I first came up with the idea of making a 3D printed rocket for the Instructables 3D Printing Contest. I wanted to make as much of the rocket out of 3D printed plastic as possible, to see how well it would work.
When I first came up with the idea, I was planning to make a solid-fuel rocket, which is the easiest to construct. However, I soon realized that this would require an oxidizer to be mixed in with the plastic, which would have been difficult. (I would have had to either pre-blend the oxidizer into the PLA filament, or blend it inside the extruder, both of which would be difficult and might damage the printer, which isn't mine.)
Therefore, I decided to switch the design to a hybrid rocket. A hybrid rocket has a solid fuel and liquid or gaseous oxidizer (or, rarely, a solid oxidizer and liquid/gaseous fuel). The advantages of hybrid rockets in this case are that the fuel and oxidiser do not have to be pre-mixed, and the oxidizer can be turned off to shut down the rocket, which is not possible with a solid rocket.
Initially, I had been planning to refill a spent CO2 cartridge with oxygen for the oxidizer, and to build the rocket into a 3D printed airplane. I found that it is possible to add a valve to a CO2 cartridge and refill it, though it is somewhat difficult. However, a stoichiometric calculation and an ideal gas law calculation showed that I could not fit anywhere near enough oxygen into the cartridge at any sane pressure. As well, I didn't have enough time before the contest deadline to design and print an airplane.
So, I am now planning to just perform a static thrust test for now, using our shop air compressor for the oxidizer source. (I have looked into 3D printed pressure vessels, and, quite surprisingly, they can be made pretty easily. So I might still make the rocket plane in the future, maybe for next year's contest.)