This was originally intended as a detailed picture of the Lunar Excursion Module, but it became clear while writing that before I could talk about the vehicle, I had to lay out it’s place in the scheme of things. This post then became a generic mission profile, and details of how the LEM worked will come in the next post.
If you Google lunar lander, you will find the LEM, but you will also find a lot of forgotten craft. Both the United States and the Russians had unmanned lunar landers and lunar crashers. That’s not a joke. Before soft landing was perfected, we learned a lot about the moon from probes which photographed all the way down to a crash landing. Those piles of rubble that dot the moon were the ancestors of Spirit and Opportunity.
That’s not good enough for a craft that was to be, in the vernacular of the day, man rated.
The LEM, or LM as it is often called today, was unlike any manned craft before or since. It has been called a “true” spacecraft, but in fact it only got half way toward that ideal. A “true” spacecraft, built in space and powered by a low force, long acting engine, would never have to endure the vicissitudes of atmospheric friction or high gravity.
The LEM did have to withstand multiple gravities during its launch from Earth, and again on landing and taking off from the moon. However, it never had to come in contact with atmospheric friction because it spent the launch hidden behind a streamlined clamshell shroud. It didn’t itself have to be streamlined, and its skin could be flimsy. The astronauts joked about being afraid of accidentally putting a boot through the side of the vessel. At least I think it was a joke.
The Saturn 5 is called a three stage rocket. It could as easily and accurately be called a six stage rocket. The first and second stages were designed to burn all their fuel and fall away. The third stage carried the rest of the vehicle into orbit and then shut down; at that point, it’s fuel was not exhausted.
If the mission was to lunar orbit or landing, the Apollo craft stayed in low earth orbit long enough to establish that all was well, then the third stage fired again to send the craft toward the moon.
On Apollo 8, there was no LEM, so in December I only described the Saturn and the CSM. Apollo 9, whose fiftieth anniversary comes in about ten days, had a LEM but never left near Earth orbit. Apollos 10 through 17 were lunar missions. They had similar flight plans and used all “six” stages.
When the Saturn third stage fired a second time, it put the entire remaining craft into a orbit toward the moon. The third stage would have gone right along with the rest to the craft, if it had been allowed to do so.
What happened next on each mission was well presented in the movie Apollo 13, but only if you already knew the what, the when, and the why. It was drama, not documentary, but with excellent animation. If you have a DVD of Apollo 13, take a look.
The LEM, and the CSM (command and service modules, treated as one) had initially been stacked vertically above the third stage, with the LEM protected by a shroud. The attached NASA drawing also shows the abort rocket above the command module, but that had already been discarded by the time the craft was actually on its way to the moon. All three astronauts were in the CM. The CSM, the LEM, the shroud, and the third stage are all still in one piece.
Now the CSM was released; it moved forward on maneuvering thrusters and turned a one-eighty. The LEM was still attached to the third stage. Now the clamshell opened up and the CSM moved carefully forward and docked with the LEM, front of CSM to top of LEM. The LEM was released from the third stage and towed away by the CSM. This position allowed the hatches on the CSM and LEM to mate so the astronauts could move freely between the two craft. The legs of the LEM, previously tucked under to fit within the shroud, now extended into lunar landing positions.
From this moment until lunar orbit was achieved and it was time for the LEM to move away from the CSM and land (or nearly land in the case of Apollo 10), the LEM/CSM were essentially one space craft. The Saturn third stage now made one last burn, changing to an orbit that would carry it out of the way.
For about two and one half days, the LEM/CSM drifted toward the moon. Upon leaving low Earth orbit, the craft had been traveling at close to 25,000 miles per hour. It should have reached the moon in ten hours, but the Earth’s gravity was pulling at it and slowing it down. Approximately six sevenths of the way to the moon, the craft was traveling at it’s slowest speed. At this point the Earth’s gravity and the moon’s gravity were in equipoise; thereafter the moon’s gravity accelerated the craft again.
At a point on the back side of the moon, the SM engine fired, slowing the combined craft enough to keep it from whipping around the moon and returning to Earth. It entered orbit of the moon. This burn, and the later one which put the CSM on its homeward trajectory, make the CSM essentially the fourth stage of the Apollo/Saturn mission.
Now it was time for the LEM to earn its keep.
What’s that you ask? Stages five and six? Where were they? The LEM itself was a two stage rocket. We’ll get details on that next post.