2 Liter Bottle Water Rockets

With five days off of school what better is there to do than make 2 liter pop bottle rockets? Not much, i think. I have a few designs of the rocket made up in CAD and the launcher too. But, i still need to go buy some of the fittings for attaching bottles together and for the launcher before i can build. I had a female-female fitting that works perfectly with two liter bottles, so that was much easier(attaching two bottles together at the "mouth"). I think ill use some nylon couplers similar to these for connecting two ends of the bottles together: