Through most of 1966, it was still reasonable to assume that NASA and the United States might enjoy an expansive post-Apollo future off the Earth. Manned missions beyond the moon were expected to evolve from programs already in place; namely, the Apollo lunar landing program, the joint NASA/Atomic Energy Commission NERVA nuclear-thermal rocket program, and the Apollo Applications Program of advanced lunar missions and Earth-orbiting space stations.

With these programs in mind, in March 1966 the American Institute of Astronautics and Aeronautics and the American Astronautical Society jointly convened the Stepping Stones to Mars conference in Baltimore, Maryland. It would, as it turned out, be the last major Mars-focused engineering meeting until the 1980s.

Attendees heard a team of engineers from NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, describe a piloted Mars mission based on both high-thrust NERVA-II nuclear-thermal rockets and low-thrust nuclear-ion (electric) propulsion. The study team's leader was veteran German-born rocketeer Ernst Stuhlinger, the director of MSFC's Research Projects Laboratory.

Stuhlinger had begun his work on ion propulsion in the 1930s. He earned a Ph.D. at age 23, then worked for Hitler's nuclear program. In spite of his science training, in 1941 he was drafted into the Wehrmacht and sent to the Russian front. After suffering wounds in the Battle of Moscow and surviving the Battle of Stalingrad, he was reassigned to Wernher von Braun's rocket team at the Baltic Sea rocket base of Peenemünde in 1943. There he worked on the guidance system for the V-2 missile. He arrived in the U.S. in 1945 courtesy of the U.S. Army with von Braun, 124 other German rocketeers, and a trainload of captured V-2 missiles. Stuhlinger resumed his ion propulsion work in Huntsville in the early 1950s, while von Braun's team was part of the Army Ballistic Missile Agency at Redstone Arsenal. Under von Braun's leadership, the Peenemünde rocketeers became the nucleus around which NASA MSFC coalesced in July 1960.

In the years before the Stepping Stones to Mars meeting, Stuhlinger had put forward several ion-drive spacecraft designs. His 1954 Sun Ship would have relied on concentrated sunlight for electrical power to drive its ion thrusters, but his other designs - the 1957 Mars and Beyond and 1959 lunar ion freight disc ships and his 1962 lunar ion freighter and 1962 spinning ion Mars spacecraft - would have employed large nuclear reactors.

The hybrid NERVA/nuclear-ion approach would, the MSFC engineers explained, magnify the benefits and mitigate the drawbacks of both propulsion methods. Efficient ion propulsion would slash the amount of the propellant required to reach and return from Mars. This would in turn reduce the number of costly rockets required to place a hybrid Mars spacecraft into Earth orbit for assembly. Five uprated Saturn V rockets would be sufficient to launch a hybrid spacecraft into Earth orbit, or about half as many as required to launch a Mars spacecraft propelled by NERVA nuclear-thermal rocket engines alone.

Cutaway of a NERVA nuclear-thermal rocket engine. The design was essentially a nuclear reactor kept from melting by the liquid hydrogen propellant that passed through it and vented from its nozzle. Image: NASA.

Nuclear-thermal rockets, for their part, would trim trip time and reduce crew radiation exposure. Nuclear ion spacecraft could escape from Earth orbit only after spiraling outward for weeks or months. Because of this, they would linger in the Van Allen radiation belts for days or weeks. Nuclear-thermal spacecraft, on the other hand, could escape from Earth orbit in hours and race through the Earth-girdling belts in minutes.

Stuhlinger and his colleagues scheduled their NERVA/nuclear-ion Mars expedition for launch in 1986, 20 years after they presented their paper, because in that year the amount of energy needed to travel from Earth to Mars and back would be relatively small and solar activity would be at an ebb. The MSFC team assumed (rather naively) that their expedition would encounter no solar flares, so they skimped on radiation shielding to reduce spacecraft mass.

They also anticipated that ion propulsion would be applied first to Earth-orbital satellite station-keeping in the late 1960s, and that enough ion propulsion research would be completed by 1974 to justify government approval of the NERVA/nuclear-ion Mars expedition. That would leave 12 years for spacecraft development and testing.

The hybrid Mars expedition would occur in three phases. Phase 1 would see nuclear-ion spacecraft components and propellant launched from Earth's surface. To enhance safety, four identical manned spacecraft would undertake the Mars voyage. If one failed, its crew could find refuge on board the remaining three spacecraft. Each spacecraft would in fact be capable of returning all 16 crewmembers to Earth in cramped conditions. For each Mars spacecraft, three uprated two-stage Saturn V rockets would launch a total of 388 tons of components and propellant into 485-kilometer-high assembly orbit. For the four-spacecraft expedition, 12 uprated Saturn Vs would launch a total of 1552 tons.

The spacecraft would each include a central module containing the nuclear-ion propulsion system, a four-person, 57-ton Mars Excursion Module (MEM) lander, and space "taxis" for crew transport between the four spacecraft. The 123-ton propulsion system would include a 20-megawatt nuclear reactor, an electricity-generating turbine-generator, electric thrusters, and a cylindrical tank holding 153 tons of xenon or cesium propellant. Twin telescoping truss-like arms extending from either side of the central module would each carry four reactor radiator panels and one drum-shaped pressurized crew module.

Proposed Mars Excursion Module (MEM) design for the 1966 NASA MSFC nuclear-thermal/nuclear-ion spacecraft. Image: NASA.

Phase 2 would see launch of four nuclear-thermal rocket stages and the Mars expedition's departure from Earth orbit. Shortly before the scheduled launch date, four uprated Saturn Vs would launch one NERVA-II nuclear-thermal propulsion module each, then four more uprated Saturn Vs would launch one liquid hydrogen tank module each. The NERVA-IIs and tank modules would dock in orbit to form four 54-meter-long, 10-meter-diameter nuclear-thermal stages, each with a mass of 309 tons. Of this mass, liquid hydrogen propellant would account for 226 tons. The nuclear-thermal stages would then each maneuver to dock with a nuclear-ion spacecraft's central module.

On 1 May 1986, the four NERVA-II engines would power up and operate for nearly 30 minutes. The spacecraft crews would, meanwhile, shelter in their MEMs. In the event of NERVA-II trouble, the MEM would serve as the crew's abort-to-Earth vehicle.

About 17 minutes after start-up, each NERVA-II engine would vent hot gas from its turbopump to spin its spacecraft once per minute, producing acceleration equal to 20% of Earth’s surface gravity in the crew modules at the ends of the twin telescoping arms. Artificial gravity would ensure, among other things, that toilets and showers would operate much as they did on Earth. The MSFC team noted, however, that "available evidence from the Gemini flight missions suggests that artificial gravity for long space missions may not be required physiologically." The longest two-man Gemini mission, Gemini VII, had lasted for 14 days in December 1965.

Ernst Stuhlinger in 1968. Image: NASA. Ernst Stuhlinger in 1968 with a photo of his 1962 nuclear-ion Mars spacecraft (on wall) and models of his 1966 nuclear-thermal/nuclear-ion Mars spacecraft (minus its NERVA-II stage), a NERVA-II Saturn V payload (right), and an Apollo Saturn V rocket. The 1966 spacecraft and Saturn V models are of the same scale. Image: NASA.

The NERVA-IIs would deplete their propellant at an altitude of 3450 kilometers, then Phase 3, the actual Mars expedition, would commence. The crews would leave their MEMs, climb down pressurized tunnels in the telescoping arms to their cabins, discard the spent NERVA rocket stages, and activate the nuclear-ion thrusters to complete spacecraft injection onto a trans-Mars trajectory. The astronauts would switch off the thrusters after an unspecified short period and the fleet would coast around the Sun along a curving Mars-bound path.

One-hundred-and-forty-five days out from Earth, the four ships would re-activate their nuclear-ion thrusters to begin deceleration. Then, on 23 September 1986, Mars's gravity would capture them into a high orbit. Their ion thrusters would continue to operate so that they would spiral down to a 1000-kilometer circular Mars orbit in 23.5 days.

During the spiral-down period, the four MEMs would undock and land on Mars, leaving the four ships unmanned. Relieved of the mass of the MEMs, the nuclear-ion ships would spiral inward toward Mars more rapidly.

The MSFC team cited data from the Mariner IV Mars probe when they proposed an "Apollo-shaped" conical MEM design. Mariner IV had flown past Mars in July 1965. It had returned data indicating that the planet's atmosphere was about 10 times thinner than expected. Because of this, winged and lifting-body Mars landers, which would rely on aerodynamic lift to reduce the amount of landing and liftoff propellants they would need, were no longer considered feasible. The Apollo-shaped MEM design had been the subject of special study by Gordon Woodcock, a member of the study team. Atmospheric drag would slow the 10-meter-wide MEM, then its heat shield would eject to expose landing retrorockets. These would slow the MEM to a halt 400 meters above Mars; the MEM pilot would then have 60 seconds to select a landing spot before exhausting his chemical descent propellants.

After a month on Mars, each MEM's 27-ton ascent stage would blast its crew back to their orbiting nuclear-ion mothership. The crews would return to the cabin modules, then the ascent stages would be cast off. Because the Mars spacecraft would no longer carry the MEMs, outward spiral from Mars would last just 17.5 days, with Mars escape taking place on 12 November 1986. Mars-Earth crossing would need 255 days; about halfway through, the spacecraft would begin deceleration. Earth-orbit capture would occur on 25 July 1987. A five-day inward spiral would place the fleet in 30,000-kilometer-high Earth parking orbit, where the ion thrusters would be turned off for the final time. A chemical-propulsion "commuter rocket" would then arrive to retrieve the Mars explorers and ferry them home to Earth. The Mars expedition ships would remain in distant Earth orbit as permanent monuments to the early days of space exploration.

The 1966 study was one of the last to look in detail at nuclear-ion propulsion until the late 1980s. Just seven years earlier, Stuhlinger had concluded his 1959 nuclear ion freighter paper by predicting that a nuclear-ion cargo ferry would serve a U.S. moon base "from 1965-70 on." When he retired from NASA in 1975, however, the U.S. had abandoned the moon and nuclear-ion propulsion was little closer to flight than it had been in 1959. The last survivor of the German rocketeers brought in 1945 to the U.S., Stuhlinger died at age 94 in May 2008.

Reference:

"Study of a NERVA-Electric Manned Mars Vehicle," Ernst Stuhlinger, Joseph King, Russell Shelton, and Gordon Woodcock, A Volume of Technical Papers Presented at the AIAA/AAS Stepping Stones to Mars Meeting, pp. 288-301; paper presented in Baltimore, Maryland, 28-30 March 1966.