The Valiant was a large flying aircraft carrier used by UNIT during the early 21st century. It contained various rooms including a conference room and many engine corridors.
History
Every detail of it was designed by "Harold Saxon", and it was used for the first Human contact with the Toclafane. (DW: The Sound of Drums) The Doctor, Martha Jones and Jack Harkness teleported onboard using Jack's Vortex manipulator at the co- ordinates 58.2 North 10.2 East.
In the following year in an erased timeline, the Valiant formed the base of operations for the Master. The Jones family (with the exception of Martha Jones, who had escaped and Leo Jones) and Captain Jack Harkness were imprisoned on the ship, as was the Doctor. It also held the paradox machine inside the Doctor's TARDIS. When the timeline was erased, the Valiant was the only point in space and time not to revert, instead moving directly through time, due to it being the "eye of the storm", that is, of the paradox machine. (DW: Last of the Time Lords)
During the Sontaran invasion of Earth in 2009, UNIT used the Valiant's turbines to clear the gas created by ATMOS and to shoot the ATMOS factory in order to allow UNIT's troops to enter and fight the Sontarans. (DW: The Poison Sky)
The Valiant was eventually destroyed when the New Dalek Empire attacked Earth following its transportation to the Medusa Cascade. A massive invasion force of Daleks swarmed the ship, decimating it with their guns. (DW: The Stolen Earth)
Behind the Scenes
The weapons system used by the Valiant had a similar appearance to the superweapon from the Jathaa sunglider and the Death Star's planetkiller from Star Wars.