The YF-220 exists as a concept or preliminary design, says Zhang Nan, president of the Beijing Aerospace Propulsion Institute, without using the name of the engine. His institute is channeling its experience in developing the YF-77 for the Long March 5 as it works on the new engine. So far, developers are tackling critical technologies and have not built parts for a flyable engine. A technology they will not attempt is staged combustion, a means of driving the pumps that, while maximizing engine efficiency, is hard to develop, especially for engines running on liquid oxygen and liquid hydrogen. In fact, it is too hard, says Zhang. The corresponding engine of the SLS, the Rocketdyne RS-25 from the Space Shuttle, does combine staged combustion with hydrogen fuel. The future Chinese engine's specific impulse (ISP)—thrust divided by fuel flow—may be as high as 430 sec., compared with 428 sec. for the YF-77, notes Zhang.
China's biggest kerosene-fuel engine, the YF-100, uses staged combustion, but applying the technology will be one of the many challenges that engineers will face in building bigger powerplants. Project managers at Xian appear to have minimized problems by adopting a plan they set out in 2011 and 2012 to first build an engine of more moderate size—300-400 tons thrust, presumably—and then doubling it for Long March 9 by feeding two of its combustion chambers with a single, more powerful propellant pump. A drawing of Long March 9 Scheme A has subtly changed since 2012 to show the extra nozzles of two-chamber engines.
Given the stated fuel loads and likely characteristics of the engines, the boosters of Scheme A are likely to burn for 160 sec. and the core for 220 sec., calculates a foreign rocket engineer. The second stage would run for 500 sec., presumably in several burns. If the Xian Institute can reproduce the efficiency of the YF-100 in the YF-660, then ISP at takeoff will be 305 sec. For Scheme B, the solid-propellant boosters may run for about 120 sec., the core first stage for 500 sec. and the second stage for 400 sec.
At 3.2%, the payload fractions of Schemes A and B are much lower than those of the Saturn V (3.9%) and SLS Block 2 (4.4%). This does not necessarily mean the Chinese design is inefficient, say engineers experienced in comparing launcher configurations; it may just reflect design choices that drive up takeoff weight but are nonetheless cost-effective. Solid-propellant boosters and their mounting structure probably account for much of Scheme B's excess of weight over Scheme A's.
The payload to LEO of the two designs suggests industry leaders here are eyeing lunar expeditions perhaps not much more ambitious than Apollo, although the mass they can deliver to the Moon's surface will also depend on how the mission is executed. Sending a crew aloft on a separate launch to join the rest of their spacecraft, carried by a Long March 9, could greatly expand the mission. The Saturn V, which lofted all Apollo modules in a single shot, had a payload to LEO of 118 tons.
At the International Astronautics Congress, the Chinese industry showed a concept for sending people to the Moon with three launches via smaller rockets. A cargo launcher, perhaps a little sibling of Long March 9, would fire a lunar-landing craft into orbit around the Moon. Then a crewed capsule would follow on an even smaller launcher, presumably a Long March 2F or Long March 7, China's current and future human-rated rockets, respectively. A propulsion unit sent on a second cargo launcher would join the capsule and propel it to lunar orbit, where it would meet the lander.
Smaller launchers are cheaper to develop, but bigger ones offer lower operating costs for their payload sizes. The economics of China's choice, then, must depend on whether it wants to sponsor heavy space missions for the long run, sending a super-heavy launcher up perhaps once a year, and not only to the Moon. If the aim is to perform a few manned lunar missions and then stop, it would surely be cheaper to execute each with multiple launches of moderately sized rockets. If more heavy-load tasks beckon, then a huge rocket is the answer, say Western engineers.
The Chinese space managers are on that wavelength. In the paper presented to the congress that detailed the Long March 9, CALT authors mentioned Moon shots, with a trans-lunar injection load of 50 tons, as only one purpose of the proposed launcher. Deep-space exploration (20 tons escaping Earth gravity), large-scale Earth-orbit missions (50 tons to geostationary transfer orbit) and new concept missions (50 tons escaping Earth gravity) were also touched upon, although the latter would require another rocket design.
Long March 9 Design Alternatives
|Scheme A||Scheme B|
|Engines||4 x YF-660||4 x unknown name|
|Thrust||4 x 650 metric tons||4 x 1,000 metric tons|
|Propellant||Liquid oxygen, kerosene||Solid|
|Tankage||4 x 320 metric tons||4 x 575 metric tons|
|Core Stage One|
|Engines||4 x YF-660||4 x YF-220|
|Thrust||4 x 650 metric tons||4 x 200 metric tons|
|Propellant||Liquid oxygen, kerosene||Liquid oxygen, liquid hydrogen|
|Tankage||1,756 metric tons||1,000 metric tons|
|Core Stage Two|
|Engines||2 x YF-220||1 x YF-220|
|Thrust||2 x 200 metric tons||1 x 200 metric tons|
|Propellant||Liquid oxygen, liquid hydrogen||Liquid oxygen, liquid hydrogen|
|Tankage||500 metric tons||200 metric tons|
|Takeoff thrust||5,200 metric tons||5,000 metric tons|
|Takeoff weight||4,100 metric tons||4,150 metric tons|
|Dry weight*||434 metric tons||517 metric tons|
|10.6% of takeoff weight||12.5% of takeoff weight|
|Total propellant*||3,666 metric tons||3,633 metric tons|
|86.2% of takeoff weight||84.3% of takeoff weight|
|Payload, LEO||130 metric tons||133 metric tons|
|3.2% of takeoff weight||3.2% of takeoff weight|
|Payload, LTO||50 metric tons||50 metric tons|
|Length||98 meters (322 ft.)||101 meters (331 ft.)|
|Source: CALT, except *Aviation Week calculations. |