This is the Apollo 9 LEM, photographed after it separated from its CSM. Photographs of either CSMs or LEMs in space are typically nose on, since each could only be photographed from the other (there wasn’t anyone else around to do it), and they only separated in lunar orbit at the outset of a landing maneuver or at rendezvous. Apollo 9 separated in low earth orbit and performed various maneuvers there, making this side-on view, right above the Earth, a rare treat.
Apollo 9 launched fifty years ago March third. That’s a Sunday, and I don’t post on Sunday, but there will be plenty on that mission the following week.
Virtually all of the missions returning from space have returned by atmospheric braking and parachute, or atmospheric braking followed by a winged landing. In the early days of science fiction movies, landings were always tail first but that was not possible on Earth until Elon Musk and SpaceX finally managed it in 2015.
On the moon, there was no choice but to land tail first, slowed by rockets, and the LEM was built around that fact. Learning how to land tail first was also a major issue; see 185. The Flying Bedstead.
The LEM was a two stage rocket. The descent stage, dot-shaded gray in this NASA drawing, made up about two thirds of the mass of the LEM. It contained a frame, tanks with fuel and oxidizer, a rocket engine, and the landing gear. It also contained storage space, accessed from the outside, for the equipment that would be used once the astronauts were on the moon.
The landing gear served multiple functions. The pads at the end of each leg were designed to keep the LEM from sinking into the lunar soil. Their size was both a compromise and a guess. No one knew either how deep the lunar dust was, nor how much structural integrity it had. Worst case scenarios had the LEM sinking hopelessly into many feet of lunar dust, the accumulation of millions of years of micrometeorites pulverizing the lunar surface. In fact, the pads only sank slightly.
The number of unknowns that faced the engineers and mission planners was immense. It it hard for people born since the seventies to imagine the depth of our ignorance before Apollo 11 landed.
The struts were designed to absorb energy, because the LEM could not fire its engines all the way to the ground. The upwash of lunar dust and rocks would have blinded the pilot and possibly knocked holes in the LEM, so the engine was designed to be cut off at a certain height above the lunar surface, letting the LEM fall the last small distance. But how high? That was another calculated estimation (guess). And how much spring would the struts need? Too little and the LEM would crash to the ground. Too much, and it would rebound with possibly disastrous results. And if one leg landed on a boulder or in a hole, the whole LEM might tip over and be unable to return to orbit.
The ascent stage contained crew space, controls, computer, radar, guidance systems, oxygen for human use, and the crew in their space suits. It also contained fuel and oxidizer and its own rocket engine, all smaller than for the descent stage since the LEM ascent stage was one third the size and mass of the complete LEM. The descent stage formed a launching platform for the ascent stage.
When Apollo 17 launched from the moon, a camera was mounted on the rover which was left behind. You can see all 36 seconds of the last ascent stage liftoff from the moon at https://www.youtube.com/watch?v=9HQfauGJaTs.
Apollo 11 proved that all this would work. Apollo 10 was a dress rehearsal of everything but the final landing. But until Apollo 9, fifty years ago this weekend, no one knew if the LEM would work at all.
More next week.