A full Naval Review took place in 4315 A.D to assess the UAS Fleet's capabilities far into the 4th Millenium. Developing threats in Tempest and the Angel Reach, along with committments made to the Six Trade Alliance placed a significant burden on existing warship hulls - some of which had been in service for hundreds of cycles, based on designs over a 1000 cycles old. One such warship - the venerable Farlease-class Light Cruiser was long due to be replaced with a much more capable, up to date design.
The review concluded that the Farlease-class vessels and derivatives such as the Elara-class, Tempest-class and Harbinger-class (which itself had a modernisation resulting in the HB-AX Artillery Light Cruiser), would not be able to provide the UAS Navy with the necessary capabilities into space beyond Aquarius. The Farlease's hull design had become restrictive of newer weapon systems for some time, the superstructure didn't support any modern B-type turrets due to internal space issues and powerplant, as a result the vessels were still equipped with 1000-cycle old FIMD weapons, which had proven to be woefully inadequate in some situations.
While a successful design, the HB-AX was unwieldy, and due to the removal of most of the existing B-type mounts, was unable to equip any mid-range weapon systems. This resulted in a warship that simply couldn't defend itself against other cruiser, or destroyer type vessels. The UAS Naval review thus put forward a requirement for a new class of warship that could fill all of the existing roles in a single, capable and vertsatile hull design.
SP-AX was born from a need for a Heavy warship with powerful short and medium-range weapons, while also carrying a stand-off axial armament to serve the roll of an Artillery Platform. While the vessel is too small to equip the powerful SPARTAN Anti-Ship Missile System, it uses a completely new design of B-type turret, using an updated Rotary MA-FIMD armament specifically designed to penetrate capital ship shields and armour. SP-AX has five such turrets, with four placed in a centralised position midships, for excellent coverage.
Combined with advances in Non-reactive gravimetric drives first introduced on Revolution-type cruisers, the Sleipnir is surprisngly agile despite having almost 50% more mass than the Farlease-class hull it was designed to replace. The designation of Heavy Destroyer was made, despite the increased mass and size, due to the ship's intended role in the fleet.
The new warship entered mass production in 4321 A.D, and saw its first combat deployment during the invasion of Angel Reach in 4349 A.D