Q:

I am curious what aspects of the Apollo program were impressive/advanced from an engineering perspective, in the 1960s and 1970s. That is, what would have made an educated engineer say, “Wow, they solved that problem?”

I ask the question because I know that as an engineering layperson I know I have very poor intuitions about what is technically difficult in spaceflight. For instance, I only learned from this website that maintaining 1 atm of pressure in a spacecraft isn't very difficult. Also, some technologies like pressure suits and rocket engines had already been developed. So it is not obvious (to me) what the actual innovations and engineering achievements of the program were.

A:

There was no one breakthrough that made it possible. The "big deal", in the mind of the world, was just that an obviously very hard thing was accomplished. And, if you doubted how hard it was, people can point out that no one has done it again in more than fifty years.

However, there are some good examples of challenging problems that had to be solved.

Problem 1: Rocket Size. Before Apollo, everyone thought we would send the top of a multi-stage rocket to the moon, it would land on its tail and launch again to return to earth. When you run the numbers on that, you end up with a pretty big lander requiring a lot of fuel, and a huge launcher to send it on its way; much larger than Saturn. The trick ended up being to only send down a little bug, and even leave part of that behind on the moon. If we had stuck with the giant lander we would never have been ready in time.

Problem 2: Rendezvous. The new method required being good at approaching and docking with another spacecraft. That's a hard enough problem that, even though the physics was well understood, they didn't really see the issues until they actually tried it. (I always get annoyed when characters in science fiction stories fail to foresee problems that the science should have told them beforehand, but sometimes that's how it works.) Wisely they tried it in Gemini in low earth orbit and had the hang of it by Apollo.

Problem 3: Rocket Size (Again). Even with the trick (called Lunar Orbit Rendezvous) used to solve Problem 1, they needed a much bigger rocket than anyone had built before. And so they built it. To get it to the launch pad, they built the crawler transporters, some of the largest land vehicles built up to that time, and to have a protected place to stack the stages, they built the Vehicle Assembly Building, one of the largest buildings by open volume in the world. I think seeing a tower the size of a 36-story skyscraper rise into the sky made a lot of people say, "Wow, they solved that problem?" I was too young at the time, but it was the initial uncrewed Apollo 4 launch of the Saturn V that made my dad think, "Huh, they might actually pull this off!"

There are many many more, but it was really the cumulative effect of solving thousands of hard problems that was the big deal. //

Number 2 is a small example of the large original research involved. A guy, later known as "Doctor Rendezvous", did his Ph.D. thesis at MIT on it in 1963. His next job was to fly it! Here's Buzz Aldrin's thesis: dspace.mit.edu/handle/1721.1/12652 –
Adam
May 19 at 1:38

The "slow down to catch up, speed up to slow down" stuff of orbital rendezvous was reportedly very confusing to the non-engineer test pilots and required someone like Buzz Aldrin to truly figure out. It's one thing to draw the equations out on paper but a whole other thing to actually do it in the cockpit. –
Jörg W Mittag
May 17 at 19:48

@JörgWMittag I think its still confusing to a lot of people today, mainly because of the terms "slow down" and "speed up" in that phrase are ... wrong, but appropriate? –
Moo
May 17 at 21:32

@Moo - If you go faster, you also go higher. Now that you're higher, you've got farther to go, so you're actually going slower –
Richard
May 18 at 18:36 //

A:

what has ALWAYS impressed the heck out of me is the sheer magnitude of scale involved... not physical size (although its size was truly impressive) but rather the huge number of complex problems that needed to be all solved in a complex optimization matrix to arrive at a suitable overall solution. This was the largest systems integration project ever to date and on a tight timeline. Project management on an unheard of scale and scope. That to me was the "Wow... they solved THAT problem" thing.

Yes. Apollo was a triumph of project management as much as, or maybe even more than, it was a technological feat. –
Wayne Conrad
May 18 at 2:40

I would argue that project management was born within the Apollo program. I don't think it even had a name beforehand. –
Vladimir Cravero
May 18 at 8:18 //

A:

It was fractally hard.

Everything they did was Voltroning hard problems together to solve other hard problems. And this was all done in a coordinated way on an incredibly tight timescale.

The long answer would fill a series of books. E.g. for a high level overview of the effort involved in the LEM alone, you can look at Tom Kelly's Moon Lander (and you should; it's great).

But to put a quick gloss on top of it, Apollo was not an aerospace engineering triumph, Apollo was a systems engineering triumph. Everyone solved hard problems in every field, but the real accomplishment was orchestrating those solutions in a way that led a seven-year program from zero to the moon.

Apollo systems engineering built upon Polaris (see en.wikipedia.org/wiki/UGM-27_Polaris). –
Jon Custer
May 18 at 20:42

@JonCuster Sort of, but it's more complicated than that. I'm not putting a history of SE in this answer though. Recommend Morris' "Management of Projects" from 1990 or so if you want an overview of the most relevant thread for Apollo/ –
fectin
May 19 at 0:55