Long March 6A: my country's first solid-liquid carrier rocket

Friends who are familiar with foreign launch vehicles must know that there are many types of launch vehicles abroad that are bundled with solid boosters, such as the United States' Atlas V launch vehicle, the European Space Agency's Ariane 5 launch vehicle, Japan's H-2B launch vehicle, India's GSLV launch vehicle, etc. The boosters bundled with my country's launch vehicles are generally liquid-powered, but the Long March 6 modified launch vehicle that was successfully launched in March this year filled this gap in my country and became my country's first launch vehicle with bundled solid rocket boosters.

Long March 6 modified carrier rocket launched into space

Achieve 14-day rapid launch target

At 17:50 on March 29, 2022, the Long March 6A carrier rocket successfully launched two artificial satellites into space and smoothly sent them into the predetermined orbit.

Unlike the Long March 6, the diameter of the core stage and the core stage of the Long March 6 Modified are both 3.35 meters. In terms of the power system, the Long March 6 Modified is still equipped with two 120-ton thrust YF-100 liquid oxygen/kerosene engines on the first stage and an 18-ton thrust YF-115 liquid oxygen/kerosene engine on the second stage. On the side of the core stage, four 2-meter-diameter solid boosters are bundled, and the boosters use a two-stage 120-ton thrust solid rocket engine. The diameter of the fairing has increased from 2.6 meters or 2.9 meters on the Long March 6 to 4.2 meters or 5.2 meters, leaving enough space for future large payloads.

The total length of the "Long March 6A" rocket is about 50 meters, with a takeoff thrust of 7230 kilonewtons and a takeoff mass of about 530 tons. It can achieve a carrying capacity of 4.5 tons to a 700-kilometer sun-synchronous orbit.

As my country's first new generation Long March series carrier rocket with solid boosters, the Long March 6A can give full play to the comprehensive advantages of liquid rocket engines with high performance and long working time, and solid rocket engines with high thrust, reliable operation and simple use and maintenance. The four solid boosters of the Long March 6A provide nearly 70% of the thrust for the entire rocket at takeoff.

By adopting solid boosters, the "Long March 6A" has achieved a modular, combined and serialized design. Subsequently, through the adjustment of the boosters, a variety of configurations can be formed to create a series of launch vehicles with a wide range of carrying capabilities, a reasonable gradient and high cost-effectiveness, to meet the diverse and intensive satellite launch needs in the future.

Due to the advantages of solid rocket engines, such as fewer components, relatively simple structure and high reliability, the onboard piping system of the "Long March 6A" has been "slimmed down" by as much as 55%; due to the advantages of solid rocket engines, such as easy operation and maintenance and long storage time, the "Long March 6A" finally realized the direct installation of solid boosters at the launch site and achieved the target of rapid launch after 14 days of preparation at the rocket target site.

Integrated engine health diagnostic system

Of course, using solid rocket engines is not all about benefits. The high thrust of solid rocket engines also brings greater vibrations. In addition, the exhaust flame of solid rocket engines contains high-temperature solid particles, which greatly enhances the radiant heat environment at the bottom of the rocket. In order to overcome the complex force and thermal environment caused by the combined operation of solid and liquid engines during flight, the design of the "Long March 6 Modified" needs to accurately predict the mechanical environment conditions of the bundled rocket and the thermal environment at the bottom of the gas-solid two-phase jet.

The use of bundled solid rocket engine boosters also brings about changes in the ignition process, that is, when the rocket is launched, the liquid rocket engine of the core stage of the carrier rocket needs to be ignited first, and then the solid rocket engines of the four boosters will be ignited. However, since the solid rocket engine cannot be shut down once it is ignited, if the liquid rocket engine of the core stage fails at this time, the rocket cannot be stopped to troubleshoot. Therefore, the "Long March 6 Modified" needs to diagnose the health status of the core stage liquid engine before the solid booster is ignited to detect possible problems as early as possible.

The four solid boosters carried by the Long March 6 modified rocket

To this end, the Long March 6 modified rocket is equipped with an integrated engine health diagnosis system that is only the size of a B5 book, which has the ability to quickly and accurately collect engine parameters and perform real-time diagnosis. The system starts engine diagnosis 2.5 seconds after the core stage liquid engine is ignited, and determines whether the engine has a fault through 0.3 seconds of continuous monitoring and diagnosis. If it is considered that the engine has a fault, it will automatically shut down the core stage liquid engine in an emergency and cut off the solid booster ignition program to prevent it from igniting.

In order to ensure the reliable operation of the engine health diagnosis system, not only a large amount of basic engine working data was collected in the early stage to ensure accurate judgment of the situation, but also three sets of identical diagnostic systems were configured on the rocket to diagnose the engine status simultaneously. Only if two or more system diagnoses simultaneously determine that a fault exists can it be determined to be an engine failure.

In addition, the engine health diagnosis system also needs to adapt to the harsh environment of the core stage of the Long March 6. Since the core stage engine of the Long March 6 is a cryogenic liquid oxygen/kerosene engine, the temperature of liquid oxygen is -183°C. Under the influence of such a low temperature environment, the environment around the engine health diagnosis system reaches -40°C. However, when the core stage engine is ignited, the temperature around the engine health diagnosis system will quickly rise to 130°C, accompanied by strong vibration, and the working environment is extremely harsh.

Schematic diagram of the solid booster of the Long March 6 modified rocket

"Smart Launch Site" provides escort

In order to ensure the reliable launch of the "Long March 6A", a "smart launch site" was specially built at the 9A launch site of the Taiyuan Satellite Launch Center. The Internet of Things technology is used to uniformly collect and integrate data from ground facilities and equipment, and the data is sorted and integrated through big data technology. It realizes full system situational awareness, full process intelligent management and control, and full process drive guarantee support, which can realize the direct installation of solid boosters at the launch site and a rapid launch within 14 days, improving the efficiency and safety of space launches.

The 9A launch station was unattended for 4 hours before the rocket was ignited, and a series of operations before the launch were remotely controlled by the launch system. This was thanks to a set of "intelligent robotic arms" that can achieve automatic docking and filling. When traditional rockets are filling or leaking propellants, they generally use manual docking and automatic detachment on site, which requires someone to be on duty at the launch site at all times, and the "intelligent robotic arm" completely replaces the manual role. This set of robotic arms can ensure accurate acquisition of the target position in complex weather environments such as rain, snow, and fog, overcome the random shaking of the rocket caused by load changes or wind, and dynamically measure and track the filling valves on the rocket in real time to ensure accurate docking.