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Freight ships are unmanned ships crafted to aid space station operations by moving rations, fuel, and other items. Tianzhou freight spacecraft (TZ for short) is an automatic Chinese cargo spacecraft built by China Academy of Space Technology as part of the country's manned space station plan. Design work on TZ started in 2010. Its main purposes are to transfer and stock up supplies for the space station, stock and land waste materials for the space station, drop in a controlled manner to a specified location, work jointly with the space station to direct the orbit and attitude of the combined structure, and support the growth of space applications and technical assessments that match the capabilities of cargo spacecraft. In a recent review article published in Space: Science & Technology, Jianyu Lei, from China Academy of Space Technology, elaborated on the design of Tianzhou Cargo Spacecraft, which included subsystem structure, function, and essential technology. The document also demonstrated the development and flight operations of TZ. A systematic understanding of the TZ cargo spacecraft was achieved through comparisons with other cargo vehicles.
Cargo vessels are unmanned crafts designed to assist space station operations by transporting food, fuel, and other supplies. The Tianzhou cargo spaceship (abbreviated as TZ) is an automated Chinese cargo spacecraft created by the China Academy of Space Technology as part of China's manned space station program. The design of TZ began in 2010, and its primary objectives are to transport and store supplies for the space station, store and release waste materials from the space station in a controlled manner, descend to a predetermined location, cooperate with the space station to control the combined body's orbit and attitude, and support the development of space applications and technical tests that are tailored to cargo spacecraft capabilities. Jianyu Lei, from the China Academy of Space Technology, provided a comprehensive description of the Tianzhou Cargo Spacecraft's design in a recent review article published in Space: Science & Technology, covering the subsystem structure, function, and crucial technology. The article also detailed the development and flight missions of TZ and compared it to other cargo vehicles, providing a systematic understanding of the TZ cargo spacecraft.
- Structural bearing and sealing subsystem:
The freight compartment has been crafted as a durable, low-leakage panel structure, designed for long-term use. It is comprised of three parts: the front cone, the column section, and the rear cone. The freight compartment's body is integrated with the cargo shelf structure, which adds only a relatively small amount of weight. The shelf structure is composed of lightweight, high-strength aluminum honeycomb panels, with carbon fiber beam reinforcement used for weak links. In total, there are 40 cargo compartments. The propulsion compartment is a metal cylinder with an outer diameter of 2.8 m and a height of 3.2 m, which is not sealed.
- Cargo Transportation and Support subsystem
All of TZ's designs are focused on its primary task of cargo transportation. To improve adaptability, a standard electromechanical heat interface for goods has been established. For loading cargo, standardized loading methods have been designed, including standard cargo shelves, cargo compartments, and standard cargo packages, along with power, information, and thermal insulation support. To transport low-temperature cargo, two space refrigerators have been created to carry a small amount of specialized cargo with low-temperature storage requirements. In addition, TZ offers reliable, affordable, and most importantly, compatible rideshares for test equipment, enabling small satellites of up to the equivalent of 1U~24U to be tested or put into orbit.
- Orbital Propellant Refueling subsystem
The adoption of orbital propellant refueling technology could result in an extended station lifespan and decreased costs. To this end, TZ and TG employ the high-pressure gas-recycling method, similar to that used by Progress and ISS. In comparison to the high-pressure gas-driven disconnectors of Progress, TZ's stepper motor-driven disconnectors are lighter, require fewer sealing links, and have no impact force during insertion. Furthermore, eight newly-developed 400L metal diaphragm tanks are divided into two functional modules for TZ: propulsion and supplement.
- Rendezvous and Docking subsystem
The successful rendezvous and docking between the cargo spacecraft and the space station is crucial for cargo and propellant replenishment, and it is a key function of the cargo spacecraft. TZ has developed a rapid autonomous navigation and guidance technology solution based on absolute positioning data for the rendezvous and docking process. The period for fully autonomous rapid rendezvous and docking has been reduced to 6.5 hours. TZ has also designed forward, backward, and radial circumnavigation schemes to enable docking with different docking ports of the space station, enhancing mission adaptability. The space docking mechanism of TZ is equipped with three active control dampers that enable the spacecraft to dock with space stations of various configurations with masses ranging from 20 t to 180 t. After docking, TZ can perform TG attitude control, debris avoidance maneuvers, and orbit reboosts for the space station to overcome atmospheric drag effects.
- Attitude and Orbit Control Subsystem
The attitude of TZ is determined using gyroscope, earth sensor, and star sensor as attitude sensors. The gyroscope measures the attitude angular velocity, and the estimated attitude is corrected based on the measured values from the earth sensor and star sensor. The main propulsion system of TZ consists of four engines, each with a thrust of 490 Newtons, and it also has 32 smaller engines (25N, 120N, 150N) for attitude control. Sixteen of these smaller engines are located near the front cone and the tail end of the spacecraft to ensure the required maneuverability. The usage mode of the engines can be configured based on the mission requirements.
- Thermal Management subsystem
TZ is faced with challenges such as uneven distribution of heat sources, significant changes in each flight stage, and a wide range of external heat flow. However, to minimize weight and cost, TZ opted not to have a fluid loop temperature control system. Rather, passive measures such as heat insulation, heat conduction, and radiation are utilized to achieve precise temperature control. To replace the traditional fluid loop thermal control system, a thermal control method that combines forced ventilation and secondary radiation is proposed. For this, three fans are positioned at the rear ball bottom for forced ventilation.
- Manned Environmental Control subsystem
Cargo spacecraft do not require complex environmental control and life support systems like manned spacecraft, but it is still necessary to ensure environmental safety when astronauts enter the cargo cabin. The integration of manned environmental control and space station design simplifies TZ's system configuration, which includes functions such as environmental monitoring, noise control, and COVID-19 detection.
- Energy and Power Supply and Distribution Subsystem
The power supply system of TZ utilizes a solar wing-lithium battery pack, maintaining a stable 100V bus voltage. Energy storage batteries consist of three groups of 60Ah lithium-ion batteries, and a high voltage direct distribution design scheme is employed for the distribution system, allowing for direct transmission of 100V bus voltage to the load end. After docking, the power systems of TZ and TG become a unified network, allowing for grid-connected power supply.
- Information Transmission and Management Subsystem
Telemetry, Tracking, and Command (TT & C) communication mainly relies on space-based systems, such as the data relay satellite system and “BeiDou” navigation satellite system. Some key flight periods are supported by land-based communication stations.
The author initiated the development of TZ to support the construction and operation of China's space stations, transporting supplies to the TG and disposing of wastes from it. The conceptual design of TZ began in 2010 and the preliminary design in late 2011, with the first flight scheduled after the Tiangong II Space Laboratory entered orbit in 2017. On May 29, 2021, TZ-2 carried out the first flight mission of the space station stage, and on September 20, 2021, TZ-3 began its routine duty of cargo transportation. In the space station stage, two TZs will be launched every year. Initially, three configurations were formulated for the TZ design: Fully-Enclosed, Semi-Enclosed, and Fully-Open, to investigate module replacement based on the type of supplies being transported, but only Semi-Enclosed TZ completed the initial development since TG did not have large non-pressurized cargo transport needs. Presently, all the missions are Fully-Enclosed TZ. The detailed timelines of the flight missions for TZ-1, TZ-2, and TZ-3 are also presented.
- TZ-1
TZ-1 launched on April 20, 2017, achieving China's premier in-orbit propellant replenishment, establishing the basis for the space station's assembly, construction, and operation. It also conducted three in-orbit propellant supplement tests. At 17:59 on September 22, a guided meteorite fell in the South Pacific.
- TZ-2
TZ-2 was launched at 20: 55: 29 on May 29, 2021, and docked with the core cabin of TG at 5: 01 on May 30, 2021. This was the first time that the Tianzhou cargo spaceship adopts fully autonomous rapid rendezvous and docking.
- TZ-3
TZ-3 blasted off at 15:10:11 on September 20, 2021 and successfully docked with the space station's core module at 22:07 on the same day. On October 17, astronauts aboard the SZ-13 spacecraft opened the cargo compartment and began to transfer supplies to the space station. Prior to SZ-13's return to Earth, TZ-2, TZ-3, and SZ-13 will remain simultaneously docked with the space station's core module.
Next, the author provided a comparison of cargo vehicles for space stations. With the retirement of the US Space Shuttle and the European ATV completing its five freight missions, the primary cargo transportation for the International Space Station currently relies on the "Progress," "HTV," "Dragon," and "Cygnus" cargo spacecraft. A table was included to compare the key performance indexes, such as length, dry mass, launch mass, payload mass, payload ratio, payload volume, and more.
Lastly, the author examined the future direction of cargo spacecraft. As the only current space cargo transport system of China, TZ's primary mission remains to support China's manned space projects. TZ meets the current task requirements, but improvements to broaden its spectrum and expand its task functions have been placed on the agenda in line with the development trend of cargo spacecraft.
1) The commercial computer information architecture is used to reduce power demand and cancel the expansion of solar wing to develop low-cost cargo spacecrafts [57];
2) Based on the current three cargo cabins, a large capacity cargo cabin and reusable cargo cabin are designed.
3) The propulsion cabin is developed into the service module with different functions such as short propulsion cabin (reducing the number of tanks), high control precision propulsion cabin, and open loading propulsion cabin (loading large antenna, large cabin load, releasing satellite);
4) Simplify the docking mechanism, reduce the type and number of rendezvous and docking sensors, expand the hatch diameter, and prepare for the overall transfer international standard payload rack (ISPR).
5) Providing the potential for a robust, free and repetitive platform for global rideshares, such as small satellite rideshares and scientific test devices.
Reference
Author: Jianyu Lei, Dongyong Jia, Mingsheng Bai, Yong Feng, Xingqian Li
Title of Original Paper: Research and Development of Tianzhou Cargo Spacecraft
Article Link: https://spj.science.org/doi/10.34133/space.0006
Journal: Space: Science & Technology
DOI: 10.34133/space.0006
Affiliations: China Academy of Space Technology, Beijing, China.
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