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Countdown! NASA Launch Vehicles and Facilities
PMS 018-B
October 1991
Section 4
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The Fixed Service Structure is located on the north side of
each pad's hardstand. It is an open framework structure about 40 feet (12.2
meters) square. A hammerhead crane on top provides hoisting services as required
in pad operations. There are 12 work levels at 20-foot (6.1-meter) intervals.
The height of the structure to the top of the tower is 247 feet (75 meters),
while the distance to the top of the hammerhead crane is 265 feet (81 meters).
The 80-foot (24-meter) fiberglass lightning mast tops off the Fixed Service
Structure at 347 feet (106 meters).
The Fixed Service Structure has three service arms. They are the
Orbiter
Access Arm, the
External Tank Hydrogen Vent Line Access Arm and the External
Tank Gaseous Oxygen Vent Arm.
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The Orbiter Access Arm swings out to the crew
compartment to provide access for personnel. The outer end of this arm supports
a small room, commonly called the "white room." This room can hold up
to six persons. The arm mates with the crew hatch to allow the astronauts to be
assisted in entering the Space Shuttle. The Orbiter Access Arm remains in the
extended position until 7 minutes and 24 seconds prior to launch, to provide an
emergency exit for the crew should one be needed. It is 65 feet (19.8 meters)
long, 5 feet (1.5 meters) wide, and 8 feet (2.4 meters) high. The Orbiter Access
Arm is attached to the Fixed Service Structure at the 147-foot (44.8-meter)
level. In an emergency, this arm can be mechanically or manually repositioned in
about 15 seconds.
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The External Tank Hydrogen Vent Line Access Arm
allows the mating of the external tank umbilicals to the pad facilities, and
provides work access to the tank area. The arm retracts several hours before
launch, leaving the umbilicals attached. At the moment the solid rocket boosters
ignite, these umbilicals release from the Shuttle and fall back against the
tower, where a curtain of sprayed water protects them from engine flame. The
External Tank Hydrogen Vent Line Access Arm is 48 feet (14.6 meters) long, and
attaches at the 167-foot (51-meter) level. The arm itself rotates 120 degrees to
its stowed position in approximately 3 minutes.
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The External Tank Gaseous Oxygen Vent Arm swings
into position over the Space Shuttle's external tank and lowers a hood, called
the beanie cap, over the top of the tank. Heated gaseous nitrogen flows into the
hood to warm the liquid oxygen vent system at the top of the external tank. This
heating action prevents vapors at the vent opening from condensing into ice that
could dislodge and damage the orbiter during launch. The vent system arm is 80
feet (24.4 meters) long, 5 feet (1.5 meters) wide, and 8 feet (2.4 meters) high.
The vent hood is 13 feet (4 meters) in diameter. The arm attaches to the Fixed
Service Structure between the 207-foot (63-meter) and 227-foot (69-meter)
levels. At 2 minutes and 30 seconds before launch, the hood lifts to clear the
external tank and the arm retracts into a "latchback" position against
the Fixed Service Structure. It can be re-extended in the event of a countdown
hold and retracted again at the same point in the countdown.
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