Long March-9 - Solid Rocket Motors
The status of this program is unclear. Large solid rocket motors were envisioned for the Long March 9 in 2021, but soon fell out of favor. Subsequently, this development effort was justified by application to a family of intermediate Kuaizhou KZ-21 / KZ-31 launch vehicles. Subsequently this scheme seems to have fallen out of favor, and the solid rocket motor development community evidently returned to the original concept of strap-on boosters for heavy rockets such as the Long March-9. However, the Long March-9 community seemingly did not get the memo, as their designs [as of 2021] did not feature such strapons. Development and testing continued, and while application to future versions of the Long March 9 cannot be excluded, it is also possible that this is a zombie project propelled by institutional inertia rather than identified requirements. This is not uncommon with technologies where development timelines are protracted and requirements are subject to frequent change.
In the United States, the 260-inch solid rocket motor program, initiated by the Air Force in 1963, was transfered to NASA in mid-1964. It was directed by the NASA Office of Advanced Research and Technology. The Air Force retained the portion of the program related to 156-inch solid motors, considered the largest practical size for land transportation, an important aspect of defense requirements. Three short-length motors were built and tested under the 260-inch program. These were called short length motors because they were approximately half as long as a motor required for an actual mission. A full length motor would range from 120 feet to 200 feet and produce 5 to 8 million pounds of thrust, depending on the particular mission. The first 260-inch motor SL-1 was fired 25 September 1965.
In 1969 the US Congress deleted funding for this program because no role had been assigned these large solid rocket motors for the near future. While no role had been assigned as yet to the 260-inch large solid rocket motor, NASA continued to regard the large solid rocket motor as one of the attractive, technically feasible alternatives for future space programs. In the FY 1970 budget presentation, no provision in either the original or the revised submission was made for any further demonstration firings of 260-inch large solid motor cases.
The configuration of the Long March 9 has gone through many plans, with two configurations that were initially used in 2012 for the primary selection. At that time, the strap-on boosters were still 3.35 meters in diameter. The debate initially focused on whether the first-stage rocket uses an all-kerosene configuration or a solid propulsion + liquid hydrogen configuration. In more detail, since both configurations require high-thrust oxyhydrogen machines, the main controversy is actually whether to develop a high-thrust kerosene engine or a high-thrust solid booster.
The structure and design of the solid booster is simpler, the technical difficulty is lower, and the thrust is easier to reach more than 1,000 tons. However, the solid booster needs to fill the fuel grain into the arrow body in advance, and the grain itself is equivalent to a barely controllable explosive. In the subsequent transfer and installation process, the staff at the launch site have been working next to a large arsenal. Once an explosion occurs, it would be very dangerous. Among the countries in the world, Europe, the United States, Japan, and India use more solid rockets.
In addition, the storage of the solid motor is also troublesome. Maintaining a posture for a long time would cause the internal grain of the solid motor to deform. This deformation is fatal, affecting the performance during use, or directly in the warehouse. Therefore, when solid engines are stored for a long time, someone needs to turn them over every once in a while to eliminate this deformation in advance.
For the high-thrust solid propulsion used in aerospace, it is generally sent to the launch site after being injected, and is not sealed for long-term storage. This has also led to the fact that the production line of the solid motor must exist at the same time as the rocket model, and the production line can not be dismantled until the last shot of the rocket before it is retired. As a result, the cost of maintaining the production line for the solid motor in the later period would remain high, which would increase the launch cost.
China will begin testing the world’s largest solid-propellant rocket engine in February, according to a senior space scientist interviewed on 24 December 2017. Liang Jiqiu, chief designer of Kuaizhou rockets at China Aerospace Science and Industry Corp, the nation’s largest missile-maker, described the engine as having a diameter of over 4 meters and a liftoff thrust of more than 1,000 metric tons. Until now, the largest solid-propellant rocket in the world — developed by Orbital ATK Inc in the United States for the Space Shuttle's Solid Rocket Booster — had a diameter of 3.7 meters, he said.
The engine would reportedly be used with Kuaizhou 21 and Kuaizhou 31 rockets, both new-generation models under development at CASIC. Kuaizhou 21 will be capable of sending a 20-ton spacecraft to low Earth orbit while Kuaizhou 31 will be able to transport a 70-ton payload, he said.
On 19 October 2021 Chinese space engineers conducted the first ignition test of the most powerful solid-propellant rocket engine in the world, which will be used on new-generation rockets, according to the Academy of Aerospace Solid Propulsion Technology in Xi'an, Shaanxi province. Researchers at the Xi'an academy have begun to design a variant based on the 500-ton-thrust type, Wang Jianru, chief designer of the engine, said, explaining that the variant will be used on the super-heavy rocket under development by Chinese engineers. Researchers at the China Academy of Launch Vehicle Technology in Beijing, another CASC subsidiary, are designing a super-heavy rocket-the Long March 9-that will likely become one of the world's largest and mightiest launch vehicles.
The engine had a diameter of 3.5 meters [smaller than the American 3.7 meter Solid Rocket Booster, and smaller than the originally advertised 4.0 meters], 150 metric tons of solid propellants and a thrust power of 500 tons, making it the world's most powerful and fuel-efficient integrated solid-propellant rocket engine, the academy said in a statement, noting that the test took place at an engine testing facility in Xi'an. Wang Jianru said that it integrates a number of advanced technologies and high-performance composite materials and has world-class capability. "The test's success means that we have achieved substantial progress in improving the capability of our solid-propellant carrier rockets. It also laid a key foundation for our research and development for a 1,000-ton-thrust engine," he said, noting that the new engines are necessary for China to build super-heavy rockets.
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