What it was: A direct-ascent lunar lander based on the Gemini capsule, which would be mated to a lower landing and ascent stage and a rocket stage that would get it to the Moon. Two men would travel in it, land on the Moon, and return to Earth in it, no docking required.
Details: One of the main factors that made the United States the winners of Kennedy’s lunar landing challenge was their focus. In contrast with the Russian program, which was constantly torn between three visionary designers (Korolev, Glushko, and Chelomei), the Americans settled on one way of getting to the Moon and stuck to it. The parameters of the Apollo program—three men launched on a Saturn V in a Lunar Orbit Rendezvous (LOR) craft—were all set by early 1963. From there it was a steady run for the next six-and-a-half years to the lunar landing, disrupted only by the Apollo 1 fire.
One thing that nearly upset this single-mindedness was the Gemini capsule. It was originally intended as a time-filling experimental craft that would use the gap between the end of Mercury capsule flights on May 15, 1963 and the first planned Apollo test flights. When Lunar Orbit Rendezvous was selected as the Moon mission’s approach, however, McDonnell Aircraft—manufacturer of the Gemini—pointed out that this would mean only two astronauts would go down to the surface. If only two were landing, why not just send two? The Gemini had fourteen days of endurance, after all, and it was only six days to the Moon and back if all went well. McDonnell worked up a way to create a lunar landing craft out of a Gemini that could be sent on a direct-ascent mission to the Moon, the approach that NASA had favoured until the surprising sea change of Spring 1961 that lead to the LOR decision.
In all McDonnell came up with three versions of the craft, dubbed the Lunar Gemini I, II, and III based on how different they were from the orbiting Gemini they were also in the process of building. The Model I was most similar, down to the use of ejector seats for the crew if something went wrong during the initial launch from Cape Canaveral. Its main difference was a modification to the side of the capsule above the left-hand crew member’s head so that there would be a bubble canopy he could look out. Model II was modified to re-enter over water (at the time, the orbital Gemini was being built to come in over land, and so was the Model I Lunar Gemini), with the resulting weight savings allowing for an upgrade to the navigation system and the installation of Apollo’s beefed-up communications system. Model III was most different, including an escape tower to replace the ejector seats, re-entry over water, and a rearrangement of the seats and windows that will be discussed later. All three were also to be equipped with the planned Apollo landing radar.
Besides the other modifications to the capsule, the Lunar Geminis would be built so they could mate to three other pieces, a terminal engine module, a landing gear stage, and a retrograde module. The whole works was then to be attached to the top of a Saturn V which would launch it into Earth orbit. Once in orbit the landing legs would be exposed by ejecting aerodynamic fairings used to protect those fragile structures from the slipstream during ascent.
The retrograde module’s engine would then fire the Lunar Gemini away from Earth towards to Moon, perform mid-course corrections, and insert it into lunar orbit. The module’s final responsibility would be to knock the craft out of lunar orbit into the descent phase, slowing it down until it was only 1800 meters above the surface.
At 1800 meters the retrograde module would have been jettisoned to crash elsewhere on the Moon and the last distance to the ground covered by the terminal landing module’s engine with the capsule and landing stage perched on top.
This touchdown would have been the most hair-raising part of the mission for the Lunar Gemini I and II. As with the regular orbiting Gemini the crew faced towards the nose of the capsule in these two, which is to say they were pointing away from the Moon as the craft backed into a landing. While the co-pilot worked the controls of the lander and watched the Moon in a deployable rear-view mirror, the pilot needed to turn around and observe the lunar surface out of the side of the capsule. That way he could look for a clear area large enough to land in and call out a course to the man working the controls. The plan was to supply him with the aforementioned bubble canopy to give him a 180-degree field of view; if engineering difficulties arose during development the alternative solution was to depressurize the cabin and have the pilot lie prone on his seat back while sticking his head out of the capsule’s opened hatch!
The Lunar Gemini III thankfully proposed rotating the crew seats so that they were side-facing and replaced the windows of the orbital Gemini so that the astronauts could see out to land—much like the Apollo LM was to do.
After touchdown, the astronauts would stay on the Moon for a day, and then the capsule and terminal landing engine would launch for return to Earth while leaving the landing gear behind. This diminished craft would be able to get home directly, without having to stop in lunar orbit like the Apollo program’s LOR.
On arrival at Earth six days after the start of the mission, the Lunar Gemini capsule would detach from the rest of the craft and bring the astronauts home. Models II and III would splashdown with the help of parachutes, while the I model would glide into an airstrip on US soil with the help of landing skids and a Rogallo wing—what we would call today a hang glider, an invention of NASA’s aerospace-focused predecessor NACA. The last remaining change to the Gemini capsule would come into play here: as a direct return from the Moon is faster than a return from low Earth orbit, all three models of the Lunar Gemini would have had a thicker heat shield than their orbital counterpart.
What happened to make it fail: NASA stayed focused and staved off all suggestions that they go with anything other than a three-man, LOR configuration. President Kennedy’s science advisor Jerome Wiesner was the highest-placed advocate of using the Lunar Gemini, but a confrontation with NASA director James Webb eventually eliminated any chance of it by the end of October 1962.
Their refusal stemmed from a couple of good objections to the two-man direct descent approach. The Lunar Gemini had much less redundancy than the Apollo CSM/LM combination, which made its missions considerably chancier. Apollo 13 proved their decision by giving Lovell, Swigert, and Haise the LM to use as a lifeboat, letting them eke out their resources on the way back to home. A Lunar Gemini crew had no such option.
NASA had also studied the Lunar Gemini I and II’s pilot-backwards landing configuration in other contexts and couldn’t come up with a way to do it that satisfied them. Lunar Gemini III was the only arrangement that went for a setup like that used by Apollo, and had to creep up in weight towards the Apollo CSM/LM combination to do it. That meant there was less incentive to move away from their initial plan.
What was necessary for it to succeed: If the Russians had been able to keep Sergei Korolev’s initial lunar landing program going, rather than having it fall into disarray during 1964-65, then the US might have been panicked into switching horses. At the very least US intelligence would have had to conclude that the Soviet Union was on track for a lunar landing in 1967 or ‘68, regardless of whether or not they actually were. This is not entirely unlikely: consider the Uragan space interceptor, which the CIA and industry insiders became convinced the Russians were developing during the 1980s. It apparently never existed.
The Jim Chamberlin-designed Gemini capsule was surprisingly capable, so the Lunar Gemini would probably have worked. The proposal suggested a landing during the first half of 1967, but it would have had to wait until no earlier than 1968 as the craft depended on the Saturn V and that rocket wasn’t ready to go until then—as late as January 1967 one of its stages exploded during testing. The shortcut to the Moon, in other words, would not actually have been that short.
The main difficulty with Lunar Gemini lay with, as mentioned earlier, its lack of redundancy. The crude landing systems of the Gemini I and II would have also produced problems. Consider the famous episode of the last few seconds of Apollo 11’s landing as Neil Armstrong worked feverishly to find a clear landing spot and only just succeeded despite flying a craft more capable than the two lesser Geminis.
So there would have been a Gemini-based lunar landing if NASA had decided to go that way, but the program would have chanced more failures and outright disasters. Mapping it on the Apollo missions has “Gemini 11” aborting at the very last second if NASA has gone with the Gemini I or II, as its pilot can’t find a place to put down. “Gemini 12” succeeds, but then “Gemini 13” is an unmitigated disaster: its astronauts have no place to go after their capsule fails and die while en route to the Moon. Whether or not there’d be a “Gemini 14” through “17” after that is an open question.