Orbiter/External Tank Disconnects
Each mated pair of 17-inch
disconnects contains two flapper valves: one on the orbiter side
and one on the external tank side. Both valves in each disconnect
pair are opened to permit propellant flow between the orbiter and
the external tank. Prior to separation from 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 external tank separation.
Valve closure on the orbiter side of each disconnect also prevents
contamination of the orbiter main propulsion system during landing
and ground operations.
Inadvertent closure of either valve in a 17-inch disconnect
during 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 result.
To prevent inadvertent closure of the 17-inch disconnect valves
during the space shuttle main engine thrusting period, a latch
mechanism was added in each orbiter half of the disconnect. 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 that
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 pressure from
the main propulsion system causes the latch actuator to rotate
the shaft in each orbiter 17-inch disconnect 90 degrees, thus
freeing the flapper valves to close as required for external tank
A backup mechanical separation capability is provided in case
a latch pneumatic actuator malfunctions. When the orbiter umbilical
initially moves away from the ET umbilical, the mechanical latch
disengages from the ET flapper valve and permits the orbiter disconnect
flapper to toggle the latch. This action permits both flappers