The Liberator-class Super Dimension Fortress, commonly referred to as the SDF-4 or the Liberator, was a next generation fold-capable battlefortress that was constructed by the Robotech Expeditionary Force's deep space shipyards, and the intended to be forward command ship of the Expeditionary Mission. It was originally featured inRobotech: The New Generation.
The Liberator design was distantly inspired by the philosophy that yielded the Izum-class and Ikazuchi-class large cruisers. The main body of the vessel retained the 'brick on its side' shape for much of its length, though it could be divided into two sub-sections: A narrow, aft central section containing the central engines and anchoring the command tower, and an enlarged and heightened forward cannon section. In addition, three additional modules were attached; two connected by a long plane to the two sides of the central body, directly to the central engine block, and one to the keel. At the front of the central body the syncro cannon was mounted.
In addition, the forward subsection of the main body shields much of the vessel from return fire, and the side bodies shielded the aft main body from flanking maneuvers. At normal battle attitudes (nose on), the Liberator's vital aft central body was usually only exposed on its top and bottom sides, and only in a very limited way forward of the engine blocks. This all combines (because of the ships' attitude) to make the central section the least vulnerable section of the ship.
The lower body contained the hangars, as well as many cargo bays, to facilitate unloading. Any troops being transported are quartered in other sections of the vessel, and the lower section has an independent anti-gravity engine block to enable it to make planetary landings.
The command citadel was located on top of a tower that rose above the main hull, above the aft central engines. The Flag Bridge was located in this citadel.
Hangar decks and crew spaces were distributed through the central and side bodies as well, though most fighter mecha are launched from the side bodies. This included no less than sixteen (in eight double housings) versions of the quick-launch bays first used on the Izum-class. There are two large Horizont bays located in the side bodies, just forward of the side aft engine sections. They were closed by large trapezoidal shutter door in the upper deck, and capable of a high turn-around rate on the shuttles.
The origin of the Liberator class Battlefortress lay during the beginning of the Third Robotech War, after the Expeditionary Forces sent most of their active combat units first to reinforce Earth against the Robotech Masters, then against the initial Invid Invasion. During those events, most large capital ships were destroyed or heavily damaged, and a new battlefortress type was authorized, the large Liberator-class.
The class was seen as the follow-up to the Super Dimension Fortress-3 Pioneer (SDF-3) design, which had several known weak points, such as too little generating power for its main armament. The final result beared little resemblance to the first three super dimension battlefortresses.
Initially the class was to be equipped with a large late-generation Reflex Cannon up front like the SDF-1 and SDF-3 Pioneer. The Synchro Cannon was only made available much later, during the final phases of the war with the Invid.
This new main Synchro Cannon technology gave less powerfull individual salvos, but could fire much faster than the large Reflex Cannon the REF was used to. This setup was better suited for anti-ship attack as both cannons had sufficient power to destroy any conceivable ship and thus the Syncro cannon's higher firing rate could take out multiple targets where a Reflex cannon could fire only once. An additional advantage was the more compact Syncro cannon installation, which did not require the entire forward hull to open up for firing, simplifying construction.
To save time, the Syncro Cannon was fitted in the place where the Reflex cannon would have been, and the freed space was given other uses, but this resulted in a larger ship than would have been strictly necessary for the armament. In addition, the heavy secondary and missile armament as well as large cargo requirements resulted in the heaviest and largest ship yet built by Earth forces, rivaling the Zentraedi Thuverl Salan-class Heavy Cruiser in volume and mass.