Quite cool and quite easy!  It goes together as shown below....

My launcher has only 6 parts and some epoxy. 

pvc 

O-rings

 pvc

 coupler 

pop bottle top 

cable ties 

valve stem (from any tire)
I was in a hurry to make it and I did some surfing to sites like these:

• http://mechtoys.home.dhs.org/

• http://www.osa.com.au/~cjh/rockets/

• http://www.scioly.org/eventpages/bottlerockets.htm
   

  They gave me the basic idea and I whipped it together in under an hour.  It shoots 1 liter bottles almost a hundred feet, and at this time I haven't yet shot a 2 liter.  Very groovy.

  DISCLAIMER:  If you make one of these, you are accepting the responsibility for your creation.
  I hold this launcher in my hand (which some might frown upon) and I still have both eyes and all my fingers, but if you make one, you would be wise to consider a remote launcher of some type.