I took one of the 3/4" PVC engines, that I have been working on, and tried to see if it could carry a fairly large mass. I made a rocket by putting the engine inside a 2 liter pop bottle. The pop bottle was then attached to a1"x2" guide stick about 5' long. 1500ml of water was added to the bottle, for weight. The engine weighted 264 grams. With the bottle, engine, stick, and added water the total take off weight was 2200 grams. That's 4.8 pounds! The fuel was 70 grams of KNO3/sugar.

Above you can see us loading the water (colored). The rocket took off and flew with a problem. It did not go very high. It looked to me like 70 feet or so. Here is a short video.

For more about the engine go to pyro.htm