SD Elite (Apocalypse)
A high powered anti-grav unit generally restricted to military or police vehicles.
Source: X-COM Apocalypse Ufopedia
While it is a good engine, it is surpassed by the SD Special and should not be used if the latter is available.
If unavailable for purchase, and if X-COM has sufficient funds, one can buy a Hawk Air Warrior and rip out the engine, selling the Hawk afterwards.