|The following is based on Fallout Tactics and some details might contradict canon.|
The Hellion is a powerful, heavily-armed aircraft, located in Buena Vista.
It is a prototype weapon which was not finished before the war, so its completion is one of the objectives given to the Calculator after the activation of its pacification protocol.
The Calculator was meant to ensure the safety of inhabitants of Vault 0 and secure the region of all the mutants and possible foreign soldiers. To do so, it required something beside the army of robots, so the Hellion was placed in Buena Vista with a working nuclear reactor, waiting for the day the Calculator will start its protocols. Many years later, the Calculator emerged as a major threat to humanity, and became the main enemy of the Brotherhood of Steel.
It is unknown whether the Hellion was in a working condition, as the Calculator may have already abandoned it for some reason, for example the lack of resources to use it. With the destruction of the nuclear reactor, the Calculator lost its chance to use the craft, and the Brotherhood surprisingly ignored its existence, likely due to the war with the robots.
If completed, the Hellion would be a vertical take-off aircraft that can reach air-speeds in excess of four hundred miles per hour. The armory would include five anti-tank bombs, eight surface-to-air missiles, and dual sixty-millimeter cannons. The Hellion was to use standard issue JP-5 fuel and can fly for 8 hours or longer without refueling.
The Hellion appears only in Fallout Tactics.