Self-Propelled Heavy Artillery
Unlike smaller walkers, such as the AT-TE, SPHA units were unable to be carried into battle by LAAT/c gunships because of their large size and weight. Instead, they were deployed directly from landed Acclamator-class assault ships, which could carry 36 of the artillery platforms.
In battle, SPHA walkers usually received orders from ground-based command posts, which provided a less isolated and more in-touch view of the battlefield than the towering walkers. In addition, SPHA units had a complement of 30 clone troopers, which could be deployed rapidly via a dorsal troop deployment hatch. In seconds, the soldiers could disembark the vehicle and secure the area immediately around the walker, providing protection from enemy infantry and other light units.
The SPHA walker chassis served as the basis for a number of different weapons platforms, though the best chronicled version was the turbolaser SPHA-T configuration. This variant's main weapon was a heavy turbolaser cannon, capable of causing extreme direct fire damage to large or slow-moving targets.
The primary disadvantage of this powerful weapon was that it was incapable of providing indirect fire support to out of sight locations, a problem remedied in other models. Another disadvantage was that the weapon could only be charged to a certain level—upon firing such shots, they had to be replaced with charged guns from the rear lines.
Some of these other variants deployed during the Clone Wars include the ion cannon (SPHA-I), anti-vehicle laser (SPHA-V), concussion missile (SPHA-C), and mass-driver (SPHA-M). Also, these units had the disadvantage of ammunition, as it would run out if not managed properly. Some of these model variations included 12 retractable antipersonnel blasters, necessitating an additional 10 gunners to the command crew of 15.
It complemented the two other key components of Republic military strategy—the AT-TE walker and the LAAT/i gunship. In a typical attack, the AT-TEs and LAATs would spearhead the assault, establishing a forward base and allowing heavier support forces (such as SPHA walkers) to consolidate gains and provide reinforcements.
In development of the system, engineers on Rothana essentially had to build their prototype around the requirements of the primary weapon: a large, dorsally-mounted external weapon assembly, a powerful reactor core to fulfill the weapon's high energy requirements, and all of the necessary support equipment and crew stations.
In order to save weight, designers rejected a complex turreting mechanism, instead relying on the entire walker to reposition in order to adjust for different firing coordinates. Because of the fixed weapon positioning, as well as the slow firing speed of the turbolaser, the SPHA was unable to track rapidly moving targets, such as speeders or small atmospheric craft. Like other walkers, the use of articulated legs prevented the SPHA from being vulnerable to anti-repulsorlift weapons, at the cost of some mobility.
The result of this development process was a walker that possessed a very capable capital ship-class weapon, but was hampered by its low mobility. This low mobility was exacerbated by the fact that SPHA walkers were equipped with heavy armor, especially in the frontal area of the vehicle around and just below the command deck. This armor was necessary to protect the vehicle's reactor core from enemy fire, as any meltdown,and subsequent explosions, would cause tremendous collateral damage.
SPHA units were used throughout the Clone Wars, seeing action in surface battles such as the First Battle of Cato Neimoidia, the Battle of Muunilinst (where many were destroyed by the nimble IG lancer droids), the Second Battle of Rhen Var, and the Second Battle of Tatooine. Many of these planetary engagements highlighted its strengths and weaknesses—massive firepower and range, but very poor maneuverability and speed.
At the suggestion of Anakin Skywalker, many Venator-class Star Destroyers of the Open Circle Fleet had SPHA-T turbolaser cannons installed inside their ventral hangar bays, where they were used effectively during the Battle of Coruscant, especially against enemy Munificent-class star frigates.
The SPHA-line walkers continued in service in the early years of the Galactic Empire, until the SPMA-line was developed, with the SPMA vehicles being faster and more maneuverable than the cumbersome and slow SPHA-Ts.
In both Attack of the Clones: The Visual Dictionary and The Complete Visual Dictionary, the SPHA-T's turbolaser is incorrectly labeled as a troop hatch.