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Overview |
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 When America rockets a new generation of explorers to the Moon aboard
NASA’s Orion crew exploration vehicle, its service module will
be the powerhouse that fuels and propels the spacecraft, and the storehouse
for air and water the astronauts use during their space travels.
The service module will be mounted directly below the cone-shaped
crew module, covering the entry heat shield during launch and in-space
activities. A spacecraft adapter will connect the service module to
the Ares I rocket, and provide structural, electrical, and data connections.
The service module will be 5 meters (16.5 feet) in diameter and will
have a mass of approximately 3,700 kilograms (8,000 pounds). It will
carry about 8,300 kilograms (18,000 pounds) of propellants.
Making its first flights early in the next decade, Orion is part of
the Constellation Program to send human explorers back to the Moon
and then onward to Mars and other destinations in the solar system. |
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Critical Services |
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Ares I launch vehicle showing location
of the Orion crew exploration vehicle’s crew module
and service module. |
The
service module’s main engine will provide
the propulsion to break the spaceship out of lunar orbit and return
it to Earth, its reactioncontrol system will provide the thrust
for vehicle maneuvering to maintain and adjust course, and its propellant
tanks will provide fuel to both systems.
Solar arrays will generate power for life support, computer, and
communications systems, and service module batteries will store
that power for use during times the vehicle is in darkness. Thermal
radiators on the service module’s
exterior will be used to shed excess heat generated by the crew
and electronic systems.
A communications system will receive signals from Earth, and beam back
data, voice, and television signals. Tanks and plumbing will supply
drinking water and air to the crew, and additional tanks will supply
water for cooling, hygiene, and space suits.
The service module’s structure also will provide places to
mount scientific experiments and cargo.
The service module will support the crew module starting before
launch until just before the two modules separate for Earth reentry.
An umbilical housing will contain the fluid, electrical and data
connections between the service module and the crew module. As the
spacecraft nears Earth, the umbilical will be disconnected. The
service module will be jettisoned just before the crew module reenters
Earth’s atmosphere. |
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The Primary Thrust |
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Orion vehicle showing major components of the service module. |
The service module main propulsion system consists
of a single rocket engine, fuel tanks, and a pressurization system.
The main propulsion system will be used to perform major thrusts to
move the entire Orion spacecraft to a new location. The engine will
be able to produce 33,000 Newtons (7,500 pounds) of thrust. The primary
use for low Earth orbit missions will be to maneuver Orion to the International
Space Station and to slow Orion down so that the crew module can safely
reenter the Earth’s atmosphere. For lunar missions, the rocket
engine will be fired for longer durations to correct Orion’s trajectory
going to and from the Moon, and send Orion from lunar orbit back to
Earth.
The fuel for the main propulsion system will be monomethyl hydrazine
(MMH) and the oxidizer will be nitrogen tetroxide (N2O4). These are
hypergolic propellants that ignite on contact with each other and need
no ignition source. This easy start and restart capability makes this
propulsion system attractive for both crewed and uncrewed spacecraft
maneuvering. Another plus is their storability—they can be stored
at room temperature without cooling. Hypergolic propellants are routinely
used in expendable rockets and the space shuttle orbiter. The propellants
will be stored in tanks within the service module. Fully loaded, these
tanks will be the heaviest component of the service module. Since gravity
cannot be used to make the propellants flow from the storage tanks to
the engines, a high-pressure helium gas system is used to force the
flow. |
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Staying the Course With the Right Attitude |
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Throughout Orion’s missions, minor maneuvering will be required
for rendezvous and docking with the International Space Station or the
lunar lander. For these operations, a reaction control system (similar
system to the space shuttle orbiter’s) will use a total of 32 small
and midsized thrusters. This system also will be used when the Orion spacecraft
needs to change its orientation (called attitude control).
Twenty-four small reaction control thrusters will be positioned around
the service module and oriented to give control in all three axes. They
will control the vehicle’s pitch, roll, and yaw. These thrusters
can be run for short durations to reposition the spacecraft or pulsed
for minor adjustments in attitude. Each thruster produces 111 Newtons
(25 pounds) of force. The system will use the same propellant as the main
propulsion system (MMH and N2O4) and be pressurized by high-pressure helium.
Eight midsized thrusters, similar to the 24 smaller thrusters, can be
used to back up the main engine for return from lunar orbit. Each of these
thrusters produces 556 Newtons (125 pounds) of force.
While the crew is on the surface of the Moon, Orion will remain in lunar
orbit in a maintenance mode without crew. During this time, mission control
will monitor and maintain a navigation state for Orion by commanding the
service module reaction control system to burn as needed for proper orbit
corrections. The final job for this system will be the controlled separation
from the crew module prior to its reentry into the Earth’s atmosphere. |
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The Power To Fly |
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Just like your car, Orion will need electrical power to run all of its
electronics such as lights, air conditioning, and radios. Unlike your
car, Orion’s electrical power will be vital to supporting the
life of its crew. The power system for the service module must generate
and store electrical power from the launch pad until the crew module
separates just prior to reentry.
Solar power will be used while in low Earth orbit, lunar orbit, and
during the flights between Earth and the Moon. Two photovoltaic solar
arrays located at the rear of the service module will be used to convert
sunlight into electricity. The solar array system includes two array
panels, deployment mechanisms, Sun sensors, and gimbals (powered joints)
to maintain optimum line-of-site with the Sun. Each array is approximately
5 meters (16 feet) in diameter and provides 9 kilowatts of power.
This size requires the arrays to be folded during launch and deployed
only after Orion reaches a stable orbit above the Earth.
From the launch pad until orbit, Orion will use lithium-ion batteries
that store electrical power. They will also be needed when the spacecraft
is in the shadow of the Earth or the Moon. When in sunlight, the arrays
will charge three lithium-ion batteries in the service module and
six in the crew module. The crew module will rely on its own batteries
after separating from the service module until it is safely on the
Earth’s
surface.
This power management and distribution system will condition and distribute
electricity throughout the Orion spacecraft. A series of switches
will have built-in microprocessors that are controlled by software
and are connected to a computer network running throughout the spacecraft. |
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Taking the Heat |
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Orion will spend most of its mission in direct sunlight. Combine that
heat with that generated by a collection of computers, radios, and
other electronic devices and four to six people living in a confined
space and things could get pretty hot in a hurry. While the crew module
and service module will have protective insulation, the spacecraft
needs a system to collect all of the heat and remove it. The service
module will use a radiator system to maintain the temperatures of
the vehicle systems and components. |
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Staying in Touch |
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Orion’s astronauts need to stay in contact with mission control
throughout their mission. When Orion achieves orbit, the service module
will deploy its high-gain antenna. This will provide primary communications
(voice, video, and data) until the crew transfers to the lunar lander
and upon their return from the lunar surface. It also will maintain
contact with mission control while Orion is operating autonomously in
lunar orbit during the crew’s lunar surface operations. |
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Mission Accomplished |
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Unlike the crew modules that can be reconditioned and used for multiple
missions, each service module will fly only one mission. Prior to
entering the Earth’s atmosphere, the service module will separate
from the crew module to be discarded. This prepares the crew module
for reentry by exposing its heat shield. Like its Apollo counterpart,
the service module will be directed to reenter the Earth’s atmosphere
so it can burn up and fall safely into a designated area of open ocean
waters. |
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Putting It All Together |
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Work on the Orion project is distributed across several NASA centers
and contractors to provide the expertise and facilities needed. Glenn
Research Center in Cleveland, Ohio, is a key member of the Orion Project
team with leadership responsibility for managing the development of
the Orion service module, from requirements development through production
and operations. Glenn’s expertise in propulsion, power, thermal,
and communication systems made it a logical choice for leading the
development activities for the service module. Engineers are actively
engaged with the prime contractor, Lockheed Martin Corporation, and
its industry teammates to design a service module that is robust,
versatile, and capable of performing the challenging missions that
lie ahead for the Orion crew exploration vehicle. The Service Module
Office at Glenn is an integral part of the Orion Project Office led
from NASA Johnson Space Center in Houston, Texas. |
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