|Image credit: NASA.|
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. As it turned out, it would 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-electric (ion) 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 electric 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.
Stuhlinger arrived in the United States courtesy of the U.S. Army. He resumed his electric propulsion work in Huntsville in the early 1950s, at which time von Braun's team was part of the Army Ballistic Missile Agency at Redstone Arsenal. He played a key role in the launch of the first U.S. satellite, Explorer 1, in January 1958. Under von Braun's leadership, the Peenemünde rocketeers became the nucleus around which NASA MSFC coalesced in July 1960.
The hybrid NERVA/nuclear-electric approach would, the MSFC engineers explained, magnify the benefits and mitigate the drawbacks of both propulsion methods. Efficient electric 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 Mars spacecraft into Earth orbit for assembly. Five uprated two-stage Saturn V rockets would be sufficient to launch all the components making up a hybrid spacecraft into Earth orbit — about half as many as required to launch a Mars spacecraft propelled by NERVA nuclear-thermal rocket engines alone.
Nuclear-thermal rockets, for their part, would trim trip time and reduce crew radiation exposure. Nuclear-electric 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 radiation belts in minutes.
Stuhlinger and his colleagues scheduled their NERVA/nuclear-electric 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 weight.
They also anticipated that electric propulsion would be applied first to Earth-orbital satellite station-keeping in the late 1960s, and that enough electric propulsion research would be completed by 1974 to justify government approval of the NERVA/nuclear-electric 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-electric spacecraft components and propellant launched from Earth's surface. To enhance safety, four identical piloted 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 crew members 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.
Phase 2 would see launch of four nuclear-thermal rocket stages and Mars expedition 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-electric 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 during Earth departure, 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 just 14 days in December 1965, so the MSFC team in fact had very little basis for its opinion.
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-electric 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 then coast around the Sun along a curving Mars-bound path.
One-hundred-and-forty-five days out from Earth, the four ships would reactivate their nuclear-electric thrusters to begin deceleration. Then, on 23 September 1986, Mars would capture them into a high orbit. Their nuclear-electric thrusters would continue to operate for 23.5 days so that they would spiral down to a 1000-kilometer circular Mars orbit.
During the spiral-down period, the four MEMs would undock and land on Mars, leaving the four ships unmanned. Relieved of the weight of the MEMs, the nuclear-electric ships could 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, returning data that indicated that the planet's atmosphere was about 10 times thinner than had been 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 MSFC study team (see "more Information" below).
Atmospheric drag would slow the 10-meter-wide MEM, then its heat shield would eject to expose chemical-propellant landing retro-rockets. 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 he exhausted the MEM's descent propellants.
After a month on Mars, the four 27-ton MEM ascent stages would blast their crews back to Mars orbit, where they would dock with their respective nuclear-electric Mars ships. The crews would transfer, then the ascent stages would be cast off. Because the Mars spacecraft would no longer carry the weight of 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 electric 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 among the last to look in detail at nuclear-electric propulsion until the late 1980s. Just seven years earlier, Stuhlinger had concluded his 1959 nuclear-electric freighter paper by predicting that a nuclear-electric 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-electric propulsion was little closer to flight than it had been in 1959. Stuhlinger died at age 94 in May 2008.
"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.
"Ernst Stuhlinger: Rocket Scientist Crucial in Space Race, is Dead at 94," John Noble Wilford, New York Times, 28 May 2008 (http://www.nytimes.com/2008/05/28/us/28stuhlinger.html - accessed 18 December 2016).
The Challenge of the Planets, Part Two: High Energy
Gumdrops on Mars (1965)
The Last Days of the Nuclear Shuttle (1971)