Stardrive

"Look, the city, as you know, is just a giant spaceship with a stardrive capable of faster-than-light travel..."

- Rodney McKay

The stardrive is a feature on city-ships that allows them to travel through space at phenomenal speeds, and consists of both a sublight and FTL drive.

Functions
The stardrive allows a city-ship travel to distant galaxies in only a matter of hours when at full power, and the sublight function of a stardrive is so fast, even when powered by nothing more than one rapidly draining ZPM, that the dialing device of a city-ship's Stargate is incapable of establishing a point of origin when the city-ship is traveling through normal space.

The stardrive requires an enormous amount of power to be exerted at once for the sublight engines to lift the city off of a planet, and so usually it can only be activated if there are at least 2 ZPMs powering the city-ship. This power requirement can be subverted, however, when an external power source supplies enough of the needed power that a single ZPM can supply the rest (such as when McKay and Sheppard powered the stardrive of Atlantis using only one ZPM and the drilling platform to supply the extra power needed to get it into the air).



The hyperdrive itself requires relatively little power (compared to what a ZPM can generate) to travel interstellar distances, and the greatest amount of power expenditure during the entire process of taking off, flying through hyperspace, and landing on another planet typically occurs when powering up the inertial dampeners and achieving the necessary escape velocity to lift the multi-megaton city off of a planet's surface. When at full speed a stardrive is actually capable of outracing even the most advanced Asgard hyperdrive, though it is unknown whether or not an Asgard hyperdrive would be able to match or even exceed the speeds a stardrive is capable of traveling at when supplied with an equal amount of power. Achieving such speeds, however, likely requires far more power than even two ZPMs used in conjunction can exert, so a city-ship's full complement of ZPMs is needed for a stardrive to operate at optimal efficiency.