Prospects for the Future Development of China’s Space Transportation System

At present, the world has developed to a new stage of large-scale access to space, which has put forward higher requirements for the development of space transportation. Facing the historical mission of building a space power, focusing on the construction of the China’s space transportation system, this paper studied the development situation of the space transportation system in the world, summarized the development status of the China’s space transportation, combined with future development requirements, and put forward prospects for the future development of the China’s space transportation system.


Introduction
As the support and foundation for a country to carry out space activities, space transportation system is an important symbol of its comprehensive national power [1]. Currently, the world spacefield has entered a new phase represented by large-scale internet constellation construction, space resources exploitation, manned lunar exploration and large-scale deep space exploration [2] . Moreover, the demands for access to space are growing rapidly, which put forward higher requirements on space transportation systems. At the same time, with the development of new technologies such as artificial intelligence, recovery, new power and materials, space transportation systems are also in a period with great opportunity for technological development.
Standing at a new historical point and facing the historical mission of building a space power, this paper presents the future development prospects of China's space transportation system(including expendable and recoverable launch vehicles, orbital transfer spacecraft etc.) from the analysis of the development trend of space transportation systems in the world and development requirements of China.

The development achievements of China's space transportation system
China's space transportation system has made significant achievements that attract worldwide attention, realizing the leap from normal propulsion to cryogenic propulsion, from tandem to strap-on launch vehicle, from launching a single satellite to multiple satellites, from launching satellites to manned spacecraft, from near-Earth orbit missions to deep space exploration, which has strongly promoted the development of national economy, science, technology and national defense construction. In recent years, under the guidance of the goal of building a space power, China's space transportation system has made new achievements in products, technologies and management.
In terms of products, a relatively complete product system of space transportation has been built. In the aspect of expendable launch vehicles, new generation launch vehicles such as LM-5, LM-6, and LM-7 have started to be applied for launch service, supporting the implementation of major projects and have placed China among top space powers. In the aspect of operationally responsiveness space, 3 LM-11 has achieved a breakthrough in the Long March series of solid launch vehicles, with operationally responsiveness launch capability and launch preparation time less than 24 hours. In addition, it achieved the China's first sea launch, providing a new launch mode for China's entry into space. In the aspect of space transportation, two generations of orbital transfer transportation systems have been developed, which realized the leap from solid to liquid for the upper stage, and the "Yuan Zheng (YZ)" series upper stage family has been formed, which played an important role in the global network construction of the BeiDou project. At present, China is continuing to carry out technical research on the new generation of manned launch vehicles, heavy launch vehicles and cryogenic upper stages. technologies [3]- [7] In terms of management improvement, substantial progress has been made in industrialization and commercialization. The launch vehicle batch-production project has been implemented, with products organized in batches for procurement, production and acceptance, and multiple rounds delivered for tests and launches on a  [8] .

The development tend of foreign space transportation system
Recent years, with the increase in demand for large-scale access to space missions, the world's major space nations are continuing to develop reliable, low-cost and efficient space transportation systems, and the capabilities and connotations of space transportation systems are changing.
Focusing on long-term development objectives, the world's major space nations are carrying out the development of next-generation launch vehicles, such as the Vulcan and New Glenn launch vehicles in the United States, the Ariane 6 launch vehicle in Europe, the Soyuz 5 launch vehicle in Russia, the H-3 launch vehicle in Japan, etc. They all pay attention to the optimization of the launch vehicle spectrum, while to achieving more prominent task adaptability and cost-effectiveness.
At the same time, heavy launch vehicles are developed for future manned lunar exploration, manned Mars exploration and other significant space missions. The United States released the "Artemis" plan, which planned to return to the Moon with astronauts in 2024 and had long-term stationing capability after 2028. It also invested in the construction of the lunar orbiting space station "Deep Space Gate" and announced the implementation of manned Mars exploration in the 2030s [9]- [13] . The Russian National Space Group successively proposed the heavy launch vehicle program "Yenisei", which is scheduled to make its first flight in 2028 with a LEO capacity of 70 tons or more. SpaceX company is also progressing the development of the "Super Heavy-Starship", which can reach more than 100 tons of LEO capacity.

Reusable technology has become an important technical development direction
for space transportation systems. The vertical takeoff and vertical landing (VTVL) recovery technology of SpaceX's Falcon-9 launch vehicle becomes more practiced , and has achieved first stage recycling dozens of times, single module has succeed in reusing up to nine times. ESA plans to adopt reusable technology on the next-generation Ariane launch vehicle, and continues to develop the "Prometheus" engine and the "Semis" VTVL prototype, while the U.K. continues to work on the technology of the combination power "Sabre" engine and "Skylon" space shuttle.
Japan and India plan to carry out experimental verifications of reusable technologies too [3]- [5] .
With regards to upper stage, ACES upper stage is developing for the Vulcan launch vehicle of the United States by applying new technologies such as advanced cryogenic propellant evaporation control and Integrated Vehicle Fluids(IVF) to extend its in-orbit service time to several weeks. With the in-orbit refueling technology, it can significantly expand the existing design concepts and application modes of space transportation systems [14]- [17] . In addition, the MiTEx upper stage with normal propellants, developed in the United States, is powered by solar cells and has the ability to work in orbit for long time [18] [19] .
The rapid progress of artificial intelligence technologies has brought new momentum to the development of the aerospace field and has become an important research direction. It leads continuous breakthroughs in technologies such as full life-cycle digital management, virtual design based on digital prototypes, rapid production and manufacturing, intelligent flight and autonomous return control [6] . incorporate new elements, and realize the important transformation of China space transportation system from "perfection" to "strength", covers from "expendable" to "recovery", from "automation" to "artificial intelligence" , from "task-driven" to "design demand-based" transformation.

2) Continue to advance the technical research and test verification of the reusable space transportation system, accelerate the process of engineering application, and realize the flight transportation
Reusable launch vehicle is an important development direction of space transportation technology, and a quickly, economically effective way to enter space.
At present, China is actively advancing the deepening demonstration, key technical research and flight test missions of the reusable of space transportation systems. The engines in recovery launch vehicle should be reused after simple treatments and the structures of recovery launch vehicle is designed to consider more factor such as fatigue.
The first is to realize the recovery of the first stage of the launch vehicle by 2025.
The key technologies for horizontal landing will be initially verified in 2021, and horizontal and vertical landing technologies will be further verified by 2025, which will initially form the ability to quickly and economically enter space.
The second is to achieve complete reuse of launch vehicles by 2035. The number of repeated use reaches 10-50 times, forming the basic ability to quickly respond to entering the space and returning on demand.
The third is to establish a full flight-based space transportation capability by 2045. The repeated use of space transportation system technology is continuously upgraded, new technologies such as intelligent configuration, new materials, and combined power are widely used, and single-stage orbit technology has achieved breakthroughs.

3) Vigorously develop high-performance space transportation technology, achieve a substantial increase in China space transfer and transportation capacity, and gain the initiative in the future competition of large-scale space resource development and utilization
The development and utilization of large-scale space resources (such as SSPS) has become a reality. China will vigorously develop efficient and environmentally friendly cryogenic space transportation systems and new power space transportation systems represented by high-power electric propulsion and nuclear power, and gradually increase large-scale and multiple orbital changes. The ability and long-term on-orbit ability expand the application fields of high-performance space transportation systems; promote the research and engineering practice of upper-level orbital applications to realize the redevelopment of the ability to enter space. The high-performance space transportation system adopts design ideas with modularization, integration, integrated and intelligence.
The first is to have long-term on-orbit, multi-ton round-trip transportation capacity by 2025. Key technologies such as cryogenic propellant evaporation control in orbit achieve breakthroughs, and the long-term in-orbit flight test verification (more than a week) of the cryogenic space transportation system will be completed.
The second is to have a larger-scale high-and low-orbit reusable transportation and space-based launch capabilities by 2035. Key technologies such as low-temperature propellant on-orbit refueling achieve breakthroughs, and on-orbit refueling, maintenance, and modular replacement will be realized.
The third is to form a series of space transportation system products by 2045.
Small nuclear-powered engine technology for nuclear thermal power engine with high-reliability and high-safety achieves breakthrough, and the on-orbit flight verification of nuclear propulsion and other new power space transportation systems will be completed. The second is to achieve the "preparation-ignition-flight-into orbit" all-up smart design with information fusion and artificial intelligence to satisfy the requirements of more reliability and safety for the next generation smart launch vehicle.

Build a world-class level
Comprehensively build an intelligent aerospace transportation system ecosystem including development and verification, production and manufacturing, testing and launch, flight and evaluation. The power system is an important support for the space transportation system, and its capability level has an important impact on the overall performance of the aerospace transportation system. At present, China aerospace propulsion technology capabilities and levels need to be continuously improved.

3) Continuously improve China aerospace propulsion technology capabilities
The first is to improve the performance of in-service engines by 2025. In view of China's current rocket engines, continuous improvements and enhancements have been implemented in key performance indicators such as specific impulse, thrust-to-weight ratio and thrust adjustment capability. Moreover, it is also required to solve the key problems such as engine multi-ignition and the low-starting pressure on turbine inlet for engine restart. These will strongly support the overall performance of China new generation of launch vehicles constantly.
The second is to apply new high-thrust (round 500 t) and reusable engines by With the increasing demand of China's capability into space, higher requirements are put forward for the development and production capacity of space transportation system. Adopting large-scale production and advanced production technology, reducing cost, improving efficiency and quality is the future development trend.
The first is to improve the construction of intelligent and digital general assembly and testing production line of launch vehicle, adopt test calibration, intelligent monitoring and other equipment, and form the technical capabilities of electronic general assembly documents, operation visualization, inspection digitization, etc., so as to realize accurate and rapid assembly and testing, and improve production efficiency and production quality.
The second is to establish and improve the production management system of launch vehicle. It will adopt the large-scale production mode, realize the mass production of launch vehicle, reduce the cost and improve the product quality. The pulsating production line will be established to meet the high-density demand for the launch vehicle production. Secondly, low-cost commercial launch vehicles represented by the SD (smart dragon) series, LM-11 series and LM-8 need to be developed. To provide high-quality services for commercial payloads, high-quality products with "high cost-effectiveness, high reliability, fast performance, and fast launch" are required. Continuously we need to explore new test-launch management mode, implement new business operation mode, improve comprehensive competitiveness, and meet the needs of the public.
3)Fully implement digital transformation, achieve collaborative and efficient development, standardized and precise management, data sharing and scientific decision-making intelligence, then promote high-quality, high-efficiency and

high-benefit development
To accelerate the construction of a world-class level space transportation system, it is necessary to seize the great opportunities of digital transformation, and use digital transformation as an important means to upgrade traditional energy and cultivate new development energy. It is also required to comprehensively promote the construction of digital aerospace, and enhance core competitiveness and value creativity, then to promote high-quality, high-efficiency and high-benefit development.
First of all, model and data-driven digital system of scientific research and production should be constructed to improve the level of launch vehicles development.
The system engineering method based on model is adopted to improve the digital collaborative development environment of aerospace model products, strengthen the applications of virtual design and simulation verification, and improve the design efficiency.
Secondly, a digital management system with big experimental data applications as the core needs to be built to improve the level of strategic management and control.
Based on advanced technologies such as cloud computing, big data, and artificial intelligence, a unified management and control platform is built to achieve centralized management and control the factors of human, financial, and material. It will comprehensively improve the level of strategic management and control by deepening the application of big data intelligent analysis, carrying out quantitative analysis, predicting and early warning of management business based on big data.`

Summary
Standing at a new historical node, facing the new requirements, opportunities and challenges, we must have the courage to assume, seize opportunities, continuous to innovate and make breakthroughs. Based on China's national conditions and the strategic development needs, China aerospace plans to carry out the launch vehicle spectrum optimization, performance improvement, commercial development and digital transformation, by adopting technical methods such as recovery and artificial intelligence, adding new functions and integrating new elements, to achieve the cross development of China's space transportation system, build a world-class level space transportation system, and support the space power construction.

Conflicts of Interest:
The author declares that there are no conflicts of interest regarding the publication of this article.