SDF-2

First Robotech War

 
 
The Super Dimenional Fortress 2 was the first human attempt to use the reverse engineered template garnered from the SDF-1.  The SDF-1 had crashed to Earth in 1999, and the alien technology was deemed to be decades, if not centuries, ahead of where humanity's was at the time.  While reconstruction and research was going on in and around the SDF-1, when enough intelligence had been gathered, the SDF-2's construction began on a Moon based facility. 
 
In addition to attempting to utilize the alien protoculture based technology from the first ship, the SDF-2 was also built to more human standards.  The SDF-1 had been built to accommodate aliens of gigantic proportion.  The SDF-2 attempted to integrate human based technologies in a modular capacity, ever ready to adapt to a new discovery from the SDF-1. 
 
On launch day of the SDF-1 and the subsequent  Zentraedi attack, the SDF-2 was only about 50% completed and useless in the defense of Earth.  After the SDF-1 disappeared, construction continued, along with the Grand Cannon.  After the rain of death attack which decimated most of the population of humanity, construction was stalled, but not stopped.  When Khyron made his suicide run on New  Macross City, the SDF-2 was fully operational and in the process of getting the Space Fold System transferred from the SDF-1.  However, both ships were destroyed.  
 
Note:  The SDF-2 is barely mentioned in Issue #36 To the Stars, and is more of an afterthought than anything else.  However, the ship is expanded on in the novel series by Jack McKinney and mentioned in the Comico series The Robotech Masters.
 

Second Robotech War

 
A very brief, but pivotal role is played by the SDF-2, or more accurately, the ruins of the SDF-2.  The ship and the Space Fold System having been destroyed, the remains of the protoculture and Flower of Life that had fueled it still lingered.  Dana Sterling stumbled into the ruins during a battle and was subjected to a number of hallucinations, which opened her eyes to the nature of protoculture and the Robotech Masters. 
 

Third Robotech War

 
While never specifically mentioned as such, it is speculated that the Flower of Life that germinated and spread over the Earth was from the Space Fold System on the SDF-2.  It was the Flower of LIfe that led to the Invid Invasion, covered in the comic series New Generation.