Each mated pair of 17-inch disconnects contains two flapper valves,
one on the orbiter side of the interface and one on the external
tank side of the interface. Both valves in each disconnect pair
are opened to permit propellant flow between the orbiter and the
external tank. Before the separation of the external tank, both
valves in each mated pair of disconnects are commanded closed
by pneumatic (helium) pressure from the main propulsion system.
The closure of both valves in each disconnect pair prevents propellant
discharge from the external tank or orbiter at separation. Valve
closure on the orbiter side of each disconnect also prevents contamination
of the orbiter main propulsion system during landing and ground
Inadvertent closure of either valve in a 17-inch disconnect during
space shuttle main engine thrusting would stop propellant flow
from the external tank to all three main engines. Catastrophic
failure of the main engines and external tank feed lines would
To prevent inadvertent closure of the 17-inch disconnect valves
during the main engine thrusting, a latch mechanism was added
in the orbiter half of the disconnects. The latch mechanism provides
a mechanical backup to the normal fluid-induced-open forces. The
latch is mounted on a shaft in the flowstream so it overlaps both
flappers and obstructs closure for any reason.
In preparation for external tank separation, both valves in each
17-inch disconnect are commanded closed. Pneumatic (helium) pressure
from the main propulsion system causes the latch actuator to rotate
the latch shaft in each orbiter 17-inch disconnect 90 degrees,
thus freeing the flapper valves to close as required for external
If the latch pneumatic actuator malfunctions, a backup mechanical
separation capability is provided. When the orbiter umbilical
initially moves away from the external tank umbilical, the mechanical
latch disengages from the external tank flapper valve and permits
the orbiter disconnect flapper to toggle the latch. This action
permits both flappers to close.
During ground mating of the external tank to the orbiter, the
latch engagement mechanism in each 17-inch disconnect provides
a go/no-go verification that flapper angle rigging is within stability
limits. Misrigged flappers will prevent full engagement of latch.
The angle of each flapper in each disconnect is still carefully
rigged within specific tolerances to assure basic stability independently
of the latch safety feature.