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A Critical History of Electric Propulsion: the First Fifty Years (1906-1956)

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A Critical History of Electric Propulsion: the First Fifty Years (1906-1956) A Critical History of Electric Propulsion: The First Fifty Years (1906-1956) Edgar Y. Choueiri∗ Princeton University Princeton, New Jersey 08544 AIAA-2004-3334† Nomenclature When writing history, it is tempting to identify the- a = Vehicle acceleration matic periods in the often continuous stream of events A = Beam cross-sectional area under review and label them as “eras”, or to point i ≡ p/V = Current per unit vehicle mass to certain achievements and call them “milestones”. j = Current density Keeping in mind that such demarcations and desig- Mv = Vehicle mass nations inevitably entail some arbitrariness, we shall m˙ = Propellant mass flow rate not resist this temptation. Indeed, the history of elec- P = Input electric power tric propulsion (EP), which now spans almost a full p ≡ P/Mv = Input electric power per unit vehicle century, particularly lends itself to a subdivision that mass epitomizes the progress of the field from its start as T = Thrust the dream realm of a few visionaries, to its transfor- uex = Rocket exhaust velocity mation into the concern of large corporations. We V = Voltage shall therefore idealize the continuous history of the η = Thrust efficiency field as a series of five essentially consecutive eras: 1. The Era of Visionaries: 1906-1945 2. The Era of Pioneers: 1946-1956 3. The Era of Diversification and Development: 1957-1979 4. The Era of Acceptance: 1980-1992 ∗Chair of AIAA’s Electric Propulsion Technical Committee, 2002-2004. Associate Fellow AIAA. Chief Scientist at Prince- 5. The Era of Application: 1993-present ton University’s Electric Propulsion and Plasma Dynamics Laboratory (EPPDyL). Associate Professor, Applied Physics This is not to say that the latter eras were lack- Group, MAE Department. e-mail: [email protected]. ing in visionaries or pioneers, nor that EP was not † Presented at the 40th AIAA/ASME/SAE/ASEE Joint used on spacecraft until 1993 or that important con- Propulsion Conference and Exhibit, Ft. Lauderdale, FL. Copy- ceptual developments did not occur at all until the right c 2004 by the author. Published by the AIAA with permission. Also published in the Journal of Propulsion and sixties, but rather that there is a discernible char- Power, Vol. 20, No. 2, pp. 193–203, MarchApril 2004. acter to the nature of EP-related exploration during 1 CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 2 these consecutive periods of EP’s relatively long his- that the associated risks were well worth taking, but tory. The preceding classification is intended to give also to the the acceptance and success EP has had in a framework to our discussion, which will be useful the Soviet Union. That the first electrically-propelled for comprehending EP’s peculiar and often checkered spacecraft to go into deep space did not do so until evolution1. The present paper, which represents the almost a century after the first EP conceptions is a first installment of our historical review, deals with fact that would have disheartened their visionary au- the first two eras, which correspond to the first fifty thors. years of the history of the field. The second and far more hindering problem that What makes the history of EP a bit unlike that stood, and remains, in the way of EP-enabled hu- of most aerospace technologies, is that despite EP’s man exploration of the planets, is the frustrating lack recent, albeit belated, acceptance by the spacecraft of high levels of electric power in space. US efforts community, it still has not been used for the appli- to develop nuclear power sources for spacecraft have cation originally foreseen in the dreams of its ear- been fraught with repeating cycles of budgetary, po- liest forefathers namely, the systematic human ex- litical and programmatic setbacks over the past five ploration of the planets. The irony of still falling decades, despite considerable technical achievements short of that exalted goal while much ingenuity has in programs that were either discontinued or did not been expended on inventing, evolving and diversify- come to fruition in a space flight2. Lyndon B. John- ing EP concepts, can be attributed to two problems son was US president when the last and, to date that were likely unforeseeable to even the most pre- only US fission3 nuclear power source was launched scient of the early originators. in space (SNAP-10A; 650 We output; launched April The first problem is EP’s decades-long role as 3, 1965). The record of the most powerful nuclear the technological “prince in waiting” of spacecraft power source in space is still held at about 5 kWe propulsion. Despite the relatively early maturity of by the 1987 flight of the Soviet Topaz 1 fission re- some EP concepts, their systematic use on commer- actor onboard the Cosmos 1818 and 1867 spacecraft. cial spacecraft was delayed until the last two decades This 5 kWe record makes the present prospects of a of the twentieth century. A measure of this forced 10 MWe electrically-propelled piloted spaceship seem detainment can be gleaned from a hypothetical con- as dim as six 100-watt lightbulbs compared to a fully- trast to the history of atmospheric flight, in which lit Yankee Stadium. the demonstration of powered flight at Kitty Hawk As much as the realization of viable nuclear power in 1903 would not have led to acceptance of the air- generation on spacecraft is critical to the fulfilment plane until 1940. This retardation is doubtless due, of EP’s ultimate role, we shall not discuss it further partially, to the technological conservatism that is en- here. Although its current chapter is unfolding now, 4 demic in the spacecraft industry, where more tradi- and not without the usual optimism , the history of tional and well-proven propulsion systems have been, placing powerful nuclear power sources in space has perhaps understandably given the immense financial not been on the whole a success story. Suffice it to stakes, difficult to supplant. Breaching this psycho- 2The SP-100 program aimed for 100 kWe output, consumed logical barrier did not fully occur in the West until half billion dollars and was terminated in 1993. The Nuclear around 1991. It was not only the result of an over- Electric Propulsion Spaceflight Test Program centered around due realization on the part of aerospace planners of the Russian Topaz II reactor (40 kWe) met the same fate around the same year. the cost-savings benefits of EP and a demonstration 3While radioisotope thermoelectric generators (RTGs) have been used reliably on 24 US spacecraft, their electric power 1The reader will soon note a measure of the vagaries of that output and specific power make them wholly inadequate for EP evolution: while the earliest thoughts and experiments related on piloted or heavy cargo missions. Even the most advanced to EP are almost all about electrostatic propulsion, the first radioisotope power systems today have specific powers below laboratory electric thruster was electrothermal and the first 10 We/kg[1]. electric thruster to ever fly in space was of the pulsed (mostly 4NASA’s Prometheus program promises to be synergistic electromagnetic) plasma type. with electric propulsion[2]. CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 3 say that when that history is documented, it would with the achievements made on EP subsystems (e.g. make that of EP, in comparison, one of steady and power conditioning, mass feeding, propellant storage, linear progress. etc.) nor can we attempt any fair accounting of the Despite these major obstacles to its development, milestones in ancillary, albeit critical, fields (e.g. low- the history of EP turned out to be a success story: thrust trajectories, mission planning, etc.). Instead, Almost two hundred solar-powered satellites in Earth we will concentrate on the evolution of the EP con- Orbit and a handful of spacecraft beyond Earth’s cepts themselves. Also, we shall focus more on tech- grvitational influence have benefited to date from the nical milestones and less on programmatic develop- mass savings engendered by EP. ments (e.g. histories of various NASA and Air Force Before starting our review of that history, we wish EP programs) even though the attainment of the for- to state some assumptions and define a few self- mer often depends on the success of the latter. imposed limitations. These may limit the scope Finally we should mention that our intent is not of our coverage, but will hopefully render the re- merely to compile a factual and dry chronicle of view easier to assimilate and bound its expansive- events and accomplishments, but rather to present ness. Specifically, we shall assume that the reader is a critical history that does not shy away from being acquainted with the major classifications of EP sys- analytical and reflective when appropriate5. tems (electrothermal, electrostatic, electromagnetic) and somewhat familiar with the basic features of the main EP concepts. The uninitiated reader might ben- 1 The Era of Visionaries: 1906- efit from reading our recent article[3] or referring to the earlier textbook[4]. In order to keep the flow 1945 of the main discussion unimpeded by mathematical It is difficult to think who in aerospace history, derivations, ancillary information, or technical and perhaps even in the history of modern science and historical details, we shall relegate these to footnotes technology, embodies the quintessential qualities of which will be frequent and often extensive. the archetypical visionary more than Konstantin Ed- Furthermore, we shall admittedly favor for inclu- uardovitch Tsiolkovsky6 (1857-1935). It is also dif- sion work performed predominantly in the United ficult to find a more vivid encapsulation of the essence States. We will however mention, without any pre- of visionary work than his own words: tension to be all-inclusive or exhaustive, a number of seminal works and important advancements that This work of mine is far from considering all occurred outside the United States and provide refer- of the aspects of the problem and does not ences, whenever possible, to publications where these solve any of the practical problems associ- developments have been described.
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