Spacecraft Systems

Apollo Command and Service modules

Zond with L3 stage in assembly hanger

artist's concept of the THEMIS A, B, C, D, E spacecraft in orbit (aka ARTEMIS)

1U ArduSat CubeSat and proposed 6U OMOTENASHI CubeSat

A spacecraft is a vehicle, or machine designed to fly in outer space. A spacecraft that only orbits Earth is typically called a satellite. For the US Apollo program to the Moon the spacecraft included the Apollo command module, service module, and lunar module. For the USSR Zond program to the Moon the Zond spacecraft was a stripped-down Soyuz 7K-L1. For the USSR N1/L3 program to the Moon the spacecraft would have included a Soyuz 7K-L3 or ("Lunniy Orbitalny Korabl", LOK), and a lunar lander or ("Lunniy Korabl", LK).

The Apollo, Zond, and N1/L3 spacecraft were designed to carry humans or animals, providing them with needs such as oxygen and water, and protecting them from heat, cold, and radiation of the space environment. The Apollo spacecraft would spin in order to change what side was facing the sun and thus distribute the heating more evenly. They and other spacecraft also provide navigation and guidance and attitude control in order to reach their destination with a minimum amount of fuel for course correction.

Most parts in spacecraft operate best between -5 and 40 degrees Celsius (20 and 120 Fahrenheit). In space, a spacecraft recieves heat directly from the Sun. To prevent the temperature from getting too high, a spacecraft must be able to eliminate excess thermal energy. Likewise, when it gets too cold a spacecraft must be able to heat itself. The simplest way to manage temperatures is by using passive thermal control systems that do not require any additional power. However, it might be necessary to use active systems such as heaters or cooling fluids pumped through the system.

If a spacecraft is to point itself in the right direction it needs to know where it is and how it is oriented in space. Common forms of navigation for spacecraft include celestial navigation, inertial navigation, and radio navigation. For celestial navigation, 'Sun sensors' indicate to a spacecraft the direction of the Sun. Similarly, 'star tracker's are optical sensors that can be used to detect a single star. Star trackers are usually used on three-axis stabilized spacecraft or on de-spun platforms of spinning spacecraft. For inertial navigation, gyroscopes tell a spaecraft the directions and velocities of its own rotations, but do not indicate orientation in space. By starting with a known attitude, and by tracking rotation using gyroscopes, the spacecraft can track its attitude while moving. Gyroscopes are often combined with accelerometers in Inertial Measurement Units or IMUs. IMUs typically have three gyroscope axis and three accelerometers, one combination of each for each spacecraft axis. Radio navigation uses direction, travel time, and doppler shift of radio signals from a base statation to determine the direction, distance, and velocity of a spacecraft. Apollo used all three of these forms of navigation.

A spacecraft also needs to control its orientation or attitude while moving through space. There are two main approaches to control the attitude of a spacecraft: spin stabilized or three-axis stabilized. Spin stabilization works by spinning the spacecraft like a top. Spin stabilization is very efficient, and little power or rocket thrust is needed to maintain a particular attitude. The downside of spin stabilization is that all the sensors are also spinning, making it difficult to point cameras, or orient antennas and solar arrays.

Many spacecraft are three-axis stabilized, which means their attitude is actively controlled around all three rotational axes. Nearly all three-axis stabilized spacecraft have a reaction control system consisting of small rocket thrusters distributed around the spacecraft. Another method for three-axis stabilitzation is the use of electrically powered reaction wheels. If a reaction wheel is set in motion with an electric motor, not only will the reaction wheel start to rotate, but the spacecraft itself will also begin to rotate in the opposite direction. Since a spacecraft has three orthogonal axes around which it can rotate, it can be controlled by three of these reaction wheels. While Apollo would spin in order to distribute heating, it used a reaction control system consisting of a series of small rocket thrusters for attitude control.

Spacecraft that have successfully orbited or landed on the Moon
Spacecraft	mission type	country of development	launch system	guidance control system technology
Luna 2	Lunar impactor	USSR	Luna 8K72 variant of R-7 Semyorka
Ranger 7, 8, 9	Lunar impactor	USA	Atlas LV-3 Agena-B
Luna 9, 13	Lunar lander	USSR	Molniya-M 8K78M
Luna 10, 11, 12, 14	Lunar orbiter	USSR	Molniya-M 8K78M
Surveyor 1, 3, 5, 6, 7	Lunar lander	USA	Atlas SLV-3C Centaur-D	no onboard computer, command remote control (R/C) by humans
Lunar Orbiter 1, 2, 3, 4, 5	Lunar orbiter	USA	Atlas SLV-3 Agena-D
Explorer 35	Lunar orbiter	USA	Delta E1
Zond 5, 6, 7, 8 (aka Soyuz 7K-L1)	circumlunar trajectory and return	USSR	Proton-K/D	Argon-11C (sometimes listed as Argon-11S ?)
Apollo 8, 10	Lunar orbiter and return	USA	Saturn V	Apollo Guidance Computer (AGC architecture)
Apollo 11, 12, 14	Lunar orbiter and lander and sample return	USA	Saturn V	Apollo Guidance Computer (AGC architecture)
Luna 16, 20, 23, 24	Lunar lander and sample return	USSR	Proton-K/D
Luna 17, 21 / Lunokhod 1, 2	Lunar lander and rover	USSR	Proton 8K82K with Blok D upper stage
Luna 19, 22	Lunar orbiter	USSR	Proton-K/D
Apollo 15, 16, 17 / with Lunar Rover	Lunar orbiter and lander and rover and sample return	USA	Saturn V	Apollo Guidance Computer (AGC architecture)
Explorer 49	Lunar orbiter	USA	Delta 1913
Hiten (MUSES-A)	Lunar orbiter	Japan	Mu-3S-II	three HD68HC000 (aka 68k) (68000 architecture)
Clementine	Lunar orbiter	USA	Titan II(23)G	1750 (MIL-STD-1750A architecture)
Lunar Prospector	Lunar orbiter	USA	Athena II	no onboard computer, command remote control (R/C) by humans
SMART-1	Lunar orbiter	ESA	Ariane 5G	Atmel TCS695E single-chip ERC32 (SPARC architecture)
ARTEMIS P1, P2 (THEMIS B, C)	Lunar orbiter	USA	Delta II 7925	low-power 8085 (8080 architecture)
SELENE (Kaguya)	Lunar orbiter	Japan	H-IIA	SuperH SH-3 (SuperH architecture)
Chang'e 1, 2	Lunar orbiter	China	Chang Zheng 3
Chandrayaan 1	Lunar orbiter	India	PSLV-XL	LEON 3FT-RTAX (SPARC architecture)
Lunar Reconnaissance Orbiter	Lunar orbiter	USA	Atlas V 401	RAD750 (POWER architecture)
LCROSS	Lunar impacter	USA	Atlas V 401	RAD750 (POWER architecture)
GRAIL A, B	Lunar orbiter	USA	Delta II 7920H	RAD750 (POWER architecture)
LADEE	Lunar orbiter	USA	Minotaur V	RAD750 (POWER architecture)
Chang'e 3, 4 / Yutu-1, 2	Lunar lander and rover	China	Long March 3B
Queqiao, Longjiang-1, 2	Lunar orbiter	China	Long March 4C
Chandrayaan 2, 3 / Vikram / Pragyan	Lunar orbiter and lander and rover	India	GSLV Mark III M1
Chang'e 5	Lunar lander and sample return	China	Long March 5
CAPSTONE	Lunar orbiter	USA	Electron
Danuri	Lunar orbiter	Korea	Falcon 9
Artemis 1	Lunar orbiter and return	USA	SLS Block 1
SLIM / LEV-1 and LEV-2	Lunar orbiter and lander and rover	Japan	H-IIA	PowerPC (POWER architecture)

CubeSats are small and light weight satellites that use an open source architecture based on a 10 x 10 x 11.35 cm cube of about 2 kg. Earth orbiting CubeSats quite often don't have attitude control at all. They're designed to have antennas and solar arrays work when oriented in almost any direction similar in some ways to spin stabilized spacecraft. An example of this would be the Earth orbiting 1U ArduSat CubeSat. However, this wouldn't work for the proposed 6U OMOTENASHI CubeSat which is intended to land on the Moon. In cases like this, miniaturized gyroscopes, accelerometers, reaction wheels, and thrusters have to be designed to track and control the CubeSats attitude.

GNC Subsystems used in CubeSats

Reaction Wheels
Star Trackers
Sun Sensors
Earth Sensors
Inertial Sensors (Gyroscopes & Accelerometers
GPS Receivers
Magnetic Torquers

Working Example

For students who want to experiment with a working example, there aren't a lot of options for building and testing working spacecraft. If you have a lot of money or are a member of a university research program, you might be able to fund a CubeSat. As stated above, CubeSats are small and light weight satellites that use an open source architecture based on a 10 x 10 x 11.35 cm cube of about 2 kg. A single 10 x 10 x 11.35 cm cube unit is known as 1U. Larger size and weight CubeSats have been created and launched including the 1.5U, 2U, and 3U. Also 6U (10 x 20 x 30 cm) and 12U (20 x 20 x 30 cm) CubeSats have been proposed and designed. The first 6U CubeSat was launched in 2014. A 1U CubeSat might cost around $10,000 dollars to build and $50,000 to $100,000 to launch into low earth orbit.

There are no electronics form factor nor communications specifications for CubeSats. Commercial off the shelf (COTS) PC/104 and PCI-104 form factor boards, which are 9.0 x 9.6 cm stackable circuit boards, fit well in 10 x 10 cm CubeSats. Stackable connectors on these boards allow for easy integration of various electronics, and many CubeSat electronics manufacturers use the same signal arrangement in the connectors. Also, controllers for CubeSats have included Arduino (ARM) and (AVR) systems, referred to as ArduSats, and smartphone (ARM) systems referred to as PhoneSats. Note that CubeSats are generally Earth orbiting satellites without attitude control and had not been launched to the Moon or deep space until Mars Cube One. Mars Cube One, or MarCO, is a pair of spacecraft that launched with NASA's InSight Mars lander to fly past Mars during InSight's entry, descent, and landing.

Some CubeSat kits are available commercially. These can be controlled by flight computers and reaction wheels similar to full size spacecraft. Some sources of CubeSat kits include:

In addition to building a CubeSat, students can work with working examples of a spacecrafts components. Many smartphones have miniature gyroscopes and accelerometers that can be programmed as inertial measurement units. Stand alone minature inertial measurement units can also be purchased from electronic suppliers.

Star tracker's might be buildable from digital cameras by combining with a star chart and appropriate software. Reaction wheels could be built from electric motors and weighted flywheels.

While a CubeSat will be a satellite in orbit, it will be a low Earth orbit (LEO). If you want to get a spacecraft to the Moon, you would need a propulsion system, and a navigation, guidance, and control system to aim it, on your CubeSat in order to enter a transfer orbit to raise it's orbit to reach the Moon.