M-Spec reentry pack
From Halopedia, the Halo wiki
The M-Spec Reentry Pack, also known as the Reentry Unit, is a piece of UNSC equipment. The reentry pack is integrated into the YSS-1000 Sabre, hooking onto the pilot's back as soon as they enter. It is possible to holster a sidearm inside the reentry pack. It was used by SPARTAN-B312 to safely drop from orbit onto the surface of Reach after Operation: UPPER CUT.
According to the Halo: Reach Legendary Edition developer commentary, the reentry pack is integrated into the YSS-1000, hooking onto the pilot's back as soon as they enter. However, during the launch cutscene, there is no reentry pack visible on the cockpit seat, or anywhere within automatic clamping distance of the pilot. Additionally, the reentry pack cannot be seen during gameplay.
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