This engine was suppose to be the final version of 3/4" PVC pipe engine, but a repeat of this test is required. The change was from a 0.25" throat to a 0.21" throat. The fuel is blue because of dye in the KNO3 fertilizer.

The photo below shows the stackup of the engine. The bulkhead is at the black mark. In the foreground from left to right is nozzle, spacer, fuel grain (5/16" core), spacer, fuel grain (1/4" core), spacer, fuel filled cylinder for smoke. The smoke cartridge was left out so that a better measurement could be made of the nozzle change. The smoke cartridge and spacers are made from turned down 1/2" PVC pipe. The smoke cartridge can also protect the bulkhead glue joint from the engine combustion effects.

The nozzle was turned from a 7/8" PVC rod and drilled with a 7/32" bit.

Clamping the nozzle area to get a better weld on the joint may not have been a good idea. The joint failed in one spot and a lot of gas went around the nozzle.

Even with the burnout the total impulse (19.4 lb*sec) was greater than the 34_3 engine at 17.8 lb*sec. It appears that the burnout problem started at about 0.4 seconds while the engine was at 46 lb of thrust. The 34_3 engine peaked at 42 lb. The specific impulse was 128 compared to 34_3's of 116.