Project Horizon (Part II): The Minimal Orbital Station and the Orbital Return Vehicle

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A “conceptual view” of the Minimal Orbital Station. Based on the mockup of the initial two-module station (linked below), the actual MOS would have looked somewhat different, but one module would be for human habitation and the rest would be gas tanks for fuelling the Moon ship and its injection stage, top. Another tanker is approaching, as it would take four fuellings in total to top up for a journey to the Moon. Image from Project Horizon: Volume I. Click for a larger view.

What it was: Project Horizon was the US Army’s full-blown proposal to put a man on the moon (and, in fact, start up a whole lunar base) by the end of 1966. While much of its focus was on the base itself, it also included extensive discussion of both the Earth-bound and orbital infrastructure they felt was necessary to reach their goal. The Minimal Orbital Station (MOS) was essentially a gas station in LEO for ships headed to the Moon, while the Orbital Return Vehicle was to be used to bring the gas station’s attendants back down to Earth when their tour of duty was done.

Details: In Project Horizon, Part I we discussed the US Army’s proposed launch facility on Christmas Island in the Pacific. Had it gone ahead, American soldiers and civilian experts would have been loaded on Saturn I rockets (or possibly a “Saturn II”, an early design of what would lead to the Saturn V) and launched into orbit.

Where would they go? To the space station, of course. The Project Horizon proposal blandly asserts “It is very likely that a previously constructed completely equipped space platform will be available in 1965 for use as housing facilities and for other support for the refueling operation.” It does then admit that it’s at least possible that 1965’s near-inevitable space station might not be suitable as a base for the Moon mission (by, for example, being in the wrong orbit). On the off chance that this happened, they proposed the Minimum Orbital Station.

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A view of the MOS mockup shown in 1960. The bottom module is a Saturn stage converted for habitation. The cone at top is the ORV. Public domain image from NASA. Click for a larger view.

Building the station would start with a launch of a Saturn I with a payload of men in the nosecone. Once in a 640 kilometer equatorial orbit (a height selected so that the station would circle the Earth in an even fraction of a 24-hour period, making it easier to return to the launch point after the mission was over) the nosecone and the upper stage of the rocket would both reach orbit, at which point the two would disengage. The upper stage’s remaining propellant would be blown out, and the aft end of the nose cone would be mated to the top of the stage—or possibly the side if this variety of Saturn I was redesigned to have an airlock there. The astronauts could then enter the empty stage and fit it out as a living and working space; this “wet workshop” concept would appear again in the designs of the Manned Orbiting Laboratory and the original Skylab.

This basic MOS was enough to house the astronauts, but not to support a Moon mission. Project Horizon assumed that the lunar landing would be a direct descent, which implies a considerably heavier load of fuel than was needed for the Lunar Orbit Rendezvous approach that NASA came to favour in the years following the Army proposal. The Saturn I was completely incapable of lifting both a direct-descent landing craft and the necessary fuel at the same time, so the two had to go up separately. Once the MOS had one empty stage housing its crew, another Saturn I would be sent up, only this time it was carrying a full load of fuel instead of men. The fuel-laden tank would attach itself to the side of MOS, and then the process would repeat: the baseline lunar landing mission in the proposal suggested four tanker launches would be needed before enough fuel was in orbit. More missions might be sent up carrying men, depending on how many orbital personnel were deemed necessary for the next step: getting the lunar craft fuelled and underway.

One final Saturn I launch would loft an unfuelled direct descent ship (that’s six launches now, if you’re keeping count), which would rendezvous with the MOS. The station’s crew would get into their spacesuits, spacewalk out into LEO, and transfer the fuel from one to the other. Three crew would then get aboard the lander and head off to the Moon. The remainder would either re-board the station for the next mission (the pace of Project Horizon’s launches, as discussed in Part I, was downright frantic), or head back to Earth.

Those headed back to Earth would use the Orbital Return Vehicle. This is where the word “minimal” really comes into play, as it was essentially just a conical capsule roughly 7 meters long and 4 meters at its base. After detaching from the MOS, a small retrorocket would knock it out of orbit and its crew would ride down to the ground. Astonishingly, the Project Horizon report suggests that it would have carried anywhere from 10 to 16 men at a time—bear in mind that the actual Gemini capsule gave its two astronauts 5.7 x 3.05 meters to play with, and no-one ever described the Gemini as “roomy”. This feat was accomplished by dividing the interior into no less than three decks (four, if you count the equipment compartment in the nose). There would be no room for sitting or standing here, so the astronauts would lie prone for the entire trip. Here’s hoping they didn’t miss their initial de-orbit burn time and have to wait 90 minutes for the next.

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An unfortunately blurry diagram of the ORV from Project Horizon: Volume II. Notice the number of astronauts depicted. Click for a larger view.

Ultimately, the plan was to turn the MOS into a destination in its own right, chaining together more and more Saturn stages converted for habitation and then eventually curving the chain back on itself to form a ring station that could be spun for artificial gravity.

What happened to make it fail: As mentioned in Part I, Project Horizon fell afoul of Eisenhower’s dislike of military activity in space. He’d already tried to pry manned space programs away from the Army, Air Force, and Navy by forming ARPA in February 1958. When that failed (ARPA then famously moving on to other projects like the development of the primitive Internet), he tried again and got them transferred to NASA.

In the particular case of the Horizon space station part of the larger Project Horizon. The Army’s main interest was militarizing the Moon, and the station was just a step toward that was made necessary by an EOR mission profile. When Eisenhower specifically took space exploration missions away from the services and gave them to NASA, the Army Moon program was dead and an Army space station to support it became superfluous.

What was necessary for it to succeed: The station was only going to go ahead if the Army had been given the green light on all of Project Horizon. Something like it might have been built if NASA had decided to use an EOR strategy for the Apollo program, but even that isn’t certain. The USSR considered a similar approach and felt that they didn’t need a station: their Moon ship would have fuelled up directly from the tanker rockets.

When NASA decided to go with a Lunar Orbit Rendezvous lunar landing in July 1962, the chances of seeing something like the MOS dropped from “maybe” to “none”. Space stations would be built in future, from Salyut to Skylab to Mir and the ISS, but none of them would serve the peculiar function of the MOS. Any similarity between them and the Horizon station was a function of position only, and not purpose.

As it was, the MOS and ORV made it to the mockup phase and no further; a diagram of one of these mockups can be found further up this page. Oddly enough it was put on display at the Daily Mail‘s 1960 Ideal Home Show in London, of all places. Somewhere between 150,000 and 200,000 visitors passed through it, and by all accounts it was one of the hits of the show.

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2 thoughts on “Project Horizon (Part II): The Minimal Orbital Station and the Orbital Return Vehicle

  1. I think that the direct-descent lunar landing is predicated on orbital rendezvous being hard, which certainly seems to have been the expectation before people started actually to do it; if that’s the case, you don’t want to be trying one in lunar orbit where a failure will mess up the mission, but one failed tanker out of four in Earth orbit can simply be replaced by another launch.

    • The problem with propellant storage in orbit is boil-off. Fail with one of the tanker launches and if you’re not quick to recover some (or even all) of your LOX has boiled away. If you’re using LH2 as your fuel, that boils off even faster. So you can’t necessarily substitute one failed launch with one replacement launch. You might conceivably have to start all over.

      It comes down to an argument between the reliability of your several of your rockets and the reliability of one of your docking procedures.

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