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 had been viewed by many as a plausible interim step between 1960s Apollo moon landings and 1980s piloted Mars landings.

Though no new manned flyby study contracts were awarded after August 1967, studies performed in 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, completed in April 1967, that had examined whether a modified NAR-built S-II rocket stage (image at top of post) could launch a piloted Mars/Venus flyby spacecraft out of Earth orbit (that is, "inject" it onto an interplanetary trajectory). They presented their results at the Fifth Space Congress in Cocoa Beach, Florida.

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 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. They together 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 S-II ignition to 17,400 miles per hour (just short of orbital velocity) at shutdown.

NAR proposed launching the S-II injection 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. An auxiliary propulsion system with three solid-propellant motors would perform orbit circularization, and eight thruster modules based on the Apollo Command and Service Module attitude control system would carry out orbital corrections, rendezvous, and docking.

The S-IIB would reach orbit with about 76 tons of LH2 fuel on board. NAR's analysis found that, if only standard S-II thermal insulation were employed, boiloff caused by solar heating would reduce this to only 25 tons in less than five days. NAR proposed to reduce boiloff 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 boiloff over 10 days - the rated lifetime of the S-IIB - to less than five tons.

The S-IIB would launch with its LOX tank empty, then separately launched automated LOX tankers would fill it in Earth orbit. NAR examined S-II-based tankers, tankers based on the Apollo Saturn S-IVB stage, which served as the Saturn V third stage and the Saturn IB second stage, and a wholly new tanker design developed by the Lockheed Corporation in a separate study.

Morita and Sandford described two S-II-based tanker designs. The first, the S-IIB/TK, would measure about 25 feet shorter than the standard 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-by-263.5-nautical-mile orbit, then fire them again are apoapsis (orbit high point) to raise its perapsis (orbit low point) and place itself into a circular 263.5-nautical-mile-high circular parking orbit. The 92 tons of LOX remaining in the S-IIB/TK after the second burn would constitute its payload. Solar heating would boil off the LOX over time, so after 163 days - the longest period the tanker would need to loiter in orbit before transferring its payload to the S-IIB injection stage - 75 tons would remain.

NAR's second S-II variant, the S-II/TK, would include a stretched LOX tank, so would measure four feet longer than the Saturn V S-II. Five J-2S engines would boost it into Earth orbit, then two would fire for a second time 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.

In Apollo lunar missions, the Douglas Aircraft Company-built Saturn V S-IVB stage placed the Apollo spacecraft into Earth parking orbit, then restarted to push it toward the moon. One S-IVB tanker variant would delete the S-IVB's LH2 tank. Another would avoid an extensive redesign by retaining - but not using - the LH2 tank. The first variant would deliver 107.5 tons of LOX to a 263.5-nautical-mile parking orbit. Of this, 92.5 tons would remain after 163 days. The second variant would deliver 110.5 tons, of which about 99 tons would remain after 163 days.

The S-IVB rocket stage served as the Saturn V third stage and, in a modified form, as the Saturn IB second stage. Of Sandford & Morita had their way, it might also have served as a liquid hydrogen tanker for loading the S-IIB interplanetary insertion stage with fuel. During Apollo moon missions, the S-IVB rocket stage served as the Saturn V rocket third stage. If Sandford & Morita had had their way, it might also have served as a tanker for filling the S-IIB interplanetary insertion stage liquid oxygen tank in Earth orbit. Image: NASA

The third concept Morita and Sandford cited was Lockheed's Orbital Tanker. Because it would be purpose-built to serve as a tanker, it would be more efficient than the NAR and Douglas tankers, but also more costly. After launch on a two-stage Saturn V, the Orbital Tanker would deliver 114.9 tons of LOX to 263.5-nautical-mile parking orbit. Of this, 110.9 tons would remain after 163 days.

NAR's proposed Mars flyby launch schedule took into account the narrow range of possible Earth-orbit departure dates for Mars flybys, the planned 10-day lifetime of the S-IIB injection stage, NASA internal piloted flyby planning, and the existence of only two Launch Complex 39 Saturn V launch pads at Kennedy Space Center (KSC). Assuming a planned 20 September 1975 Earth-orbit departure, a 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 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 required), the piloted flyby spacecraft, and the S-IIB injection stage. NAR estimated that KSC workers would need to work no more than one eight-hour shift per day to ready the pads in time for the piloted flyby spacecraft and S-IIB launches in September 1975. More shifts would be added if the backup tanker became necessary.

On 15 September 1975, the S-IIB injection stage would lift off, followed within 24 hours by the piloted flyby spacecraft. Morita and Sandford noted that either the piloted spacecraft or the S-IIB injection stage could launch first. The flyby spacecraft and the S-IIB would rendezvous and dock within 12 hours of S-IIB launch, then the combination would set out after the waiting tankers.

The piloted flyby spacecraft/S-IIB stage combination would dock with the LOX tankers in turn beginning about 24 hours after the S-IIB stage reached orbit. Each would link up with the aft end of the S-IIB, transfer its LOX, then be discarded. Dockings would occur about 12 hours apart. The astronauts and mission controllers on Earth would then check out the assembled spacecraft.

If all checked out as planned, the piloted Mars flyby mission would be certified ready for interplanetary flight five days after S-IIB launch, just as the launch window opened for a minimum-energy Earth-Mars transfer. The amount of propellant required to depart Earth orbit for Mars would steadily increase from the moment the launch window opened on 20 September. Assuming a 15 September launch into Earth orbit, the S-IIB would, Morita and Sanford calculated, retain sufficient LH2 to launch the flyby spacecraft out of Earth orbit for five days after the launch window opened; that is, until 25 September 1975.

Reference:

"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 1970's, pp. 10.1-1 - 10.1-22; paper presented in Cocoa Beach, Florida, 11-14 March 1968.