I made this model in Tinkercad, and then cut it into two pieces and added a threaded connector to the two pieces.

I will be printing this on a Printrbot Simple, so I only had a small build area so I wanted a way to make a larger rocket.

Finally got a working model to print, took me four different tries to fix the bottoms of the side rocket fin things. They kept snapping off either mid print, or after it was printed. Also, the screw thread idea had to be scrapped because my printer is not perfectly calibrated. I opted for trying the pin system from a user on thingiverse. Mixed results on the pin connector, but it got it together which is all I wanted.

Phew. Many trials and errors, but I do like the final model.
Instructions
Printed on a Printrbot Simple.

1.75 MM PLA filament from Matterhackers
205 temp
50% infill