At 7 o’clock on the morning on April 4, 1968, Apollo 6 thundered off the launch pad. Everything looked perfect until the rocket started vibrating in flight so hard it was almost bouncing up and down. The so-called pogo effect (it’s almost like the rocket is bouncing on a pogo stick) increased g-forces in the spacecraft and shook the rocket’s frame so hard that loose structural panels on the lunar module adapter section fell off. The irregular launch also destroyed the flight path, leaving the spacecraft in a highly elliptical orbit rather than the planned circular one. Luckily Apollo 6 was unmanned, but the problem was bigger than one mission. NASA had spent years solving the pogo problem only to see it return, this time threatening manned lunar missions.