Another story on forgotten requirements in the space program. During NASA’s Constellation program, we all knew the Apollo Lunar Modules had a requirement that they couldn’t land on terrain more sloped than 11 degrees. (Or was it 10.5? I’m no longer sure.) But nobody knew why. 1/n
2/n Even the lead designer for the Altair lunar lander told me that he didn’t know why Apollo Lunar Landers had that requirement, although he knew that it was in fact their requirement. It was another requirement whose rationale was long lost in the mists of time.
3/n One reason we talked a lot about that requirement was because it almost got violated during one of the Moon landings: Apollo 15. That mission, lunar module blew so much more dust than most other landings (except Apollo 12) that they couldn’t see what was beneath them.
4/n. They ended up landing right on the rim of a broad, shallow crater. The engine nozzle got so close to the crater’s raised rim that the back pressure (choked flow) forced the nozzle to crumple as a huge blast of dense soil shot out. See the crinkle? 
5/n. You can read more about these blast effects in this paper I wrote here: onlinelibrary.wiley.com/doi/10.1029/20… . But getting back to the ~11 degree tilt limit imposed on the lunar modules... After landing, the back footpad of the lunar module was suspended in free space over the crater.
6/n. They shut off the engine as planned, then the lunar module starting tipping backward into the crater. It tilted back about 10.5 or 11 degrees then the back footpad hit the ground inside the crater. One of the crew (Scott or Worden) said “BAM” on the radio when it hit.
7/n. So they were right at the design limit of the lunar module. During Constellation, this became one of the canonical examples for why it is important to control blowing dust, so the crew can see what they’re landing on. We discussed this a lot.
Supposedly, the mission could have failed if the ground was just a little more tilted, if that shallow crater was just a tinier bit deeper than it was. But the key word is “supposedly”, because nobody knew any more where the requirement came from or what it really meant.
9/n. One theory was that it was the limit for how much side-loading the release pins could handle on the Ascent Module. Too much side-load and the friction would be too great, they wouldn’t be able to release, the crew wouldn’t be able to come home. That was just our hypothesis.
10/n. I strongly suspect that it wasn’t a real failure point for the mission. More likely just a design bogey. Maybe the chief systems engineer told the team, “Let’s assume it could tilt up to...uhh...let’s say 11 degrees. Yeah. Design it to work even up to that amount.”
11/n. So it’s possible that every piece of the lunar module was designed to work at even greater tilt angles than 11 degrees. Apparently, though, there was no /guarantee/ the mission would work if it tilted more than 11 degrees. We might never know.
12/n. You might wonder, why not just go get the design documents and look up the rationale? We never had much luck trying to find old specifications. Some were buried in contractor tech doc libraries, and those contractors are long gone. Nobody knows the fate of their libraries.
13/n. A company gets bought by another company so they merge their libraries, but where? Then the relevant division gets spun off and re-merges with another company, which later goes bankrupt. So where did all the old documents end up? Nobody has funds to track this all down.
14/15. Even the telemetry data tapes from the lunar landings are in disrepair and will soon be unreadable. And we can’t interpret the data format from those tapes if the specs cannot be re-located. It’s sad how quickly we are losing this history.
15/15. We ended up doing forensic analysis of the artifacts. Example: we measured the leftover Lunar Modules with photographic techniques to make up for the lack of documents. Studying Apollo is like solving mysteries even though it was just 40 years ago.
