This video shows where to find and how to complete all of Rocket Raccoon's missions. Completing the 3rd and final mission unlocks Raccoon Rocket for purchase. The first and second missions unlock the Garbage Truck as well as Star-Lord. This video also shows a quick look at some gameplay with Raccoon Rocket.
LEGO Marvel Super Heroes 100% Walkthrough Playlist:
http://www.youtube.com/playlist?list=PLYpDU5ElRBfndtPu_cRrrqCy9Ddzbwbwc