Reusable launch vehicle refers to a rocket that takes off from the ground and completes the scheduled launch mission, returns in whole or in part and lands safely, and can perform the launch mission again after maintenance and refueling. The concept of reusable launch vehicles is relative to single-use launch vehicles.

The advantage and ultimate goal of reuse is to reduce the waste caused by the abandonment of expensive arrows, engines and electrical equipment after a single use of the vehicle, and to reduce the production and launch costs of the vehicle through multiple use and cost sharing.

Reusing launch vehicles is an important means to reduce the cost of space transportation, and it is also an effective way to improve the rapid response capability of space, which has the possibility of becoming a potential strategic deterrent. In addition, the reusable launch vehicle also represents the highest level in the field of aerospace science and technology, and its technology spillover and industrial upgrading effect will significantly enhance the technical level and innovation ability in related fields, and promote the growth of the national economy. Therefore, the space powers are the development of reusable launch vehicles as the focus of future development.

　　key technology

　　(1) Precise return flight and safe landing control technology

After the separation of the first and second stages of RLV, the first sub-stage carries out the return landing flight, which goes through the process of gradually descending from suborbital altitude to the ground, and the flight environment is complex and there are many random disturbance factors. In this process, the engine works many times, using thrust vector control (TVC), RCS, pneumatic grid rudder surface and other control mechanisms to overcome various internal and external disturbances, carry out attitude, position, deceleration and dynamic control, accurately return and safely land to the designated landing site. In order to achieve the goal of precise return and safe landing, it is necessary to make an accurate mathematical description of the vehicle's own characteristics, flight environment and disturbance, carry out multi-round GNC control algorithm simulation and principle flight test, and realize the optimal matching between the control algorithm and the actuator. The precise return flight and safe landing control technology can be transformed into the flight control of active rocket debris to solve the problem of landing zone safety, or applied to the fixed-point soft landing of spacecraft. NASA's Jet Propulsion Laboratory has developed a fuel-optimized transfer guidance algorithm (G-FOLD) to provide a technical reserve for RLV return landing and lunar/Mars fixed-point soft landing. China successfully implemented the lunar soft landing in the lunar exploration project Chang'e -3 mission, and initially mastered the control method. RLV return landing also involves the optimal trajectory planning problem, mainly studying when the RCS system works and duration, engine working time and thrust size, how to determine the speed and position constraints of each segment start and end point when controlling segments in different flight phases, and the determination of the opening time of the grid rudder and landing buffer mechanism. The purpose of trajectory optimization is to minimize propellant consumption, while attenuating aerodynamic heating effects and ensuring high-precision landing. Under a variety of constraints, the traditional theory and method of trajectory optimal planning will encounter the problem of convergence or convergence to a suboptimal solution. SpaceX GNC Chief Engineer LarsBlackmore and University of Texas Acikmese[30] proposed a lossless convex optimization theory that has the advantages of fast convergence, insensitivity to initial values, and the resulting solution is the global optimal solution. The research results of Masten's Xombie aircraft using lossless convex optimization in vertical return guidance control have won the first place in the "Lunar Lander Challenge Program" Grand Prix initiated by NASA. SpaceX has not disclosed the technical details of a sub-level recovery, and it is speculated that the results of lossless convex optimization may also be used.

　　(2) Variable thrust reusable engine technology

Realizing RLV return landing requires the engine to have multiple starting capabilities; At the same time, during the return process, the residual amount of propellant in the tank is less than 10%, the tank pressure decreases, and the engine ignition starting conditions (inlet pressure and temperature conditions) also deviate from the normal range. The engine must have the ignition starting capability under wide inlet conditions. In the landing section, it is required to reduce the speed to the allowable conditions for landing through engine thrust adjustment. The large-scale variable thrust of the engine needs to be achieved through multiple adjusting components, and the adjustment and control laws are complicated. At the same time, key components such as the injector, regenerative cooling body, and turbine pump must also have the ability to work reliably under corresponding conditions. A lot of research and test work is required. In addition, the traditional liquid rocket engine is a one-time use, without considering the health monitoring and remaining life assessment related to repeated use. The technical requirements of repeated use put forward by vertical take-off and landing need to carry out engine health monitoring and remaining life assessment technology, involving engine data acquisition and processing, fault diagnosis and control, intelligent impairment and life assessment and other related technologies. For example, Blue Source's new Shepard rocket booster stage uses a 50-ton thrust BE-3 liquid hydrogen liquid oxygen engine, which can continuously adjust thrust in the range of 18% ~ 98% and has the ability to be reused many times. The company is developing a 250-ton thrust BE-4 engine with better performance, which uses staged combustion and liquid oxygen methane propellant and has been tested for thermal testing.

　　(3) High reliable landing buffer mechanism technology

In the process of rocket recovery, a highly reliable landing buffer mechanism must be used to reduce the impact overload at the moment of landing, so that the rocket body can land smoothly. The landing buffer mechanism should have better strength and buffer function, as well as the ability to adapt to the tilt attitude and residual speed, so as to ensure the stability of the landing. Since the landing support is close to the rocket engine, it also needs to have the thermal protection ability to withstand the anti-invasion heat flow. The landing buffer mechanism adopts the outrigger soft landing mechanism, which has the advantages of reclosable, reusable, high buffer efficiency, good landing stability and less space occupation. The outrigger detector is widely used in the soft landing detection of exoplanet detectors, and the most critical buffer technology to absorb the landing impact load has been verified in engineering. The landing buffer mechanism used in RLV recovery needs to bear the landing weight of several tons or even more than ten tons of the arrow body, and the technical difficulty is far more than the landing buffer mechanism used by hundreds of kilograms of detectors.

　　(4) Rapid inspection and maintenance technology for return to the field

Recycled RLV can be reused after simple repair and refueling is the key to reduce costs and improve rapid response capabilities. After returning to the field, the state of the arrow body needs to be detected, analyzed and repaired in a short time. However, the traditional method mainly detects the products on the arrow one by one by disassembling. The failure mainly depends on manpower for failure analysis and positioning, and a large number of repeated tests are usually needed to reproduce the problem, which has a great impact on the launch cycle and cost. In order to shorten the launch cycle and reduce costs, RLV return retest needs to be based on the principle of fast, intelligent and efficient, using non-disassembly rapid detection and maintenance technology. By combining the rapid retest of the expert system on the arrow, the health status and life expectancy of the full arrow of the vehicle are automatically evaluated and analyzed, and the components that fail or have a low life expectancy are repaired or replaced in a timely manner, giving the conclusion that the arrow can be launched again in the short term. Arrow on the system in the design of the need to integrate the concept of health management, overall design.