From Halopedia, the Halo wiki
The Ballista bears resemblance to the Onager mass drivers employed as anti-ship defense. They are mounted on Isabel's modified G81 Condor gunships, affixed to the bottom of the craft. They can be upgraded to fire up to three rounds in rapid succession, though the capacitors and equipment to do so do require the removal of the craft's slipspace drive.
List of appearances