LF9 ion engine
From Wookieepedia, the Star Wars wiki.
The LF9 ion engines were the primary engines of the Victory-class Star Destroyer.
[edit] Behind the scenes
The original Star Wars Essential Guide to Vehicles and Vessels stipulates, "The Victory Star Destroyer's biggest weakness is found in ship-to-ship combat. Its LF9 ion engines cannot produce sufficient speed for deep-space actions, and most modern combat starships can simply outrun them."
It also underscores that the VicStars "were designed with three missions in mind: planetary defense, planetary assault and ground-troop support, and ship-to-ship combat."
Alongside that it goes on to say that "To achieve success, Victory captains must first establish a superior position, for example, by pinning enemy ships in a planetary gravity well so they cannot escape into hyperspace. Victories can best surmount their shortcomings if they are assigned Interdictor cruisers, escort frigates, or light cruisers for support."
The dossier on the VicStar concludes with, "The Victory is an excellent ship for planetary actions because it is one of the largest capital starships that can operate effectively in a planetary atmosphere. While most modern combat starships are restricted to orbital bombardment, the Victory can bring the battle directly to an enemy on the surface, giving the ship both a tactical and psychological advantage."
From all this one can deduce that the LF9 ion engines are at home most in a planet's atmosphere with spatial flight, while being essential is something of an afterthought for them. Everything about the VicStar from the description of the LF9s to the self-evident presence of a 900-meter-long capital ship coming stock with atmospheric maneuvering surfaces suggest that the ImpStar's predecessor was crafted with the intent to get in the face of a planet's defenses and to uproot them at a distance that is extremely atypical of a capital starship of this size with smaller capships like the 195-meter-long Marauder corvette being more the norm for this type of combat role.
