"A heavier ballistic system using advanced power systems to magnetically hurl larger rounds that have the kinetic power to knock Destroyers off course while damaging them heavily."
Mass Drivers unlocks the weapon system of the same name, Mass Driver(weapon). This is the medium mount basic Ballistic Weapon. This node is the point where the tree branches into truely interesting technologies.
Mass Drivers will cost as much as 15,000 or as little as 5,000 research points to unlock, depending on chance.