Gudersnipe School Ship Mercury was the first Mercury-class warship commissioned. She was built in haste early in the Golden Age, and was on active-duty by A.Y. 1201. Many defects and design flaws kept G.S.S. Mercury from preforming anywhere near her targeted effiency, but she was still the single largest ship constructed to date.
Mercury would set the standard for that class of ships, being equipped with a heavily armored hull (two miles thick in places) and centered around a single main weapon.
The main reactor of the Mercury was quite a bit different from what one would expect, and completely unique among starships. At the time, most vessels used nuclear fission reactors for power, with a few employing fusion (The Nugen Reactor had yet to be concieved).
G.S.S. Mercury used an internal combustion engine.
The single-stroke engine was the largest ever constructed, and used hydrogen bombs anywhere from 1 to 10 megatons as its fuel source. The energy released at detonation was used to drive a piston which spun a turbine and generated electricity to power the ship and weapons.