Among the simplest rockets to build is the 2-liter pop bottle with some fins and weighted nose cone.  The longest lasting rocket of mine is made from 3 2-liter bottles connected with Robinson Couplers.  That is, the bottles are connected together with hollow, threaded lamp rod, with O-rings and washers to seal them air-tight.  The bottom (opposite nozzle end) of the lowest bottle is connected to a bottle-cap.  Select a washer size that is small enough diameter so the second bottle can just screw onto this lid.  The top bottle is connected to the second bottle in the same manner.  One nice thing about this arrangement is that if the rocket ever lawn darts with no parachute, it usually just crumples the topmost bottle.  No problem just unscrew and toss it, and screw a new one in place.  Very easy.  I believe a 3-section rocket of this type is ideal.  The lower section gets filled with water and the top 2 are pressurized.  Be sure to get any water out of middle section before pressurizing rocket or the rocket will have a very short flight – water does not go through the coupler quickly enough during “the burn”. 

At the top of the rocket is a separate payload section made from another bottle.  It has room for the “passenger” as well as horizontal parachute deployment system with Tomy timer.  Robert Youen’s gets credit for this (as far as I’m aware).   The right photo shows a loaded chute.  This was my deployment system of this type – Not too pretty with all the strapping tape and rubber bands, but it works very reliably.  I preload the timer with 1 full turn before inserted the small eye-screw attached to the end of the rubber band.  It gets 3/4 of a turn more with the rubber band attached, and it is held in place by the wooden trigger. This wooden trigger is connected to the launcher with fishing line.  When the rocket takes off, the trigger is pulled and timer starts – delay is about 3 – 3.5 seconds before the rubber band comes loose, unwinds rapidly and the chute is ejected out the side.  The 16″ chute is made from a plastic garbage bag, cotton string for parachute shrouds, and there is an elastic shock cord connecting the chute to the top bottle cap.   The holes in the chute, where shrouds are connected, are reinforced with a bit of strapping tape on each one.

This rocket is a real crowd pleaser.  While it doesn’t go as high as the FTC rockets, it sounds really cool, and is powerful.