What it was: A tentative plan to explore the long-term biological effects of free fall on primates before sending human beings into space for extended periods. It was in essence a tiny, automated space station inhabited solely by monkeys.
Details: By December 1967 the longest manned flights in space had still fallen far short of what would be necessary for long-term habitation of space stations, let alone trips to Mars or Venus. The record holder, Gemini 7, had lasted for 13 days and 18 hours and only three other missions had stayed in orbit even as long as four days.
Accordingly while there were hints that there were biological consequences to being in microgravity for extended periods of time their exact nature was a mystery. As part of one approach to solving the problem NASA’s Langley Research Center gave a contract to Northrop Corporation (and to Lockheed, which developed a similar alternative) to study ways of conducting automated and teleoperated animal experiments over a year-long period in orbit.
The result was an Apollo Applications Program proposal, the Orbiting Primate Spacecraft (OPS). The plan was to build a small pressurized compartment big enough for two rhesus monkeys and their perishable supplies, and attach it to an unpressurized section based on the LM&SS truss; this would carry life support equipment such as a water tank and a lithium hydroxide CO2 scrubber.
If the mission had gone ahead the OPS would have been put in the upper stage of a Saturn IB, in the spot normally reserved for the LEM. An Apollo CSM carrying either two or three astronauts would have been placed above it as per usual and then the rocket launched from Cape Canaveral. The upper stage, the OPS, and the CSM would end up in a 185 kilometer, 28.5° orbit, at which point the CSM would undock, turn around, and then re-dock nose-first with the exposed OPS—again, much like an LM would be treated. The CSM would then fire its engine and boost itself and its scientific payload to a more long-lasting 460 kilometer orbit where the OPS would be released and the astronauts could get on with the rest of their mission.
Inside it, the two monkeys would be in individual pods sealed off from the rest of the craft and each other, to prevent fighting, with a small opening for some social contact. This design prevented stray waste or food from gumming up the instruments during a year-long mission with no humans on board. Northrop decided to further solve the latter problem by by conditioning each monkey to respond to a food light flicking on by going to its feeder and placing its lips over its outlet—a pellet of their usual monkey chow would then be put into its mouth so that food wouldn’t fly off in zero-g. The proposal does discuss feeding them with pastes or other more tractable foods, but that approach was rejected over worries that the subjects would be sufficiently unhappy with the change from their usual Earth diet that it would affect the results of the experiments.
“Monkey waste” was noted as the much more difficult issue. Without gravity or a human to keep the life pods clean, Northrop had to spend considerable thought on their design and still felt their solution needed more work. One wall of the pod was made into an open wire grid (which not incidentally gave the monkeys a “floor” to hang on to if they wanted one) over a storage receptacle; a forced airflow would have been passed through the pod and pushing waste toward it whenever needed—the monkeys were to be under observation by television camera anyway. If something stuck, another wall was movable and Ground Control could command it to scrape the compartment.
The movable wall was primarily for another purpose, though: forcing the monkey into a recovery capsule attached to the bottom of the OPS once the mission was over. As the capsules could be sealed off from the rest of the spacecraft one could also have also been used to store and isolate a monkey that had died for whatever reason prior to the end of the mission.
Approximately one year after the Orbiting Primate Spacecraft was set floating on its own, a second Apollo CSM would be sent up to retrieve the its simian inhabitants. Once again the ship would dock nose-first with the station, and the monkeys (or the mortal remains of one or both of them) would be retrieved by a spacewalking astronaut. After a short capsule-enclosed spacewalk of their own the two subjects would be stowed aboard the CSM and returned to Earth, while the OPS would be deactivated and left in orbit. The monkeys would then provide a wealth of data to supplement what they had already given—the plan was to test them onboard using a lights panel they had been conditioned to obey prior to the mission, as well as possibly surgically attaching various wireless telemetry probes to them (though that technology was quite advanced for the time). One part of getting this data actually turned out to be the other major problem past monkey waste that Northrop couldn’t crack: how to weigh—or technically, mass—the monkeys in zero-gravity without a human on board to do it.
Though the Orbital Primate Spacecraft never made it past a fairly speculative stage, it was positioned quite clearly in NASA’s schedule as it existed in December 1967. Northrop examined the various Apollo missions that weren’t headed to the Moon (primarily Apollo astronaut training and the building and supplying of Skylab at its subsidiary stations as it was then conceived) and settled on Saturn/Apollo Applications flight 218, then scheduled for late 1970. Retrieval was more nebulously scheduled beyond the basic parameter of keeping the experiment running for six to twelve months, and was relegated to any one of the resupply missions SAA 221 to 228.
While the Orbiting Primate Spacecraft weightlessness experiment was the primary mission described, Northrop also briefly described a few other possible uses for a variant OPS, such as mounting it on an LM landing stage and studying the biology of the rhesus monkeys under lunar conditions, or putting one at the far end of a tether attached to a (presumably unmanned) CSM acting as a counterweight for spinning up the arrangement and running artificial gravity experiments. They also somewhat bizarrely tout it as a possible habitation module extension for a spacecraft built around the Gemini B being developed for the Air Force’s Manned Orbiting Laboratory, and even joining two of them in sequence as an emergency shelter in orbit.
What happened to make it fail: The Apollo schedule was very much in flux by the time Northrop submitted their study to NASA in December 1967. The Apollo 1 fire in February of that year had suddenly made it that much harder to reach the goal of landing a man on the Moon by the end of 1969 and NASA refocused their efforts much to the detriment of anything that didn’t directly lead up to Apollo 11. The Apollo Applications Program suffered accordingly.
By the time the AAP got back on track, budget was a problem. Several AAP proposals were cancelled because there were no more Saturn Vs available no matter what money could be thrown at them, but this wasn’t one of them. Had the money been there, there wouldn’t have been much difficulty coming up with a Saturn IB, a CSM, and the OPS. But this project was going to have to wait until after Skylab no matter what, and considering that that didn’t get going until May 1973—five years after the flight date assumed for the proto-Skylab in the proposal—this mission would have flown during the absolute nadir of NASA funding. By then the Space Shuttle program was absorbing everything they had.
The author also has a suspicion that the mission never went very far because it lacked dignity—while intellectually the OPS is a reasonable experiment, it’s hard to not react to it emotionally as “monkeys in a can”. Shepherding a mission from conception to flight is a political process, and what ex-military man or sober scientist is going to champion it and expose himself to ridicule (unfair or not) when there are so many other impressive or even heroic space missions to choose from?
As it was, it never advanced beyond the Phase A stage, which is to say simply determining how the mission would work in general and not serious design work, and NASA settled on getting their biological data by gradually extending the length of human missions. Skylab 2 lasted 28 days; Skylab 3 lasted 59. The longest American stay in space to date is still only 215 days (Michael López-Alegría, on ISS in 2006-2007), while only two human beings (Valeri Polyakov and Sergei Avdeyev) have exceeded the year that this experiment would have lasted.
What was necessary for it to succeed: The Apollo 1 fire was obviously a major problem, but simply eliminating it probably doesn’t help—it was a symptom of a wider problem at NASA and something else would have come along to disrupt the AAP schedule (and delay the OPS) instead.
The best bet would have been some greater commitment to space stations in the 1970s than NASA actually had. After Skylab the American record for consecutive time in space languished for a very long time until the ISS was up and running. Under those circumstances there was no need to push forward long-term microgravity knowledge. By the time it became necessary, the US even had some access to Russian data, as the USSR had had several long manned missions to the Salyut stations and Mir and the two countries were co-operating to an extent after the end of the Cold War.
Having the US follow the same path as the Soviets wouldn’t have been too hard. The Integrated Program Plan of 1969-70 had several components and the Space Shuttle and a space station were the two likeliest to go forward once it became clear that the entire IPP was a political impossibility. NASA chose the Shuttle, but that was a somewhat surprising decision at the time. Get them to pick a station as their primary goal instead and suddenly the Orbiting Primate Spacecraft starts to look a lot more attractive.