Spacecraft Engineering

Summary

Spacecraft engineering is an interdisciplinary engineering field concerned with the design, development, and operation of unmanned satellites, interplanetary probes, and manned spacecraft. Unmanned satellite missions include commercial communications and remote sensing (including meteorological satellites), scientific research, military communications, navigation, and reconnaissance. Interplanetary probes are exclusively confined to scientific and exploratory missions. Manned spacecraft missions are currently confined to long-duration assignments at the International Space Station (ISS) and to short-duration flights supporting the ISS on either the United States Space Transportation System (STS) or the Russian Federation Soyuz spacecraft. However, manned missions returning to the moon or manned flights to Mars are in the planning stages and scheduled to begin in the 2020s and 2030s.

Definition and Basic Principles

Spacecraft engineering is the process of designing, constructing, and testing vehicles for deployment and operation in the full expanse of space above the Earth's atmosphere, generally regarded as beginning at an altitude of fifty miles. Launch vehicles and rocket propulsion are considered part of the related but separate field of rocketry.

Spacecraft have to be robust enough to survive several harsh environments. The vibrational loads generated by the launch vehicle will damage or destroy weak or poorly designed structures. The electronic components must function reliably in a high-radiation environment. All parts of the spacecraft cycle from extreme heat to extreme cold in each orbit as the spacecraft moves into and out of the Earth's shadow. Outgassing in a vacuum can contaminate solar panels and camera optics. Motion through the plasma of the ionosphere creates potentially damaging static-electric charges.

Energy is expensive in space. It must be gathered from sunlight from solar panels in a limited area or generated from onboard fuel supplies, the exhaustion of which will end the mission. Batteries wear out through repeated charging and discharging and must be managed carefully to last as long as possible. Electrical components must operate with high efficiency and draw as little power as possible. Inactive components have to be kept off or in a low-power standby state. Everything must be designed with the knowledge that repairs or replacements will be impossible at worst and difficult, dangerous, and expensive at best.

Batteries, fuel cells, pressure vessels, propulsion systems, and nuclear power modules are all inherently hazardous devices. A launch failure can be catastrophic. All launch ranges operate under rigidly enforced safety standards for protecting the spacecraft, the launch vehicle, and the personnel around them. Reliability, survivability, and safety must be designed into the spacecraft from conception. Rigorous testing throughout the development and manufacturing process plays a major role in spacecraft engineering.

Background and History

Spacecraft engineering did not exist before the development of the A-4 (more popularly known as the V-2) rocket by the German army during World War II. The United States and the U.S.S.R. integrated technology from captured German scientists, engineers, and rocket hardware into their domestic long-range ballistic missile development efforts. The primary goal was the development of intercontinental-range ballistic missiles for the delivery of nuclear warheads in case of war. The A-4, however, flew very poorly without the ton of high explosives it was initially designed to carry. The ordnance payload was replaced with scientific instruments for exploring atmospheric conditions at high altitudes and astronomical observations unimpeded by atmospheric interference. The extreme conditions of rocket flight and the harsh space environment posed new challenges to the instrument developers. Techniques that evolved to meet those challenges laid the foundation for the new field of spacecraft engineering.

In the early years, spacecraft were of necessity small and lightweight. The main challenges were miniaturizing components and operating on small amounts of electrical power. The newly invented transistor and its packaging into integrated circuits were adopted by spacecraft engineers immediately despite their initially high cost. Power came from compact, high-performance batteries supplemented by recently developed silicon solar cells to produce electricity from sunlight. The initiation of manned spaceflight added the challenges of providing a livable environment for the crew and returning them safely to Earth. Spacecraft engineering had to confront the biological issues of providing air, water, and food while disposing of waste products. Long-duration manned missions raised quality-of-life issues such as comfort, privacy, personal hygiene, and physical fitness.

In the 1970s, interplanetary spacecraft ranged far from the sun, straining the capabilities of solar panels to provide sufficient electrical power. Spacecraft engineers harnessed the energy of radioactive decay to power deep-space missions and keep the spacecraft warm so far from the sun. Missions to Mercury and Venus, so near the sun, posed the opposite challenge of keeping the spacecraft from overheating.

Command, control, and data acquisition from remote platforms with limited power available for broadcasting has always been and continues to be a major communications challenge for spacecraft engineers.

How It Works

Spacecraft. Spacecraft are composed of a payload and a bus. The bus is designed as a major system comprising seven or more subsystems. The electrical power system (EPS) provides the power to operate the active components. The communications system (Comms) maintains contact with ground control. The command and data handling system (C&DH) issues electronic commands to all onboard units and collects data from each unit for transmission to the ground. The thermal control system (TCS) regulates the temperatures of all onboard units to keep them within acceptable operating ranges. The attitude control system (ACS) controls the rotational dynamics of the spacecraft to achieve and maintain the required orientation in space. The propulsion system (PS) changes the spacecraft's trajectory to keep it on course. The structure holds all spacecraft components and provides the mechanical support necessary during manufacture, transport, and launch. Manned spacecraft include additional environmental control and life-support systems (ECLSS). Redundant (duplicate) components and subsystems are used as much as possible to maximize reliability.

Electrical Power. The electrical power system is responsible for power generation, capture, or storage, plus delivery of conditioned electrical power to all parts of the spacecraft. Power may be generated by onboard reactors such as fuel cells or captured from sunlight by solar panels. Power from sunlight is stored in rechargeable batteries for later use when the spacecraft is in eclipse or when power demand temporarily exceeds the total available from solar panels alone. Power-conditioning circuits are necessary to provide electricity at the voltage, current, and stability required by individual components.

Communications (Comms). Communications consist of antennas, transmitters, receivers, amplifiers, modulators, and demodulators necessary for communications with the ground. Comms may also include equipment for encrypting and decrypting the signal to prevent interception and to block attempts at illegally seizing control of the spacecraft with false messages. Communication must be maintained across distances that may stretch billions of miles in the case of interplanetary probes using signals of modest strength because of the limited amount of electrical power available. Static-free frequency modulation (FM) signals are preferred to minimize transmission errors. High frequencies, on the order of billions of cycles per second, allow large amounts of data to be moved quickly. Spacecraft engineers are also experimenting with internet-type communications protocols, as well as laser communication systems that encode data onto a beam of light.

Command and Data Handling. The command and data handling system centers on the flight computer. The computer monitors the status of all components, turns them on and off according to schedules transmitted from the ground, and collects housekeeping data from all units and science data from any science packages onboard.

Attitude Control System. High-gain antennas must be accurately pointed toward the Earth to maintain communications with ground control. Remote-sensing instruments and science packages need to be pointed at their study targets. Manned spacecraft must maintain a proper attitude for safe reentry. To achieve all of these, the altitude control system senses the orientation of the spacecraft relative to the fixed stars and reorients the spacecraft as necessary to fulfill the mission.

Spacecraft orientation is determined by reference to the sun, the Earth, and the brightest stars. The sun and the bright stars can be located optically. The Earth can be sensed even during an eclipse by the infrared radiation emitted by its warm surface. Interplanetary probes can locate the Earth by homing in on the radio signal coming from ground control.

Some spacecraft require three-axis stability where rotation about any axis must be rigorously suppressed. If the spacecraft begins to rotate in an undesired manner, onboard gyroscopes are spun up to absorb the additional rotational momentum and, by reaction, leave the spacecraft as a whole stationary. Many other spacecraft maintain stability by rotation about a fixed axis. Control of the altitude control system is the responsibility of the command and data handling system. When the total rotational momentum of the spacecraft gets too large, the excess is eliminated by firing the attitude thrusters in the propulsion system.

Propulsion System. The propulsion system performs occasional maneuvers to keep Earth-orbiting satellites on station or interplanetary probes on course. The propulsion system comprises rocket thrusters, propellants, pumps, valves, and pressure vessels. Attitude control thrusters control the rotational dynamics of the spacecraft. Course correction thrusters change the speed or direction of motion of the spacecraft. The propellant must be storable for long periods under harsh space conditions. Special pressurization techniques are necessary to move liquid propellants from tanks to thrusters in zero gravity. The spacecraft must carry enough propellant for the planned mission lifetime plus a reserve necessary for deorbit at the end of life.

Structure. Spacecraft structures must satisfy the competing requirements of strength and low weight. Spacecraft structures must not bend or sag under the acceleration loads experienced during launch. Nuts and bolts must contain a locking feature to prevent them from loosening under vibrational loads. The structure's mass plays a passive role in thermal control by conducting heat from warmer to colder parts of the spacecraft.

Thermal Control System. The thermal control system uses active and passive methods of moving heat to maintain normal operating temperatures for critical components. Active methods include using heaters to warm cold objects and pump-driven fluids to move heat from hot to cold areas. Passive devices include reflective coatings and insulation.

Environmental Control and Life-Support Systems (ECLSS). Life-support systems manage air, water, food, and waste. Humans require a pressurized atmosphere that provides oxygen for respiration and humidity for comfort while removing carbon dioxide. Too little oxygen leads to hypoxia, while too much leads to oxygen toxicity. Too much carbon dioxide leads to carbon dioxide poisoning. Too little humidity leads to extreme crew discomfort and possibly dangerous electrostatic discharges. Too much humidity leads to condensation, which can interfere with electrical systems and nurture the growth of bacteria and fungi. Humans need about seven pounds of water daily for proper hydration and seven pounds for hygiene and housekeeping. Active adults require 2,500 to 5,000 calories a day to function without losing body mass. Corresponding amounts of waste are generated.

Captured waste products must be recycled, returned to Earth, or dumped overboard. Short-duration flights can be open-loop—all the required consumables can be onboard at launch, and the waste products can be dumped overboard or disposed of after landing. The mass involved is prohibitive for long-duration flights. Open-loop systems can continue to be used if regular resupply flights are possible, as is done for the ISS. For manned missions to Mars, by contrast, resupply will be difficult, if not impossible, driving the ECLSS design toward full regenerative recycling, where carbon dioxide is taken up by plants that provide food and oxygen and wastewater is purified and reused. The ISS has an important role as a test bed for these emerging technologies.

Applications and Products

Commercial Spacecraft. To date, all commercial spacecraft have been communications satellites or remote-sensing satellites. Geosynchronous satellites appear stationary in the sky to an observer on Earth and make radio contact through fixed antennas possible. Television and radio programming sent up from the ground are amplified and rebroadcast to anyone with a line of sight to the spacecraft. Because the satellite never sets, the signal is uninterrupted. These satellites also provide radio and telephone communications for ships at sea and people living in remote locations. The market for these satellites supports many spacecraft manufacturers worldwide.

Remote-sensing satellites observe the surface of the Earth across a broad range of the electromagnetic spectrum, which stretches from radio waves through the infrared and optical bands and into the ultraviolet. The oldest and most mature application of remote sensing is weather forecasting. Infrared and optical photography from space is used in land-use planning, mapping, crop surveys, and pollution monitoring.

In 2011, the National Aeronautics and Space Administration (NASA) shut down the space shuttle program, which provided manned spaceflights to the ISS, so that commercial firms could take over this function. Companies like Boeing and SpaceX received funding from NASA for this purpose, and in 2018, SpaceX provided the first private spacecraft to reach the ISS. In 2020, SpaceX brought the first NASA crew to the ISS. In 2021, the first private passenger-carrying suborbital spacecraft carried groups of tourists to the threshold of space. The flights were carried out by Space X, Virgin Galactic—owned by British billionaire Richard Branson—and Blue Origin—owned by Amazon founder Jeff Bezos—all took paying customers on flights into space. Space tourism to the ISS and sub-orbital flights continued in the 2020s, with companies like Axiom Space and Blue Origin joining the industry.

Scientific Spacecraft. The first US spacecraft, Explorer I, made the first major scientific discovery of the space age when it discovered belts of protons and electrons trapped by the Earth's geomagnetic field. Discoveries by scientific spacecraft of all nations have profoundly changed mankind's knowledge of the Earth and its place in the universe. Interplanetary probes have mapped almost every significant body in the solar system and discovered dozens of planetary moons undetectable from Earthbound telescopes. The Hubble Space Telescope has photographed stars at birth and death and a black hole at the center of the Milky Way. Energy that does not penetrate the Earth's atmosphere, such as X-rays, gamma rays, and high-energy ultraviolet rays, can be studied only from space platforms.

Military Spacecraft. Military spacecraft include communications satellites, reconnaissance and surveillance satellites, missile-attack early-warning satellites, and navigation-support satellites such as the Global Positioning System (GPS) constellation. Military spacecraft are considered force-enhancement or force-support assets. They do not directly engage in hostilities.

Careers and Course Work

Undergraduate study in almost any engineering field is sufficient entry-level training. However, this should be followed by graduate study at an institution with a strong spacecraft design program—preferably one with faculty involved in an ongoing spacecraft project. Entry-level jobs are available with spacecraft manufacturers, government space agencies, intelligence agencies, and military departments. Entry into the field is also possible through appointment as a commissioned officer in the armed forces who has been assigned space missions and responsibilities.

Upper- and mid-level management jobs require administrative and budgeting skills that can be acquired on the job but are significantly enhanced by business or financial management courses. These courses can be completed as part of a secondary master's degree, through professional development training, or by continuing education. Strong written and oral communication skills are essential.

Most spacecraft development work involves trade secrets and access to classified information. Individuals who cannot qualify for a security clearance, have a bad credit history, a criminal record, or are not citizens of the country where they reside will not readily find employment in this field.

Social Context and Future Prospects

Spacecraft and the services they provide are now part of everyday life. Airliners navigating across oceans and pedestrians navigating city streets rely on GPS devices to find their destinations, and embedded GPS chips in cell phones allow parents to track their children anywhere. Satellites bring television straight to the home and radio straight to the automobile. Public databases allow anyone connected to the Internet to acquire a satellite photo of almost any spot on Earth. Accurate long-range weather forecasting exerts a daily influence on almost all business and personal planning. Spacecraft are now an indispensable part of the global economic and social infrastructure. This infrastructure must be replaced as it ages and wears out, creating a continuing demand for the services of the spacecraft engineer.

Spacecraft have become so numerous in low-Earth orbit that spacecraft disposal at the end of life is a major design challenge for modern spacecraft engineers. On February 10, 2009, Iridium-33, a US communications satellite, collided with Cosmos-2251, a defunct Russian military communications satellite. The debris generated by the collision, as well as other debris, threatens other satellites at the same orbital altitude. Future collisions are certain to happen more often as the population of spacecraft increases. They are best prevented by deliberately deorbiting nonoperational spacecraft so that they burn up in the atmosphere in reentry or by moving them to orbits at seldom-used altitudes. Those not burning up on reentry are often disposed of in the South Pacific Ocean. Tougher regulation by the United States and international agencies continues as space exploration increases. At the Group of Seven (G7) Hiroshima Summit in 2023, the issue of space debris was a topic of concern. Leaders agreed on the importance of implementing the guidelines outlined by the UN Committee on the Peaceful Uses of Outer Space to limit space debris.

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