Among the challenges Chinese engineers had to overcome in achieving a landing on the Moon, one of the trickiest was developing a powerful variable-thrust engine for the job—one that could responsively throttle up and down as needed to get the probe to lunar orbit, then deliver it to its landing point, stopping on the way to survey the terrain.

Chinese development engineers overcame many roadblocks to bring the engine to fruition. It is the main powerplant for the Chang'e 3 lunar probe, says the chief designer for the propulsion program, Jin Guangming. Generating 7.5 kilonewtons (1,690 lb.) of thrust, it is three times as powerful as the main powerplant of China's Shenzhou manned spacecraft, Jin says, emphasizing that the main difficulty in development was not the large increase in output but, rather, achieving a wide range of variable thrust, precision control, high performance (presumably meaning efficiency) and durability.

The Academy of Aerospace Propulsion Technology, part of China Aerospace Science and Technology Corp., the country's main supplier of launchers and spacecraft, spearheaded the engine's design. Chang'e 3 arrived on the Moon on Dec. 14 GMT.

Chang'e 3 is the first mission in which the engine has been used for a probe, Jin tells state news agency China News Service, leaving open the possibility that it has had other applications. Its characteristics include optimal performance at full thrust, he says, while in the range of 1.5-5 kilonewtons it must meet the challenge of reliably shifting to any needed output. The engine was required to fire during the journey to the Moon—for course correction and to demonstrate its functionality—then to relight to bring the probe into a lunar orbit and brake it for the controlled descent, during which it had to responsively deliver variable thrust. It had to power up again to arrest descent so the probe could assess and adjust its landing point, and then take the craft down for a controlled descent until shutting down at an altitude of 3 meters. The descent was autonomous, without ground control.

The academy began researching variable-thrust engines in 1996-2000, says Jin. Development of the engine began in 2008, and many problems were encountered. The engine was tested 100 times with an accumulated firing period exceeding 60,000 sec. The aim was to ensure it could perform without fault for 20 min. Ground testing was complete by the time of the November 2012 Zhuhai air show, where the engine was exhibited.

Based on analysis of a photo of a display version of the engine, one U.S. propulsion expert says it is most like a throttleable monopropellant hydrazine engine, although “there is not enough evidence in the photo to rule out a dual-propellant approach burning hydrazine with nitrogen tetroxide as the oxidizer.”

In development, “the first difficulty was controlling weight,” says Jin. The engineers would have liked to have increased the size of the nozzle, to extract as much efficiency as possible in generating the high thrust, but they had to deal with weight and dimension limits. In particular, a large nozzle would have demanded longer landing legs, which would have increased the chance of an unstable touchdown. As exhibited at Zhuhai, the engine's nozzle was not at all small.

Another problem was dissipating heat during operation. To avoid interference with the equipment on the spacecraft, a heat shield was needed, but that obstructed the dissipation of heat, thereby affecting the engine's reliability. The developers dedicated innumerable research, testing and repeated redesign hours into overcoming this difficulty, says Jin.

The nozzle of the engine's thrust chamber uses a special alloy plus coatings to resist oxidation at high temperatures. The nozzle's walls are as thin as several hundred microns. Thickness and shape vary. To ensure the precision of the shape, the nozzle was formed and worked in a single piece, says Jin.