Spacecraft are all kinds of aircraft outside the earth’s atmosphere that basically operate according to the laws of celestial mechanics, also known as space vehicles. Spacecraft can be divided into two categories: unmanned spacecraft and manned spacecraft. Unmanned spacecraft include artificial earth satellites and space probes. Artificial earth satellites orbit the earth, accounting for more than 90% of the spacecraft, and are divided into three categories: scientific satellites, application satellites and technology test satellites. Application satellites are satellites that directly serve the national economy and military, such as communication satellites, broadcasting satellites, weather satellites, navigation satellites, earth resource satellites and reconnaissance satellites. Space probes are divided into lunar probes, planetary and interplanetary probes, etc. Manned spacecraft include manned spacecraft, space stations and space shuttles.
The spacecraft orbiting the earth is called earth orbiting spacecraft, which is the largest number of spacecraft, such as artificial earth satellites, space stations, spacecraft, etc. Earth orbits include: geosynchronous orbit, 35786km above the ground; medium earth orbit, 2000~20000km above the ground; low earth orbit, 200~2000km above the ground; and large elliptical orbits. During the flight stage of the spacecraft in the atmosphere, the working environmental conditions of its power supply system are similar to those of the aircraft power supply system, but its flight speed is extremely fast, the acceleration in the mechanical environmental factors is large, and the climatic and environmental conditions change extremely quickly. The requirement that spacecraft electrical equipment can ensure normal operation in a weightless working environment and any position conditions brings complexity to the structural design of spacecraft electrical equipment.
The power supply system is the system used in the spacecraft to generate, store and distribute electrical energy. Most spacecraft work for a long time, requiring a large capacity of the power supply, and the weight of the power supply accounts for 15% to 25% of the weight of the entire spacecraft. These power sources are divided into three categories according to different energy sources: chemical power sources, solar cell power sources and nuclear power sources. The output current of the converter and voltage stabilizer realizes voltage/current conversion and voltage stabilization, realizes electrical insulation to ground and power busbar protection, and eliminates the transient changes from the power busbar and the influence of electrical noise on electrical equipment. The distributor completes the distribution of the power supply.
The choice of spacecraft power system depends on the working life of the power system, load characteristics and load requirements (average load and peak load), solar irradiation, working environment, weight, volume and structure, etc.
Early launched satellites used chemical power sources, such as zinc-mercury batteries, zinc-silver batteries, and cadmium-nickel batteries. The discharge current of the zinc-mercury battery is small, and the working voltage is not stable. Nickel-cadmium batteries can output greater power, but the specific energy is slightly lower. In the 1950s and 1960s, the scientific experiment satellites, space probes and return-type satellites mostly used zinc-silver batteries, which have large discharge current and specific energy, and are the main power source for short-term flying spacecraft. Manned spacecraft and space shuttles mostly use hydrogen-oxygen fuel cells. The peak power of each battery of this power source is as high as 12kW, the maintenance-free working time can reach 2500h, and it has multiple start and stop functions. Nickel-cadmium batteries, silver-cadmium batteries and nickel-hydrogen batteries are often used as accumulators for solar arrays.
In the outer space of the earth, the solar radiation intensity (1360W/m²) is 1.3~1.7 times that of the ground. The use of solar cells can reduce the weight of the spacecraft, but the solar cell array-battery pack power system must be formed together with the battery to solve the power supply problem when the spacecraft enters the shadow area. This power system has a working life of up to 10 years and is the most commonly used power source for Earth-orbiting spacecraft. About 60% of the world’s launched spacecraft use this power source. During the flight of the space probe, the intensity of sunlight will change significantly, affecting the power generation of the solar cell array. This power source cannot be used in situations where there is no sunlight or the light intensity is extremely weak.
The nuclear power sources used in spacecraft include radioisotope thermoelectric generators, nuclear reactor thermoelectric generators and thermionic generators, which all use the energy released by the mutation (fission or decay) of the nucleus to generate electricity. This energy is output in the form of heat, which is converted into electrical energy by a thermoelectric converter. This kind of nuclear power source has a long service life, reliable operation, strong ability to withstand nuclear radiation, strong charged particle field and micrometeor bombardment, etc., and is often used in interplanetary probes and some military satellites. Nuclear power is expensive and unsafe.
In 1978 and 1982, the Soviet Union’s “Cosmos” 954 and “Cosmos” 1402 satellites caused radioactive contamination after they were loaded into the atmosphere. The United States, Russia and other countries continue to develop kilowatts and hundreds of kilowatts of nuclear power sources to meet the growing demand for power consumption. High-efficiency solar cells, concentrating solar cells and nuclear power sources for reactors are under development.
Since April 24, 1970, China has successfully launched the first artificial earth satellite DFH-1 (Dongfanghong 1) with the first CZ-1 (Long March-1) carrier rocket, and successfully launched 6 new types of payloads with the CZ-7 carrier rocket on June 25, 2016; since the test flight of the first short-range missile on June 29, 1964, the first combined flight of the two missiles in October 1966, the launch of a long-range rocket to the Pacific Ocean in May 1980, and the successful launch of an underwater missile in September 1982 to date, China already has a variety of series of ground-to-ground, ground-to-air, shore-to-ship, and air-to-air missiles. All the power supplies on the arrow, on the star and on the bomb are all developed by China. Among them, the power sources on the arrow and the bomb are mainly zinc-silver batteries, and the power sources on the satellite include zinc-silver batteries, solar batteries, cadmium-nickel batteries, etc.
The working state of the power supply system of the earth orbit spacecraft is normal and emergency. A spacecraft with solar cells as the main power source has two states of illumination and shadow in the normal operation of its power supply system. When the spacecraft is transferred from the lighted area to the shadowed area or from the shadowed area to the lighted area, its working state will change, and the working conditions of the equipment in its power supply system and its electrical load will be different before and after the changeover.
The bus voltage is an important parameter of the spacecraft power supply system. There are three typical primary power bus voltages for spacecraft, namely 28V low voltage, 42V medium voltage and 100V high voltage, and there are also spacecraft using 50V, 70V and 160V voltages.
The main factors to be considered in selecting the busbar voltage level are: 1. The power demand level of the spacecraft. The power demand level is usually expressed in terms of the load (characteristic) curve of the spacecraft. ②The quality and loss of the spacecraft power grid. ③ The withstand voltage level of the components used in the system. ④ The uneven charging of the spacecraft surface in the space ion environment causes the risk of electrostatic breakdown and the plasma leakage of the solar cell array. ⑤ Input voltage requirements for the operation of various DC-DC converters in spacecraft loads (especially large spacecraft payloads). ⑥ Influence on the conversion efficiency and reliability of the secondary power supply (DC-DC converter).
Generally speaking, when the electrical power demand of the spacecraft is below 2kW, the 28V busbar voltage is used; when the spacecraft is 3~5kW, the 42V (or 50V, 70V) busbar voltage is used; when it exceeds 6kW, the 100V or slightly higher busbar voltage is used. The power consumption of China Dongfanghong-4 satellite and the future space laboratory reaches 10kW, using 100V bus voltage, and most of the remaining spacecraft use 42V or 28V power supply voltage. The power supply voltage above 100V will also cause many problems in the solar cell array, which must be prevented one by one. For example, during orbital operation, the highly insulating cover sheet on the surface of the solar array is not uniformly charged (sometimes as high as tens of thousands of volts), which can generate electrostatic discharges and excite plasma channels. When the adjacent solar cell strings have a voltage of 100V and the cell strings have a current of more than 1.5A, the arc discharge will continue, generating high heat and burning the polyimide layer on the substrate, make the solar cell circuit and the carbon fiber mesh laid on the surface of the solar cell substrate conduct, short-circuit to the satellite “ground”, and part or all of the power of the solar cell array will be lost. In this regard, corresponding preventive measures should be taken.
Spacecraft of the future are moving towards larger size and long-term work, so the power consumption will further increase, and it is very urgent to develop high-efficiency solar cells, concentrating solar cells, solar thermal power sources, renewable fuel cells and space nuclear power sources of hundreds of kilowatts. At the same time, technologies such as power control, switching, protection and self-recovery after failure should also be developed accordingly. Engineering researchers in the field of space power sources are actively developing new space power sources, among which thermoelectric conversion devices based on thermoelectric cycle theory are one of the areas of interest. It can be matched with solar radiation energy or nuclear energy to form a new type of solar thermal power supply system or nuclear reactor power supply system, and its energy conversion efficiency is high; at the same time, the storage of energy has also changed the traditional form of chemical batteries and turned to the thermal energy storage form of phase change materials (such as lithium fluoride). In short, with the continuous development and application of new technologies, the spacecraft power supply and its power supply will also continue to develop and improve, thereby promoting the development of spacecraft and the entire space technology.