From Halopedia, the Halo wiki
The D20 Heron is very large and roughly square in shape, featuring a protruding area at the front of the ship which most likely serves as the bridge. The Heron features four tilt engines, two mounted on opposite sides of the ship, and four conventional, rear-mounted engines. The configuration allows for easy maneuverability in space, such as among debris fields. The Heron was originally intended to deploy the infrastructure required to begin and sustain humanity's colonization efforts such as the colony starter unit. The necessities of the Human-Covenant War forced the UNSC to re-purpose the craft for a logistical support role.
The D20 is capable of transporting and placing a prefabricated firebase "pod" - the military use variants of the civilian colony starter units - on a suitable landing zone designated by friendly ground forces. When deploying a firebase pod, a circular structure amidships spins clockwise, possibly unscrewing to detach the payload. The vessel is capable of operating in both atmosphere and vacuum. Up to ten Herons can be docked on the exterior of a Phoenix-class support vessel, with five being berthed on either side. The craft's unstreamlined nature has earned it the nickname "big brick" among UNSC personnel.
Following the departure of the CAA First Reverie and a hundred other colony ships in 2362, D20 Herons would be used by many colony ships during the period later referred to as the Domus Diaspora to establish functional infrastructure and an active presence on a new world.
The converted Phoenix-class colony ship UNSC Spirit of Fire embarked ten D20 Herons Febuary 2531. These dropships were used extensively to construct firebases on Harvest, Arcadia, and Trove in February 2531. During the Battle of Trove, a collision between Spirit of Fire and the Covenant destroyer Proclamation's Tithe resulted in the destruction of all five of the human vessel's portside Herons. By the time Spirit of Fire arrived at Installation 00 in March 2559, the lost Herons had been replaced. However, the ship's crew primarily used D81-LRT Condors in place of the Herons for building firebases on Installation 00; the Herons were instead used exclusively for mass deployment of cargo and troops in secured rear areas.
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