Tank Separation System
The external tank is separated from the orbiter at three structural
attach points. Separation from the orbiter occurs before orbit
insertion and is automatically controlled by the orbiter's general-purpose
computers. External tank separation can be manually initiated
by the flight crew using the same jettison circuits as the automatic
sequence. Separation is controlled by the ET separation auto,
man switch on panel C3 and the sep push button on panel C3. In
the auto position, the onboard GPCs initiate separation. To manually
initiate separation, the ET separation switch is positioned to
man and the sep push button is depressed.
The forward structural attachment consists of a shear bolt unit
mounted in a spherical bearing. The bolt separates at a break
area when two pressure cartridges are initiated. The pressure
from one or both cartridges drives one of a pair of pistons to
shear the bolt, with the second piston acting as a hole plugger
to fill the cavity left by the sheared bolt. A centering mechanism
rotates the unit from the displacement position to a centered
position, aligning the bearing flush with the adjacent thermal
protection system mold line.
The aft structural attachment consists of two special bolts and
pyrotechnically actuated frangible nuts that attach the external
tank strut hemisphere to the orbiter's left- and right-side cavities.
At separation the frangible nuts are split by a booster cartridge
initiated by a detonator cartridge. The attach bolts are driven
by the separation forces and a spring into a cavity in the tank
strut. The frangible nut, cartridge fragments and hot gases are
contained within a cover assembly, and a hole plugger isolates
the fragments in the container.
The aft separation involves right and left umbilical assemblies.
Each assembly contains three dual-detonator frangible nut and
bolt combinations that hold the orbiter and external tank umbilical
plates together during mated flight. Each bolt has a retraction
spring that, after release of the nut, retracts the bolt to the
external tank side of the interface. On the orbiter side, each
frangible nut and its detonators are enclosed in a debris container
that captures nut fragments and hot gases generated by the operation
of the detonators, either of which will fracture the nut.
The right aft umbilical assembly consists of an electrical disconnect,
the gaseous oxygen 2-inch pressurization disconnect used for pressurization
of the external tank's oxygen tank and the 17-inch liquid oxygen
The left aft umbilical assembly consists of an electrical disconnect
plate, the gaseous hydrogen 2-inch pressurization disconnect used
for pressurization of the external tank's hydrogen tank, the 4-inch
recirculation disconnect used during prelaunch to precondition
the main engine and the 17-inch liquid hydrogen disconnect.
After release of the three frangible nuts and bolts at each aft
umbilical, three lateral support arms at each orbiter umbilical
plate hold the plates in the lateral position when the external
tank separates from the umbilical plates. Each 17-inch disconnect
has been commanded closed. The orbiter umbilical plates are retracted
inside the orbiter aft fuselage approximately 2.5 inches by three
hydraulic actuators and locked to permit closure of the umbilical
doors in the bottom of the aft fuselage. Hydraulic system 1 source
pressure is supplied to one actuator at each umbilical, hydraulic
system 2 source pressure is supplied to the second actuator at
each umbilical, and hydraulic system 3 source pressure is supplied
to a third actuator at each umbilical.
The retraction of each umbilical disconnects the external tank
and orbiter electrical umbilical in the first 0.5 of an inch of
travel and releases any fluids trapped between the 17-inch disconnect