When the external tank and Orbiter are connected how are the bolts
installed and what is their size?

Dear kohalanui-ga

They are 14 inches according to this extract from a fault report on a
space shuttle: "Following routine film analysis of external tank
separation during the STS-106 launch, engineers noted a 2 ¼ - inch
protrusion of the aft attach bolt following tank separation. At
separation, a frangible nut inside the orbiter releases the 14-inch
bolt which is supposed to fully retract into the external tank's bolt
housing. On STS-106, that bolt did not fully retract."
Source:
http://www-pao.ksc.nasa.gov/kscpao/status/stsstat/2000/oct/10-05-00.htm
Photographs at:
http://www.space.com/missionlaunches/launches/sts92_scrub_001005.html

The installation is as follows:
"Each of the two aft external tank umbilical plates mate with a
corresponding plate on the orbiter. The plates help maintain alignment
among the umbilicals. Physical strength at the umbilical plates is
provided by bolting corresponding umbilical plates together. When the
orbiter GPCs command external tank separation, the bolts are severed
by pyrotechnic devices."
http://www.shuttlepresskit.com/STS-96/REF66.htm

Further explanation of their installation and how they work can be
found at:
http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/sep/sepsystem.html
http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/et.html

Please ask for clarification of this research, or if the links do not
work, before rating the answer.

answerfinder-ga

Search strategy
orbiter "external tank" bolt OR bolts
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