In February 1991, six engineers with McDonnell Douglas Space Systems Company in Houston, Texas, unveiled a Mars transportation scenario which, they explained, was "driven by the desire to explore Mars in a continuous fashion rather than designing a series of independent, Apollo-style missions. . .then phasing out the program." Specifically, they proposed using the Cislunar Libration Point (CLP) and the Cismartian Libration Point (CMP) as "parking orbit transportation nodes" for a pair of Nuclear-Electric Propulsion (NEP) Mars spacecraft. The CLP, better known as Earth-moon L1, is situated about 70,000 kilometers from the center of the moon's Earth-facing Nearside hemisphere (that is, about 327,000 kilometers from Earth). The CMP, better known as Sun-Mars L1, is located 1.083 million kilometers Sunward of Mars. The McDonnell Douglas team assumed that, by the time their Mars program began, piloted Lunar Transfer Vehicles (LTVs) would already be using the CLP as a staging area for lunar landing missions.
The McDonnell Douglas engineers based their proposed NEP spacecraft design on one developed at NASA's Lewis Research Center in Cleveland, Ohio (image above). A 10-megawatt nuclear reactor on the spacecraft's nose would power its tail-mounted electric thruster clusters, which would ionize and expel xenon atoms to generate thrust. Electric propulsion spacecraft accelerate slowly but continuously and use much less propellant than equivalent chemical-propulsion and nuclear-thermal-propulsion spacecraft. The McDonnell Douglas engineers calculated that their reusable NEP Mars spacecraft could haul double the payload of an equivalent expendable chemical-propulsion Mars spacecraft while using 42% less propellant.
They based their Mars mission program schedule on the timeline for the Space Exploration Initiative (SEI) President George H. W. Bush had announced on May 11, 1990, as part of his commencement address at Texas A & M University. When Bush first unveiled SEI on July 20, 1989, the 20th anniversary of the Apollo 11 moon landing, he called for a return to the moon followed by an expedition to Mars, but he established no deadlines for accomplishing these feats. Bush's May 1990 timeline had the first Americans setting foot on Mars ahead of the 50th anniversary of Apollo 11 in 2019.
In late 2015 or early 2016, NEP Spacecraft 1 would transfer from 1000-kilometer-high Low-Earth Orbit (LEO) to its parking place at the CLP. It would have a mass of 455 metric tons when it started its five-month "maiden spiral" outward from Earth. During its slow voyage outward, Spacecraft 1 would orbit the Earth many times, gradually gaining altitude, and would pass repeatedly through the Earth-girdling Van Allen Radiation Belts. The repeated slow Van Allen Belt crossings would expose any astronauts on board to a lethal dose of radiation. Because of this, Spacecraft 1 would travel from LEO to CLP without a crew. It would expend 43 metric tons of propellant, reducing its mass to 412 metric tons.
Making the CLP and CMP destinations for the NEP spacecraft would eliminate the need for long spirals in and out of low planetary orbits at the start of each transfer between the Red and Blue planets. Spacecraft 1 and its sister ship Spacecraft 2 would each depart LEO once for the CLP and would never approach nearer to Mars than the CMP.
In early 2016, about a month after Spacecraft 1 reached the CLP, astronauts would arrive from LEO in a fast chemical-propulsion LTV. Rapid transfer from LEO to CLP would limit crew radiation exposure to levels similar to those experienced by Apollo crews.
The astronauts would perform exhaustive checks of Spacecraft 1's systems; then, if all checked out as normal, they would activate Spacecraft 1's NEP thrusters to take advantage of the 2016 minimum-energy Earth-Mars transfer opportunity. Such opportunities occur every 26 months. Spacecraft 1 would need 18 days to escape the Earth-moon system, then about 220 days to reach the CMP. This transfer duration would compare favorably with those of chemical-propulsion or nuclear-thermal-propulsion Mars spacecraft.
The transfer would include a coast period during which the electric thrusters would be switched off. The longer the coast period, the less propellant the spacecraft would need (but the longer the overall trip duration). Conversely, a shorter coast period would mean a shorter trip time (but greater propellant expenditure). After the coast period, Spacecraft 1 would point its thrusters in its direction of motion to slow itself. The McDonnell Douglas team assumed that Spacecraft 1 would arrive at the CMP with a mass of 350 metric tons.
Spacecraft 1 would park at the CMP for from 500 to 600 days to await the next minimum-energy Mars-Earth transfer opportunity, which would occur in mid 2018. During the wait period, the crew would separate from Spacecraft 1 in a two-stage expendable Mars lander. The journey from the CMP to the martian surface would require from 15 to 30 days, the McDonnell Douglas engineers estimated. They calculated that a Mars lander that needed 15 days to reach Mars from the CMP and could deliver 50 tons of payload to Mars's surface would have a mass of 150 tons.
After a surface stay of unspecified duration, the astronauts would lift off in the lander's ascent stage, leaving its expended descent stage and its payload behind on Mars. The journey from Mars back to the CMP would last from 15 to 30 days. The crew would dock with Spacecraft 1, transfer their Mars samples and data from the ascent stage, then cast off the ascent stage.
Meanwhile, back in the Earth-moon system, Spacecraft 2 would perform its unmanned maiden spiral from LEO to the CLP. It would park at the CLP until a crew arrived in an LTV. In mid 2018, about 26 months after Spacecraft 1 left the CLP, Spacecraft 2 would set out on a near-copy of Spacecraft 1's Mars voyage. Because the 2018 Earth-Mars transfer opportunity would be more favorable (that is, it would need less energy) than its 2016 counterpart, Spacecraft 2 would arrive at the CMP with slightly more mass (356 metric tons) than had Spacecraft 1. Its crew would then separate in a lander to explore a new landing site on Mars.
In mid 2018, at about the time Spacecraft 2 departed the CLP, Spacecraft 1 would leave the CMP. Having left behind its lander and payload, Spacecraft 1 would have a mass of 173 metric tons when it departed the CMP and 138 metric tons when it returned to the CLP. Travel from the CMP to the CLP would need less time than from the CLP to the CMP because the NEP spacecraft would have less mass during the CMP-CLP transfer.
Upon arrival at the CLP in late 2018, Spacecraft 1's crew would board a waiting LTV and return to the LEO space station. In the from 500 to 600 days that followed, automated cislunar NEP freighters would deliver to unmanned Spacecraft 1 xenon propellant, life support supplies, spare parts, and an expendable lander for its next Mars voyage.
The McDonnell Douglas engineers noted that the CLP is an unstable libration point, so any spacecraft parked there would need to hold position using thrusters or risk ejection from the Earth-moon system. Fortunately, the occasional stationkeeping maneuvers would be small.
In mid 2020, near the end of its nearly two-year parked period, Spacecraft 1 would receive an LTV bearing the Mars program's third crew. The authors noted as an aside that cargo and astronauts bound for Mars could originate on the moon. After system checks, Spacecraft 1 would depart the CLP with a mass of 412 metric tons.
Spacecraft 2 would park at the CMP until mid 2020, then would begin its first transfer back to the CLP. It would depart the CMP with a mass of 173 metric tons and arrive at the CLP in late 2020 with a mass of 137 metric tons. Spacecraft 1, meanwhile, would arrive at the CMP for the second time in early 2021 with a mass of 366 metric tons. The twin NEP spacecraft could trade CLP and CMP parking places indefinitely, never meeting, the McDonnell Douglas team wrote.
"Optimal Cycling Between Cislunar and Cismartian Libration Points With Reusable Nuclear Electric Transfer Vehicles," Steven J. Sponaugle, Brian H. Rishikof, Steven F. Davis, Douglas A. Pesek, Diane R. Walyus, and Victor R. Bond, AAS 91-104; paper presented at the AAS/AIAA Spaceflight Mechanics Meeting held in Houston, Texas, February 11-13, 1991.
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