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The Orion Service Module
Orion Approaches ISSWhen 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.
  Critical Services

Vehicle Separation

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.
  The Primary Thrust
Service Module Components
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.
  Staying the Course With the Right Attitude
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.

The Power To Fly

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.
  Taking the Heat
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.
  Staying in Touch
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.
  Mission Accomplished
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.
  Putting It All Together
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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Tim Reckart
Editor: Elizabeth B. Wagner
NASA Official: Elias Naffah
Last Updated: October 5, 2011
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