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September 1989
FLTSATCOM-8
VERSATILE UHF/EHF MILITARY SATELLITE COMMUNICATIONS SYSTEM SHARED BY
THE U.S. NAVY, AIR FORCE, AND DEPARTMENT OF DEFENSE
PROVIDES RELIABLE, SECURE COMMUNICATIONS FOR SHIPS AND SUBMARINES AT
SEA, PLANES IN THE AIR, AND MILITARY GROUND UNITS THROUGHOUT THE
WORLD
PROVIDES INSTANT COMMUNICATIONS BETWEEN PRESIDENT AND COMMANDING
OFFICERS IN THE UNITED STATES AND REMOTE UNITS ANYWHERE IN THE WORLD
FLTSATCOM (pronounced FleetSatCom, for Fleet Satellite
Communications) is a versatile, high-capacity worldwide military
communications system operated by the United States. NASA previously
launched seven of these spacecraft for the military services, all on
Atlas/Centaur vehicles. With FLTSATCOM-8, the last to be launched
under NASA auspices, there will be six FLTSATCOM satellites operating
in orbit.
FLTSATCOM provides instant communications between the President and
commanding officers in the United States and remote units stationed
anywhere in the world. FLTSATCOM-1, operating since February 1978,
provides service from Southeast Asia across the Pacific to the West
Coast of the United States. FLTSATCOM-2, launched in May 1979, covers
the Indian Ocean area from Africa to the Phillipines. FLTSATCOM-3,
launched in January 1980, provides service from the middle of the
United States across the Atlantic and the Mediterranean. FLTSATCOM-4,
launched in October 1980, is co-located with FLTSATCOM-1 and provides
coverage over the Pacific. FLTSATCOM-5, launched in August 1981, was
damaged during launch, and was never declared operational.
FLTSATCOM-7, launched in December 1986, was placed in orbit
co-located with FLTSATCOM-1. It now provides service over the United
States. The most recent, FLTSATCOM-6, was lost after being struck by
lightning shortly after launch in March 1987.
In addition to the Ultra High Frequency (UHF) capability of the
earlier satellites in this series, FLTSATCOMs 7 and 8 carry an
Extremely High Frequency (EHF) communications package. This package
serves as a test bed for the new MILSTAR terminals.
FLTSATCOMS are launched on Atlas/Centaurs from Launch Complex 36 on
the Cape Canaveral Air Force Station in Florida. The Atlas stage will
complete its burn and fall into the ocean. The first burn of the
Centaur injects the spacecraft into a parking orbit, at a perigee
altitude of about 92 statute miles (148 kilometers) and apogee of
approximately 229 statute miles (369 kilometers). After a coast
period of about 14 minutes, the Centaur engines ignite again and
place the spacecraft into a highly elliptical, or egg-shaped,
"transfer orbit" with an apogee of about 22,362 miles (35,988
kilometers).
The Centaur releases the spacecraft and, as its final act, performs a
retromaneuver which takes it safely out of the flight path. The U.S.
Air Force Space Systems Division (SSD) then assumes charge of the
satellite, operating through its Consolidated Space Test Center
(CSTC) at Onizuka Air Force Base, Sunnyvale, CA. NASA tracking
stations throughout the world, together with the Air Force Satellite
Control Network Remote Tracking Stations, provide range and
range-rate measurement support to assist the CSTC controllers in
bringing the satellite on station.
The elliptical transfer orbit is designed so that the satellite will
reach its apogee while over the equator. To convert the orbit from an
elliptical to a circular one, and change the angle of inclination so
that the flight path will be more nearly above the equator, CSTC
operators will correctly aim the spacecraft and fire an onboard solid
propellant motor at a selected apogee. This final burn "transfers"
the satellite into a circular "drift" orbit, almost at synchronous
altitude and with the angle of inclination reduced to 5 degrees. The
FLTSATCOM then drifts to its assigned place in the global network,
where the CSTC controllers fire the small thrusters of the onboard
hydrazine reaction control system to stop the drift motion.
When a satellite is located above and in line with the equator at an
altitude of about 22,238 miles (35,789 kilometers), and given a
velocity of 6,879 miles (11,071 kilometers) per hour, its movement
becomes "synchronized" with that of the Earth below. It appears to
remain stationary in the sky, while actually completing one orbit
every 24 hours. All fully geosynchronous satellites, including those
for commercial communications, weather observation, and military
communications, are stationed above the equator at the same altitude,
spaced around a circle about 165,000 miles (266,000 kilometers) in
circumference. They are carefully separated by distance or by
assigned radio frequencies to prevent interference between their
individual communications systems.
Since FLTSATCOM-8, like its predecessors, will initially be inclined
to the equator, it will appear from the ground to be moving back and
forth from north to south. At the same time, it will appear to move
slightly east and west from the centerpoint, and so trace a constant
figure "8" across the equator in the sky.
The Navy portion of the FLTSATCOM shared system provides
communications between naval aircraft, ships, submarines, and ground
stations. The Air Force portion of each satellite is part of the
USAF Satellite Communications System (AFSATCOM). AFSATCOM links the
National Command Authority with Strategic Air Command units, and
other arms of the Air Force. A FLTSATCOM provides 23 UHF channels.
FLTSATCOM-8, with an 81-pound (37-kilogram) adapter for connection to
the vehicle, weighs about 5,061 pounds (2,296 kilograms) on the
ground, and has a mass of about 2,696 pounds (1,223 kilograms) in
space after burning up the apogee motor propellants. It measures 43.4
feet (13.2 meters) from tip to tip of the fully extended solar
panels. The main body is 7.5 feet (2.3 meters) wide, and 21.6 feet
(6.6 meters) high from the bottom of the body to the tip of the
offset spiral antenna mast. Both the spiral antenna and the solar
panels are in a retracted configuration for launch, as is the
16-foot-(4.9-meter) diameter, silver-filled stainless steel mesh UHF
antenna.
The main body consists of three attached hexagonal modules called the
payload module, the spacecraft module, and the EHF module, or FEP.
The solar arrays extend from the spacecraft module, which also
contains the hydrazine-fueled reaction control system thrusters and
tanks, Sun and Earth sensors, a reaction wheel which spins to hold
the spacecraft steady in its operating attitude, and the other
systems needed for control and operation of the spacecraft. The
payload module contains the three antenna systems, the transponders
for the 23 channels, and all the associated electronics required to
support the communications functions. The offset mast is the UHF
transmit antenna. A small, separate conical helix antenna atop the
central mast serves as the S-band Tracking, Telemetry and Control
antenna, used to command and monitor the spacecraft. The superhigh
frequency antenna horn protrudes through a hole cut into the UHF
antenna mesh.
The FEP contains the Extremely High Frequency communications package,
with its 30 (maximum) voice channels. The package was designed and
built by the MIT Lincoln Laboratory in Lexington, Massachusetts. The
EHF antenna, consisting of a 5 degree steerable spot beam and an
Earth coverage aperture, look through cutouts in the center portion
of the UHF transmitting antenna.
In operation, the momentum wheel provides a means to control the
spacecraft attitude so that the antennas are always aimed at the
Earth. The two solar arrays rotate on their extended arms so that
they constantly face the Sun. These two arrays contain three panels
each, with a total of 23,000 solar cells, each 0.79 by 1.57 inches (2
by 4 centimeters) in size, which will produce about 2,200 watts at
the beginning of their orbital life. Three 24-cell nickel-cadmium
batteries provide power when the spacecraft must operate in the
Earth's shadow; 2,150 of the solar cells are reserved for battery
charging.
FLTSATCOMs 1 through 4 and 7 have accumulated some 40 years of
on-orbit service. Four of the five orbiting satellites already have
lasted longer than their five-year design life. The current estimate
of the expected life of the UHF communications capability is in
excess of 10 years. The Extremely High Frequency package on
FLTSATCOM-8 is expected to last more than two years.
The spacecraft are built by TRW Space & Technology Group, Redondo
Beach, CA. The U.S. Navy manages the overall program, and the U.S.
Air Force Space Systems Division is the contracting agency for the
space segment. The military agencies reimburse NASA for the cost of
the Atlas/Centaur launch vehicle and associated launch services.
ATLAS/CENTAUR
137.6 FEET (41.9 METERS) TALL; 10 FEET (3 METERS) IN DIAMETER
WITH PAYLOAD, WEIGHS APPROXIMATELY 360,856 POUNDS (163,684 KILOGRAMS)
AT LIFTOFF
ATLAS THRUST 438,416 POUNDS (1,950,074 NEWTONS) AT LIFTOFF
CENTAUR THRUST 33,000 POUNDS (146,784 NEWTONS) IN A VACUUM FOR 7 1/2
MINUTES
Atlas/Centaur vehicles are built by General Dynamics/Space Systems
Division (GDSS). FLTSATCOM-8 and its launch vehicle will be launched
by a team from NASA, GDSS and the U.S. Air Force. This will be the
last Atlas/Centaur launch by NASA. In the future, NASA will contract
with either the U.S. Air Force or the vehicle manufacturer to procure
Expendable Launch Vehicles (ELVs) such as the Atlas/Centaur, and
related launch services. NASA will retain oversight responsibilities
for those vehicles which carry NASA payloads.
The two-stage, liquid-fueled Atlas/Centaur has been used to launch a
variety of scientific and technological spacecraft. These have
included Surveyors to the moon, Mariners to Venus, Mercury and Mars,
and Pioneers to Jupiter and Saturn. It has placed Applications
Technology Satellites, and COMSTAR, INTELSAT, and FLTSATCOM
communications satellites into geosynchronous transfer orbits. In
1984, it was upgraded by lengthening the Atlas stage to provide
larger propellant tanks. The Centaur stage has been improved by
substituting attitude control thrusters powered by hydrazine (used as
a monopropellant) for ones powered by hydrogen peroxide, and
replacing the oxygen and hydrogen propellant pumps by pressure-fed
systems.
The 76.3-foot (23.3-meter) long first stage is an uprated version of
the flight-proven Atlas vehicle used in the national space program
since 1959. The Rockwell International/Rocketdyne MA-5 engine system
burns RP-1, a highly refined kerosene, and liquid oxygen. The MA-5
uses two main engines, a 377,500-pound (1,679,120-newton) thrust
booster engine with two thrust chambers, and a smaller sustainer with
a single thrust chamber that produces approximately 60,000 pounds
(266,900 newtons) of thrust. The sustainer nozzle is located between
the two larger ones of the booster engine. Two small vernier engines
which help control the vehicle in flight also are burning at liftoff,
for a total thrust of 438,416 pounds (1,950,074 newtons). Total
weight at liftoff is about 360,856 pounds (163,684 kilograms).
An unusual feature of the Atlas vehicle is its "stage-and-a-half"
construction. All five thrust chambers are burning at liftoff. After
more than 2 1/2 minutes of flight, the booster engine cuts off. This
engine and its supporting structures are jettisoned, deleting a large
portion of the structural weight of this stage. The sustainer and
vernier engines continue to burn until the propellants are gone, at
about 4 1/2 minutes. This means an Atlas retains most of the weight
reduction advantage gained by jettisoning a used-up stage, but does
not have to ignite its engines in flight, as a separate stage must.
The only radio frequency system on the Atlas is a range safety
command system, consisting of two receivers, a power control unit,
and a destruct unit. The Atlas can be destroyed in flight by ground
control if necessary, but otherwise receives all its control
directions from the Centaur stage.
The Centaur stage sits above the Atlas on a barrel-shaped interstage
adapter. The Atlas and Centaur separate two or three seconds after
the Atlas burns out. Eight small retrorockets near the bottom of the
Atlas fuel tank then back this stage away from the Centaur.
The Centaur stage is 30 feet (9.1 meters) in length without the
fairing on top. Exclusive of payload, it weighs about 39,000 pounds
(17,700 kilograms) when loaded with propellants. The main propulsion
system consists of two Pratt & Whitney engines burning liquid oxygen
and liquid hydrogen, producing 33,000 pounds (146,784 newtons) thrust
in the vacuum of space in which they are designed to operate. These
engines can be stopped and restarted, allowing the Centaur to coast
to the best point from which to achieve its final trajectory before
igniting for another burn. While coasting, the stage is controlled by
12 small thruster engines, powered by hydrazine. These hold the stage
steady and provide a small constant thrust to keep the propellants
settled in the bottom of their tanks, a necessity for a second or
third burn.
A cylindrical nose fairing with a conical top sits on the Centaur and
protects the spacecraft. Total vehicle height is 137.6 feet (41.9
meters). Both stages are 10 feet (3 meters) in diameter.
The Centaur electronic packages are mounted in a circle around a
conical equipment module, located above the upper tank. An adapter on
top of this module connects to the payload adapter on the bottom of
the spacecraft. These electronic packages provide an integrated
flight control system which performs the navigation, guidance,
autopilot, attitude control, sequence of events, and telemetry and
data management functions for both the Atlas and Centaur stages. The
heart of this system is a Digital Computer Unit (DCU), built by
Teledyne. The DCU sends commands to control most planned actions,
including all but items one, two, and five in the table, shown below.
The DCU receives guidance information from a combination of sensors
called the Inertial Measurement Group, built by Honeywell, and sends
steering commands to all Atlas and Centaur engines. The Centaur also
has a ground-controlled destruct system similar to that on the Atlas,
in case the vehicle must be destroyed in flight.
The Centaur uses the most powerful propellant combination available,
has a lightweight structure, and an engine burn time of up to 7 1/2
minutes, the longest of any upper stage now in service. This gives it
the most total energy for its size of any stage yet built.
The following table provides a list of the major events that will
occur during the flight.
Event Time After Distance
Liftoff Altitude Downrange Velocity
MIN:SEC MI/KM MI/KM MPH/KPH
Liftoff T+0 --/-- --/-- --/--
Atlas Booster
Engine Cutoff 2:35 37/60 55/89 5,703/9,178
Jettison Atlas
Booster Engine 2:38 39/63 60/97 5,764/9,276
Jettison Centaur
Insulation Panels 3:0 51/82 93/150 6,124/9,856
Jettison Nose
Fairing 3:43 71/114 169/272 7,055/11,354
Atlas Sustainer/
Vernier Engines
Cutoff 4:30 89/143 266/428 8,466/13,625
Atlas/Centaur
Separation 4:32 89/143 271/436 8,469/13,630
First Centaur Main
Engines Start 4:43 97/156 295/475 8,441/13,584
Centaur Main
Engines Cutoff 9:55 102/164 1,294/2,082 16,652/26,799
Second Centaur Main
Engines Start 23:56 101/163 5,103/8,212 16,686/26,854
Second Centaur Main
Engines Cutoff 25:32 110/177 5,600/9,012 22,013/35,426
Centaur/Spacecraft
Separation 27:47 179/288 6,391/10,285 21,791/35,069
Centaur/Spacecraft
Separation 27:47 179/288 6,391/10,285 21,791/35,069
These numbers may vary, depending on exact launch date, launch time,
and spacecraft weight.
NOTE: The final velocity of 22,013 miles (35,426 kilometers) per hour
places the spacecraft in a transfer orbit, with an apogee of 22,362
miles (35,988 kilometers) and a perigee of 104 miles (167
kilometers). The U.S. Air Force then assumes control of the
spacecraft. At an apogee chosen by Air Force controllers, the onboard
apogee motor will be fired to make the orbit more circular at
geosynchronous altitude, about 22,238 miles (35,789 kilometers) above
the equator. It will then be "drifted" to its assigned place in the
FLTSATCOM global network. The spacecraft will have a final velocity
of about 6,879 miles (11,071 kilometers) per hour. It will complete
one orbit every 24 hours, and so move back and forth above the same
area on both sides of the equator.