Later this year, China will mark the 15th anniversary of its first human spaceflight. On October 15, 2003, Yang Liwei launched into space on a Long March 2F rocket. After making 14 orbits around Earth, Liwei returned to the planet as China received congratulations from countries around the world. It had succeeded where only the United States and Russia had before.

At the time, the secretive Chinese government released few technical details about the spaceflight. But apparently there were some serious problems, especially during the launch of the rocket. In a new interview with Xinhua, the official Chinese news media, Yang revealed that he experienced extreme vibrations between 30 and 40km above the ground.

"I thought I was going to die," Yang said. At the time, sitting in the seat of his cramped Shenzhou spacecraft, he recalled telling himself, "Hold on! Just hold on for a bit longer."

The low, thrumming vibrations lasted for 26 seconds before the rocket finally calmed down. The rest of the flight proceeded more nominally.

Although the Xinhua article does not state so explicity, this vibration appears to be a manifestation of "pogo," a metaphor used because the action of the rocket is something akin to the vibrating of a pogo stick. This is caused when the engines of a rocket thrust at slightly different levels, causing differences in acceleration across the rocket, and introducing variations in the flow of propellants.

NASA experienced this problem most acutely during the launch of Apollo 6, the second uncrewed test flight of the Saturn V rocket. For about 30 seconds during the launch, the rocket shook violently, damaging some of its upper-stage engines. After this flight, NASA took steps to dampen these vibrations and had enough confidence in these efforts to place humans on the next launch of the Saturn V rocket, and, indeed, to send them around the Moon in the historic Apollo 8 mission.