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One of the signs accompanying the upper stage mockup.

It reads

Horizontal Upper Stage
PADD
Performance, Analysis & Design Demonstrator
Marshall Space Flight Center - Human Engineering Analysis Team

The Upper Stage (US) is the middle portion of the Ares I vehicle. It is located between the First Stage and the Crew Exploration Vehicle (CEV). The US contains the Instrument Unit, Liquid Hydrogen Tank, Liquid Oxygen Tank, Aft Skirt, Interstage, Thrust Cone, and J-2X engine. The purpose of the US is to continue propelling and directing the CEV into space after the First Stage falls away.