For fun, we built a Pop Bottle Rocket launcher. It took a few attempts to get it working, but once it did, it was a big hit.
[Note the use of goggles. We are crazy, not stupid.]
Once we had the redesigned inlet, test #3 (shown below) was highly successful.
The launch was able to clear the tops of the trees, which is somewhere in the range of 65-85 feet. In future launches we will probable look at adding fins and a nose cone to decrease wind resistance, and also look at optimizing water load to get maximum height. (We also might try to increase pressure and/or tank capacity. (I was only half kidding about building a wind tunnel in the basement.)
The details of the launcher are listed below.
The locking mechanism for the launcher consisted of a bulge in the 1/2 inch schedule 40 PVC pipe. The bulge was achieved by heating the pipe with a candle and then pushing it together with a bit of a wiggle. While this weakened the pipe, it seemed likely that the 70 PSI to which the system would be subjected would still be acceptable. (http://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html) Once the bottle was butted up to the bulge, the zip-tie assembly was positioned to cover the neck, and was locked into place using a hose clamp covered in duct tape. (The handyman's secret weapon.)
In the next picture, you can see the launcher set up in a dry-loaded configuration. The spring (cross-section of pop bottle with holes in it) is holding the collar up with is locking the cable-tie latch in place.
The pull cord for the trigger is routed along the launcher shaft into an auxiliary tube which both guides the pull-cord and adds additional stiffness to the main shaft. Also of interest is the use of strapping tape on the bottle in order to protect the bottle from deformation and bursting due to pressure. According to what little data is available, a non-reinforced 2-liter pop bottle has a failure point around 70 PSI, which is very close to the pressures involved. (http://answers.yahoo.com/question/index?qid=20061204051635AAoQDT9) Given that the benefits of re-enforcement vary wildly, and that non-reinforced bottle is near its failure point, I believe that safety goggles are a good idea.