SD5 stands for Silent Dawn 5. Those large turrets are mass driver turrets of a large caliber, and in order to propell a projectile at devestating speed a large barrel is needed and so it uses large turrets.
This is a sort of sub mod/campaign for my Silent Dawn storyline. As a repost, a Silent Dawn is a ferocious war that knows very little parallel. It made WWII look like a petty skirmish with SD4 (The Dark Return) having an estimated 4 billion dead. This number, however, is mostly because the Homeworlders blew up their enemie's home planet and hense that huge number. In short, there are a total of 5 SD wars spanning 2200 years. This specific war, The Containment War, is much like the Earth-Minbari War of Babylon 5 in that one side is faced up against a faction of superior technology and numbers. The Terrans of Earth barely stand a chance against the Homeworlders of Homeworld. The Terrans are not a unified force, if you notice the title of the ship it is called the UNA Athens. UNA stands for Unified Nations of America meaning that the first force that met the opposition were only of American construction. Right from the start, the Homeworlders had the upper hand (which is like 10 stories higher than the Terrans' hands) since the Homeworlders had like a dozen planets and hundreds of ships all unified under one banner versus the fractured Terrans with a fraction of the Homeworlder's ships and dozens of countries still un-unified and only 3 colonies. So these Terrans are basicly in deep sh*# unless they can mount a unified defence. But as I said earlier, that name changes to UNE (United Nations of Earth) which means they eventually DO unite. Oh, the Terrans are also willing to resort to some serious use of nuclear weapons. Weapons in which the Homeworlders outlawed and disallows in their warfare teachings.
This ship was slightly inspired from a Wing Commander Prologue ship, you can probably notice it. However that inspiration is only used for a small sillouette rather than the total design.