 China is advancing its space capabilities by developing staged combustion, an engine technology that is likely to offer greater performance for the Long March 6 and 7, two of a family of launchers that the country will field around the middle of the decade. The smaller of the two, the Long March 6, may be the first to go into service, beating the flagship third member of the family, the Long March 5 heavy launcher. A new 18-metric-ton-thrust engine “is a high-altitude liquid oxygen and kerosene engine with a staged combustion cycle and has been indigenously designed by China,” says national space contractor CASC. If successfully executed, this technology would offer a high specific impulse, a key measure of rocket performance that compares the duration and level of thrust with the mass of fuel consumed in generating it. The practical result should be a greater payload to orbit for a launcher of a given size. The improved performance will probably be essential for China’s next generation of launchers to be competitive as the technology becomes increasingly common in the future. As a liquid-fuel engine, the powerplant has limited meaning for China’s military capabilities. Modern missiles generally have solid propellants. But the development underscores the country’s ability to catch up with advanced foreign aerospace technology. It can be assumed that development of the staged-combustion engine is going well, because CASC would not discuss a problem-ridden program that had hitherto received minimal attention, and because two rocket-building subsidiaries, SAST and CALT, are speaking optimistically of getting the related launchers into service within three or four years. Shanghai-based SAST says its development of the Long March 6 is progressing smoothly and that the light launcher may become operational before the Long March 5. The Long March 6 will be able to loft 1,000-kg. (2,200-lb.) payloads to an orbit of 600 km. (370 mi.) altitude, says SAST Vice President Meng Guang. The first launch of the Long March 5 heavy rocket is due in 2014 after a development program that began in 2007. Development of Long March 6 began in 2009, exploiting engines and stage modules already designed for the Long March 5. The core of Long March 6 will use the standard 3.35-meter (11-ft.) diameter of the current Long March series, Meng and other SAST officials say. That means it will be based on the K3 stage module, the larger of two kerosene-fueled modules that SAST and CALT are developing for the launcher family. A scheme for the family discussed in 2007 suggested that the smaller of its three launchers—now identified as Long March 6—would be based on the smallest, 2.25-meter-dia. module, K2, but the plan has obviously changed. The modules can serve as boosters or core stages. Despite the rise in diameter, the first stage of Long March 6 will presumably be propelled by the single YF100 engine generating 120 metric tons (265,000 lb.) of thrust previously associated with the light rocket. The upper stage will have a smaller engine, says Meng, without giving details. CALT Vice President Hao Zhaoping has said that the medium-heavy Long March 7 will use an 18-ton engine for its second stage. Since CASC says the staged-combustion engine will have a thrust of 18 tons and will be used on “new-generation launch vehicles,” it should be the powerplant for the second stages of Long March 6 and 7. One foreign rocket engineer calculates that the Long March 7 would need two of the 18-ton engines for its second stage. The Long March 5 has core stages fed by liquid hydrogen and liquid oxygen, another technology route to high specific impulse. Kerosene-fueled modules with YF100s serve as its boosters. Staged combustion avoids the usual waste of fuel or oxygen used to drive the pumps in an engine with the conventional gas-generator cycle. To avoid overheating, the mixture in that process in any liquid-fuel rocket is deliberately not optimal. It must have either too much oxygen or too much fuel, one of which is therefore partly wasted as the exhaust is dumped overboard. In staged combustion, that still-usable exhaust is instead fed into the main combustion chamber, driving up pressure and burning a second time to maximize impulse from the available tankage. The principle sounds simple, but in practice it presents great challenges in handling the hot, high-pressure exhaust. Staged combustion engines in service include the Russian RD-180, used on the Atlas V, and the Space Shuttle Main Engine. Chinese engineers probably chose to use the technology first in a smaller engine because high specific impulses in upper stages have the greatest effect on payload, and because a smaller engine would be easier to develop. The foreign engineer estimates that the engine will offer 15-20% higher specific impulse than an otherwise equivalent kerosene engine, and 10-15% greater payload to low Earth orbit, although another rocket propulsion specialist, from the U.S., thinks that with only a second stage using staged combustion, the payload advantage is likely to be 5-10%. It is not known whether the Chinese engineers have chosen an oxygen- or kerosene-rich mixture for the pre-combustion. The former has advantages but is considered harder to develop. The Chinese staged-combustion engine “adopts many advanced technologies, such as forced start and optimal stage transfer,” says CASC, giving neither the engine’s name nor the identity of the institute that developed it. The engine can operate for a long time, it adds. “It can regulate the thrust mixture ratio, supply a working medium for tank pressurization and provide a power supply for the servomechanism,” the hydraulics. Optimal stage transfer implies that the engine is just the right size to propel its stage. Forced starting may mean restarting, which an upper-stage engine will often have to do, while the regulated mixture ratio implies that the Chinese powerplant has an advanced capability to accept propellants at imperfect ratios—maximizing its use of tankage even if the oxygen supply is reduced by boiling off. A second U.S. rocket engineer interprets the reference to servomechanism power as meaning that the engine feeds high-pressure kerosene from the pump outlet, uses it as a working fluid in the actuators, and then sends it back for combustion. Several Russian engines have such a function, instead of a completely separate hydraulic system. The design of the engine in general shows signs of Russian practice. It is not known whether that is because Russia has helped China with the program or because Chinese engineers simply like Russian features. Manufacture of the Long March 7 has not yet been assigned to either SAST (Shanghai Academy of Spaceflight Technology) or CALT (China Academy of Launch Vehicle Technology), both subsidiaries of CASC (China Aerospace Science & Technology Corp.). Photo Credit: CASC source | 
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