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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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