This rocket ejects water at a high-speed to propel it. A 2-liter bottle is filled half-way with water, and then pumped up to about 100psi. When the bottle is released, the air pushes the water out, propelling the bottle.
Here is the first quick connect rocket design. The two-liter lid has a quick-connect fitting, as is shown on the right. The bottle can be filled half full with water it make a mess and improves distance :-). The bottle is then connected to an air hose with 100PSI. When the quick connect is released, the bottle is propelled forward by the air pressure.
The main disadvantage of the above method it that one needs to be right behind the rocket when firing. This has obvious dissadvantages when the rocket is full of water, and also is not the safest place to be when the rocket is full of air. The new quick connect design overcomes these problems. I took apart a quick connect to see how it works. Orriginally there was a sleeve that one pulled back to release the fitting. I replaced the sleeve with a rod bent in a 'U'. Now when I pull the 'U' out with a string, the rocket lifts off.
The fins on this rocket help it fly much more stably, and not tumble.
This rocket launching pad was designed to clamp the 2-litre to an air hose, and release it when the string was pulled. It was put on the back burner when the above mentioned quick-connect release was invented.