orbital replacement unit (HST)

{{about|orbital replacement units of the Hubble Space Telescope|orbital replacement units of the ISS|orbital replacement unit}}

An orbital replacement unit or orbital replaceable unit is a modular component of spacecraft that can be replaced upon failure either by robot or by extravehicular activity. The Hubble Space Telescope (HST) was designed with 70 such parts,{{cite web |url=http://setas-www.larc.nasa.gov/HUBBLE/HARDWARE/hubble_ORU.html |title=Returned Hardware: Orbital Replacement Units |work=Hubble Space Telescope Archive System |publisher=NASA{{\}}Langley Research Center |first=William H. |last=Kinard |accessdate=27 March 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20130219022932/http://setas-www.larc.nasa.gov/HUBBLE/HARDWARE/hubble_ORU.html |archivedate=19 February 2013 }} including scientific instruments and limited-life items such as batteries.

On HST some parts were designed from the start as ORUs and all used captive bolts with a standard 7/16" double-height hex head;{{cite web |url=https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/SM3A-MediaGuide.pdf |title=Hubble Space Telescope Servicing Mission 3A Media Reference Guide |publisher=NASA |agency=Lockheed Martin Missiles and Space |accessdate=21 September 2010}} Section 2.2. later when it was decided to avoid returning HST to Earth for repair, more systems and modules were designated as ORUs (but used a wider variety of fasteners). HST servicing mission 3A (SM3A) replaced (or added) 15 ORUs,{{rp|fig 2-1}} e.g. it replaced the DF-224 computer with the Advanced Computer.

The electrical system of the International Space Station also has such subsystems that provide power generation, power distribution and energy storage.{{cite web |url=http://spaceflightsystems.grc.nasa.gov/print/eps_prt.htm |title=Glenn Leads Sustaining Engineering of Power System Hardware |publisher=NASA{{\}}Glenn Research Center |date=14 February 2008 |accessdate=27 March 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20120222024245/http://spaceflightsystems.grc.nasa.gov/print/eps_prt.htm |archivedate=22 February 2012 }}