From Halopedia, the Halo wiki
Planet-breakers were able to carry over five thousand Warrior-Servants, an armory stocked with spare combat skins, thousands of hand weapons and a plethora of other military devices, hundreds of war sphinxes, as well as armed scout craft and orbital picket cruisers. A planet-breaker also possessed a facility suited for performing brevet mutations, as well as machinery required to produce personal armor.
A planet-breaker changed its overall shape and configuration depending on the situation. In spaceflight, the ship was ovoid-shaped and approximately 800 meters long. When entering a planet's atmosphere, the ship would configure into a more aerodynamic shape, bowing out at the sides, growing landing pads and activating fluxor shields to protect against the heating from atmospheric entry. Only approximately half of such as ship was actual matter, one third being fuel and reaction mass and the rest being comprised of various forms of hard light, allowing the captain to reconfigure the ship's layout or change the interior decor at will. Instead of having actual windows, adjustable, convincing projections of the exterior could be projected to the ship's interior at will.
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