Its descent and ascent engine systems, its propulsion systems, and its restart systems had to work to safely and reliably set astronauts on the Moon and return them back into orbit about the Moon.

According to the Kennedy Space Center’s Spaceport News (January 11, 2008), Don Phillips, NASA’s test supervisor at the time of the launch said, “We were in the height of the Cold War and had all the schedule pressure you can imagine to meet John Kennedy’s ‘before this decade is out’ mandate. Our job was the total integration of the vehicle with the facilities and the Range. We also interfaced with mission control in Houston. We had a very dedicated and veteran launch team made up of NASA and contractor personnel.”

The Saturn rocket inserted the second stage and the LM into a 101 x 138 mile (163 x 222 kilometer) orbit about the Earth. The LM separated from the second stage about 45 minutes later. It circled the Earth two times, during which time the LM separated from the LM adapter, before performing a planned 39 second burn of its descent propulsion system (DPS) to simulate a descending approach and landing onto the lunar surface.

The only glitch in the mission came at this point when the LM's guidance computer stopped the DPS burn four seconds into the burn.

NASA mission controllers concluded that the computer’s programming software was at fault (the propellant tanks had not pressurized completely after four seconds, so the engine had not reached full thrust by the time, which caused the computer to sense that something was wrong, when in fact everything was ok).

Now in a 104 x 138 mile (167 x 222 kilometer) orbit, NASA mission controllers quickly modified their original plan and turned off the guidance computer in order to begin a sequence of burns to fire the descent and ascent engines.

This procedure fired the descent engine twice (once for 26 seconds at a thrust level of 10% and again for seven seconds at 100% thrust). At this point, the descent engine became the first “throttleable rocket engine” fired in space.

A third DPS burn was performed 32 seconds later, consisting of a 26-second burn at 10% thrust and a two-second burn at 100% thrust.

With the DPS system tested, NASA then moved on to the ascent engine and its "fire in the hole" burns.