How difficult is it to develop reusable space transportation systems?

Reusable space transportation system refers to a space transportation system that can travel back and forth between the ground and space orbit multiple times, with typical characteristics of "free access to space, on-demand return to the ground, and repeated use". Reuse technology is the forefront direction of space transportation development, an effective way to achieve safe, reliable, fast, free, and low-cost access to space, and has important significance in meeting future space development and reducing launch costs. It is one of the important signs of China's transition from a space power to a space powerhouse.

As early as the 1950s and 1960s, humans began exploring technologies related to the reuse of space transportation systems. After decades of development, major aerospace powers and regions have accumulated varying degrees of reusable technology reserves, forming various typical reusable launch vehicle schemes. At present, countries are adopting a more pragmatic and cautious attitude towards developing reusable space transportation systems, with a focus on two-stage orbital systems and actively exploring single-stage orbital solutions.

There are various classification methods for reusable space transportation systems: according to the level of the system, it can be divided into multi-stage orbital reusable launch vehicles and single-stage orbital reusable launch vehicles; According to the takeoff and landing mode, it can be divided into vertical takeoff and horizontal landing reusable vehicles, vertical takeoff and horizontal landing reusable vehicles, and horizontal takeoff and horizontal landing reusable vehicles; According to the adopted power form, it can be divided into rocket powered reusable carriers and combined powered reusable carriers.

Among them, rocket engine technology has become relatively mature and is currently the preferred propulsion method for engineering applications. Combination power is an important development direction in the future, and key technology research and experimental verification are being carried out around the combination cycle engine technology. The first stage of the rocket powered two-stage orbital reusable space transportation system is currently the focus of attention for reusable technology both domestically and internationally.

For the first stage of a rocket powered two-stage reusable space transportation system, research work is mainly carried out domestically and internationally through two technical approaches: disposable rocket configuration and lift type configuration.

Reusable launch vehicles based on disposable rocket configurations have attracted much attention abroad and have been continuously developed, mainly including vertical return and parachute descent recovery types. A typical case of vertical return is SpaceX's Falcon 9 carrier rocket. As of now, the Falcon 9 rocket has completed more than 80 land and sea vertical landing and recovery missions, verifying key technologies related to vertical takeoff and landing, and has been applied in the commercial launch market.

On July 20th of this year, Amazon founder Jeff Bezos entered "space" aboard his Blue Origin company's "New Shepard" rocket and spacecraft. The rocket completed a vertical landing and the spacecraft completed an parachute landing.

For our country, traditional carrier rockets can solve the problem of airspace safety through precise recovery technology; In the future, new carrier rockets can use vertical takeoff and landing technology to achieve the recovery and reuse of sub stages/boosters.

The rocket powered reusable launch vehicle with lift configuration can be divided into suborbital stage and orbital stage. The US space shuttle orbiter is a typical orbital level reusable carrier. In the 21st century, the United States proposed the XS-1 program, a suborbital transport vehicle technology validation aircraft with rocket power, autonomous vertical takeoff, and horizontal landing, focusing on verifying low-cost, fast turnaround, and fast launch technologies. The XS-1 project demonstrates obvious inheritance, compatibility, and complementarity. The suspension of the project in 2020 combines the urgency of the situation and the complexity of the technology, and does not affect the development and low-cost launch of winged reusable aircraft in the United States.

On July 11th, the "SpaceShip-2" suborbital spacecraft developed by Virgin Galactic in the UK was launched at high altitude by the mother aircraft "White Knight II", and then the rocket engine was started to fly to an altitude of about 86 kilometers above the ground. After re-entry and return, it landed horizontally.

The flight demonstration and verification project of China's suborbital reusable carrier was ignited and launched on time at the Jiuquan Satellite Launch Center on July 16th, and landed smoothly and horizontally at the Alxa Right Banner Airport. The maiden flight was a complete success. By prioritizing the development of suborbital reusable launch vehicles, a reusable space infrastructure platform can be constructed in advance.

The reuse of carrier rockets mainly involves technical difficulties such as the design of parachute descent recovery systems and high-precision control of vertical return; The suborbital reusable launch vehicle mainly involves technical difficulties such as aerodynamic heating, flight control, reusable structure, and reusable evaluation.

This type of reusable launch vehicle has minimal impact on the overall design of existing configuration launch vehicles. In terms of parachute descent recovery, the sub stage return section uses parachutes to decelerate and ultimately achieve recovery, with the highest technological maturity and minimal loss of carrying capacity. In terms of vertical recovery, the sub stage return section uses the main engine to restart and reverse thrust deceleration, and implements precise landing recovery using high-precision control methods, which has certain technical difficulties; Meanwhile, in order to meet the operational requirements of the vertical return engine, it is necessary to reserve propellant, which has a significant impact on the carrying capacity.

(1) Umbrella descent recycling technology for large group umbrellas and large cushion airbags. The main umbrella system consists of multiple main umbrellas, each with a nominal area exceeding thousands of square meters. The group umbrella system exhibits asynchronous opening of umbrellas, resulting in significant unevenness in the distribution of umbrella load. On land landing, buffer airbags can be used for landing buffering. After each airbag is filled, its volume can reach several tens of cubic meters, which determines the landing buffering performance of a sub level, the total mass and volume of the recovery system. At the same time, there are also issues with the material strength of the airbags themselves.

(2) Vertical return high-precision control technology. The vertical return high-precision control technology ensures that the rocket sub stage flies back to the predetermined landing site in a stable posture according to the predetermined trajectory, involving multiple specialties such as return mission planning, trajectory, attitude control, and navigation guidance. It has the characteristics of strict boundary constraints and high landing accuracy requirements.

(3) Vertical return engine wide range thrust adjustment technology. During the vertical return process of a rocket sub stage, due to the depletion of its propellant and the low weight of the sub stage, the engine needs to have a wide range of thrust adjustment capabilities; Compared to existing fixed thrust or small-scale variable thrust engines, there are more regulating components, the working range of key components is wide, and the regulation and control laws are complex.

For rocket powered reusable launch vehicles with lift configurations, the recent development focus is on a two-stage orbital approach, where the suborbital reusable launch vehicle serves as the first stage and is combined with the second stage of a disposable launch vehicle to form a fast and low-cost space entry capability. Compared with single-stage orbit insertion, the two-stage orbit insertion method can discard useless weight after the first stage work is completed, and the carrying efficiency is significantly improved.

The suborbital reusable carrier combines the technical characteristics of both aircraft and spacecraft, using liquid oxygen hydrocarbon rocket engines as the main power source, adopting a lift type configuration, vertical takeoff and horizontal landing, and can achieve accurate landing and complete reuse through automatic approach landing. Compared with space shuttle orbiters or other orbital stage vehicles, suborbital reusable launch vehicles have significantly reduced re-entry and return speeds, greatly improved thermal environments during re-entry and return, significantly reduced main engine operating time, and are more likely to achieve low-cost goals. However, there are also many technical difficulties.

(1) Aerodynamic thermal technology for round-trip between heaven and earth: Vehicles fly in large airspace and wide speed ranges, with complex trajectories. The fuselage layout usually adopts wing body fusion or wing body combination, and the control surface layout needs to consider multiple methods. For such vehicles, the requirement to increase the lift to drag ratio of the aircraft is often contradictory to the requirement to reduce the heat flux density. At the same time, the carrier also needs to balance high-speed re-entry and return with low-speed approach and landing, stability/maneuverability, and has high requirements for heat protection and control.

(2) Aerospace round-trip flight control technology: Due to the lift type configuration of the carrier, there is a strong coupling relationship between the pitch channel, roll channel, and yaw channel, which poses multiple constraints such as complex mechanical environment and uncertain control throughout the flight. In the ascending phase, wind interference is significant and there is a large amount of takeoff drift. During the return phase, the coupling between yaw and roll of the carrier is severe. Therefore, how to achieve full process stable control is a major technical challenge faced by the carrier.

(3) Lightweight and high-strength structural technology: To further improve the carrying capacity, reducing the structural coefficient is one of the challenges that need to be solved for repeated use and efficiency improvement. Therefore, it is necessary to conduct research on lightweight and high-strength structural technology to significantly reduce the structural coefficient of the carrier. This can be achieved from two aspects: optimizing the storage tank structure and optimizing the main load-bearing structure, and composite materials can be used.

(4) Reuse evaluation technology: In order to achieve low-cost and high reliability reuse space transportation, research institutions need to establish design criteria and standards, evaluation systems applicable to reusable carriers, and have the ability to accurately determine whether the carrier has the ability to reliably complete the flight mission again during two flights. For the repeated use of thermal protection materials and lightweight structures, it is necessary to effectively detect materials in different parts and structures through non-destructive testing and other methods to evaluate the reliability level of structural materials for the next flight. For the repeated use of engines, it is important to clarify the methods for rapid detection, evaluation, and maintenance.

From the perspective of development trends, the development of reusable space transportation systems will start with rocket powered suborbital reusable launch vehicles, following the development law from partial reuse to complete reuse, from rocket power to combined power, and from multi-stage orbit to single-stage orbit, with step-by-step verification and step-by-step formation of capabilities.

The author believes that the successful completion of the verification test of suborbital reusable launch vehicle technology has laid a solid technical foundation, reduced the risk of subsequent lift rocket powered reusable flight tests, minimized the technological leap in the development of large-scale reusable launch vehicles, and laid a technical foundation for the development of reusable space transportation systems.

Firstly, the development of reusable technologies represented by suborbital reusable launch vehicles can significantly reduce the cost of entering space. Currently, space transportation is shifting from "solving how to enter space" to "solving how to enter space at low cost". Reusing technology is an effective way to solve this problem, with enormous potential to reduce costs by an order of magnitude and effectively enhance China's international competitiveness in space launches.

Secondly, developing reusable technologies can fundamentally solve aviation safety issues and promote green aerospace. The safety of the landing area of carrier rockets has become a widely concerned issue in the international community. Currently, the scattered area of debris from land launch sub stages in China is 2100 square kilometers, with a population of nearly 300000. The surrounding sea environment of the Hainan launch landing area is complex. The suborbital reusable carrier can effectively eliminate safety hazards in space launches by achieving return to the original launch site and horizontal landing recovery.

Thirdly, the development of reusable technology can achieve rapid and high-frequency access to space. Ensuring safe, reliable, fast, and low-cost access to space is a prerequisite for large-scale development and utilization of space resources. The suborbital reusable launch vehicle achieves low-cost and rapid turnover of launch and exit space, effectively supporting high-density launch activities such as large-scale launches and emergency launches in the future.

Fourthly, developing reusable technologies can lead scientific and technological innovation, promote independent and controllable aerospace technology, and drive the transformation of the aerospace industry and the construction of the national economy. The significant leap from disposable to reusable space transportation will effectively drive basic disciplines such as advanced aerospace power, high-temperature resistant lightweight materials, advanced manufacturing and testing to enter the world's advanced level, promote independent and controllable space technology, realize the transformation of China's space transportation technology from following to leading, and drive the overall improvement of China's scientific and technological innovation capabilities.

Fifth, the development of reusable technologies will greatly promote revolutionary changes in the space industry. The suborbital reusable carrier can be applied in the future for high-speed passenger transportation, fast cargo transportation, space tourism, etc. It can also be used as a basis to build comfortable spaceports, achieve space vacations, and carry out product processing, biopharmaceuticals, etc. in weightless, vacuum, and sterile environments, greatly changing human production and lifestyle.

Source: China Aerospace News