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=--=--=START=--=--= NASA Spacelink File Name:6_2_2_46_3_9.TXT
TRANSFER ORBIT STAGE FOR THE STS-51 MISSION
The Transfer Orbit Stage (TOS) will boost NASA's Advanced
Communications Technology Satellite from low-Earth orbit into
geosynchronous transfer orbit with a maximum altitude of 21,519
nautical miles (34,624 km). This will be the second mission of the
Transfer Orbit Stage and the first time it has flown on a Space
Shuttle mission.
The Transfer Orbit Stage was first used in September 1992 as the
upper stage booster for NASA's Mars Observer mission. Following
launch on an expendable rocket, the TOS successfully propelled the
spacecraft on a trajectory from Earth orbit to the red planet.
The Space Systems Projects Office at NASA's Marshall Space
Flight Center, Huntsville, Ala., manages the TOS program for NASA.
That role involves ensuring TOS compliance with over all mission
requirements, including those for integration with the launch vehicle
and satellite and flight safety requirements.
Transfer Orbit Stage Description
The Transfer Orbit Stage, built by Martin Marietta Astronautics
Group in Denver, for Orbital Sciences Corp., Dulles, Va., is a
single-stage, solid-propellant rocket system. It is the latest
addition to NASA's upper stage fleet, which includes a range of
vehicles to boost satellites or spacecraft in the second step of
their journey to geostationary orbit or toward interplanetary
destinations.
TOS, constructed primarily of high-strength aluminum alloy,
weighs 20,780 pounds (9,426 kg) including solid propellant fuel. It
is almost 11 feet (3.3 m) long and about 7.5 feet (2.3 m) in
diameter. The satellite, weighing 6,108 pounds (2,771 kg), is
mounted on top of the Transfer Orbit Stage. Portions of both the
satellite and TOS are covered with gold foil multi-layered insulation
for thermal protection from the Sun.
Major elements of the TOS system are a solid rocket main
propulsion system, a navigation and guidance system, a reaction
control system which is used to adjust TOS attitude or local pointing
and an airborne support equipment cradle that holds the satellite and
upper stage in the Shuttle cargo bay and facilitates deployment from
the orbiter.
The ORBUS-21 solid rocket motor main propulsion system,
manufactured by United Technologies Chemical Systems Division, San
Jose, Calif., will give the primary thrust for the 110 seconds of
powered flight. To provide the 59,000 pounds of thrust (262,445
newtons) to inject the satellite into its transfer orbit, the motor
will use 18,013 pounds (8,171 kg) of the solid rocket propellant
HTPB (hydroxyl terminated polybutadiene).
Pitch (maneuvering upward or downward) and yaw (turning to the
left or right) will be controlled during the burn by gimballing the
nozzle of the solid rocket motor with two thrust vector control
actuators. Roll control is provided by the reaction control system
during motor burn.
TOS guidance and control avionics are based on a laser inertial
navigation system manufactured by Honeywell, Inc., Clearwater, Fla.
It acts as the brains of the vehicle, computing location and
providing signals to the propulsion system to maintain the proper
trajectory. All TOS operations are performed autonomously with no
ground commanding required. The guidance system uses laser
gyroscopes with no moving parts, thus reducing chances for
malfunctions in space. A telemetry and encoder unit records
performance data from all on-board electronics and sends it to
ground control at KSC.
The reaction control system thruster assembly, manufactured by
UTC/Hamilton Standard Division, Windsor Locks, Conn., correctly
positions the TOS and its payload, based on information from the
laser inertial navigation system. The three-axis control system
uses 12 small maneuvering rockets, which rely on decomposed
hydrazine as their propellant, to fine-tune the orientation of the
vehicle and its payload before solid rocket motor ignition.
The reaction control system also slowly turns the satellite-TOS
for thermal control to avoid overheating from the sun. The reaction
control system makes final attitude adjustments before TOS
separation from the satellite.
The equipment needed to adapt the satellite-TOS to the Space
Shuttle is called the airborne support equipment. This equipment is
manufactured by Martin Marietta. Prior to deployment, the TOS rests
in the aft cradle and is clamped firmly in the Shuttle's cargo bay
by the forward cradle.
ACTS/TOS deployment scenario
During the STS-51 mission, Discovery crew members will initiate
a predeployment checkout to ensure that all critical TOS systems are
healthy and ready to deploy. The upper forward cradle, similar to a
clamp, will then be unlatched and rotated open. The satellite-
booster will be elevated 45 degrees out of the cargo bay. If any
problems are detected in the combined payload up to this point, it
can be lowered, relatched and returned to Earth at the end of the
mission. If no anomalies are detected, a pyrotechnic system will
release the satellite-TOS and springs on the cradle will gently
nudge it out of the orbiter. The satellite-TOS will coast for 45
minutes while the Shuttle maneuvers to a safe distance, 11.7 miles
(18.8 km) away, to avoid a possible collision or damage from the TOS
solid rocket exhaust plume.
Once the Transfer Orbit Stage has positioned the satellite in
the proper attitude, the TOS solid rocket motor will fire for 110
seconds, accelerating to the 22,800-mph velocity (36,685 km/hr)
necessary to boost the satellite into its geosynchronous transfer
orbit. Then the Transfer Orbit Stage will make final attitude
adjustments as the satellite speeds toward apogee, the point
farthest from the Earth in its orbit.
Shortly after rocket burnout, the satellite will separate from
the TOS and the TOS will make a perpendicular turn to avoid being in
the satellite's path. Later, thrusters and a solid rocket motor on
the satellite itself will fire to place the satellite into its final
geosynchronous orbit. The actual timing of the satellite burn is
controlled by commands from the ground.
Extra-Vehicular Activity Tools
If a mechanical problem with the TOS airborne support equipment
were to develop prior to or after deployment of the satellite-TOS,
two astronauts can use one or more specially designed tools to
correct it. The tools were designed at Marshall Space Flight Center
and tested under simulated weightless conditions in the center's
Neutral Buoyancy Simulator water tank. The actual use of these
devices is considered unlikely since the airborne support equipment
itself is fully redundant, with all systems having built-in back-
ups.
Source:NASA Spacelink Modem:205-895-0028 Internet:192.149.89.61
=--=--=-END-=--=--=