The Self-Propelled Heavy Artillery - Turbolaser (SPHA-T) is a field artillery piece mounted on the body of a 12-legged heavy walker. It was developed by Rothana Heavy Engineering (RHE), a local subsidiary of Kuat Drive Yards (KDY) for the Grand Army of the Republic.
The drivers and gunners sit in a control room at the front of the walker. A wide panoramic viewport provides a view for the commanding officer and some of his crew. On each side of the vehicle there is a doorway with a round-topped frame (roman style). The starboard hatch is on the same deck as the window, and leads out onto the platform below the main cannon. The portside hatch is on a lower deck, below and behind the cylindrical side-mount of a smaller laser cannon. Ladder rungs are visible on several parts of the hull. Eleven rungs curve around the port side of the main cabin, possibly for crew to evacuate the upper deck in an emergency (assuming that the viewport or roof has been opened). There are five rungs underneath the cylindrical mount of the portside gun; perhaps they're used for maintenance. On the starboard side, seven more rungs run up from the gun platform onto the roof of the control cabin.
In addition to the main gun and the side gun, some sources [e.g. NEGVV p.138] claim that some SPHA-Ts have retractable defensive antipersonnel blasters. These must be hidden beneath recessed panels; they aren't visible in external views of the walker [during the movie]. During the Battle of Geonosis, AT-TE walkers and footsoldiers guarded the approaches to each SPHA-T.
Each SPHA-T cannon exceeds the size and the instantaneous firepower of all the guns on an Accalamator transport ship. This is unsurprising, since the Acclamator is simply a well-protected military barge with some secondary capability for surface bombardment, and not a true combat warship. The Acclamators that landed on Geonosis could not hope to shoot through the shields protecting Trade Federation core ships, but the combined, maximum firepower of several SPHA-T guns overcame these shields (one target at a time). If a SPHA-T were integrated into the body of a star frigate, destroyer or larger vessel (with adequate, dedicated power feeds) then it would compare with the heavy turbolaser turret of a star destroyer (a gun of similar volume when the internal root of the turret is included).
The disadvantage of a SPHA-T (compared to ship-mounted guns) is that is exhaustible. It isn't linked continually to a ship's reactor system and fuel reserves. A SPHA-T can only fire a few times at maximum firepower before it must return to the mothership for recharging. If the SPHA-T has an internal reactor, the small size of this device must limit output to the levels of a starship in the 30m - 50m size range: insufficient to support a full-powered heavy turbolaser indefinitely. Of course a SPHA-T could conserve its charge and prolong its operational time by using lower power settings most of the time.
The usual method of deployment is not yet observed. Animatics portray the lowering of two SPHA-T guns onto a forward-opening ramp under the bow of a transport ship that superficially resembles an Acclamator. However this is not possible in the final [filmed] version of an Acclamator, in which the two hangar cavities are in the wings. Essential power conduits and reactor fuel silos run through the longitudinal axis of the ship, right through the middle of the space where this SPHA-T cavity and forward ramp would appear. The real side ramto pass a SPHA-T, but a tight fit might be possible if the body and the gun deploy as separate pieces, for outside assembly. The hangars of an Acclamator are supposed to carry some SPHA-T equipment [annotations in AOTC:ICS]. It is possible that larger classes of transport ships and landing barges exist that can ferry SPHA-T guns whole, without the need for vulnerable, complex external assembly.
Side view of several SPHA-T cannons using their maximum firepower to shoot at the nearest Trade Federation core ship. [PAL DVD widescreen version of AOTC]
Beams from a group of SPHA-T guns penetrate the shields of a launching core ship and slice into its hull. [PAL DVD widescreen version of AOTC]
Closeup of clone crewmen in the upper deck control room of a SPHA-T, behind the window. [PAL DVD widescreen version of AOTC]
Early animatic of the deployment of SPHA-T artillery from transport ships on Geonosis. [AOTC DVD documtenaries]
Early animatic view of the front of a SPHA-T. An AT-TE walker appears to the right side. [AOTC DVD documtenaries]
Trade Federation core ships provide the best quantitative clues to the performance of a SPHA-T turbolaser.
The maximum continuous heat disposal by the shield system of a core ship is rated as 6×1023 W [statistics in AOTC:ICS]. Up to this total limit, continuous attacks from all directions can be absorbed into the shield mechanisms and re-radiated in harmless forms, while the energy field comprising the shield is simultaneously replenished. A sufficiently strong and abrupt surge could deplete the shield's potential energy locally (exposing a region of the hull), but the duration must be short. Firstly there is a naturally time-scale of feedback between the generators and the affected location of the shielded space. Secondly, there may be diffusion or wave propagation between neighbouring regions of the field. The shields, being massless, are probably mediated by light-speed phenomena, in which case local shield effects would only be apparent for surges of less than a few microseconds (width of the core ship's shields divided by c). Thus local shield failure isn't a plausible explanation for the success of SPHA-T attacks lasting a second or so. For long-duration attacks, we must compare total firepower and shield power directly.
The maximum reactor output of an Accalamator-class military transport is 2×1023 W. Thus the maximum continously sustainable firepower of an Acclamator is less than a third of what the core ship can shrug off. In a one-on-one test of guns against shields, the core ship would not suffer any unsustainable accumulation in its shields' internal heat-sinks.
In the Battle of Geonosis it was therefore unfeasible for Acclamators to attack core ships directly. Furthermore, it was tactically impossible to settle close to the Separatist landing fields, and any inter-ship fire would have been attenuated and scattered by the Geonosians' theatre shields over the intervening tens of kilometres. Rather, the Acclamators would deploy their lumbering, pre-charged SPHA-T artillery to crawl across the open battlefield and concentrate on the enemy ships and fortifications from more advantageous positions.
Approximately five SPHA-T guns were able to penetrate the shields and cut into a single launching core ship. If the core ships were fully shielded (as a sane and cowardly Neimoidian captain must command when evacuating an invasion zone), then we infer a lower limit on the output of an individual SPHA-T. By this reckoning, each gun yielded an instantaneous maximum firepower of around ~1.2×1023 W. However this rate was only sustainable for a little over a second before the SPHA-Ts were exhausted. This implies that the gun's reactor initially carries, and eventually annihilates, the equivalent of around a million tons of fuel. This much mass-energy, in whatever exotic form it takes, must contribute significantly to the effective weight of the walker.
The recoil force implied by a beam of this power is on the order of 4×1014 N, a horrendous and almost incredible magnitude for a vehicle of this size. It's enough to propel a million-ton object with an acceleration of 40000G, or a 2km wide iron ball at 1G. This is a severe problem. One possible solution would be a principle like that of a “recoilless rifle”: a mechanism that applies an equal counterforce, e.g. a harmless neutrino counter-beam firing in the opposite direction. Otherwise, the gun requires some form of extensive force-field anchorage to the surrounding terrain (perhaps akin to the tensor fields and inertial compensators that hold an accelerating starship together). The way walkers and troops leave areas of clear ground behind a SPHA-T may be a clue to where the recoil pressure is distributed.
The recoil problem highlights the SPHA-T as one of the most interesting and potentially revealling studies in the physics and engineering of STAR WARS. There is much to be learned from this example.
Sources [NEGV&V] state that the SPHA-T requires an unusually “huge reactor”. They imply, but do not explicitly state, that the reactor is within the vehicle itself. It could alternatively mean that a SPHA-T needs access to recharge from a powerful reactor, such as that of an Accalamator transport ship. Firing depletes the SPHA-T's internal store of energy; they need to cycle back to their motherships for recharging during battle [ITW:AOTC]. This facility was probably an important design constraint of the Acclamator and other military transport ships that are more specifially adapted for SPHA-T support.
Peculiarly, the twelve SPHA-T guns typically carried aboard an Acclamator give the ship the theoretical possibility of firepower exceeding that of a star destroyer with eight heavy turrets. However this is only a capability in theory. The main reactor of the Acclamator is inadequate to keep the SPHA-Ts continuously charged, and they are not mounted externally. Their mounting and fire control systems are not suited to the accelerations involved in space combat.
The SPHA walker module was used as the base of several different classes of mobile heavy artillery.
The SPHA-T, seen in action on Geonosis, is a turbolaser cannon. The SPHA-T is essentially a line-of-sight gun: it can shoot anything that it can see. It fires massless beams that are essentially the same as the fire from turbolasers, laser cannons and blasters. The beam itself is an invisible form of energy that propagates at the speed of light. The bright light emission, which appears to drift at distinctly subluminal speeds, is a byproduct of the decay of some patches of the beam into ordinary photons. Its apparently slow propagation is essentially a strobe effect upon more rapid fluctuations within the beam, as seen by the [virtual] movie camera. Air has a high degree of transparency to this turbolaser beam (like blaster energy generally) but solid matter does not. The beam passes harmlessly through air with little collateral heating, but dumps intense power within the first opaque material that it meets. Indeed at its highest intensity, the beam may melt a thin column all the way through its target.
The main weapon of the SPHA-I is an ion cannon. This means it fires bursts of a plasma or charged particles. Damage effects may be electromagnetic as well as thermal, however the shots propagate slower than light and thus could be dodged more easily than turbolaser fire across astronomical distances (perhaps not a concern in a ground engagement).
Other versions carry rocket or projectile weapons. The walker artillery deployed with General Kenobi's armies on Muunilust [Clone Wars cartoon] fired physical projectiles, perhaps by an electromagnetic mass-driver mechanism. Their fire arced under the effects of gravity and air resistence, unlike the ray-like, line-of-sight fire of the SPHA-T turbolaser guns. Projectile (passive) and missile (active) weapons have the advantage of being able to reach targets over the horizon or behind cover. The commensurate disadvantages include: the enemy's ability to intercept incoming shots in flight; the eventual exhaustion of physical ammunition; the ability of pure energy weapons to deliver more damaging energy for a given weight of ammunition and support machinery.
Loyalist SPHA artillery firing projectiles in battle on Muunilust. The refire rate is about one second (24 frames of animation). The parabolic arcs of fire prove the material, projectile nature of the shots. One single, long shot takes about 34 frames (~1.42s) to rise and fall. In this amount of time, on Earth, a rising and dropping projectile could reach only about 2.45m at the top of its arc. The greater spatial scale of the scene implies that this animation shot is time-compressed; the real-time interval must be many times greater than 1.42s. [Clone Wars cartoon, DivX AVI clip]
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