Ernst Stuhlinger was one of Wernher von Braun's compatriots at Nazi Germany's Baltic Sea rocket base of Peenemünde. He worked on missile guidance systems. At the end of the Second World War, he became one of 126 German rocketeers spirited to New Mexico by the U.S. Army. He worked beside von Braun at the Army Ballistic Missile Agency at Redstone Arsenal in Huntsville, Alabama, and helped to lead the team that launched the first successful U.S. satellite, Explorer 1, on 31 January 1958. In mid-1960, he transferred to NASA with the rest of von Braun's group to form the nucleus of NASA's Marshall Space Flight Center.
Stuhlinger had worked in Hitler's nuclear and missile programs, but ion propulsion was his first love. In a paper presented in Japan a year before his transfer to NASA, Stuhlinger proposed an innovative split lunar transportation system which would see passengers reach a moon base, 238,000 miles away, in from 40 to 60 hours on board high-thrust chemical-propulsion spacecraft. These would "pierce through the Van Allen radiation belts in a sufficiently short time to keep the passengers safe." Cargo, meanwhile, would reach the moon on unmanned nuclear-powered ion freighters, which Stuhlinger dubbed "cargo ferries." These low-thrust spacecraft would, Stuhlinger explained, "need several weeks for the one-way trip, but [would] offer a payload-to-weight ratio which is superior. . .to that of high-thrust vehicles." In other words, an ion-propelled spacecraft could deliver a lot of cargo while expending minimal propellant. Both the fast chemical and slow ion spacecraft would depart for the moon from a space station in 600-kilometer-high Earth orbit.
Stuhlinger's 1959 lunar cargo ferry design included a nuclear reactor at the end of a long slender boom. The reactor would heat a working fluid which would drive a turbine. The turbine would turn a generator that would provide the cargo ferry's ion drive with electricity. The ion drive, a conical cluster of thrust chambers positioned to thrust at a 90° angle relative to the reactor boom, would draw propellant (probably cesium; Stuhlinger was not specific) from a spherical propellant tank. A pair of radio dish antennas on booms would enable operators at the Earth-orbiting station and the lunar surface base to remotely control the cargo ferry, which would include little or no automation. A crane-like arm would support a cylindrical cargo lander.
The cargo ferry's most distinctive feature was probably its disc-shaped radiator, which was designed to cool the working fluid after it passed through the turbine. The radiator was positioned so that it would be edge on to the Sun as the cargo ferry moved through space, so that most of it would not be in direct sunlight. The radiator would rotate to drive working fluid to its outer edge, where it would be pumped back to the reactor.
Stuhlinger described a typical lunar cargo ferry mission. The spacecraft would gradually spiral out from space-station orbit and after several weeks would fly past the moon at a distance of a few hundred kilometers, not capturing into lunar orbit. The cargo lander would detach during the flyby and fall toward the lunar surface base, firing chemical-propulsion rocket motors to reduce speed. The cargo cylinder would separate from the lander a few feet above the moon's surface and drop to a rough impact. Relieved of the cargo cylinder's mass, the lander, motors still firing, would reverse direction, ascend until it exhausted its propellants, and crash a safe distance away from the lunar surface base. The ion cargo ferry, meanwhile, would point its thrust chamber forward to slow down and begin spiraling back to low-Earth orbit. Back at the space station, the ferry would be refurbished, refueled, and loaded with another cargo lander for a new trip to the moon.
Stuhlinger offered design data for four lunar cargo ferries, two of which are described here. All four would carry a 50-ton cargo lander. Design 1, the smallest of the four, would have a total mass of about 20 tons without the lander. Of this, its two-megawatt nuclear reactor would account for 10 tons, three tons would be structure, and propellant would amount to 6.8 tons. The ion thrust chamber would produce 5.2 kilograms of thrust. The voyage from 600-kilometer Earth orbit to the moon and back would last 116 days.
The largest of Stuhlinger's lunar cargo ferries was Design 4, which would have a total mass of about 78 tons without the cargo lander. Of this, a 12-megawatt reactor would account for 60 tons, 5.5 tons would be structure, and propellant would amount to 12.7 tons. Design 4's large ion thrust chamber would produce 25 kilograms of thrust during its 58-day roundtrip lunar voyage.
Stuhlinger calculated that using ion propulsion for lunar cargo deliveries would dramatically reduce the mass of propellants that would need to be launched into Earth orbit. For 10 round trips of a single reusable Design 4 ferry, for example, 193 tons would need to be launched into Earth orbit (exclusive of cargo). For comparison, 2470 tons would need to be launched into Earth orbit (again, exclusive of cargo) for 10 round trips of a chemical-propellant, high-thrust cargo vehicle capable of a 40-hour voyage from the Earth to the moon.
The lunar cargo ferry design Stuhlinger presented in Japan underwent a major revision in 1962. The image at right shows a model of a nuclear ion Mars expedition spacecraft built as a prop for the 1957 Walt Disney television program Mars and Beyond. The model included elements of the 1959 lunar cargo ferry design, the most obvious of which were its disc-shaped radiator and boom-mounted reactor. The 1962 lunar cargo ferry design, shown at the top of this post, had features in common with Stuhlinger and Joseph King's 1962 design for a piloted Mars spacecraft: most notably, twin boom-mounted ion drive units and triangular radiator panels arrayed along the boom holding the reactor. The 1962 manned Mars spacecraft will be the subject of tomorrow's Beyond Apollo post.
References:
"Lunar Ferry with Electric Propulsion System," Ernst Stuhlinger, First Symposium (International) on Rockets and Astronautics, Tokyo, 1959, Proceedings, M. Sanuki, editor, 1960, pp. 224-234.
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