Space Shuttle missions designated STS-3xx (officially called Launch On Need (LON) missions) were rescue missions which would have been mounted to rescue the crew of a Space Shuttle if their vehicle was damaged and deemed unable to make a successful reentry. Such a mission would have been flown if Mission Controldetermined that the heat shielding tiles and reinforced carbon-carbon panels of a currently flying orbiter were damaged beyond the repair capabilities of the available on-orbit repair methods. These missions were also referred to as Launch on Demand (LOD) and Contingency Shuttle Crew Support.[Who? No mission of this type was launched during the space shuttle programme.
The Procedure:
The orbiter and four of the crew which were due to fly the next planned mission would be retasked to the rescue mission. The planning and training processes for a rescue flight would allow NASA to launch the mission within a period of 40 days of its being called up. During that time the damaged (or disabled) shuttle's crew would have to take refuge on the International Space Station (ISS). The ISS is able to support both crews for around 80 days, with oxygen supply being the limiting factor. Within NASA, this plan for maintaining the shuttle crew at the ISS is known as Contingency Shuttle Crew Support (CSCS) operations. Up to STS-121 all rescue missions were to be designated STS-300.
After STS-124 the Shuttle missions to the ISS no longer need such a mission since due to the increased capabilities of the ISS, a stranded crew would have sufficient on-board resources to wait for the next Shuttle mission.
In the case of an abort to orbit, where the shuttle is unable to reach the ISS orbit and the thermal protection system inspections suggest the shuttle cannot return to Earth safely, the ISS may be capable of descent down to meet the shuttle. Such a procedure is known as a joint underspeed recovery.

The STS-3xx missions were developed in the aftermath of the loss of Columbia However, NASA spent some effort researching rescue options even before the disaster. Before the ISS was launched, or in the event of the shuttle being unable to reach the station, shuttle crews would have had to transfer directly between shuttles. The orbiters would have been unable to dock, so while they used their RMS arms to grapple each other, the crew would have made an EVA between the shuttles. This would have been carried out using the two EVA-designated mission specialists wearing the Shuttle/ISS Extravehicular Mobility Unit (EMU) spacesuits, while the remaining crew would have been sealed up in pressurized Personal Rescue Enclosure and carried over either by hand, or using a pulley system (akin to that of a clothesline pulley) like that employed in the Apolloprogram for lifting samples from the Moon's surface into the Lunar Module

Flight	Rescue Flight
STS-114 (Discovery	STS-300 (Atlantis)
STS-121 (Discovery)	STS-300 (Atlantis)
STS-115 (Atlantis)	STS-301 (Discovery)
STS-116 (Discovery)	STS-317 (Atlantis)
STS-117 (Atlantis)	STS-318 (Endeavour)
STS-118 (Endeavour)	STS-322 (Discovery)
STS-120 (Discovery)	STS-320 (Atlantis)
STS-122 (Atlantis)	STS-323 (Discovery*)
STS-123 (Endeavour)	STS-324 (Discovery)
STS-124 (Discovery)	STS-326 (Endeavour)
STS-125 (Atlantis)	STS-400 (Endeavour)
STS-134 (Endeavour)	STS-335 (Atlantis)