Service Module(SM)

The Service Module supplies the electrical power, water, oxygen, and life support necessary for the CM and astronauts for the trip from the Earth to the Moon. The SM also controls the flight path of the CSM/LM stack. The SPS engine of the SM provides the thrust necessary to leave Earth orbit to begin translunar coast, and then to leave lunar orbit and return to Earth.
  • Height : 24ft 5in (7.4422m)
  •   Body:14ft 8in (4.4704m)
  •   SPS nozzle :9ft 9in (2.9718m)
  • Diameter : 12ft 10in (3.9116m)
  • Maximum Weight : 55000lb (24.97ton)
  • Manufacturer : North American Rockwell Corp. Space Div., Downey, Calif.


The Service Module (SM) is connected to the Command Module (CM) until just before re-entry, and it supplies oxygen, water and electric power to the Command Module. The SM is equipped with the SPS (Service Propulsion Subsystem) engine, four quads of four (16 elements) RCS (Reaction Control Subsystem) engines, the fuel, oxidizer, helium which is necessary for these.
These engines are used to control the flight path until CM/SM separation just before re-entry. The SPS engine and its fuel accounts for about 75% of the weight of the SM.
The SM body is made of aluminum honeycomb core between two aluminum face sheets, with a thickness of about one inch. The "spine" of the SM is made of a 44-inch (1.11m) diameter cylinder installed in the center of the SM, with 6 sections (Sector 1 to Sector 6) surrounding it. The 360 degrees around this center section is divided among two 50-degree sections (Sectors 1 and 4), two 60-degree sections (Sectors 3 and 6), and two 70-degree sections (Sectors 2 and 5).

Center Section

Two 40-inch diameter globular helium tanks and the SPS engine are located here. Each helium tank carries 19.6 cubic feet of helium gas, pressurized at 3600psi. These are used to "pump" fuel and oxygen for the SPS engine. The fuel is "pumped" to the engine from the tanks by injecting this inert gas.

Sector 1

A weight to help balance the SM was carried in this sector from Apollo 1 through Apollo 14. For Apollo 15 through 17, this was used as the SIM (Scientific Instrumentation Module) Bay. A high-resolution camera as well as radiation measurement devices were carried here to probe the surface of the moon from lunar orbit.

Sector 2

An RCS engine quad and a radiator panel are installed on the outside panel of this sector. An oxidizer sump tank for the SPS engine, and an RCS fuel tank and conduits are installed inside. The oxidizer sump tank can hold 13923lb of nitrogen tetroxide, and is manufactured of titanium, with a height 153.8 inches, and a diameter of 51 inches.

Sector 3

An RCS engine quad and a radiator panel are installed on the outside panel of this sector. An oxidizer tank for the SPS engine, and an RCS fuel tank and conduits are installed inside. The oxidizer tank is a little smaller than the oxidizer sump tank, and carries 11284lb of oxidizer, with a height 154.47 inches, and a diameter of 45 inches.

Sector 4

Three fuel cell power plants, two cryogenic oxygen tanks, two cryogenic hydrogen tanks, and the related infrastructure is installed here. The three fuel cell power plants are located at the top of the sector, and each has a height of 44 inches, a diameter of 22 inches, and weighs 245lb. These supply electric power and drinking water to the spacecraft.
The center of this sector contains the two cryogenic oxygen tanks, and the two cryogenic hydrogen tanks, arranged with the oxygen tanks at the top, and the hydrogen tanks at the bottom. All of these tanks are spherical and supply oxygen and hydrogen to the fuel cell power plants.
The cryogenic oxygen tank is made of about 26-inch diameter Inconel (an alloy made of nickel and iron). It carries 326lb of super-cooled oxygen, in a semi-liquid, semi-gas state.

The cryogenic hydrogen tank is made of about 31.75-inch diameter titanium. It carries a little over 29lb of super-cooled hydrogen, in a semi-liquid, semi-gas state.
The Apollo 13 emergency was due to the explosion of the Number 2 cryogenic oxygen tank carried here. Therefore, one spare cryogenic oxygen tank in Sector 1, and one spare battery in Sector 4 were added after Apollo 13.

Sector 5

An RCS engine quad and an environmental control radiator panel are installed on the outside panel of this sector. A fuel sump tank for the SPS engine, and an RCS fuel tank and conduits are installed inside. The fuel sump tank can hold 8708lb of propellant (a 50-50 mixture of hydrazine and unsymmetrical dimethylhydrazine), and is manufactured of titanium with a height of 153.8 inches, and a diameter of 51 inches.

Sector 6

An RCS engine quad and an environmental control radiator panel are installed on the outside panel of this sector. A fuel storage tank for the SPS engine, and an RCS fuel tank and conduits are installed inside. The size of the fuel storage tank is same as the oxidizer tank in Sector 3, and carries 7058lb of propellant, and has a height of 154.47 inches, and a diameter of 45 inches.


There are two antenna systems, for communications and telemetry transmissions with NASA. One antenna, the S-band high-gain antenna was located off to the side of the SPS engine. The other was comprised of two sets of VHF omnidirectional antennas on the center of the SM body.
The high-gain antenna is composed of four 31-inch diameter round reflectors surrounding an 11-inch square reflector. This is a directional antenna, and it cannot communicate if it is not precisely facing the direction of the Earth. In contrast, the omnidirectional antennas do not need to be pointed at the Earth. These were called "scimitars" due to their shape. The INCO (Instrumental and Communication Officer) would select which antenna to use based on the spacecraft's orientation, typically using the high-gain whenever it was facing the earth. The omnidirectional antenna is made of 1/100-inch stainless steel, and the entire length is about 13.5 inches.

RCS engine

An RCS (reaction control subsystem) engine is a small rocket engine which is used to control the orientation of the spacecraft. The SM has four RCS quads installed (16 total engines), and these are installed with a 7 degree offset toward the +Z, -Z, +Y, and -Y axes. Monomethyl hydrazine (MMH) is used as fuel, and nitrogen tetroxide (N2O4) is used as oxidizer. Each of these engines has an output of 100 pounds of thrust.
Two fuel tanks, two oxidizer tanks, and one helium tank are installed inside of the SM body. The oxidizer is mixed with the fuel in a 2:1 ratio, and ignites due to spontaneous combustion, and does not need any sort of spark to ignite.

SPS engine

The SPS engine supplies the thrust necessary for Lunar Orbit Insertion (LOI), transearth injection (TEI), and midcourse corrections during flight. The SPS engine itself is 3 feet 5 inches, and has an engine bell or nozzle is 9 feet 4 inches, made of columbium and titanium attached. The entire weight of the SPS with fuel tanks is 41500 pounds, and occupies 75% of the SM. Its output is 20500 pounds of thrust. A 50:50 mix of hydrazine and unsymmetrical dimethylhydrazine (UDMH) is used as fuel, with nitrogen tetroxide used as oxidizer. No spark is necessary to light the engine, as these materials spontaneously combust when mixed. There is no throttle in this engine. Changing the ignition time (about 750 seconds from 0.4 seconds) allows for adjustment of the engine output. The SPS is rated to be started about 50 times. SPS ignition occurs after short firing (ullage maneuver) by the RCS engines, to help settle the fuel at the bottom of the tanks in the zero gravity environment. The SPS engine is installed on a gimbal ring, and can rotate +10 to -10 degrees around the Z (Yaw) axis, and +6 to -6 degrees around Y (Pitch) axis.
Because the SM center of gravity changes during flight (due to fuel, water, oxygen, etc. usage), "straight" is actually +1 degree around the Z axis and -2 degrees around the Y axis. The engine is produced by AerojetGeneral Corp. in Sacramento, California.

A heat shield in installed on the end of the SM where the SPS engine bell is attached, protecting the SM from the heat of the SPS engine.

Connection, separation with CM

Six beams made of aluminum alloys are arranged at the forward bulkhead. Compression pads are installed in three of these. Tension ties that connect the CM and SM are installed with compression pads in the other three. The CM and SM are connected by these tension ties and the CM/SM umbilical which supplies electric power, oxygen, water, and so on.

The separation of the SM and CM is done automatically by a controller installed in the SM. Cutting of the connectors, the transfer of the control of the electric system and jetting of the SM RCS engine to draw CM and SM apart are done almost instantaneously at the time of the separation.
First, before the separation, astronauts pressurize the CM RCS by using electrical power provided by the SM. The separation sequence begins when either one of two switches on the main console is toggled. The electrical transfer through the CM-SM umbilical is disabled first. After 10 seconds, the tension ties that connect the SM with the CM are separated. At the same time, the stainless steel guillotine cutter is fired by explosives, cutting the cables and wires in the CM-SM umbilical. Finally, the RCS engines of the SM fire for five seconds, the flight course of SM is changed to avoid a collision with the CM. Until the fuel is used up or electric power is expended, the SM RCS keeps firing to ensure that the SM's flight path stays away from the CM.


Thanks to Jonathan Long for revice my poor English.