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Figure ES-1
Location of assessed upgrades.
shuttle upgrades program should solicit additional upgrade proposals for protecting the shuttle from meteoroids and orbital debris.
Phase III Upgrades
Auxiliary Power Unit
Every shuttle orbiter has three auxiliary power units (APUs) to pressurize the vehicle's hydraulic systems during ascent and reentry. NASA is studying a number of options for replacing the current APUs—which use toxic hydrazine propellant—with an electric system that would be safer and easier to maintain. NASA should continue studying potential modifications to the APUs to determine the costs, benefits, and appropriate scope of each upgrade. The
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