What were the differences between the STS Orbiters?

Question

There were six STS orbiter vehicles, of which five were operational for space flight:

• OV-099: Space Shuttle Challenger
• OV-101: Space Shuttle Enterprise
• OV-102: Space Shuttle Columbia
• OV-103: Space Shuttle Discovery
• OV-104: Space Shuttle Atlantis
• OV-105: Space Shuttle Endeavour

Challenger and Enterprise were used for tests, with Challenger later refitted for space flight. Columbia was the first to fly, on April 12, 1981 and Endeavour the last one to have its maiden flight on May 7, 1992.

Technology progressed during the 11 years between Columbia and Endeavour and the orbiters received various upgrades during service. As a consequence, they were not identical, making them (un)suitable for specific missions:

• Columbia kept its internal air lock: this made it suitable for working Hubble service missions and launching the Chandra space telescope, but this plus other differences made it too heavy for ISS missions.
• Atlantis was fitted with equipment for Mir missions.

I'm interested to know what the major differences between all of the five flight-ready orbiters were, i.e. differences that affected their suitability for certain mission types. This would include things like unique equipment installed to facilitate certain missions (like the Atlantis example), but also structural differences that were a result from more advanced technologies or design applied to the newer orbiters (such as with Columbia, that had a heavier wing and fuselage spars).

Answer 1

The Orbiters changed over time so that, for example, Columbia when it was destroyed was quite a bit different from when it was delivered. When writing this up I've tried ignore the temporal differences and to list the major differences that had an effect on operations.

All the Orbiters had different things failed in them at various times that were not deemed worthy of fixing: gas flow sensors in the life support system, quantity probes in the propulsion system, etc, that led to operational differences; I have not listed those. All the Orbiters had minor external differences on top of the major ones: their names, locations of Thermal Protection System (TPS) repairs, differences in decals, wear and tear, etc.

OV-102 Columbia

• External differences
  • Black chines
  • Shuttle Infrared Leeside Temperature Sensing (SILTS) pod on tip of vertical tail

• Internal airlock
• Original wing carry-through structural design
• Electrical Power Reactants Supply and Distribution (PRSD) differences
  • O2 & H2 tank sets 4 and 5 shared a heater controller and switches; O2 single heaters, H2 dual heaters
  • H2 tanks 4 and 5 shared a check valve
• Extended Duration Orbiter (EDO) capable
  • scarred for Regenerative Carbon Dioxide Removal System (RCRS)
  • scarred for single EDO PRSD pallet in payload bay
• Scarred for Orbital Maneuvering System propellant kit pallet in payload bay

• Orbiter Experiments (OEX) recorder and associated Developmental Flight Instrumentation sensors
• Vent doors 4 and 7 removed
• Original TPS design (including unique features in the wing leading edge)
• originally had ejection seats (2) which were removed. Bailout system added.

OV-099 Challenger

• 2889 lbs lighter than OV-102

• Original wing carry-through structural design

• Internal airlock

• Missing all the fixes that were incorporated during the stand-down after the STS-51L failure (see here for information)

• Modified TPS design

• no bailout system

• Centaur capable

OV-103 Discovery

• 6870 lbs lighter than OV-102
• External airlock
• Revised wing carry-through structural design which had to be re-modified later
• Space Station Power Transfer System (SSPTS) for electrically connecting to International Space Station (ISS)
• Not EDO capable
• Centaur capable
• Vent doors 4 and 7 removed
• Modified TPS design for Department of Defense missions from Vandenberg
• bailout system

OV-104 Atlantis

• 6870 lbs lighter than OV-102
• Revised wing carry-through structural design which had to be re-modified later
• External airlock
• Auxiliary Power Conversion Unit for electrically connecting to ISS
• EDO capable (single pallet), switches on panels A11 and A15
• Centaur capable
• Vent doors 4 and 7 removed
• Modified TPS design
• bailout system

OV-105 Endeavour

• Final wing carry-through structural design
• 6000 lbs lighter than OV-102 ¹
• SSPTS for electrically connecting to ISS
• External airlock
• Modified TPS design
• EDO capable
• scarred for Regenerative Carbon Dioxide Removal System (RCRS)
• scarred for dual EDO PRSD pallet in payload bay, switches on panel A15
• PRSD meters on panel O2 allow Tank 5 to be selected
• Vent doors 4 and 7 never installed
• bailout system

I'm sure I've missed stuff.

¹ Thanks to dan04 for finding the weight difference

Sources

• All photos credit NASA, annotated by me
• Space Shuttle, Dennis Jenkins, 1992 edition
• 1982 Press Reference Manual
• Shuttle Crew Operations Manual
• personal notes
• science.ksc.nasa.gov pages on Columbia and Endeavour, found by dan04