Between August 1963 and November 1964, a 13-member team at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, conducted a detailed in-house study of Mars and Venus flyby missions. These would see a manned flyby spacecraft depart Earth orbit, coast past its target planet, and return to Earth. Only small course-correction maneuvers would be necessary after Earth-orbit departure.

The study, led by Harry O. Ruppe of the MSFC Future Projects Office (FPO), was a follow-on to the Early Manned Planetary-Interplanetary Roundtrip Expeditions (EMPIRE) study, which had lasted from May 1962 to February 1963. MSFC FPO had directed EMPIRE contractors Ford Aeronutronic, Lockheed, and General Dynamics to study manned Mars and Venus flyby and orbiter missions in the early 1970s as a means of justifying early development of nuclear-thermal rockets and launch vehicles more powerful than the Apollo Saturn V. MSFC FPO stressed these technologies in EMPIRE because MSFC was NASA's lead center for large rocket development and because it was involved in the joint NASA/Atomic Energy Commission nuclear propulsion program through the Reactor In-Flight Test (RIFT), which sought to launch a nuclear rocket into space in 1967.

The new manned planetary flyby study acknowledged changes in the advance planning environment within NASA. Whereas the EMPIRE contractors had been instructed to "attempt" to use Apollo hardware in their spacecraft designs - and had responded by designing all-new systems with little Apollo heritage - the MSFC in-house study adhered strictly to the rule that Apollo technology should be used everywhere possible. This reflected increasing restrictions placed on NASA advanced technology development by President John F. Kennedy and his successor, President Lyndon Baines Johnson. Put succinctly, NASA planners had begun to realize that a commitment to the goal of a man on the moon did not imply a commitment to the goal of a man on Mars.

The MSFC team declared, nonetheless, that it was "inconceivable" that the "tremendous" technology that NASA had developed for Apollo would not lead eventually to a manned Mars landing. It was simply a matter of which course NASA should follow to get there. An Earth-orbiting space station or a moonbase were 1970s goals that could use Apollo hardware and provide "training" for manned Mars landings; like Apollo, however, these would operate "within the Earth's 'sphere of activity.'" Manned Mars/Venus flybys in the mid-to-late 1970s, on the other hand, could be based on Apollo systems, yet would venture beyond the safe harbor of the Earth-moon system.

Little was known of Mars's atmosphere or surface conditions when Ruppe's engineers performed their study. A manned Mars flyby in the 1970s could, they argued, provide data they would need to design a 1980s Mars landing mission. They proposed that, in addition to exploring Mars closeup with remote-sensing instruments mounted on their spacecraft, flyby astronauts should serve as caretakers for a small armada of automated probes. These would include "landers, atmospheric floaters, skippers, orbiters, and possibly probes. . .to perform aerodynamic entry tests [of spacecraft] designs and materials."

Automated probes would need caretakers, the MSFC team believed, because they had had a checkered history. Mariner II had flown past Venus successfully in December 1962, near EMPIRE's end, confirming what astronomers had already begun to suspect: that the planet's dense clouds hid a hellish surface. The Ranger VII moon probe had returned images of southeast Oceanus Procellarum as it plunged toward planned destructive impact in July 1964, thereby providing engineers designing the Apollo Lunar Module lander with essential data on the moon's surface. Mariners I and III had, however, failed, as had the first six Rangers. Mariner IV had been launched toward Mars on 28 November 1964, as editing began on the Ruppe team's study report. As it saw print in February 1965, the 261-kilogram solar-powered robot remained healthy. It was, however, anyone's guess whether Mariner IV would survive until its planned Mars flyby in July 1965.

The MSFC engineers believed that "the major emphasis of the manned flyby-unmanned probe combination" should "be focused on assisting later [Mars] landing missions." Engineers who lacked data on Mars conditions would, they explained, have little choice but to design the Mars landing spacecraft for "worst conditions." This would tend to increase its mass and thus the number of costly booster rockets necessary to place its components into Earth orbit for assembly.

Conversely, adequate knowledge of Mars would enable engineers to cut costs by taking advantage of the conditions there. Of particular importance, they wrote, would be probes that would test propellant-saving aerodynamic braking maneuvers in the martian atmosphere and prospect for "usable indigenous materials. . .such as water" on Mars's surface. They estimated that, lacking adequate prior knowledge of Mars, the first manned landing mission "would probably transport 2 or 3 men to the surface of Mars for a few days. . .[at a cost of] a billion dollars per man-day on Mars." If on the other hand, "the physical properties of Mars were well known, we could think. . .of the first landing as a long-duration base, reducing cost to less than 10 million dollars per man-day."

Stacked atop its two-stage Saturn V rocket, the 1965 MSFC Mars/Venus manned flyby spacecraft would have resembled Skylab, shown here on its way to the launch pad in 1973. Image: NASA.

The MSFC team consulted Ruppe's published launch opportunity tables to determine that several Mars and Venus flyby launch windows would open in the mid-to-late 1970s. Because Venus has a nearly circular orbit around the Sun, opportunities to reach it would vary little in terms of amount of energy required, mission duration, and Earth-return velocity (all critical factors in interplanetary mission design). Mars, on the other hand, has a noticeably eccentric (elliptical) orbit, which means that these factors vary considerably from one launch opportunity to the next. For their detailed analysis, the MSFC engineers opted for a "typical" Mars flyby that would leave Earth orbit in September 1975, and a corresponding "typical" Venus flyby that would depart Earth orbit in August 1978.

An "improved" two-stage variant of the Apollo Saturn V would serve as the manned flyby program's workhorse Earth-to-orbit booster. The first payload it would place into orbit for any flyby mission would be the 125-ton flyby spacecraft with a multipurpose "aft skirt assembly." Stacked atop the two-stage Saturn V and covered with a streamlined launch shroud, the flyby spacecraft/aft skirt assembly would outwardly resemble the Skylab Orbital Workshop, which was launched on a two-stage Saturn V in May 1973, eight years after Ruppe's team completed its study.

The three-stage Saturn V configured for Apollo moon flights stood 363 feet tall, while the two-stage Saturn V with Skylab on top measured 333.6 feet tall. The two-stage Saturn V with the flyby spacecraft/aft skirt assembly combination on top would stand 332 feet tall. Skylab measured 84.5 feet long at launch, while the flyby spacecraft/aft skirt assembly would measure 89 feet long with its launch shroud (A in the drawing below) and 81.6 feet long in orbit, after its shroud had been discarded.

Simplified cutaway drawing of MSFC FPO's 1965 Mars/Venus manned flyby spacecraft design. The letters are identified in the text. Image: NASA MSFC.

The MSFC engineers tapped as their rocket engine for course corrections the Apollo Lunar Module Descent Engine (B). It would draw hypergolic propellants (that is, fuel and oxidizer that ignite on contact with each other) from four spherical tanks (I). The tanks were designed to hold enough propellants to change the flyby spacecraft's speed by 500 meters per second (mps). The 0.5-kilometer-per-second course change would need 26,272 pounds of propellants for the 1975 Mars flyby and 20,583 pounds for the 1978 Venus flyby.

A pair of 5000-pound "radioisotope power supply systems" would be mounted to the flyby spacecraft near the course-correction engine, well away from the spherical, 20-foot-diameter Lab/Crew Living area (M). During ascent to Earth orbit, these would remain folded inside the launch shroud (C). Some time after shroud separation, they would pivot outward to their flight positions (D) and begin to make electricity.

Apollo Command and Service Module (CSM) in lunar orbit. Image: NASA.

The flyby spacecraft's pressurized Hangar (E) would fill the space between the course-correction engine and the course-correction propellant tanks. The three-man flyby crew would reach the Hangar from their main living area via an airlock tube (J). The Hangar would contain at its center a modified Apollo Command and Service Module (CSM). The Ruppe team felt it necessary to cocoon the CSM within the Hangar to protect it from "micrometeoroids, outgassing, and other detrimental effects" of long space exposure.

The CSM warranted special protection for two reasons. First and foremost, it was the flyby crew's end-of-mission Earth-atmosphere reentry vehicle. The astronauts would ride in its conical Command Module (CM) (F) and would use the Service Propulsion System (SPS) main engine (H) on its drum-shaped Service Module (SM) (G) to slow to Apollo lunar-return speed of 11 kilometers per second (kps) before they reached Earth's atmosphere. Cocooning the CSM in the Hangar would also limit the amount of costly redesign and retesting the CSM would need before it could be used for manned Mars/Venus flyby missions. The CM for flyby missions would lack a docking unit, but otherwise would closely resemble the Apollo lunar CM. It would, therefore, need no new testing.

For Venus flybys, the SM also could remain unchanged. The Mars flyby SM, on the other hand, would approach Earth moving fast enough that its SPS engine would need to fire for up to 536 seconds longer than the Apollo lunar SPS and burn up to 2790 pounds more propellants than the Apollo lunar SM could hold. The Mars flyby SM would thus need longer propellant tanks and either a redesigned SPS or a pair of conventional SPSs operating in tandem or in series. A new engine rated for a longer burn time was also a possibility, though that option would not be in keeping with the MSFC team's goal of reliance on Apollo hardware.

In addition to the Earth-atmosphere reentry CSM, the flyby spacecraft Hangar would house five tons of automated probes destined for release near the mission's target planet. As noted above, the astronauts' main job would be to ensure that the probes remained functional until they reached Mars or Venus. The crew would thus have available within the Hangar 1000 pounds of tools and supplies for servicing the probes. The MSFC engineers also placed an airlock for spacewalks in the Hangar, though they doubted that it would see much use, as well as a stock of emergency life support provisions.

When not attending to their cargo of probes, the three flyby astronauts would live and work in the Lab/Crew Living Area, where they would breathe a half-oxygen, half-nitrogen atmosphere at a pressure of 10 pounds per square inch. The Lab/Crew Living Area and the Hangar could each be repressurized 12 times during a Mars flyby mission and eight times during a Venus flyby mission. Repressurization would occur in the event that a meteoroid punctured the spacecraft hull or after scheduled periodic air dumps that would purge the atmosphere of toxic trace gases outgassed from furnishings and equipment and generated by experiments and cooking. Each repressurization would need 1885 pounds of gases, bringing the total breathing gas carried to 22,650 pounds for the typical Mars flyby spacecraft and 15,050 pounds for the Venus flyby spacecraft. A system for recycling air between purges would have a mass of 1800 pounds on both the Mars and Venus flyby spacecraft.

The Ruppe team's engineers cited a study by the MSFC Research Projects Laboratory (RPL) when they rejected specialized radiation shielding for the flyby spacecraft's bottle-shaped emergency shelter (K). The RPL had found that solar flares powerful enough to harm flyby crews were unlikely to occur in the mid-to-late 1970s. In place of 1000 pounds of shielding, the MSFC team proposed a double-walled shelter with the crew's water supply stored between its walls. Two 500-pound water reclamation systems (main and spare) would recycle cabin air moisture, wash water, and urine. Equipment and food would be arranged around the shelter's exterior to provide additional radiation protection. The crew would sleep inside the shelter to minimize their exposure to cosmic rays. In the event of fire, catastrophic pressure loss, or other emergency, the shelter, which would contain a duplicate set of spacecraft controls, could be sealed off from the rest of the flyby spacecraft.

The MSFC engineers calculated that building the flyby spacecraft so that it could spin to create artificial gravity would add 69,000 pounds to its mass. The engineers rejected this in favor of providing a small centrifuge (L) capable of holding two astronauts at a time. Support arms would attach the twin centrifuge gondolas to a motorized ring around the hatch leading into the emergency shelter.

The Lab/Crew Living Area would nestle in a bowl-shaped recess in the aft skirt assembly (O). At its front end, the aft skirt assembly would match the 22-foot diameter of the flyby spacecraft; at its aft end, it would match the 33-foot diameter of the S-II second stage of the Saturn V that would boost it and the flyby spacecraft into 185-kilometer-high Earth orbit. S-II separation would reveal twin RL-10 rendezvous and docking rocket motors (P) and a large socket-like docking structure (N) on the aft skirt assembly's aft end. At its front end, the aft skirt assembly would contain a ring-shaped, 22-foot-diameter Saturn V Instrument Unit (IU). In addition to guiding the Saturn V carrying the flyby spacecraft during its ascent to Earth orbit, the IU would provide guidance control for Earth-orbital assembly maneuvers and for Earth-orbit departure.

The number of two-stage Saturn V rockets required to place into Earth orbit the flyby spacecraft, its S-IIB Orbital Launch Vehicle (OLV), and liquid oxygen (LOX) for the S-IIB OLV would depend on the amount of energy required to place the flyby spacecraft on course for its target planet. Even in the least demanding opportunities, Mars flybys would require more energy than Venus flybys, so would need more Saturn V rockets.

The MSFC engineers described in detail the assembly campaign for the Mars flyby mission that would leave Earth orbit in September 1975, during a launch opportunity lasting 28 days. The first two-stage Saturn V in the assembly campaign would lift off from one of the two Complex 39 Saturn V launch pads at Cape Kennedy, Florida, on 28 April 1975. If the Saturn V rocket failed and the flyby spacecraft/aft skirt assembly it carried was destroyed, then a backup would lift off on 24 June 1975.

The next Saturn V in the series would launch on 28 June 1975, bearing the first of four LOX tankers to 185-kilometer orbit. The Ruppe team's tanker could transport about 95 tons of LOX. Three more successful tanker launches would be needed; these would occur on 6 July and 7 July and 3 September 1975. A single backup tanker would be available in the event of a launch failure; if it became necessary, then it would launch on 6 September 1975.

With a Mars flyby spacecraft/aft skirt assembly and four LOX tankers safely orbiting the Earth, the sixth and last Saturn V would launch the S-IIB OLV into a 485-kilometer-high orbit on 13 September 1975. As its name implies, the S-IIB OLV would be a derivative of the Saturn V S-II second stage. Modifications would include deletion of two of its five J-2 engines and improved insulation to retard boil-off and escape of the roughly 80 tons of liquid hydrogen it would carry into orbit. The MSFC engineers expected that an S-IIB OLV could be developed that would retain enough liquid hydrogen for flyby spacecraft Earth-orbit departure 72 hours after its launch from Complex 39, but aimed for an Earth-orbit departure 50 hours after Earth launch.

Using the twin RL-10 engines in its aft skirt assembly, the unmanned flyby spacecraft would climb to a 485-kilometer circular orbit and rendezvous with the S-IIB OLV as soon as the latter was confirmed to be safely in orbit. It would then back up and dock with the S-IIB OLV. Next, the four LOX tankers would climb to 485-kilometer orbit and dock one at a time with the S-IIB OLV. Each would pump its cargo into the S-IIB OLV's LOX tank, then would undock and move away, clearing the way for the next in the series.

The astronauts would board the Mars flyby spacecraft 20 hours before planned launch from Earth orbit. If NASA had a space station in Earth orbit in 1975, they might board from that. An alternate plan would see the flyby astronauts reach their spacecraft on board an Apollo CSM launched from Earth on a Saturn IB rocket. After entering the flyby spacecraft and checking out its systems, they would cast off the CSM.

The S-IIB OLV's three J-2 engines would burn for about eight minutes on September 26, 1975 to push the flyby spacecraft/aft skirt assembly combination out of 485-kilometer Earth orbit and place it on course for Mars. The burn would add about five kps to its speed. After the flyby spacecraft/aft skirt assembly combination separated from the S-IIB, the RL-10 engines in the aft skirt assembly would be used to fine-tune the flyby spacecraft's course. The aft skirt assembly, its work done, could then be cast off or retained for at least part of the mission to provide additional radiation/meteoroid shielding for the Lab/Crew Living Area.

Image: NASA MSFC.

Ruppe's team provided an example heliocentric orbital plot for a manned Mars flyby mission leaving Earth on September 26, 1975. The dashed line on the plot represents the flyby spacecraft's path around the Sun. Flight to Mars would require 130 days. Halfway to Mars, on November 30, 1975, the crew would adjust their spacecraft's course using the course-correction engine. The MSFC engineers budgeted enough propellants for the first midcourse burn to change the flyby spacecraft's speed by 150 mps. The crew would eject "consumed life support" (that is, body and food waste, saturated absorbent charcoal, used filters, and other trash) shortly before the course-correction burn so that it would continue on the flyby spacecraft's original course and not intersect Mars.

Mars flyby would occur on 3 February 1976, when Mars and the flyby spacecraft were 0.86 Astronomical Units (AU) - that is, 0.86 times the Earth-Sun distance - from Earth. The flyby spacecraft would approach Mars's day side, reaching a distance of 200,000 kilometers from the planet's center 6.5 hours before closest approach. It would pass 792 kilometers from Mars's surface moving at about 11 kps relative to the planet, then would retreat from Mars's night side. During approach to the planet, the astronauts would release 2.5 tons of robot probes and carry out continuous observations. Near closest approach, they would ignite the course-correction engine a second time.

During retreat from Mars, the astronauts would release an additional 2.5 tons of probes. While the flyby spacecraft remained close to Mars, it would relay data from the probes to Earth at a high data rate. The flyby spacecraft would, however, spend only one hour within 18,250 kilometers of Mars's center. Five and a half hours after closest approach, it would pass beyond 164,000 kilometers from the planet's center, and shortly after that the Mars probes would switch to direct transmission to Earth at a low data rate. The crew would then begin a grueling 539-day journey home.

A few weeks later, the crew would become the first humans to enter the Asteroid Belt. Maximum distance from Earth (3.21 AU) would be attained on September 13, 1976, about one year into their mission. At about the same time, Earth would move behind the Sun as viewed from the flyby spacecraft. The crew would then perform the mission's final course-correction burn, changing their spacecraft's speed by up to 200 mps.

The flyby spacecraft would pass inside of Mars's orbit on 31 May 1977 at a distance of 0.353 AU from Earth. Over the following two months, it would gradually catch up with the homeworld. On 19 July 1977, six days before planned Earth atmosphere reentry, the crew would transfer to the modified Apollo CSM in the Hangar and check out its systems. Two days before reentry, the CSM would emerge from its cocoon and abandon the flyby spacecraft. On 25 July, with Earth looming outside its small windows, the crew would turn the CSM so that its engine or engines pointed in its direction of flight. A burn lasting up to 19.4 minutes would reduce the CSM's speed from up to 15.8 kps to Apollo lunar-return speed of 11 kps, then the conical CM would detach and, using small rocket motors, orient its bowl-shaped heat shield for reentry. Minutes later, the CM would deploy three parachutes and lower gently into the ocean.

Image: NASA MSFC.

The Ruppe team also prepared an orbital plot for the Venus flyby mission departing Earth in August 1978. A shortened S-IIB OLV would add about 3.8 kps to the Venus flyby spacecraft's speed. The mission would be of shorter duration than the Mars mission - only one year - with Venus flyby occurring low over the planet's day side on 11 December 1978. The spacecraft would attain its greatest distance from Earth - 0.674 AU - on 15 April 1979. After leaving the Hangar, the CSM's main engine would trim about 2.6 kps from its Earth-approach speed. Reentry and splashdown would occur on 16 August 1979.

The MSFC engineers outlined a hardware development schedule based (inexplicably) on a Venus flyby in late 1975 and a Mars flyby in 1978 (that is, the reverse of the program detailed above). They also estimated the probable cost of the flyby program. They assumed that no new-start funding for the program would become available in NASA's budget before Fiscal Year (FY) 1969, after the first successful Apollo lunar landing, which in 1965 was scheduled to take place during 1968. Detailed flyby program planning would begin in mid-1968 and last a year.

LOX tanker, flyby spacecraft, and interplanetary avionics development would commence in the last quarter of 1968. LOX tanker development, at a cost of $380 million, would be completed in late 1974. A pair of LOX tanker flight tests would launch on two-stage Saturn V rockets in 1973 and mid-1974. A flyby spacecraft development test unit would reach Earth orbit on a two-stage Saturn V in 1974; among other things, it would be used for crew training. The flyby spacecraft would cost more to develop than any other hardware element ($1.563 billion). Avionics development (total cost: $325 million) would include a Saturn IB-launched flight test.

S-IIB OLV development (total cost: $425 million) would start in late 1969 and conclude in 1974. S-IIB OLV flight tests would take place in 1973-1974. Apollo SM modifications (total cost: $115 milion) would begin in mid-1970 and end in 1974, and aft skirt assembly development (total cost: $165 million) would span late 1970 through early 1975. An aft skirt assembly flight test using a Saturn IB launch vehicle would take place in 1974.

Science probe development for the 1975 Venus flyby would begin in mid-1970 and continue through the last quarter of 1975. Mars probe development would start in the last quarter of 1973 and run through 1977. Probe development would cost $220 million for each mission.

The MSFC engineers based their operational cost estimates on learning curves developed through the many Saturn V and Saturn IB launches that they expected would occur by the mid-1970s. They estimated that 62 three-stage and two-stage Saturn Vs would be launched prior to the first Venus flyby Saturn V launch, so that each Saturn V for the Venus flyby would cost $70 million. Fifty-two Saturn IB launches would take place before the first Venus flyby Saturn IB launch, leading to a cost per Venus flyby Saturn IB of $22 million. They assumed that 70 Apollo CSMs would have flown before the first Venus flyby CSM, leading to a Venus flyby CSM cost of $72 million.

For the 1978 Mars flyby, the MSFC engineers assumed that NASA would have already launched 98 three-stage and two-stage Saturn V rockets by the time the first Mars flyby Saturn V lifted off, lowering the cost per Mars flyby Saturn V to only $65 million. Seventy Saturn IB launches would have taken place, reducing the cost for each Mars flyby Saturn IB to $20 million. One hundred CSMs would have flown ahead of the first Mars flyby CSM, reducing the flyby CSM cost to $69 million.

Design and development cost would peak in FY 1972 at $895 million. Operational cost would peak at $497 million in FY 1974. The peak funding year for the program would be FY 1973, when operational and development costs would total $1.222 billion. Development costs would total $3.75 billion between FY 1969 and FY 1978. Operational costs would total $2.671 billion between FY 1971 and FY 1978. The entire manned flyby program would cost $6.421 billion. They estimated that by providing data to engineers, the manned flyby program would reduce by about $4 billion the cost of a follow-on Mars landing mission.

The MSFC engineers also conducted what they called a "mission worth analysis." They first assumed an undefined "basic space program" for the 1970s and 1980s. Manned Venus flyby missions could, they calculated, be deleted from the program with only a 2% impact on total space program worth and only a 10% reduction in planetary program worth because "it is not possible to land on Venus." Leaving the Venus flybys in place but deleting the Mars flyby and landing missions would reduce total space program worth by 9% and planetary program worth by half. Deleting all manned planetary missions and relying only on robotic probes would reduce total space program worth by 12% and planetary program worth by 63%.

Mariner IV spacecraft. Image: NASA.

Mariner IV triumphantly flew past Mars on 14-15 July 1965, five months after the MSFC team's study report saw print. It returned 21 black-and-white images of the planet's cratered surface and conducted a radio-diffraction experiment that indicated a martian atmospheric pressure ten times less than expected. Mariner IV revealed a Mars apparently inhospitable to life. The mission also showed that robots could cross the gulf between Earth and Mars and return useful data without help from astronaut caretakers.

Oddly enough, neither Mariner IV's success nor its discouraging Mars findings undermined the manned flyby concept. The flyby program goal of putting Saturn-Apollo hardware to new uses remained attractive to many in NASA. In April 1966, NASA Associate Administrator for Manned Space Flight George Mueller launched a new manned flyby study under the auspices of the Planetary Joint Action Group (JAG), which drew members from MSFC, the Manned Spacecraft Center in Houston, Texas, Kennedy Space Center in Florida, NASA Headquarters, and NASA planning contractor Bellcomm. The new study, which emphasized the 1975 manned Mars flyby opportunity, sought to flesh out automated probe and on-board instrument designs and to further explore the interplanetary potential of Apollo technology and techniques.

References:

Manned Planetary Reconnaissance Mission Study: Venus/Mars Flyby, NASA TM X-53205, Harry O. Ruppe, Future Projects Office, NASA Marshall Space Flight Center, 5 February 1965.