UPDATE 2013/08/07
Added files for B/C size engines (and some A's)
Tested 2 stage rockets with both A and B size engines -- They both fly great.

Videos:
https://www.youtube.com/watch?v=mGV658F-S14
https://www.youtube.com/watch?v=yqqz5XzA2GQ
https://www.youtube.com/watch?v=YmJDoERuGF8
https://www.youtube.com/watch?v=OEV92Uavh04

Just a simple nose cone and tail fins that can be attached directly to a model rocket engine to become a complete rocket. (Thanks to my friend Chris for the idea).

Files are included for both 1/2A size, and B/C size.

This thing flies great as long as you use the guide tubes on a proper rocket launch pad. I just used a bamboo skewer in a block of wood. Just make sure it slides off easily.

The single stage version shot off the launcher, and was amost out of sight in seconds, until the ejection charge created a puff of smoke in the distance.

Don't launch direcly from the ground without a guide rod (don't ask me how I know... sorry Chris)

Don't plan on recovering the pieces, since they will probably not survive. That is why it is called a "Disposable Rocket". It only costs around 25 cents in material to make, and 15-20 minutes to print.

I am in no way responsible for how you use this, so please use good judjement in launching it, and please don't launch near people, buildings, or other important objects.

Print 1 each of the nose cone and fins for a single stage rocket. Pretty much any settings should work, it is designed to only be 2 extruder widths thick almost everywhere, and layer height is not critical.

Attach the nose cone to the top of the rocket engine, and the tail fins to the bottom, and make sure the launch guides line up with each other.

For a 2 stage rocket just print an extra fin section, and add to the bottom of another rocket with a second engine.

Depending on how your printer does on the first layer you may need to shave a little off around the openings to get it to fit over the engine. If it is too loose, just shim with a little tape.