Research on Overall Optimization Technology of Long March 6A Launch Vehicle

HONG Gang,NING Guofu,YANG Fan,SHEN Zili,ZHANG Liang

Shanghai Academy of Spaceflight Technology,Shanghai 201108

Abstract:On March 29,2022,the Long March 6A launch vehicle developed by the Shanghai Academy of Spaceflight Technology successfully made its maiden flight at the Taiyuan Satellite Launch Center.The Long March 6A launch vehicle was China’s first solid bundled rocket.It uses non-toxic and non-polluting propellants,and has a 700 km sun-synchronous orbit carrying capacity of no less than 4.5 t,which is more than 50% higher than that of a LM-4C launch vehicle.During the development of the LM-6A launch vehicle,the matching relationship between systems and the overall performance of the entire rocket improved significantly through continuous overall optimization work.This paper introduces the typical overall optimization work during the development of the LM-6A launch vehicle.

Key words:Long March 6A,solid bundled rocket,overall optimization

1 INTRODUCTION

On March 29,2022,the Long March 6A launch vehicle successfully made its maiden flight from the Taiyuan Satellite Launch Center,signifying a breakthrough in solid bundled rocket technology in China.It has great significance in improving China’s aerospace transportation system and to meet the high-density launch of satellites in the future.

As a new generation of medium-sized launch vehicles in China,it was the first time solid boosters were used in the Chinese launch field.The LM-6A carrier rocket had new technical characteristics,high capability requirements,but with little experience for reference.In order to improve the comprehensive performance of the whole rocket and improve the reliability and safety for the launch,overall optimization work was carried out continuously throughout the improvement of the carrying capacity and the optimization of the system scheme to ensure that the technical performance of the whole rocket can meet the requirement.

2 FIRST STAGE LENGTH OPTIMIZATION

A launch vehicle is commonly transported to the launch site by railway in China.Due to the restriction of railway transportation,the overall transportation length of the first stage was limited to 24.4 m in the initial stage of the project,while the flight time of the first stage had to be about 188 s,hence this did not give full play to the capability of the core stage dual engine.After demonstration and optimization,it was decided to lengthen the first stage,while adopting for the first time in China a scheme of segmental railway transportation and assemble the sections at the launch site.So,the final length of the first stage was lengthened to 30.5 m,enabling a flight time for the first stage of about 235 s.Through the adjustment of the length of the first stage,the carrying capacity into 700 km sun-synchronous orbit was increased about 1.0 t,and the launch cost performance was also improved greatly.

After the length of the first stage was increased,the front auxiliary binding point of the solid booster was adjusted from the position of the corresponding first-stage oxygen tank to the position of the corresponding first-stage inter-tank section,which reduced the difficulty of tank design,avoided the adverse effects of the low temperature environment in the binding connection and separation device while improved the reliability and safety of the structural product and booster separation.

3 SECOND STAGE POWER CONFIGURATION OPTIMIZAITON

The first core stage of the LM-6A launch vehicle adopted two YF-100 engines.In the initial stage of the project,there were two power schemes considered for the second stage,a single YF-115 engine or two YF-115 engines,as shown in Figure 1 and Figure 2.

Figure 1 Second stage scheme of a single YF-115 engine

Figure 2 Second stage scheme of two YF-115 engines

After the closed-loop demonstration of the overall ballistic-control load structure,the dual-engine scheme was about 1.3 t heavier than the single-engine scheme at the time of engine shutdown.Through calculation of the carrying capacity into different orbits,we can see that when the orbit height is higher than 500 km,the carrying capacity of a single-engine scheme is greater,and the higher the orbit height,the more obvious the carrying capacity advantage of a single-engine scheme.

The dual-engine scheme for the second stage was a new configuration,and the engine related design work needed to be re-engineered.The single-engine scheme had been applied on the second stage of the LM-6 launch vehicle,and the technology was mature which meant the whole rocket configuration and the product matching to the rocket could be simplified.It is also about 3 m shorter than the dual-engine scheme,which was conducive to reducing the flight load.The main task of the LM-6A launch vehicle was launching payloads into a sun-synchronous orbit of more than 500 km.Comprehensive consideration from the perspectives of carrying capacity,inheritance,economy,reliability,the second-stage power system finally led to the adoption of a single YF-115 engine scheme.

4 OPTIMIZATION OF ROCKET BODY SUPPORT AND BINDING FORCE TRANSMISSION METHOD

The support and force transmission method for the rocket body was the focus of the demonstration of the bundled rocket,which had a direct impact on the design of the whole rocket structure,take-off safety,the assembly process of the rocket body,and the launch pad.The Space Shuttle,Ariane 5,H-2,LM-5 and other launch vehicles use booster support,with front main transmission method,while the Atlas 5 series,Delta 4 series,Arian 3/4,LM-2F,LM-3B/C and H-2A/2B adopt core-stage support and rear main force transmission.

According to the overall scheme of the LM-6A launch vehicle,a comprehensive comparison and demonstration of the two methods,booster support with front main force transmission and core stage support with rear main force transmission,was completed.The core-stage structure of booster support with front main force transmission mode is lighter in weight and has a larger carrying capacity,but the entire rocket assembly process is complex,the launch pad scale is large,and the safety requirements are high.The advantages of the core-stage support with rear main force transmission method are mainly reflected in that the whole rocket assembly process is simple,the launch pad is less complex,and the economy and extensibility are good.However,the core-stage structure is of higher weight,and the carrying capacity is about 100 kg less than that of the booster support with front main force transmission method.After comprehensive comparison,the development work was finally carried out according to the core-stage support with rear main force transmission method,as shown in Figure 3.

Figure 3 LM-6A carrier rocket is erected on the launch pad

5 OPTIMIZATION OF NOZZLE EXPANSION RATIO OF THE SOLID BOOSTER MOTOR

The nozzle expansion ratio is an important parameter in solid motor design,which affects the structural design,charge design and flight total impulse of the solid motor.At the beginning of the project,the designed nozzle expansion ratio was 10.9.The average external pressure of the booster flight section of the LM-6A launch vehicle was about 0.035 Mpa,and the corresponding altitude performance was about 8 km.The best expansion ratio of the solid booster nozzle was about 22,and the outer diameter of the nozzle outlet was about Ф1900 mm.It was proposed to increase the expansion ratio of the nozzle of the solid booster motor.According to the production and processing capacity,the equipment layout in the tail section of the solid booster,the final design height of the solid motor nozzle is 5.3 km,the expansion ratio is about 16,and the outer diameter of the nozzle outlet is Ф1640 mm.

Before optimization,the solid motor worked in an under-expanded state throughout the entire process and did not exert its best performance.After optimization,the expansion ratio of the solid booster nozzle was about 16,the carrying capacity was increased by 250 kg,the zero position length of the electric servo mechanism was also increased,and the space layout was more convenient.

6 INTERNAL BALLISTIC OPTIMIZATION OF SOLID BOOSTER MOTOR

Different from the liquid engine,the thrust of the solid booster motor can be simply adjusted.In the initial stage of the project,the internal ballistic design requirements were proposed,on the premise of ensuring the charge amount,a large thrust is provided at the initial stage of take-off,so that the rocket can take off quickly and leave the launch pad;the thrust of the solid motor begins to decrease after flying for 15 s,the purpose was to reduce the maximum dynamic pressure and the flight load;the thrust of the solid motor could be increased after flying for 50 s to 80 s,giving full play to the propulsion effect of the solid booster,and enabling separation as soon as possible;after flying for 80 s until the charge is exhausted,the thrust is gradually reduced,the thrust difference caused by the deviation of the operating time is reduced,and the design pressure of the attitude control system is reduced.

Based on the above requirements,a number of internal ballistics were designed.After simulation calculation,the saddle-shaped internal ballistic curve was finally selected,as shown in Figure 4.

Figure 4 Internal ballistic curve of solid booster motor

7 OPTIMIZATION OF THE OVERALL SCHEME OF SOLID BOOSTER SEPARATION

Different from the controllable shutdown of a liquid engine,the solid booster does not have a command shutdown function.After ignition,it can only burn out.There are differences between the actual burnout time of the four solid booster motors which means a reasonable,feasible,safe and reliable solid booster separation condition and separation schemes must be formulated.

Three separation startup conditions are set,namely allowable separation,conditional separation,and timing separation,enabling a cooperative booster separation function.The conditional separation includes chamber pressure conditions and range conditions (as shown in Figure 5).The purpose of adopting the chamber pressure condition is to judge that the thrust of the motor has dropped to a specified range through the chamber pressure of the solid motor combustion chamber to ensure the safety of the separation process,which is a necessary condition for the separation of the solid booster.The purpose of adopting the range condition is to reduce the landing point deviation.To narrow the scope of the landing zone,the logical relationship between the range condition and the chamber pressure condition is: the range condition is reached but the chamber pressure condition is not reached,so they cannot be separated;the chamber pressure condition is reached but the range condition is not reached,the booster will fly until the range condition is reached;the chamber pressure condition is reached but the range condition has passed,it will directly separate.

Figure 5 Booster separation conditions

The separation of the 4 solid boosters has two schemes:two-by-two separation and simultaneous separation.The simultaneous separation scheme has higher requirements for the instantaneous high-current discharge capacity of the battery on the rocket,but the separation safety,attitude controllability,landing point distribution range and carrying capacity are superior to the two-by-two separation scheme,and it was finally decided to adopt the simultaneous separation scheme of four solid boosters.

8 CONCLUSION

During the development of the LM-6A launch vehicle,there were many overall optimization projects initiated.This article only lists a few representative ones.Through continuous overall optimization work,the LM-6A launch vehicle adopted a simplified configuration,reduced system complexity,optimized system matching relationships,improved the carrying capacity,and laid a foundation for meeting the technical targets of the whole rocket.The LM-6A launch vehicle Y1 flight test verified the reasonable and suitability of the overall scheme selection.