But it “must be built, must be launched, must be a success”, said a scientist involved in the project who requested anonymity, “otherwise the Chinese space programme will always live as a dwarf in the shadow of giants”.

The CZ-5 can lift 25 tonnes of cargo into low-earth orbit, two and a half times as much as the previous most powerful Long March rocket. To do that it required a whole new family of engines. The first stage of the CZ-5 consists of two large YF-77 liquid hydrogen/oxygen engines and the four boosters each have a pair of YF-100 kerosene/oxygen engines.

The rocket engine development and testing took place at a facility hidden in a valley to the south of Beijing. Residents living nearby, already used to ground tremors accompanied by deep, explosive blasts, detected a noticeable increase in testing activity in recent years.

According to official accounts, the first test-firing of the main engine was a failure, as was the second, third and fourth.

“Each failure dealt a heavy blow to the whole rocket team,” CZ-5 designer in chief Li Dong said in a documentary film released by China Central Television in April.

The first two prototypes exploded immediately after ignition, the others perished in flame and smoke due to fuel leaks.

“Some foreign experts said we would never be able to bring the engine from the drawing board to life,” Li said.

Meanwhile, other teams suffered disheartening setbacks as well. The CZ-5 is five metres in diameter, among the world’s largest rockets in service today, but to control its weight its fuselage had to be as thin as 3mm.

In a stress test, the fuselage team spotted a crack as thick as a hair near a welded area, deputy chief designer Yang Hujun said in the documentary film. It took them more than three years to improve the welding method to eliminate the threat, otherwise “the entire rocket may veer off course due to the loss of balance after launch,” he said.

The CZ-5 belongs to a new generation of Long March rockets for China’s future space projects. The other two, the medium-sized, solid-fuel CZ-7 and CZ-11 – which can be mounted and launched in 24 hours – have already been launched successfully.

But the CZ-5 could also be viewed as the oldest model in the family, dating back to as early as 1986, when initial research was funded by the military-controlled 863 Projects, according to state media reports.

At the time, China and the United States were in a “honeymoon” period that saw a lot of cooperation in space research, which boosted China’s confidence in building a large rocket with American assistance.

But the imposition of a hi-tech embargo following the 1989 Tiananmen crackdown severely impeded the development of large rockets in China. It was not until 2006, three years after Colonel Yang Liwei became the nation’s first astronaut, that the central government gave the CZ-5 project the official go-ahead.

The success of the manned space flight, and China’s long isolation from international space programmes, prompted a plan to build its own space station, which would not be possible without a heavy-duty launch vehicle. The core module of the Chinese space station will weigh more than 20 tonnes, while the CZ-2, most powerful rocket in the existing fleet, could not even lift half that amount.

The CZ-5 will also be used in China’s lunar and Mars exploration programmes, with the latter commencing as soon as in four years’ time, according to People’s Daily.

Civilian demand for such big rockets is limited, but the CZ-5 already has plenty of military missions lined up, according to mainland space experts familiar with the project.

The Delta IV Heavy, the CZ-5’s counterpart in the US, which can lift 28 tonnes into low-earth orbit, has four launches scheduled for this year: two spy satellites, one space surveillance satellite and one military communications satellite.

“That makes a good JD (job description) for CZ-5 in the future,” said a mainland satellite scientist who declined to be named.

At the China Academy of Space Technology, also known as the 5th Institute, established by legendary rocket scientist Qian Xuesen in the 1950s, a brand new satellite platform called Dongfanghong-5 was recently completed.

Satellites built on the DFH-5 platform will weigh at least 10 tonnes, heavier than any Chinese satellite in orbit today. The first of them, a communications satellite for the military, will be launched by the CZ-5 into a very high geostationary orbit next year, according to publicly accessible reports on the mainland.

Military satellites are bulkier because they have higher performance requirements than civilian satellites. Military communications satellites, for instance, carry a large number of heavy-duty transmitters to handle many channels of encrypted communication at the same time, while military reconnaissance satellites carry very large optical telescopes or radar to generate extremely high definition images on ground targets.

Previous models in the Long March series used new technology in no more than 30 per cent of the total design, with traditional wisdom holding that a greater percentage of proven technology was safer.

The CZ-5 is different, however, with Li saying “almost 100 per cent of the design is new”.

The rocket’s unprecedented size meant almost every previous component, even the simplest screws, had to be redesigned, and the CZ-5 team quickly realised that they had to start almost everything from scratch, with the research and development phase lasting almost twice as long as their initial estimate.

Computer simulations conducted at the end of the assembly estimate the launch has a 98 per cent chance of success, better than the 97 per cent for the safest previous Long March rocket.

Li said that was because the CZ-5 had two backup systems for each of its core components, while other Chinese rockets had just one.

Liang Xiaohong, a senior Chinese rocket researcher, said earlier this year that an even bigger rocket was being designed by China, with development of a 10-metre-diameter CZ-9 already under way.

When completed, the CZ-9 would match or even surpass America’s Saturn V, the biggest rocket ever built, and be capable of taking Chinese astronauts to the moon.