to retrieve a rocket while still holding the launch button.
We ended up building both the paper rockets from Make Volume 15, and the foam rockets from Volume 29. Both are inexpensive and easy to quickly build, a plus for short attention-span youngsters. The paper rockets fly considerably higher, and take a bit of damage on landing. I used a 1" length of 7/8" dowel and attached a half of a rubber ball to the top with some success, but most rockets were unusable after several high flights. We also determined experimentally that any launch pressure over 60 psi would likely lead to a spectacular catastrophic failure.
My other real contribution to this project for future Makers is a template for the rocket body. The original author supplied an 8 x 14 template. I wanted to use 8 x 11.5 card stock, so I created my own design, which I will gladly share with you!
This was a fun project for all ages, and my test subjects ranged from my three year old grandson, up to my teenage nephews. Even my in-law engineers took keen interest. The amount of physics you can teach and experiment with is nearly endless.