After many years of lurking on instructables, I feel it is time to finally post one myself :-)
Sorry about the lack of photos - I only realised this would make a good instructable after completing the launcher!
I recently intended to construct a compressed air rocket 'for my daughter', however noted concerning comments from many about the brittle nature of PVC piping.
Rummaging around under my house, I discovered an old mini-keg that seemed too useful to throw away. This turned out to be the perfect pressure vessel for my rocket launcher.
Step 1: The Pressure Vessel
The particular keg I have was purchased from a craft beer brewery, but it appears similar in construction to this Grolsch mini keg.
I removed the plug from the top of the keg, taking care not to damage the rubber gasket. By complete chance, the sprinkler solenoid I ripped from an abandoned irrigation project in my garden still had 13mm barbed poly hose connectors (see example in photo) on each end and these connectors fit perfectly into the rubber gasket on top! This connection is strong enough to hold around 50psi pressure - i've not been brave enough to go higher yet without safety modifications.
Step 2: Air Inlet
The input valve was a bit more tricky, you'll have to be a bit inventive depending on what you have on hand. I used the kegs pouring spout as the air input. I ripped a schrader valve from an old wheel barrow wheel (leave about a 2inch radius of rubber surrounding the valve). I cut off the 'tap' section of the keg's spout, leaving around 2inches of tube protruding. I moulded a big lump of epoxy putty around the spout to make a bigger outside diameter, smeared the whole area with silicone (careful not to block the tube), then wrapped the rubber circle surrounding the shrader valve around the epoxy. I didn't have any jubilee clips on hand, so I just wrapped wire tightly around the whole assembly to keep it on (I carved a barb into the epoxy plug to ensure the wire keeps the rubber valve in place).
For the rocket, I simply used a $1 bubble wand that fit snugly over the 13mm irrigation house outlet. I'll spend a bit more time refining the rocket design (maybe one for my own play and a soft foam rocket for when the daughter wants to play).
I feel this design is more durable than the pvc variety. One safety improvement I plan to make is to tether the sprinkler valve to the keg body. This is a likely failing point of the system and i don't want this heavy valve to go flying! The air inlet is a bit of a bodge, but it's holding up fine.
I'm also making a battery controlled remote switch (on a much longer wire) so i can fire it in a more remote area - i'm sick of climbing on the roof to collect stranded rockets!
If you don't have a 5l mini keg handy, I urge you to go out and get one and empty it responsibly!