Valiant was a name given to a large Earth-bound flying aircraft carrier used by UNIT during the early 21st century. (DW: The Sound of Drums, The Poison Sky, and others)
21st century
Every detail of the 21st century Valiant 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 coordinates 58.2 North 10.02 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 Tenth 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)
The Valiant was fitted with a mounted energy weapon almost identical to the gun used to destroy the Sycorax, meaning the technology could have been salvaged from the remains of the Torchwood Institute.
During the Sontaran invasion of Earth in 2009 Colonel Mace called in the Valiant, which was referred to as Hawk Major. The powerful turbines were used to clear the Sontaran's fog away from the ATMOS factory and the energy weapon was used to blast apart the factory, killing many Sontarans. It was unknown if the weapon was installed after the Master's defeat or if Harry Saxon himself had it incorporated into the initial design. (DW: The Poison Sky)
The Valiant was specifically targeted when the New Dalek Empire attacked Earth following its transportation to the Medusa Cascade. (DW: The Stolen Earth) It took heavy damage and when it was forced over heavily populated areas, the decision was taken to land on empty farmland, avoiding a further loss of life. However, the attack still continued (DWBIT: Dalek Wars) and a massive force of Daleks swarmed the ship, taking out its shields. The order was given to abandon ship, and the Valiant was destroyed. (DW: The Stolen Earth)
Propulsion
The Valiant used what appeared to be large Turbofan engines. The Valiant seems to be able to rotate these engines to lower itself toward the ground as seen in (DW: The Poison Sky) As it was designed by the Master, it was more advanced than any current Earth vehicle due to the fact that it has been previously mentioned to have had the reverse-engineered Jathaa sunglider technology built into it. As a back-up it may have had some alternative form of travel, like Anti-gravity propulsion. These allowed it to not only hover high in up in the Earth's atmosphere, but to move across the globe to any destination deemed necessary.
Weapons
It was unknown what sort of weapons the Valiant utilised, apart from the reverse-engineered Jathaa Sunglider weapon. It may have featured a series of CIWS turrets and short range missiles, but as a United Nations ship it might not as it is stated to be neutral territory (DW: The Sound of Drums) The Battles in Time: Dalek Wars depiction of the battle showed fighter jets were tasked to the Valiant.
As Jack Harkness overheard a soldier reporting that the shields were down, the Valiant may have had some kind of protective force-shield.
- The Valiant's design was very similar to that of Cloudbase/Skybase, the headquarters of Spectrum from Captain Scarlet TV shows (the Captain Scarlet cartoon was a contemporary of the Who revival) and to Helicarriers used by the fictional Marvel Comics organization SHIELD. The Valiant's weapon system looks like the one that was salvaged from a Jathaa sunglider and the Death Star's planetkilling weapon system from Star Wars. A similar-looking vessel, commanded by an elite British air corps, also appears in the fantasy film Sky Captain and the World of Tomorrow.