Rabu, 10 Mei 2017

Stuhlinger's Cosmic Butterfly (1954)

Ernst Stuhlinger (bottom image above) owed his place in the Guided Missile Development Division at Redstone Arsenal in Huntsville, Alabama, to Operation Paperclip, the U.S. Army's effort to retrieve rocket engineers and V-2 missiles and assembly hardware from the smoking ruins of the Nazi German empire. The U.S. military was (of course) mainly interested in tapping their talents to build missiles, but the Germans did their energetic best to cultivate other aspects of rocketry in the United States. For example, the most famous of the German rocketeers, Wernher von Braun, set out in the 1950s with help from Collier's magazine and Walt Disney to "sell" space stations and moon and Mars expeditions to the American citizenry.

In a paper presented at the Fifth International Astronautical Federation Congress in 1954, Stuhlinger pitched interplanetary travel using low-thrust ion (electric) propulsion. The spacecraft design he proposed comprised three major parts: the crew/payload compartment at the ship's center; a 146.4-ton multi-unit solar-electric power system; and a multi-chamber low-thrust ion drive system.

Stuhlinger provided no details about the layout of his ship's crew/payload compartment, other than that it would carry up to 50 tons of crew and cargo. He did, however, offer abundant details on his solar-electric power and ion drive systems.

The former would include two 350-meter-wide "wings," each comprising 19 independent electricity-generating "sub-units." A dish-shaped mirror 50 meters wide would form the largest component of each 4400-kilogram sub-unit. Stuhlinger wrote that his spacecraft would gain speed very slowly, accelerating at a rate equal only to about 1/1000th of Earth's surface gravity. At such a low rate of acceleration, a fork dropped in the ship's messroom would need more than five minutes to strike the floor. The low acceleration would mean that the mirrors would have no need of robust construction; they might comprise "thin aluminum foil with a very light supporting frame."

Each 450-kilogram, 2000-square-meter mirror would concentrate sunlight onto a boiler, causing a working fluid within it to turn to steam. The steam would drive a turbine, which would in turn drive a generator capable of producing 200 kilowatts of electricity. The steam, meanwhile, would enter a disk-shaped radiator cooler and condense back into fluid. Boiler, turbine/generator, and cooler would revolve together as a unit, completing one revolution every 10 seconds. This would generate acceleration that would cause the working fluid to flow to the cooler's outer rim, from which it would be pumped back to the boiler.

The multi-unit solar-electric power system would have built-in redundancy, Stuhlinger noted. Even if a large "meteor" hit the ship, he wrote, "the total loss of one or two sub-units would mean only a minor reduction of the capacity of the power plant."

Stuhlinger rejected an "atomic pile" as a heat source; in addition to having a mass of "hundreds of tons," a reactor would emit harmful radiation that would demand heavy shielding and make in-flight repair difficult. He added, however, that "an atomic pile will be a very promising power source for an electrically propelled space ship as soon as the mass problem, the shielding problem, and the maintenance problem have been solved satisfactorily."

The third major part of Stuhlinger's ship, the ion drive, would consist of many clustered thrust chambers. Within each, electricity from the solar-electric power system would ionize cesium or rubidium vapor using heated platinum grids and paired positive and negative electrodes. The cesium or rubidium ions would then depart the chamber through an opening at a large fraction of the speed of light to push the ship through space.

Stuhlinger wrote that cesium would be a more efficient propellant than rubidium. A cesium-fueled ship would need only 1833 thrust chambers to produce as much thrust as a rubidium-fueled ship with 2200 chambers. He noted, however, that cesium is "a rare element which might not be available in quantities as required for space ships."

Despite its large number of thrust chambers, Stuhlinger's ion drive would generate at most nine kilograms of thrust. This would, however, be applied continuously for long periods. Assuming no interference from planetary or solar gravity, Stuhlinger's ship could in a year travel 183 million kilometers in a straight line and reach a velocity of 12 kilometers per second.

Stuhlinger calculated that his ship would need just 18.6 tons of rubidium to accelerate continuously for one year. Even with its elaborate solar-electric power and ion drive systems, his ship's mass would total just 280 tons. To reach the same 12-kilometer-per-second velocity, a chemical-propulsion Mars spaceship would need a mass of about 820 tons, most of which would comprise propellants. For his calculations, Stuhlinger assumed that the chemical ship's rocket motors would burn nitric acid oxidizer and hydrazine fuel. He also assumed that both the ion and chemical Mars ships would be assembled in Earth orbit from components launched atop chemical-propulsion cargo rockets; his ship's lesser mass meant that it would need about a third as many cargo launches for assembly as would its chemical counterpart.

An ion drive spaceship would, of course, not travel between Earth and Mars in a straight line; it would instead gradually spiral out of Earth orbit into solar orbit, follow a curved course around the Sun to Mars, capture into a distant Mars orbit, spiral gradually down to a low Mars parking orbit, spiral out of Mars orbit, follow a curved course around the Sun back to Earth, capture into distant Earth orbit, and gradually spiral down to low Earth parking orbit. Halfway to Mars and again halfway to Earth the ship would turn end for end to face its thrust chambers forward and begin a slow deceleration. Stuhlinger determined, nonetheless, that his low-thrust solar-electric ion drive spaceship could travel from Earth orbit to Mars orbit and back in just two or three years; that is, in approximately the same period of time that a high-thrust chemical spaceship would need.

Stuhlinger did not call his spaceship the Cosmic Butterfly; that name originated with Frank Tinsley (1899-1965), an artist, cartoonist, and author famed for his futuristic technical illustrations. Tinsley used the term "gigantic butterfly" in reference to Stuhlinger's design in a 1956 article in Modern Mechanix magazine. The illustration at the top of this blog post, which Tinsley painted in 1959 for an American Bosch Arma Corporation advertisement titled "Cosmic Butterfly," depicts a ship little different from Stuhlinger's 1954 design.

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