Sometimes we do these water rocket afternoons at the Santa Fe Childrens' Museum. You end up helping the children pump all afternoon, and this is exhausting. Also, pumping up big things like 2-liter bottles or Illy cans with a bicycle pump gets tiring very fast. Following Gordon's lead, I made a CO₂ rig.

closed >>>   The box is 8.5" x 10.5", and 15" high. The holes in one of the side walls form a handle. The walls are made of 3/4" ply, except for the wall opposite the handle, which consists of only 2x2's on the corners. These are shaved down a bit to accomodate the round bottle. The two round things in the front panel are T-nuts, because front and back come off, and:

base plate >>>  The front and back plates form a base plate. Some threaded rod and wingnuts, plus the T-nuts in the panels, tie them together in the field. The added utility is that the top of the box makes a perfect platform to put the water tank on top of.

the bottle >>>   I got a (5-pound size) aluminum bottle, had it pressure-tested and valved, and filled with CO2. Now I can tie the bottle down, and protect the regulator. From the regulator I go to a couple of turns of 1/4" copper, a small valve an a brass hosebarb for my 1/4" plastic hose.

drawhasps >>>  The bottom of the front and back panels have 1/4" holes in them (visible on the previous picture, and these slip over short 1/4" dowels. The tops are held in place with drawhasps, which are recessed to there is a smooth top.

The rig in use in the field. Niels is filling the big espresso can with a gallon or two of water. See it fly!