Solid Rocket Motors - China

Although the precise history of the development of rocket propulsion is obscure, the first rockets were developed in ancient China. Key to the development of the early Chinese rocket was the invention of something similar to black powder, a mixture of saltpeter (the oxidizer), charcoal and sulfur that burns rapidly and furiously when ignited. The first crude firecracker probably was a sealed bamboo tube filled with black powder. When ignited, the black powder exploded violently. If only one end of the tube was sealed, the burning black powder forced a mass of flames, gas and particles out of the unsealed open end. In reaction, the bamboo tube would move rapidly in the opposite direction. This device was a crude rocket.

The black powder used by the Chinese was a form of solid propellant (even though it was a loose powder). In holiday fireworks, the spectacular skyrocket displays are launched by solid propellants. In recent times, solid propellant rocket motors have been used in anti-tank weapons, air-to-air and air-to-ground rockets, and intercontinental ballistic missiles, as well as launch vehicles for spacecraft. In these motors the propellant is not a powder but a semi-liquid substance that is cast in the rocket body. The propellant then hardens into a rubber-like substance that will burn in a controlled way when ignited.

The development of China's large-thrust solid rocket motors started late. It is understood that solid propellant engines have the advantages of simple structure, high reliability, and good maneuverability. Nowadays, the world's major spacefaring nations have all developed mature all-solid rockets. There was a certain gap between China and the international advanced level in this field. China had a significant gap with the international advanced level.

Solid propellant is a solid mixture composed of oxidant, fuel (flammable agent) and other additives. It can be divided into single-base propellant, double-base propellant, and composite propellant according to the nature of the formulation components. The Double Base Propellant has a specific impulse of 170 to 220 seconds. The double-base propellant is mainly composed of nitrocellulose, nitroglycerine (NG) and some additives. Composite Propellants have a specific impulse of 225 to 265 seconds. The composite propellant uses a single flammable agent and oxidant material, and a liquid high-molecular polymer binder as a fuel. To increase energy and density, some powdered light metal materials can also be added as flammable agents, such as aluminum powder. Polysulfide rubber, polyurethane (PU), polybutadiene - acrylonitrile (PBAN), carboxyl- terminated polybutadiene (CTPB) Hydroxyl-terminated polybutadiene (HTPB), and hydroxyl- terminated polyether (HTPE), are commonly used.

Solid rocket motors are a very complex system and play a decisive role in the design of weapon systems. The engine performance index is mainly analyzed through tests. It is based on the complexity of current engine test system and the reliability of test data.

In the early days, the secrets of solid rocket technology were closely guarded by other countries. The ingredients of solid propellant were only known to contain a solid oxidizer and a bonding agent which was used as fuel. The oxidizer was considered to be potassium perchlorate and ammonium perchlorate; but there was no information on the bonding agent. In this country, knowledge about the key ingredients was practically non-existent. Only through the efforts of a team of young pioneers digging through foreign literature and piecing the puzzle together that a general concept of building a solid- propellant rocket gradually took shape.

The research work began with the raw materials. But due to the lack of experience, ignorance aboutthe danger of composite propellants, and poor working conditions, many pioneers sacrificed their lives during the exploratory work. On 6 December 1962, while filling propellant into a 300-mm engine, the fuel mixture ignited and exploded, killing Chen Sumei, Han Yuying instantly, and critically injring Wang Zhenkao, Liu Enke, who later also lost their lives.

In 1967, an order was issued by high officials to develop a large solid rocket engine. During the process of developing a prototype engine, many technical difficulties were encountered, including the problem of bonding separation between the fuel and the shell structure, the sealing problem, and the problem of structural strength. Thus, developing a reliable rocket engine was in the critical path of solid rocket development. On 16 March 1974, director of the Propellant Accessaries Factory, Wang Lin, returned to the shop after work to try out a very dangerous mixing procedure. While operating the horizontal mixing machine, the propellant exploded, demolishing the steel-reinforced protective wall and killing comrade Wang Lin.

The Mohaupt fusing cable used for stage separation was also a key element in rocket testing. Because of inadequate assembly and test facilities, a batch of fusing cables were manufactured by hand, and the first full-scale stage separation test carried out by hanging the test article on a large tree in an open field. The development of the Mohaupt fusing cable was a technical breakthrough in stage separation of China's multi-stage rockets.

On 01 July 1, 1962, under a three year plan on solid propulsion, the Fifth Academy set up an independent institute at Hohhot in Inner Mongolia called the Solid Rocket Motor Institute under Xiao Gan. Through the next two years the SLBM program was oriented firmly towards solid propulsion laying the foundation for expanded R&D on both the SLBM and the solid rocket. The JL-1 program was named in 1964 when the Solid Rocket Motor Institute was made the Fourth Sub-academy under the Fifth Academy.

One of its early motors was used as the third stage of the CZ-1 space launcher that launched China’s first satellite in 1970. It also developed China’s first generation Submarine Launch Ballistic Missile (SLBM) JL-1 and its land based variant the DF-21. It then bagan work on the second-generation SLBM, the JL-2. In addition, the Fourth Academy is also the designer of China’s other solid ballistic missiles including the DF-11 (M-11), DF – 15 (M-9), DF-21, DF-31 and the escape rocket on China’s Shenzhou spaceship.

Academy Director Tian Weiping stated the staff "worked hard, overcame numerous obstacles, overcame difficulties, and developed a road of independent innovation and development of a solid missile weapon power system with Chinese characteristics, and promoted the realization of China’s aerospace solid motor business. The historic leap from scratch, from small to large, from weak to strong, successfully completed the research, production, test and flight tasks of various models..."

Lewis and Litai in their book discuss some of the details of the JL-2 / DF-31 development. They state - quoting a Chinese reference - that the first two stages of the 3-stage JL-2 have 2 meter diameter solid rocket motors and the third stage has a 1 m diameter solid rocket motor. The project to build the JL-2 and the advanced submarine was apparently delayed due to Deng Xiaoping who wanted a greater focus on economic development. In 1986 the priorities shifted towards the development of the land-based version of the JL-2 called the DF-31. So unlike the JL-1 which converted a missile developed for launch by a submarine to a land based DF-21 launcher, the JL-2 / DF-31 program envisages a movement from a land-based platform towards a sea-based submarine platform.

Before 2015, China did not use solid engines for the rocket's main power, but used it as an auxiliary power in a small area and applied it to the apogee engine of Dongfanghong No.1, the brake engine of the return satellite, and Fengyun-2 meteorological satellite escape engine and spaceship escape engines.

In 2012, the state officially established a large-thrust solid rocket engine. In Beijing, the young Wang Jianru represented the design team. After that, he led the team, conquered a series of difficulties, and worked tirelessly. Wang Jianru is the chief technical expert of the 41th Institute of the Fourth Institute of China Aerospace Science and Technology Corporation, and is also the chief designer of the large-thrust engine of the trial. Although less than 40 years old, he has been working on solid rocket motors for more than a decade and is one of the leaders in solid-propelled power technology in China.

In 1978, Wang Jianru was born in a peasant family in western Shaanxi Province. The poverty made him diligent and independent. In 1996, he was admitted to the School of Navigation Engineering at Northwestern Polytechnical University to study mechanical and electrical engineering. Four years of hard work allowed him to successfully enter the fourth aeronautical space at the time of graduation, and was assigned to a newly established pre-research room for solid engine design and analysis. In less than four years, he was able to independently manage the pre-research projects and successfully applied for the first case of the Fourth Institute to explore the first generation project.

The experience was blank and the difficulties encountered can be imagined. He said that he had almost read all the information he could find, discussed it with experts, research teams and so on for many times, and finally developed an effective technical route and verification method. His team went from the theoretical pre-research in 2005 to the success of the 2015 Long March 11 flight. In 2015 the all-solid Long March 11 rocket launcher made its first flight. The 2 meter, 3 segmented solid engine with a diameter was successfully developed.

Speaking of the features and functions of solid rocket motors, in 2016 Wang Jianru said: "The solid-state engine itself is simple in structure and the development cycle is very short, because the simple structure will inevitably lead to the development of each component is relatively easy. In addition, it is capable of long-term storage. It is convenient for quick emergency launching such a function.In addition, one of the biggest features of solid-state engines is that it is easy to realize large thrust. In the world at present, the field of space launch vehicles is mainly used as the power for take-off level, generally after the rocket takes off and erupts from the atmosphere."

On 02 August 2016 the domestic solid rocket motor with the largest diameter, the largest amount of charge, and the largest thrust was successfully tested in Xi'an, Shaanxi. This was the first solid rocket engine with a diameter of 3 meters in China. It successfully operated for 100 seconds according to a predetermined target, and achieved a successful ground hot test.

The so-called "trial test" indicates that China has mastered the key technologies for large-scale solid rocket booster engines, laying a solid foundation for meeting China's future aerospace demand for manned moon landing and deep space exploration. Since 2008, Wang Jianru and his colleagues have successively developed a series of relatively small, large-thrust, solid rocket motors.

The 3m diameter is a basic threshold for a space launch vehicle to achieve a large launch vehicle or heavy launch vehicle. to achieve a capacity of more than 300 tons of thrust, the diameter must be expanded to 3 meters. It would be very difficult to achieve this thrust without a diameter of 3 meters. Without enough thrust, large launch vehicles cannot go into space. Only large solid rocket motors can provide large thrust, but the key technologies for its design and manufacture have always been firmly in the hands of foreigners.

Wang Jianru said in 2016: "From an international point of view, the United States, Europe, India, and Japan are all making great efforts to develop high-thrust solid-state engines as boosters. In particular, the United States, Europe, and India have all successfully developed a large thrust segment of more than three meters in diameter. Solid-state engines have already been successfully launched, and our country’s launch vehicles are mainly based on liquid power. Solids are just starting, but with the development and maturity of our national solid-state engine technology over the past few years, we have reached unanimous recognition for the solid engine in the space transportation field, and feel that it is necessary to make the solid engine play a greater role and get better applications in our country's space transportation field."

The "3m diameter" is the lowest threshold for "large solid rocket motors." To make a solid engine output more thrust, it must carry more fuel; to load more fuel, it must be made larger; and there are only two ways to increase the volume: either increase length or increase diameter. Wang Jianru stated that if we want to achieve 300 tons of thrust with a diameter of 2 meters, then our engine may be very slender, and it is basically impossible to design and make. So you need to increase the diameter to 3 meters. Since 2014, Wang Jianru led his research team of less than 20 people and began to work hard to create a “3m diameter solid engine”.

Wang Jianru said in 2016: “Because our previous solid engines were basically built around a design that was 2 meters in diameter or smaller, this time we went directly to the diameter of 3 meters. It should be said that the steps are still quite large. Welding, because the diameter of the weld is very long, we must ensure that the weld is 100% reliable quality, there is no defect. Such a requirement for how to guarantee the process, the difficulty is very large. The difficulty in the process of some sectional butt joints and heat insulation in the combustion chamber is very great. In the development process, due to the unexpected problems caused by the size, the processing plant spent a lot of energy.”

A good beginning is half the battle. Wang Jianru said that the successful 3-m diameter solid-state engine will lay a solid foundation for the development of large-scale high-thrust solid rocket motors in China. But at the same time, there are still more research work waiting for them to further improve, and they need to stride forward and catch up.