Air Command's latest update (#132) covers the latest flights of their Inverter Rocket. I think this is one cool looking rocket!
One of the many recurring questions on TRF is, "can I mix different types of propellant grains in one motor?"* Well, that is kinda what these guys have done in the water rocket world. Check out this montage and click over to read all about the flights.
* The oversimplified answer is: In theory, yes. However, you have to know what you are doing and other motor mods might be required. Not to mention, the resultant motor will not only be uncertified, it won't be kosher under TRA 'research' rules, and the flame will not have multiple distinct colors. Sounds like it is a lot easier to do with water. Not to mention prettier.