Review and Prospect of LM-3A Series Launch Vehicle As Space Express for BDS

ZHANG Yipu,ZHOU Tianshuai,LIU Lidong,LI Dan,HU Wei

Beijing Institute of Astronautical Systems Engineering,Beijing 100076

Abstract:The LM-3A series launch vehicle was used for all launch missions for the BeiDou Navigation Satellite System (BDS) project,including BDS-1,BDS-2,and BDS-3.So it is known as Space Express for BDS.During the 26 years' development period for the BDS project,a series of key technological breakthroughs with the LM-3A series of launch vehicles were made,improving the launch capability of different payloads into GTO,IGTO and MTO,from sending one satellites into transfer orbit to sending two satellites into transfer orbit,to sending two satellites into target orbit directly.A total of 59 satellites in 44 launches were launched using the LM-3A series launch vehicle for the BDS project,achieving 100% success.

Key words:LM-3A,launch vehicle,BeiDou Navigation Satellite System (BDS)

1 INTRODUCTION

On June 23,2020,the 55th global navigation satellite of the BDS project,which was also the last satellite of the BDS-3 system,was successfully launched by the LM-3B Y68 carrier rocket from the Xichang Satellite Launch Center,marking the completion of the deployment of China's global navigation system.

According to the three-step development strategy,China's navigation satellite system construction involved a three-phase development,namely BDS-1,BDS-2,and BDS-3.The 44 launches via the LM-3A series launch vehicle for the BDS project,including 4 launches for the BDS-1 system,18 for the BDS-2 system,and 22 for the BDS-3 system,were completed with a 100% success rate.

LM-3A series has three basic configurations (see Figure 1).LM-3B and LM-3C were developed on the basis of the LM-3A,with 4 and 2 strap-on boosters respectively.There are also some derivative configurations[1-3]for LM-3B and LM-3C.Seven configurations of LM-3A series,namely LM-3A,LM-3B/G1,LM-3B/G3,LM-3B/YZ-1,LM-3C,LM-3C/G2,and LM-3C/YZ-1,were used to carry out all flights for the BDS project.

This paper gives a brief introduction of LM-3A series launch vehicle,summarizes technological developments in the past 26 years for the BDS project,and outlines prospects for its future technological development.

2 BRIEF INTRODUCTION OF LM-3A SERIES LAUNCH VEHICLE

Figure 1 Configurations of LM-3A launch vehicle series

The LM-3A series of rockets are three-stage liquid-propellant medium-lift rockets,and are mainly used for MEO,GTO,and deep space missions.The third stage engine with a twicestart capability was developed for the LM-3A series launch vehicle.

According to an overall theory of“strapping boosters after changing on third stages,insistance on seriation,combination,generalization,and develop with coordination”,the rocket evolved gradually.The first step of seriation was developing a hydrogen-oxygen third stage for the LM-3A launch vehicle based on the LM-2C launch vehicle.Then LM-3B and LM-3C were developed based on LM-3A core stage with 4 and 2 strap-on boosters respectively.

China's new generation of communications satellite system project was envisioned in February 1986.The LM-3A rocket development was initiated officially as part of this project.Based on the inherited mature technologies from LM-3 rocket,there were solid technological breakthroughs on more than one hundred projects,represented under four key areas namely,highthrust hydrogen/oxygen engine,dynamically tuned gyro four-axis inertial platform,cryogenic helium heating pressurization and the hydrogen power double pendulum servo mechanism,enabling China's launch vehicle technology developing to a higher level.The carrying capacity of China's launch vehicle for GTO mission has reached 2600 kg.The maiden flight of the LM-3A achieved complete success,sending the SJ-4 satellite and a dummy satellite into orbit.The main launch missions LM-3A has been used for include DFH-3 platform communications satellites,BDS-1 satellites,BDS-2 IGSO satellites,Chang'e 1 orbiter,FY-2 meteorological satellite,and others.China's first lunar orbiter Chang'e 1 was successfully launched on October 10,2007.There were 12 BeiDou satellites launched by the LM-3A carrier rockets.All 27 launches of LM-3A were completed successfully.

The LM-3B launch vehicle was developed on the basis of the LM-3A,with modifications to the third stage plus adding 4 strap-on boosters.After years of improvement,5 configurations,from LM-3B/G1 to LM-3B/G5,have been derived.Although the maiden flight for the International 708 satellite mission failed on February 15,1996,LM-3B resumed launches after overall quality rectifications based on thorough fault analysis.The LM-3B launch vehicles have successfully completed many subsequent international commercial satellite launch missions including Mabuhay,Apstar-2R,ChinaSat-1,SinoSat-1 and Apstar 6,winning a position in international space commercial launch market for China.LM-3B and its improved configurations are the main force in the LM-3A series,mainly used to launch GEO communications satellites,BDS-2 MEO satellites,BDS-3 satellites,FY-4 meteorological satellites,etc.The LM-3B launch vehicle was used to launch Chang'e 3 lunar explorer into space on December 2,2013,thus laying a solid foundation for soft landing,exploration,and surface survey on the lunar surface.LM-3B was also used to launch the Chang'e 4 lunar probe into space on December 2,2018,which became the first ever probe making a soft-landing on the far side of the moon.As of December 2020,LM-3B had completed 72 successful launches.

In support of China's first generation of relay satellite project,the LM-3C launch vehicle was developed.The maiden flight launching Tianlian 1-01 satellite achieved complete success on April 25,2008,signifying China had made a breakthrough in non-axisymmetric rocket design.The distribution of launch capability for high orbit got more reasonable.The goals of actual seriation and combination for LM-3A series were accomplished.The carrying capacity of LM-3C launch vehicle to GTO reached 3900 kg.As of December 2020,LM-3C had completed all 17 launch missions successfully.The range of configurations of the LM-3A series in service is shown in Figure 2.The carrying capacity and number of launches for each configuration are shown in Table 1.

Figure 2 Configurations of the LM-3A series of launch vehicles

Table 1 Launch statistics and launch capability of LM-3A series

The lift-off mass of LM-3A,LM-3B,LM-3C are 243 t,456 t,and 366 t,respectively.The overall length of these configurations are 52.5 m,56.5 m,and 56.3 m,respectively.The diameter of the booster is 2.25 m.The diameters of the first and the second stages are both 3.35 m,while the third stage's diameter is 3 m.The column section diameters of payload fairing are 3.35 m,4.2 m,and 4 m,respectively.The engines of boosters,first and second stages,use N2O4/UDMH as propellant.The engine of the third stage uses cryogenic LOX/LH2 as propellant.

3 TECHNOLOGY DEVELOPMENTS FOR BDS-1 PROJECT

In 1994,after full appraisal the BDS-1 project,an active positioning experimental system with two satellites,was initiated.Based on the DFH-3 platform,the BDS-1 satellite with a mass of 2300 kg needed to be launched into GTO.LM-3 and LM-3A were capable of carrying out the high orbit launch missions at that time in China.Since the carrying capacity to GTO of the LM-3 launch vehicle was only 1400 kg,LM-3A became the only choice.

After its maiden flight in 1994,the LM-3A launch vehicle completed three launch missions with DFH-3 platform communications satellites.The new journey of launching BeiDou satellites started from LM-3A Y5 rocket.

Four BDS-1 satellites were launched by the LM-3A Y5 rocket and Y6 rocket in 2000,LM-3A Y7 rocket in 2003,and LM-3A Y12 rocket in 2007.The LM-3A launch vehicle took the first step in redundant design control system by applying redundant rate gyroscopes.The telemetry system increased its transmission rate to 2 Mbit/s in S-band.The capacity of the data transmission was three times as much as before,significantly improving the ability of data acquisition.The laser strapdown inertial measurement unit (LSIMU) was adopted from the LM-3A Y7 rocket to conduct flight verification for the subsequent redundancy scheme of combination inertial platform plus LSIMU.The operation at the launch site was substantially optimized by canceling the subsystem test and overall test in the processing facility,as the result of adding the complete rocket final test before delivery to launch site.By the time the Y12 rocket launched the 4th satellite of BDS-1,the LM-3A launch vehicle had grown to be a highly reliable cryogenic propulsion rocket for launching high orbit satellites,with a system level redundant control configuration,a 2 Mbit/s transmission rate TT&C system in S-band,a reduced preparation time of 30 days at launch site,and strong data acquisition capability thus laying a solid foundation for the LM-3A series launch vehicle becoming the Space Express for the BDS project in the future.

4 TECHNOLOGY DEVELOPMENTS DURING BDS-2 PROJECT

BDS-2 is a regional navigation satellite system with 16 satellites operating in GEO,IGSO,and MEO,which needed to be launched into three different transfer orbits over 14 flights.Starting with the experimental satellite of BDS-2 launched in April 2017,the LM-3A family then began to conduct launch missions for the BDS-2 project.

4.1 LM-3A Launch Vehicle

The LM-3A Y13 rocket achieved 8 ‘firsts' when launching the experiment satellite of BDS-2,namely the first time for launching with a remote control model,the first time using No.3 launch pad at XSLC,the first time applying lift-off rolling technology,the first time pre-setting all the year's trajectories for selection,the first time using a bidirectional wind compensation algorithm,the first time using a once start-up mode for the third stage engine,the first time using a southeast flight direction,and the first time using a new debris impact zone.

Above all,it was the first time designing a trajectory towards the southeast for the LM-3A launch vehicle.New trajectories for the flight of the carrier rocket and impact zones for each separated stage were redrawn.The previous impact zones on land was reduced.The impact zones for the faring and second stage were set out at sea to mitigate the difficulty of guaranteeing a safe land falling area.The new trajectory in the direction of the southeast satisfied the engineering requirements for carrying capacity and deploying satellites into IGSO/MEO.Combined with the technology of an east flight direction for GTO,the rocket included breakthroughs in satellite launch deployment for constellations in multi-orbit planes,thus laying a solid foundation for the successful networking of BDS-2.

Secondly,it was the first breakthrough for using a bidirectional wind compensation algorithm.By introducing three different typical wind fields,the attack angle and sideslip angle caused by inlight wind were reduced by optimizing the pitch and yaw angles in the first stage flight segment,thus effectively reducing the aerodynamic loading.The technology of bidirectional wind compensation not only assured the safety and reliability for the rocket flights,but also increased the probability of launching on time.

Thirdly,the LM-3A series launch vehicle took the lead in adopting the technology of rolling on ascent to a specific direction.A fixed aiming direction on the ground with rolling on ascent solved the problem of aiming direction when not in line with the trajectory,thus avoiding the reconstruction of the ground aiming site and made full use of the available resources provided at the launch site.A rational and reliable design for the roll program angle,enabled the LM-3A series launch vehicle to have the capability of heading in different flight directions,and met the requirement for BDS-2 project constellation missions.

The rolling technology made it possible to continue using the existing optical sighting method and process.The onboard inertial platform lined up with the available sight azimuth angle,then a gyro torque was applied to the inertial platform's roll channel,which allowed the platform to align to the actual flight azimuth.The rocket's first and third quadrant axis was adjusted to the expected plane by rolling the rocket once it lifted off from the launch pad.The mature design approach and results were fully demonstrated as the state of the rocket after rolling and orientation was consistent with the expected state,confirming the reliability of launch and flight.

The last new technology,the single-burn operation mode,was used for the first time for the third stage engine.The air range to target orbit was reduced from 8000 km to 4000 km.Previously using the mode of two start-ups,the satellite would be located over central Australia when it separated from the rocket,which needed the deployment of space tracking ships near Australia's territorial water or the need to rent foreign measurement and control stations,both of which were complicated and difficult TT&C modes.Through a single-burn scheme,only one TT&C ship met the requirement of the TT&C task for the injection segment of the flight,which greatly simplified the TT&C plan and improved the feasibility and economy.It laid a foundation for the smooth implementation of the project.

LM-3A launch vehicle launched an experimental satellite to MEO and several satellites to IGSO.

4.2 LM-3B Launch Vehicle

The weight of a BDS-2 satellite was more than 2 t.A LM-3A launch vehicle can meet the launch requirement to IGSO.For MEO satellites in the same plane,it would have been a waste of carrying capacity to send only one satellite on a LM-3A rocket.Thus,it made more economic sense to launch two satellites with one LM-3B rocket.

Based on these mission requirements,a newly developed model named LM-3B/G1 with a modified fairing diameter from 4 m to 3.7 m,could launch two satellites.This made breakthroughs in several key technologies,such as external support technology for large tandem dual-satellite,equipment bay design technology for an integral hoist cryogenic upper stage,process optimization technology for combination operation with two satellites,and separation design technology for tandem dual-satellites.The new rocket model filled the domestic rocket spectrum gap enabling the launch of two high orbit satellites with one rocket.

It was the first time to adopt a tandem dual-satellite external support technology.The new payload fairing was developed for an integral hoist.The rocket structural appearance was optimized to reduce the aerodynamic load in flight and improve its carrying capacity.The new technology benefits of the design of the separation system,reduced the complexity of operation at the launch site,and improved the modularization of the launch vehicle.The external support structure for tandem dual-satellites is shown in Figure 3.

With the technical breakthrough in lightweight equipment bay and low-temperature protection,a new integral hoisting equipment bay based on the ‘K' shape beam was developed,which achieved the optimization of the layout of equipment bay at low temperature with rational and effective utilization of space.The technology of a safe separation design applied to a large platform with tandem dual-satellites was creatively introduced,which solved the problem of safely separating two satellites and far field security.A new far field analysis technology was applied.

The LM-3B rocket carried out the launch missions of BDS-2 MEO satellites.

4.3 LM-3C Launch Vehicle

LM-3C launch vehicle was the first non-axisymmetric large liquid-fueled rocket in China.The aerodynamic characteristics of pitch and yaw channels are quite different.The attitude control system was redesigned.The LM-3C launch vehicle was also the first rocket model with two boosters in China.

LM-3C rocket carried out the launch missions of BDS-2 GEO satellites.

4.4 Distributed Remote Test Launch Control Technology for Cryogenic Liquid Rocket

For the BDS-2 project,the distributed remote test launch control system for LM-3A series launch vehicle which was first applied in 2007 was adopted.Optimized to the characteristics of the cryogenic rocket,the standard Gigabit Ethernet was used to transmit data.The redundancy design of architectures,devices,lines,and ports assured high reliability and low latency[4].The integrated goal of centralized control,unified management and information resource sharing were achieved by managing and controlling the subsystems through the redundant network.All test data for each subsystem was processed and issued in real-time,so it could be browsed and monitored on different terminals.It had the characteristics such as automatic command and test,wireless signal transmission over optical fiber,paperless online interpretation of data,a redundant design and security control network,which laid the groundwork for a cryogenic fueled launch vehicle remote test launch control system in China.

4.5 Other Technology Developments

For the BDS-2 project,LM-3A series launch vehicle adopted integrated navigation systems based on dual-SIMUs and GNSS,greatly increasing the flight reliability and orbit injection accuracy.A triple-modular redundancy design was used for satellite navigation receivers,supporting GPS,GLONASS,and BDS-2 satellite navigation systems at the same time.The semi-major axis injection accuracy of GTO was improved from dozens of kilometers to 10 km.

Restricted by number and position of the TT&C ships,the information of the carrier rocket for GEO satellite operation could not be captured in real-time due the wireless transmission blind zone during the flight.This deficiency was overcome by using a relay communications satellite system so that all the flight segment telemetry data could be received in real-time.The LM-3A series pioneered the use of the space-based telemetry technique among China's launch vehicles when introduced for the BDS-2 project.The telemetry data was transmitted in real time via relay satellites,reducing the pressure on measurement ships,thus laying a good foundation for the application of spacebased telemetry technology for China's launch vehicles[5],and effectively guaranteed the smooth implementation of high-density launch missions for the BDS-2 project.

5 TECHNOLOGICAL DEVELOPMENTS DURING BDS-3 PROJECT

The BDS-3 is a global navigation satellite system consists of 24 MEO satellites which were directly injected by 12 LM-3B/YZ-1 rockets,3 GEO satellites,and 3 IGSO satellites launched on 6 LM-3B/G3 rockets into GTO.

For the construction of BDS-3 project,China's first general upper stage matched with the basic stage was developed to launch satellites into medium or high orbits directly.A BDS-3 IGSO experiment satellite was successfully launched in 2015.It contributed to the technological development of the direct injection of a navigation satellite platform,providing China with the capability for quick deployment and establishment of a satellite constellation.

5.1 LM-3B/YZ-1 and LM-3C/YZ-1 Launch Vehicles

To meet the requirements of launching BDS-3 MEO satellites,LM-3B/YZ-1 and LM-3C/YZ-1 launch vehicles were developed with basic stages and an upper stage.The LM-3B launch vehicle can launch the YZ-1 upper stage with two satellites into MTO.While the LM-3C launch vehicle is capable of launching the YZ-1 upper stage with one satellite into MTO.The YZ-1 upper stage will maneuver at apogee after a long time coasting and inject the satellites to MEO[6].The parallel dual-satellite structure is shown in Figure 4.

5.2 YZ-1 Upper Stage

The YZ-1 upper stage met the mission requirement of launching two BDS-3 MEO satellites or one IGSO satellite with the 3.35 m diameter series rocket,with the ability of multiple re-starts,launching the satellites into MEO and IGSO directly,quick orbit maneuvering and launching multiple satellites atop one upper stage.It was the first domestic general upper stage capable to inject satellites into medium or high orbits.

YZ-1 upper stage's mission attribute is that of something between a vehicle and a spacecraft with the technical features of both.Compared with launch vehicle's dozens of minutes of operation time,the YZ-1 upper stage can operate for 6.5 h.It transits the space environment similar to a spacecraft.It uses autonomous navigation primarily and ground telemetry and telecommand as auxiliary.The technique of multiple re-starts increases the adaptability of missions.The high-thrust engine increases the maneuver capability.The independent electrical system makes it more flexible and highly optimized for general use.

The YZ-1 upper stage consists of a rocket structure,a propulsion system,a control system,a telemetry system,a thermal control system and a power supply system.The structure of equipment bay is built into a vertical wall shell.The control system implements a 1553B bus and sensor level redundancy scheme for a single machine to control the flight.A unified power supply and distribution scheme is adopted to supply power to the control system,telemetry system,and for the thermal control system.Incoherent measurement and Turbo codes were used to achieve telemetry data transmission functionality and track measurement at long distance and at a high bit rate.The trajectory can be replanned according to the data and uplink commands[7].A passive thermal control is utilized to control the thermal environment during the flight,with the assistant of active electrical thermal control.

YZ-1 general upper stage matching with the LM-2C launch vehicle was completed for the SSO satellite mission,effectively boosting the LM-2C launch vehicle's carrying capacity.In the future,it will also be adapted for use with LM-2F and LM-7 launch vehicles to provide launch services covering various orbits and payload capacities to users.

5.3 Others

With BDS-3's short-message function,research on booster debris localization was carried out.The time to locate the debris was reduced from a few hours or even a half month down to 30 min.Mass data such as temperature,attitude,overload,and video gained in this process laid a solid foundation for the future parachute-based falling area control.

In the construction process of the BDS-3 project,the LM-3A series has optimized the unification of configurations and de-tasking.The configurations of LM-3B and LM-3C have been unified to greatly improve production efficiency and to effectively control the technical states and process quality.Batch production has been achieved in the true sense.De-tasking is to disconnect the sequence code of rocket from the actual payload.The goal of independent rocket segment assembly,whole rocket test and task interchange,have been achieved which solved the problem previously of prepared products having to enter long-term storage due to task postponement while the new products were still on a tight schedule[8,9].

6 ANALYSIS ON BDS PROJECT LAUNCH MISSION

The BDS project brought the requirement for one type of rocket launching satellites into different orbital planes.The LM-3A series needed to have the abilities to launch satellites into medium and high orbits along in the direction of east and southeast.To ensure the time requirement for the deployment of the satellite constellation and reduce the cost of engineering construction,the LM-3B/G1 launch vehicle was developed during the BDS-2 project,making breakthroughs in dual-satellite external support technology.The LM-3B/YZ-1 launch vehicle was developed during the BDS-3 project,with a breakthrough in technology for injecting tandem dual-satellite into MEO orbit directly.There were also many major breakthroughs achieved in structural design,separation technology,and for environmentcontrol,providing significant references for the development of other new types of launch vehicles.

Table 2 Launches of LM-3A series for BDS satellite missions

All BDS satellite launch missions carried out by the LM-3A series launch vehicle are listed in Table 2.All launch missions for the three phases of the BDS project were carried out through 44 LM-3A series flights with a 100% success rate.Upon quantitative evaluation,the flying reliability of the LM-3A series launch vehicle reached 0.95 (confidence 0.7),which reaches an international advanced level.The LM-3A series launch vehicle has carried out 107 launch missions from January 2000 to December 2020,where the BDS satellite missions accounted for over 40% of the total.The annual statistical results are shown in Figure 5.Among the high-density launches,30%-40% of annual launch missions were for the BDS project.Particularly,the launches for the BDS project accounted for more than 70% of the total missions in 2018.

Figure 5 Annual launches of LM-3A series for BDS project

7 FUTURE PROSPECTS

The development of the LM-3A series started in the 1980s.The continuous technical innovations and engineering applications of reliability designs enabled the LM-3A series of launch vehicles to become the first domestically made rockets achieved seriation,generalization and combination.

Continuous improvements for the LM-3A series will focus on reliability,safety,and mission adaptability.

7.1 Improvement of Reliability and Safety

Limited by the technological level of the 1980s,large numbers of sensitive electric explosive devices (EEDs) are used in LM-3A series launch vehicle.Although validated by more than one hundred successful flights,the reliability and safety still have the potential for improvement.Insensitive EEDs developed for new-generation launch vehicles will replace these sensitive EEDs to provide better capabilities for anti-electromagnetic interference in transportation,ground test and flight.Development of parachute-based booster falling area control will continue.The boosters' theoretical impact zone can be reduced to 10% of the previous area by setting safe and feasible impact points.Process optimization has to be carried out continually at the launch site under the premise of high-density launches to improve the automation of pre-launch operation and reduce the overall man-hour requirement at the front-end[10].

7.2 Improvement of Mission Adaptability

To support the BDS-3 project,two configurations of a fourstage rocket,LM-3B/YZ-1 and LM-3C/YZ-1 were developed for launching satellites directly into medium and high orbits.Expanding the adaptability to different missions will play an important role in the fields of medium orbit or high orbit scientific detection,deep space exploration,orbit transfer,space debris clean-up,and new technology demonstration.Under the condition of high-density launches for the BDS project,technical conditions and manufacture processes of LM-3B/YZ-1 and LM-3C/YZ-1 were stable to guarantee the products' good consistency.In order to meet the needs of the satellite users,the electrical system will be improved and the carrying capability will be slightly upgraded.In addition to the generalization of booster,the first and the second stages of different configurations,the structures of the entire wave transparent payload faring and equipment bay will be unified as well.Research on active load shedding technology based on SIMU data,full guidance control in the powered phase and iterative guidance technology have been conducted.According to the typical failures of main engines,terminal correction engines,and auxiliary attitude control engines,research on fault detection and trajectory reconstruction is carried out to increase the adaptability under failure conditions.On-board wireless sensing systems have been applied to realize quick configuration of data acquisition ability.