The M1A3 "Retrofit" is an experimental Abrams main battle tank variant outfitted with alien erestal crystal technology. It originally started off as a wild experiment by tordenchi combat technicians during the invasion of Earth to make use of human tanks that ran out of fuel or ammunition after their crews extensively engaged the rynar invaders.

Almost immediately following the invasion, an official design was refined and implemented in small numbers through joint human and UTO efforts. The main 120mm smoothbore gun was replaced by a plasma cannon with stopping power on par with most standard plasma rifles used by the United Treaty Organization. A plasma machine gun with multiple firing modes also replaced the M2 Browning machine gun mounted on the commander’s hatch, but since humans are abnormally small compared to other species in the universe, it’s mostly used for suppression or smaller targets such as drones. This plasma machine gun can either be directly controlled by a human or remotely controlled from inside the tank itself. The co-axial M240 machine gun was outright removed altogether with a replacement deemed unnecessary, much to the dissatisfaction of human tank crews.

The engine was completely replaced by an erestal power core with cooling racks and coils placed along the tank’s rear and any weapon barrels. This power core simultaneously provided fuel and energy for hydrogen ammunition. Combined with heat-absorbing ceramic armor and attachments replacing its conventional armor, the weight of the tank was significantly reduced to under 60 tons.

Reception to the tank variant was mixed. Although it was a cheap and quick way to bring human military hardware up to UTO technological standards, new military vehicle designs developed years later alongside widespread mech adoption quickly rendered the retrofit obsolete. Human tank crews on Earth especially disdained them since depleted power cores could be impossible to replace on the battlefield without nonhuman assistance, and conventional ballistic weaponry proved to be far more capable at killing the rynar since their soldiers specifically wore armor to protect themselves against far more common plasma weapons.

Many considered the experiment impractical or a failure, despite learning from it.

Ironically, the tank variant found a second life on human colony worlds. Whereas the retrofit could be impractical on Earth since its solar system completely lacks erestal energy and is incapable of recharging erestal crystals through solar power, a vast majority of other solar systems lack this issue. This means a tank in a compatible solar system can have practically infinite fuel and energy following a brief recharging period where the power core is simply exposed to the sun, albeit this renders the tank vulnerable. The eventual development and adoption of ballistic and composite armor to counter human ballistic weaponry also made plasma weapons far more pragmatic compared to the past for humanity.

As a direct result, human military forces on colony worlds have not only made extensive use of the Abrams variant, but also other retrofitted tanks. They’re extremely inexpensive compared to other options, making them very appealing when operating on a shoe-string budget. Nonhuman pirates and raiders are regularly caught by complete surprise when humans fend them off with dozens of tanks, especially if they previously considered the small humans easy targets to pillage or kidnap. Retrofitted infantry fighting vehicles transporting entire human infantry squads behind the tanks also made for a deadly duo. With Abrams firing plasma cannons equivalent to nonhuman plasma rifles and Bradleys firing autocannons with potency on par with nonhuman plasma pistols, it creates complete chaos for comparatively gigantic alien warriors already engaging entrenched human soldiers on the ground.

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Credit for this eye candy goes to  Drunk_ferret. The cute mouse included is Alice starring in  Piershey's Of Mice And Men series!