High Reliability LM-2F Launch Vehicle for Space Station Mission

QIN Tong ,RONG Yi ,HU Yanchen ,LI Wenqing

1 Beijing Institute of Astronautical Systems Engineering,Beijing 100076

2 China Academy of Launch Vehicle Technology,Beijing 100076

Abstract: As China’s only manned launch vehicle,the LM-2F Launch vehicle successfully launched the Shenzhou 12 manned spacecraft into its predetermined orbit on June 12,2021,signifying the successful completion of the first manned launch mission in the construction and key technology test stage of China Space Station (CSS).From the launch of the Shenzhou 11 manned spacecraft on October 12,2016 to the launch of Shenzhou 12 in 2021,over the past five years,the LM-2F launch vehicle has been continuously improved in terms of product reliability through technological innovation and design improvement,and its flight reliability index has increased from 0.97 to 0.98.Based on the new launch mission requirements for the construction stage of CSS,this paper introduces the technological innovation and reliability improvement methods of the LM-2F from the aspects of design improvement,research methods and process optimization.The LM-2F launch vehicle will make greater contributions in supporting the construction of CSS with higher reliability and perfect launch success rate.

Key words: space station mission,high reliability,technological innovation,design improvement

1 INTRODUCTION

On June 17,2021,the LM-2F launch vehicle successfully launched the Shenzhou 12 manned spacecraft carrying NIE Haisheng,LIU Boming and TANG Hongbo into its predetermined orbit,marking the complete success of the first manned launch mission in the construction and key technology verification stage of Chinas Space Station (CSS).On October 16,2021,the LM-2F launch vehicle completed the Shenzhou 13 mission successfully and sent astronauts ZHAI Zhigang,WANG Yaping and YE Guangfu up to the CSS where they have stayed for over 6 months to complete the long term residence mission.

Currently,the LM-2F launch vehicle is the only on-service launch vehicle developed for manned space mission in China.Since the beginning of China’s Manned Space program in 1992,the design work of LM-2F launch vehicle has been conducted with the goal of achieving high reliability and high safety and in accordance with certain reliability design principles.The reliability requirement of LM-2F was increased from 0.91 to 0.97,which realized the transformation of the launch vehicle from a functional design to reliability design.At the same time,in order to further ensure the safety of astronauts,a fault detection and processing system and an escape system were added to the LM-2F launch vehicle,enabling escape and life-saving capabilities to ensure the safety of astronauts in case of a launch vehicle failure from the launch pad to fight.The validation of the escape system was verified through tests from zero altitude escape and rescue and in the simulated flight of the maximum speed head(Q) of the escape spacecraft.The safety requirement of LM-2F meets 0.997,which has reached the world advanced level comparing with foreign manned launch vehicles.

Since 2011,the second phase of the development of LM-2F,the team has kept forging ahead.Through a series of technological innovations,while ensuring the successful completion of major tasks such as rendezvous and docking with the space laboratory (Tiangong),reliability of LM-2F continues to improve.Taking the LM-2F launch vehicle Y12 mission (launch of the Shenzhou 12 manned spacecraft) as an example,compared with the Y11 mission,there were 109 technical state changes,most of which were to improve the reliability of the rocket.Through a series of technological innovations and design improvements,the reliability of the LM-2F launch vehicle has been improved from 0.97 to 0.98,which provides a firm basis for the smooth implementation of the key technology verification and construction stage of the space station (referred to as the space station construction stage).Combined with the new mission requirements of the LM-2F launch vehicle in the space station construction stage,this paper focuses on the technical improvement,research methods and process optimization of the LM-2F launch vehicle to maintain high reliability.

2 IMPROVE THE IGNITION STRATEGY OF THE ESCAPE CONTROL ENGINES

Low altitude escape,especially escape from the launch pad,is the most complex of the four escape procedures of the LM-2F launch vehicle.Although the zero altitude escape test was carried out in the early development process,the impact of low-level winds in different directions on the escape needs to be further analyzed.

The LM-2F launch vehicle development team analyzed the impact of historically measured low altitude wind fields on low altitude escape.It was found that under the action of a small amount of low altitude wind,when escape occurs on the launch pad,the landing point of the return module is less than 500 m away from the launch point,which may cause the return module to fall back into the explosion range of the faulty launch vehicle and threaten the safety of astronauts (see Figure 1).

Figure 1 Sketch of escape control engine ignition direction on the launch pad

In order to further increase the reliability of the escape system and ensure the safety of astronauts,when there is a need to escape from the launch pad under the condition of wind,this paper proposes to change the control engine ignition strategy of the escape spacecraft to improve the adaptability to the low altitude wind field.Changing the original state of ignition with only control engine II to choose control engine II or III ignition according to the low-level wind before launch it can effectively reduce the possibility of the return module falling back into the dangerous area.According to the analysis results,the measures of changing the ignition strategy of the escape engine and making decisions to control the ignition state of the engine before firing are taken to improve the probability that the landing point of the return module is far away from the potentially dangerous area under the action of low altitude wind.

The developers from the spacecraft system,launch vehicle system and launch site system have carried out simulation analysis on the escape under the conditions of controlling the ignition of engine II or III in order to increase the ignition function of escape aircraft.For the five limit conditions of “no dumping,yaw dumping -6°,yaw dumping 6°,pitch dumping 84° and pitch dumping 96°”,they calculated the secondary separation point parameters of controlled firing II and controlled firing III under the standard state respectively.The spacecraft system staff simulated a combination of 210 groups of low altitude wind fields.The results show that this solution can control the return module from falling into the dangerous area.

Taking the worst working conditions as an example,the distribution of spacecraft landing points is shown in Figure 2.The ignition of controlled engine II or controlled engine III cannot completely prevent the return capsule from falling into the dangerous area,but individual cases falling into the dangerous area belong to different wind fields.In the actual emergency situation,the ignition control engine can be selected according to the calculation results of the measured wind field to avoid falling into the dangerous area.

Figure 2 Dispersion of return capsule falling points

3 THERMAL INSULATION AND ENVIRONMENTAL IMPROVEMENT OF ESCAPE TOWER

The escape tower is a unique component of the LM-2F launch vehicle,which is primarily used for astronauts’ escape and life-saving device before dropping the tower during the launch vehicle ascend stage.The escape tower is composed of an escape main engine,a separation engine and escape control engines,see Figure 3.All three types of engines are solid propellant engines.In order to ensure the working performance of solid propellant engines,when the temperature is lower than 0° in winter and higher than 30° in summer,a thermal insulation layer is laid outside the escape main engine and separation engine,and the escape tower will be clad with thermal insulation to ensure that the charging temperature of the engine is within the range of 10° -30° during launch.

Figure 3 Sketch of lattice fins and heat insulator

On February 1,2021,after the launch of the SQX-1 Y2 launch vehicle developed by the Beijing Interstellar Glory Space Technology Company,insulation foam that should have fallen off but fell on the lattice rudder IV,causing the rudder steering to be locked under the action of the air pressure.The foam was again blown off from the lattice rudder IV during the subsequent flight,and control over the lattice rudder was resumed following instructions.The rudder deflection angle was deflected more than 30° in a short time,resulting in the sudden change of rocket attitude,which led to the failure of flight test.

The LM-2F launch vehicle is the only launcher equipped with lattice fins in the LM series of launch vehicles at present.It can only be used in the process of executing the tower escape modes.Therefore,under normal flight conditions,the insulation layer will not fall into the lattice fins,but when the lattice fins are open,there is a small possibility that the insulation layer may fall into the lattice fins.After careful analysis by the development team and coordination with the launch site staff,the launch site can ensure that the ambient temperature of the launch vehicle is kept within the allowable range of the charging temperature of the engine during the closure of the rotary platform in the launch area.At the same time,through relevant tests of the solid engines,it is considered that after the rotary platform is opened,the solid engines are exposed to the outside air,resulting in the change of internal charge temperature of no more than 2°.In addition,if under extreme temperature conditions (the launch temperature is allowed),and the charge temperature of the solid engines can be further controlled within the range of 12°-28° through the thermal insulation on the escape tower.Therefore,through the improvement of the environmental temperature control of the launch area and with the detailed analysis of the development team,it is considered that the launch mission requirements can still be met without the thermal insulation of the escape tower.Thus,the risk of lattice fin failure would be eliminated and the safety of astronauts is improved.

4 ANALYSIS OF THE SEQUENCE LINK DURING LAUNCH VEHICLE FLIGHT

For key products such as control systems,pyrotechnics and engines,as well as essential links such as ignition and separation that affect flight success,considerable effort is spent on carrying out closed inspections of design indicators,focusing on the realization of critical design indicators.The primary working ideas and methods are as follows.

Using the event chain method,according to the physical transmission link realized under the system function,comprehensively review the products involved in the realization of the system function,including stand-alone equipment,cables and connectors,to ensure that there are no omissions in the products.

For the products reviewed,identify the product failure mode analysis as an indicator,sorting out the critical design indicators of the product,including performance,reliability,environmental adaptability,interface and maintainability,to ensure that the key design indicators are identified without omissions.Taking environmental adaptability as an example,attention should be paid to the specific installation location of cables and connectors,whether the cables are effectively protected,and whether the connectors could be abnormally disconnected in a vibration environment.

According to the critical design indicators sorted out,check the actual measurement results of the rocket products one by one to check whether they meet the requirements of the design requirements.

For the 6 normal flight sequence functions (ignition-takeoff,including variable energy accumulator operation,escape tower separation,booster separation,interstage separation,fairing separation,spaceship and vehicle separation),for the 4 escape procedures and the flight sequence function of the spaceship and vehicle emergency separation,a total of 11 critical links are reviewed to check the closure of design indicators.Taking LM-2F’s unique escape program I as an example (see Figure 4),in addition to checking the coordination and matching of timing actions between systems,it also checks and confirms the performance and installation status of important products such as fault detection systems,pyrotechnics,and engines.

Figure 4 Link sketch of escape program I (ignition the control engine III)

Through design correctness of the pre-launch ground workflow and flight sequence of each system of the LM-2F launch vehicle for the Y12 mission,the coordination and matching of timing actions between systems,design status of important products such as control systems,pyrotechnics,and engines,actual indicators and other important factors to carry out the chain of events were all reviewed and confirmed thus ensuring the stability and reliability of the flight actions of the launch vehicle.

5 EMERGENCY RESCUE PROCESS SETTINGS

In order to ensure the safety of astronauts during their work at the space station,if the manned spacecraft docked at the space station fails to return safely,an emergency launch mission of a manned spacecraft is started on the ground.Therefore,while the astronauts are working at the space station,we need to prepare an emergency rescue rocket at the Jiuquan Satellite Launch Center and to be on standby at any time to meet the needs of emergency rescue mission.

The emergency launch process of the manned spacecraft has two options:8.5 days and 16 days.The conditions for the two options are as follows:

1) The space station and its docked loading spacecraft have failed,and the backup systems related to key functions have been activated.After an evaluation and decision,an 8.5-day emergency launch plan will be implemented;

2) The space station and its docked loading spacecraft failed,but all functions were basically normal,hence a 16-day emergency launch plan will be implemented without affecting the safety of astronauts.

5.1 8.5-day Emergency Rescue Launch Plan

The 8.5-day emergency launch plan is shown in Figure 5.It takes 2 days for personnel concerned to gather at the launch site,and the effective working time at the launch site is about 6.5 days.After the launch vehicle has undergone visual inspection and state recovery in the technical areas,it will be transferred to the launch area for testing.The spacecraft cover assembly and escape tower will be docked with the launch vehicle at the launch area,and the follow-up emergency rescue launch mission will be completed.

Figure 5 Main task of 8.5-day emergency rescue

Figure 6 Main progress of LM-2F Y12 /Y13 missions

5.2 16-day Emergency Rescue Launch Plan

The 16-day emergency launch plan is shown in Figure 6.After the personnel are assembled at the launch site,the related work on the manned spacecraft will be completed on the technical areas in the workshop within 11 days.Next,within 3 days,the spaceship and faring assembly and escape tower will be docked with the rocket in the technical area workshop,the assembly of the spaceship,rocket and escape tower will be transferred to the launch area,the function and interface inspection will be finished at the launch area,and the rocket propellant filling and launching will be completed.

During the execution of the Y12 launch mission,in order to prepare for the emergency rescue rocket,the two launch vehicles for Y12 and Y13 missions completed the factory assembly at the same time,and after arriving at the Jiuquan Satellite Launch Center one after another,the product recovery,hoisting,and testing of the emergency rescue duty of the launch vehicle for Y13 mission were carried out first,the basic stage being sealed in an erect state.After that,the formal process of the Y12 launch vehicle was carried out to execute the launch mission of the Shenzhou 12 manned spacecraft.After the successful launch of the Y12 launch vehicle,the Y13 launch vehicle immediately entered the emergency rescue duty state(see Figure 7).The Y13 launch vehicle was released from the emergency rescue duty state and entered the normal launch process after the astronauts of the Shenzhou 12 mission returned safely.

Figure 7 LM-2F Y13 launch vehicle on the duty of emergency rescue

During the verification and construction of the key technologies of the entire CSS,the LM-2F launch vehicle carried out the Shenzhou spacecraft launch missions in accordance with the state of “launching one and preparing one” and “rolling backup”.The preparation for emergency rescue can effectively deal with possible risks at the space station and its docking manned spacecraft,and further improve the safety of astronauts during their missions at the space station.

6 CONCLUSION

Combined with the new mission requirements of the LM-2F launch vehicle during the construction phase of the CSS,this paper focuses on the technological innovation and reliability improvement methods from the aspect of technological improvement,research methods and process optimization since the launch of the Shenzhou 11 manned spacecraft in 2016.

By adding the ignition function of the escape spacecraft to control engine II,the escape safety for the astronauts is further ensured;by canceling the thermal insulation and improving the environmental condition of the escape tower,the risk of lattice fin failure was eliminated and the safety for the astronauts improved;by formulating two emergency rescue launch plans of 8.5 days and 16 days,this can meet the needs of emergency rescue and rescue duty tasks,and further improve the safety for astronauts during their mission at the space station.

The LM-2F launch vehicle development team will continue to maintain the work style of strict,careful and meticulous,continuous improvement,be brave in innovation,never stop striving,and always take high reliability and high safety as the objective to struggle forward so as to provide adequate guarantees for the smooth implementation of the CSS mission.