Flyby's Last Gasp: North American Rockwell's S-IIB Interplanetary Booster (1968)

Stacking a Saturn V rocket: inside the Vertical Assembly Building at Kennedy Space Center, a giant crane gingerly lowers an S-II second stage onto an S-IC first stage. Image credit: NASA.
NASA abandoned work toward piloted Mars and Venus flyby missions based on hardware developed for Apollo and its planned successor, the Apollo Applications Program, during the final months of the pivotal year 1967. Until August of that year, however, the concept was viewed by many as a plausible interim step between 1960s Apollo Moon landings and 1980s piloted Mars landings.

Though NASA awarded no new piloted flyby study contracts, studies performed in 1965, 1966, and 1967 continued to report out at aerospace conferences and in NASA briefings during 1968 and 1969. In March 1968, for example, North American Rockwell (NAR) engineers W. Morita and J. Sandford summed up a study they completed in April 1967 for NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama. Their study looked at how a modified NAR-built S-II rocket stage might be used to boost a piloted flyby spacecraft out of Earth orbit (that is, "inject" it onto an interplanetary trajectory). They presented results of their study at the Fifth Space Congress in Cocoa Beach, Florida.

Image credit: NASA.
The 33-foot-diameter, 81.5-foot-long S-II, the second stage of the Apollo Saturn V rocket, weighed about 40 tons empty. A single propellant tank divided by a dome-shaped "common bulkhead" held a total of more than 400 tons of liquid oxygen (LOX) and liquid hydrogen (LH2) propellants. LH2 is of low density, so the LH2 section in the top/front part of the tank measured more than twice as long as the LOX section.

The propellants fed a cluster of five J-2 rocket engines, each producing 200,000 pounds of thrust. Together they consumed more than a ton of propellants per second during their 6.5 minutes (390 seconds) of operation, boosting the Saturn V's speed from 6000 miles per hour at separation from the Saturn V S-IC first stage to 17,400 miles per hour (just short of Earth-orbital velocity) at S-II shutdown.

NAR proposed to launch the S-II interplanetary boost stage, which it designated the S-IIB, into Earth orbit on a two-stage Saturn V. The S-IIB would include two or three improved J-2S engines in place of the S-II's five J-2s. After separation from the spent S-II, the J-2S engines would fire briefly to place the S-IIB into an elliptical Earth orbit. An auxiliary propulsion system made up of three solid-propellant motors would perform orbit circularization, and eight thruster modules based on the Apollo Command and Service Module (CSM) attitude control system would carry out orbit corrections and rendezvous and docking with the piloted flyby spacecraft.

Proposed North American Rockwell-built piloted flyby payloads are shown in red. Image credit: NAR/DSFPortree.
The S-IIB would reach orbit with about 76 tons of LH2 fuel on board. NAR's analysis determined that, if only standard S-II thermal insulation were employed, boil-off caused by solar heating in orbit would reduce this to only 25 tons in less than five days. NAR proposed to reduce boil-off by installing a hydrogen gas-filled "vapor barrier" between the LH2 and LOX sections of the propellant tank and by applying "super-insulation" panels to the stage exterior. These modifications would reduce total LH2 boil-off over 10 days — the rated orbital lifetime of the S-IIB — to less than five tons.

The S-IIB would need to lift off with its LOX tank empty if the two-stage Saturn V was to place it in Earth orbit. Separately launched automated LOX tankers would then dock with it to fill the tank. The NAR engineers examined S-II-based tankers, tankers based on the Apollo Saturn S-IVB stage, and a wholly new tanker Lockheed Corporation designed in a separate study for MSFC.

LOX tankers considered in the North American Rockwell study. Green represents each design's LOX cargo volume. Image credit: NAR/DSFPortree.
Morita and Sandford described two S-II-based tankers. The first, the S-IIB/TK, would measure about 25 feet shorter than the standard Saturn V S-II stage. It would separate from the S-II second stage of the two-stage Saturn V that launched it, fire its twin J-2S engines for 3.5 minutes to attain a 100-nautical-mile-by-263.5-nautical-mile orbit, then fire them again at apogee (the high point in its orbit about the Earth) to raise its perigee (the low point in its orbit about the Earth). The circularization burn would leave the S-IIB/TK into a 263.5-nautical-mile-high parking orbit.

The 92 tons of LOX remaining after the circularization burn would constitute the tanker's payload. Solar heating would cause the LOX to boil off over time, so after 163 days — the longest period the tanker would need to loiter in Earth orbit before transferring its payload to the S-IIB injection stage — 75 tons would remain.

NAR's second S-II tanker variant, the S-II/TK, would have a LOX tank four feet longer than that of the standard Saturn V S-II. It would serve double-duty as a Saturn V second stage and a tanker. After it separated from the S-IC first stage, its five J-2S engines would boost it into a 100-nautical-mile-by-263.5-nautical-mile orbit, Earth orbit, then two engines would fire a second time at apogee to circularize its orbit. The S-II/TK would retain about 105 tons of LOX after the circularization burn and about 82 tons after 163 days in orbit.

Sandford and Morita next examined tankers based on the Douglas Aircraft Company-built S-IVB stage. The 22-foot-diameter S-IVB served as the the second stage of the Saturn IB rocket and the third stage of the Saturn V moon rocket.

The first S-IVB tanker design would trim cost by retaining — but leaving empty — the S-IVB stage LH2 tank. The second would delete the LH2 tank, making for a tanker that was shorter and lighter, but more heavily modified and thus more costly. The first design would deliver 110.5 tons of LOX to 263.5-nautical-mile orbit, of which about 99 tons would remain after 163 days. The second S-IVB-based design would deliver 107.5 tons to a 263.5-nautical-mile circular parking orbit. Of this, 92.5 tons would remain after 163 days.

The third tanker Morita and Sandford investigated was Lockheed's Orbital Tanker. Because it would be purpose-built to serve as a tanker, it would be more efficient than the NAR S-II and Douglas S-IVB tankers, but also more costly. Efficiency in this case would be measured in terms of the expected amount of LOX boil-off.

After launch on a two-stage Saturn V, the Orbital Tanker would fire LH2/LOX or solid-propellant rocket motors to place itself into a 263.5-nautical-mile-high parking orbit. The Orbital Tanker would reach orbit bearing 114.9 tons of LOX in an insulated spherical tank. Of this, 110.9 tons would remain after 163 days.

Sandford and Morita looked at Mars and Venus flybys, but emphasized a Mars flyby that would leave Earth orbit in late September 1975. Their proposed Mars flyby launch schedule took into account the narrow range of Earth-orbit departure dates, the planned 10-day lifetime in Earth orbit of the S-IIB injection stage, and the existence of only two Launch Complex 39 Saturn V launch pads at NASA's Kennedy Space Center in Florida.

Assuming an Earth-orbit departure date of 20 September 1975, the piloted Mars flyby mission would begin with three LOX tanker launches in April-May 1975. They would lift off between 153 and 130 days before the scheduled launch to Earth orbit of the S-IIB injection stage. A Saturn V bearing a fourth, backup tanker would be held in reserve.

Following the launch of the third LOX tanker in May 1975, KSC ground teams would refurbish the twin Launch Complex 39 pads for launch of the backup tanker (if necessary), the piloted flyby spacecraft, and the S-IIB injection stage. NAR estimated that KSC workers would need no more than one eight-hour shift per day to ready the pads in time for the piloted flyby spacecraft and S-IIB stage launches in September 1975. More shifts would be added if the backup tanker became necessary; that is, if one of the first three tankers failed to reach orbit or malfunctioned in orbit while awaiting arrival of the spacecraft and S-IIB stage.

On 15 September 1975, the S-IIB injection stage would lift off, followed within 24 hours by the piloted flyby spacecraft. Spacecraft and stage would rendezvous and dock within 12 hours, then the combination would set out in pursuit of the waiting tankers.

The piloted flyby spacecraft/S-IIB combination would dock with the three LOX tankers about 12 hours apart. Each would in turn link up with the aft end of the S-IIB, transfer its LOX cargo, and detach.

The piloted flyby astronauts and mission controllers on Earth would then perform a detailed systems check of the piloted flyby spacecraft/S-IIB stage combination. If all checked out as normal, they would be certified ready to depart Earth orbit on 20 September, just as the launch window opened for a minimum-energy Earth-Mars free-return transfer.

The quantity of propellants required to depart Earth orbit on a Mars flyby trajectory would increase steadily from the moment the launch window opened. At the same time, boil-off would cause the quantity of propellants in the S-IIB stage to steadily decrease. Morita and Sandford calculated that the S-IIB stage would retain sufficient LH2 to boost the Mars flyby spacecraft out of Earth orbit toward Mars for five days after the launch window opened; that is, until 25 September 1975.


"The S-II Injection Stage for the Mars/Venus Flyby Mission," W. H. Morita and J. W. Sandford, Proceedings, Fifth Space Congress: The Challenge of the 1970s, pp. 10.1-1 – 10.1-22; paper presented in Cocoa Beach, Florida, 11-14 March 1968.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Apollo Ends at Venus: A 1967 for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple-Flyby: Venus/Mars/Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)


  1. Here is another take: On the SIVb Tanker one, why not load LOX into the empty Hydrogen tank to the limit of what could be lifted? Boil off should be lower than the LOX tank, since the hydrogen tank was designed for a liquid 200 degrees colder, and more volume could be boosted to orbit.


  2. I suspect that the added LOX would exceed the structural limits of the S-IVB. Also, I wonder about the effects of a partial cargo of LOX moving around in the LH2 tank. And - could it be pumped out of the tank without some sort of ullage system that would cause it to settle but could complicate transfer after the S-IVB tanker docked with the S-IIB boost stage/piloted flyby combination?

    Just some thoughts.


  3. I thought the J-2 produced 232,000 pounds of thrust (in a vacuum).

    Please allow me an aside: There's a scene in the Stephen Baxter novel voyage in which a crew is told their Skylab mission will take place in lunar orbit. The commander says this is nonsense because the J-2S wasn't going to be ready for a while, and thus they couldn't be flying to a lunar lunar orbit station. It turned out that they would use the wet workshop concept, with the S-IVB stage having a docking adapter, and after it was empty the Apollo CSM would brake the new station into lunar orbit.


    1. Phil:

      I got the 200K figure from the study. Sea-level thrust was closer to 100K, so perhaps they shot down the middle.


  4. I can't help thinking that Elon Musk and the design folks at SpaceX must have read some of this report and research that led to it. The idea of the refueling tankers for ITS seem like they were taken right from this report or from the research that produced it.

    1. Hi, Ocean:

      I don't think the Newspace folks in general read a lot of history; certainly nothing as obscure as this study. In any case, if they do draw from history, they don't (as far as I've seen) cite what they read. No, I think they are doing as many before them have done - reinventing the wheel. This would be fine, except it means that they cannot draw upon the lessons history can teach us.

      For example, we have no evidence that reusability is a cost-saver. In fact, we've seen the opposite. I firmly believe that one cannot build a cheap Earth-to-orbit space vehicle and reuse it cheaply. Shuttle taught us that. One has to spend money up front if one wants to get the ops costs down. In other words, one has to build a costly but robust vehicle and hope to save money over time by flying it at low cost. This is why space programs that are not flush with cash (that is, all of them) do not in general aspire to routine reusability.


  5. If NASA did not go towards the Space Shuttle and a focus on earth observation post Apollo, it makes you wonder what of all its ambitious plans it could really afford. The progression Moon 1960s to Flyby 1970s to Mars landing 1980s is a new concept for me, but it makes sense. I also remember though the plans for a permanent Moon base in the 1970s. You have also shared the plans about permanent Space Station, that in the end came to fruition this century with the ISS. No Kennedy style blank check was to come so, what would get prioritized and how? If Apollo in the end felt too expensive, how would Space Station + Moon Base + Mars Landing feel to the public? Just food for thought.

    1. I'm going to correct a couple of what I perceive to be misconceptions in your comment before I respond to it. I don't meant to be critical.

      NASA did not have a blank check during Apollo. JFK and LBJ (but especially JFK) were very conscious of the costs of spaceflight and were eager to keep those costs under control. Early efforts to tack on projects not directly related to the moon landing goal were slapped down. For example, nuclear rockets. Though ground-based experimentation continued until Nixon, JFK terminated a plan to fly a nuclear Saturn V upper stage by 1967 and LBJ refused to restore its funding. That nuclear rocketry continued at all is attributable to the intervention of LBJ's long-time ally Clinton Anderson, Senator from New Mexico, home of Los Alamos and nuclear test sites.

      The Apollo mission mode (LOR) was all about keeping it fast and cheap. It was not inherently a very flexible mission mode. I'll say more about that if you want. For now, let me just note that, in 1966, when NASA conducted five internal studies in an effort to define its future, the lunar base study group planned to abandon LOR in favor of Direct Ascent as soon as they could. This would have required investment and would have led to the demise of the Lunar Module.

      LBJ sought a $5.1-billion NASA budget in FY 1968 because he sought to get the Apollo Applications Program rolling. Congress refused to fund it, and cut his request by half a billion dollars. LBJ might have fought - he was known as a fighter - but he acquiesced to the cuts. James Webb, who had vowed to fight for NASA's future, was instructed to back off.

      NASA Administrator James Webb is an interesting case - he purposely kept spending on post-Apollo development almost at zero for fear of arousing the ire of Congress and causing a backlash against Apollo. He also did everything he could to reduce the cost of Apollo without endangering its success. A lot more test flights were planned for Apollo as it began than were eventually carried out. Apparently Webb planned to push for an Apollo-based post-Apollo program in earnest beginning in 1967-1968, but the Apollo 1 fire intervened. He publicly lamented in 1967 and 1968 that NASA had been unable to lay the groundwork for its future. LBJ accepted his resignation in 1968. By some accounts, Webb went to see LBJ about NASA's future - Apollo looked set to succeed, so it was time to get working - and mentioned resignation, and LBJ surprised him by taking him up on the suggestion.

      The difficulties NASA faced under Nixon were 1) a mandate to build a wholly new kind of spacecraft, 2) a "new" role as a jobs program, and 3) funding in the range of only $3.3 billion per year during a period of rapid inflation. This was down more than a billion dollars from the Apollo peak. In those days, a billion dollars was worth something. Apollo was a jobs program, too, to be honest, but not to the degree that Shuttle was - Apollo had a clear overriding geopolitical goal. Shuttle did not, except that it would be all things to all possible constituencies.


    2. (Ran a little long - this is part 2 of my 2-part reply to pir34, 30 July 2017 01:00)

      I like to imagine what NASA might have accomplished in the 1970s with an annual budget of ~$4 billion/year and Apollo-based technology (that is, not major new development such as Shuttle). We would certainly have seen a series of increasingly complex and productive Skylabs. Imagine Skylab following a Salyut-type path in the 1970s, but with ten times the Soviet Salyut budget.

      I do not believe we could have pulled off a Mars flyby before the end of the 1970s decade, but that's OK, our Skylab experience would have laid the groundwork for interplanetary flight in any case. I think we might have been able to pull off some more Apollo lunar missions during the 1970s, but we'd also have seen pressure to avoid failure by staying closer to home. Perhaps we'd have done a month-long Apollo lunar polar orbiter mission with a specialized instrument carrier in place of the LM in 1976 to celebrate the Bicentennial. We'd have confirmed ice at the moon's poles ~20 years sooner. Hard to predict the effect of that discovery.

      As the 1980s dawned, we'd not be starting fresh with a wholly new system (Shuttle) after a flight gap lasting from 1975 to 1981, we'd have made space familiar and have accrued thousands of man-days of space experience. We'd still have the Saturn V. Where would we go from there?


    3. No need to apologize, constructive criticism is well taken. I think the last time anyone got a blank check from the government was when Roosevelt told the Army Air Force to set their own production target for 1944. It just feels like a blank check compared to tightness of the NASA budget afterwards. The 1960s were not just a great time for human spaceflight, it was a great time for robotic spacecraft too. True, mission creep has made it impossible to maintain this sort of cadence any more but still, many more science missions were sent out into space during the space race than at any other comparative time frame.
      What I remember from Chris Craft's excellent book about Apollo 13 was that the tanks that exploded already had a large legacy of missions before they failed in an unexpected way. Apollo 13 is also considered the turning point when public opinion turned against further lunar landings. Could the US have decided to follow a path similar to the USSR of building capacity in LEO before embarking to flybys and landings beyond LEO? Sure, after all this did happen in the Shuttle Era. Perhaps the Soviet path of incremental improvement of existing technology was better than the American path of seeking new capabilities by pursuing technological leaps

    4. Some suggestions of what we could have accomplished had we continued with the Apollo/Saturn hardware:

      Amazing Stories of the Space Age, by Rod Pyle, says that "It is estimated that the same money [spent on the shuttle program] could have paid for six launches of the Saturn V each year, with two of those flights being lunar missions."

      This is an excerpt from Michael Griffin's essay, Human Space Exploration: The Next 50 Years.

      "Let’s assume that we had kept flying with the systems we had at the time, that we had continued to execute two manned Apollo lunar missions every year, as was done in 1971-72.  This would have cost about $4.8 billion annually in Fiscal 2000 dollars. 

      "Further, let us assume that we had established a continuing program of space station activities in Earth orbit, built on the Apollo CSM, Saturn I-B, and Skylab systems.  Four crew rotation launches per year, plus a new Skylab cluster every five years to augment or replace existing modules, would have cost about $1.5 billion/year.  This entire program of six manned flights per year, two of them to the Moon, would have cost about $6.3 billion annually in Fiscal 2000 dollars.  The average annual NASA budget in the 15 difficult years from 1974-88 was $10.5 billion; with 60% of it allocated to human spaceflight, there would have been sufficient funding to continue a stable program of lunar exploration as well as the development of Earth orbital infrastructure.  I suggest that this would have been a better strategic alternative than the choices that were in fact made, almost 40 years ago."

    5. Phil:

      From an economic, scientific, and engineering standpoint, it would have made sense to incrementally develop Apollo. The trouble is, the political environment - which ideally reflects society's interests, but as often as not fails to do so - was against it. Not because space was especially costly, but because it was highly visible - an easy target.

      Engineers like Griffin ignore the overwhelming role irrational behavior has in dictating what flies and what does not. This applies to the robotic side of the house as much as it does to the piloted side. Why did we build a giant atomic Mars rover when we could have surveyed many more sites using MER-type rovers? Why must we build another giant atomic rover? Why have we not launched a new Mars orbiter? The ones in Mars orbit are getting quite old now.

      This is why I do not have a hard point of divergence in my "Apollo continues" Dreaming a Different Apollo series. There's no cause to suppose a rational decision about what we do next.

      I'm playing with another timeline in which Apollo is barely kept alive by President Humphrey, who directs NASA to focus its main effort on supersonic transports instead. The aeronautical side of the house takes charge. I have some research to do to make sure this works. Ultimately, it leads to single-stage-to-orbit transportation with meaningful payloads using just-slightly-beefed-up civilian hypersonic transports. Space Shuttle by the back door, and growing out of routine airline-type operations.


  6. My parents wedding was on September 20, 1975 and I've born in 1982 - according to von Braun August 1969 plan, man landing on Mars was to happen on August 15, 1982 - exactly 35 years ago this day. That was the "all out", Apollo-like option.
    The "relaxed budget" option had the Mars landing happening in spring 1986 - and we got STS-51L instead. :(

  7. A:

    Nixon's staff tacked on a fourth, even more "relaxed" option - that's described in the foreword to the September 1969 Space Task Group Report to Nixon, which was mostly written by NASA Administrator Tom Paine. Paine's three options had the same program, more or less, but with different timetables, and Nixon didn't want to endorse Mars, moon bases, and all that, so the fourth option made all that very indefinite. We followed the fourth option - and we're still following it.

    Some of my posts relate to the STG and Paine's efforts to push Nixon toward commitment to ambitious goals for NASA.



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