A critical History of Electric Propulsion: The First Fifty Years (1906-1956)
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positively charged atoms had been known, the pro- ton had been discovered, and he had recognized the better suitability of ions to propulsion)[8, p. 222]: It is quite probable that electrons and ions can be used, i.e. cathode and especially an- ode rays. The force of electricity is unlim- ited and can, therefore, produce a powerful flux of ionized helium to serve a spaceship. However, we shall leave these dreams for a while and return to our prosaic explosives 13 .
ciently versed in electricity and magnetism, let alone the newly burgeoning field of gaseous electronics, to tackle the problem of EP. That problem was first addressed, and at an even earlier date than Tsi- olkovsky’s first qualitative speculations, by a young American visionary who was trained precisely in these nascent branches of physics and who shared a passion for space travel with the “dreamer from Kaluga”, despite never having heard of him or his 12 One example is the little recognized fact that he had clearly anticipated laser propulsion (another EP concept of sorts) in this quote from 1926[8, p. 134]:“We may have a case when, in addition to the energy of ejected material, we also have an influx of energy from the outside. This influx may be supplied from Earth during motion of the craft in the form of radiant energy of some wavelength; . . . ”. 13 The word “explosives” in Tsiolkovsky’s parlance refers to liquid chemical propellants. CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 6 ideas 14 . Robert Hutchings Goddard 15 ’s (1882-1945) early career as a young academic physicist was divided be- tween his official research work on electricity and his personal passion for propulsion 16 . That this would lead him to think of electric propulsion was natural if not inevitable. Aside from an amusing anecdote 17 about his “ear- liest recollection of a scientific experiment” at the age of five[17] –and which, incidentally, involved the use of “electricity” for “propulsion”– the first doc- 14 There seems to be no evidence to doubt the claims made by each of Tsiolkovsky, Oberth and Goddard of having arrived at many of their early findings regarding chemical rockets in- dependently. Oberth stated in a letter addressed to Goddard and dated May 3, 1922[14], that he had just learned of God- dard’s work as he was preparing the manuscript of his book Die Rakete zu den Planetenr¨ aumen for publication. In response Goddard sent him a copy of his famous 1919 monograph “A Method of Reaching Extreme Altitudes” which Oberth subse- quently cited in an appendix of his book. This letter therefore fixes 1922-1923 as the date when both men became aware of each other’s work. It is most likely that Tsiolkovsky learned of his Western counterparts’ works not long after the publica- tion of Oberth’s well-disseminated book in 1923, well after the (limited) publication of the first two versions (1903 and 1911) of his own “Investigation of Universal Space by Means of Re- active Devices”, but before his extensive work of 1926 carrying the same title. Goddard and Oberth seem to have remained unaware of the work of the Russian visionary until around 1927, the year of the Moscow exhibition on “Interplanetary Apparatus and Devices” where Tsiolkovsky was hailed as the Russian “father of rocketry” . (Goddard’s wife Esther wrote on a souvenir scrapbook of that exhibition notes that decried the insufficient recognition given to her husband’s work[15].) We had already mentionned in Footnote 8 Tsiolkovsky’s reac- tion, as early as 1924, to Oberth’s popularity. Later, Oberth wrote to Tsiolkovsky “I would certainly be much further in my own work today. . . had I taken into account your superior work.”[14]. 15 Refs. [14, 16, 15] are three of the biographical books on Goddard. 16 This duality of interest is epitomized by Goddard’s work habits during his memorable stay at Princeton University as a research fellow in electricity and magnetism during the aca- demic year of 1912-1913. During the day he worked on dis- placement current experiments, his official research project (a by-product of which led to a patent that was instrumental in the development of the radio tube), and he spent his evenings working on the theory of rocket propulsion[14, 16]. 17 The story involves a five-year old Goddard attempting to propel himself upwards after rubbing zinc from a battery on his shoes and scuffing them vigorously on a gravel walk to cause electric sparks[14, 17]. Figure 1: Robert H. Goddard. umented instance in which Goddard considered the possibility of electric propulsion dates to September 6, 1906. On that day the twenty-four-year old God- dard set out to address the problem of producing “reaction with electrons moving with the velocity of light” and wrote down his thoughts on this problem in his notebook. In particular he posed the question: At enormous potentials can electrons be lib- erated at the speed of light, and if the po- tential is still further increased will the reac- tion increase (to what extent) or will radio- activity be produced?[18, p. 84] Goddard quickly demonstrated in these handwrit- ten pages[18, pp. 82-88] (dated September 6 and 9, 1906) that he was quite aware of the most recent de- velopments in physics concerning the nature of cath- ode rays 18 . However the incomplete state of that 18 This was not the first time the young Goddard considered the application of cathode rays to propulsion. A few months earlier, in another entry in the same notebook[18, pp. 38-41], dated February 18, 1906, he conceived a device (which he also illustrated schematically) in which two parallel tubes, one pro- ducing (negative) cathode rays and the other (positive) canal rays, were thought to yield a net reactive force. This would seem to be the earliest documentation of an electric rocket con- cept. However, a close examination of his notes reveals that he did not discuss the device in terms of the rocket effect, i.e.
CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 7 knowledge hindered him from answering his ques- tions. Despite highly educated attempts he was not able to calculate the levels of required energy or power nor resolve the issue of what happens when the electrons reach the speed of light and the accel- erating potential is raised further. His notes and cal- culations on September 9 demonstrate that he was well aware of Walter Kaufmann’s careful measure- ments, published in 1901, which indicated that the inferred mass of the electron increased as its speed neared that of light. While he was, apparently, not yet aware of Einstein’s special relativity theory, which was published only a few months before and had not yet gained much acceptance 19 , Goddard found him- self contending with the conjecture that the electron’s inertia at the speed of light might be infinite. He did remain hopeful, however, that experiments might de- termine “the voltage necessary to give a speed equal to the velocity of light”. It is interesting to consider why, at that early stage, Goddard was more concerned with the electrostatic acceleration of electrons rather than ions despite his knowledge of canal rays, and why these early ideas, not surprisingly, still fell short of a workable thruster concept. We can suggest five reasons: 1. As we already mentioned in Footnote 11, the na- ture of these rays was still debated at that time and the ionization physics underlying the pro- duction of electron-ion pairs was not clear. reaction due to mass expulsion, but rather in terms of cre- ating a momentum imbalance. Specifically, Goddard stated that the cathode and anode rays would “simply serve as ways to increase an effect which is unbalanced”. These ideas of propulsion through unbalanced internal forces were constant on his mind since his first thoughts on the subject while in a cherry tree[15] in 1899. He did not totally give up such a concept, it seems, until March 4, 1907 when, after conceiving another device where charged particle acceleration in opposing directions was to produce a momentum imbalance, he wrote in his notebook[18, p. 150]: “The device . . . cannot be used, as the two opposite accelerations on each end of the condenser battery would neutralize each other”, and concluded with the insight: “A simpler plan would be to expel the electrons after they had acquired a significantly great velocity.”. 19 It was not until Planck and Minkowski published their ideas on special relativity in 1908 that Einstein’s famous 1905 publications on the subject were taken seriously. In 1905 Ein- stein was only a “technical expert third class” at the Bern patent office. Figure 2: An excerpt of the the entry dated Septem- ber 6, 1906 in Goddard’s handwritten notebook showing some of the questions he attempted to an- swer quantitatively in order to assess the feasibility of electric propulsion using electrostatic potentials to accelerate electrons to the speed of light (represented by the symbol Λ). 2. There was the implicit belief in these early writ- ings that high accelerating voltages (and not high beam currents) were the main technical dif- ficulty. This, consequently, favored electrons as the propellant needed to reach extremely high velocities. 3. There was still a lack of appreciation of the im- mense difficulty, stipulated by the laws of spe- cial relativity, in accelerating a particle having a finite rest mass to a speed very near that of light
20 . 4. It is doubtful that Goddard, at this early time, had fully appreciated the practical (i.e. system- related) penalty incurred by an electric rocket 20 While even late-nineteenth century cathode ray tubes ac- celerated electrons to speeds that are a fraction of that of light, the technology of powerful radio-frequency sources capable of accelerating electrons through linear resonance accelerators to speeds very close to that of light was not developed until after 1940.
CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 8 with an exceedingly high exhaust velocity 21 . 5. There is another system-related penalty that must have been far from Goddard’s mind. Elec- trostatic acceleration of lighter atoms, let alone electrons, while less demanding on the voltage, results in beam currents which, because of space charge limitation, incur adverse demands on the required area (and therefore size and mass) of the accelerator 22 . These five problems which confounded Goddard’s first thoughts on EP, were eventually dealt with one 21 This penalty can be seen by expressing the thrust-to- power ratio, T /P , of an EP system as a function of its exhaust velocity. Using the definition of thrust efficiency η ≡
mu 2 ex P (1)
and T = ˙
mu ex (2) we can write T P = 2η u ex (3)
which shows how raising the exhaust velocity, even at a max- imum thrust efficiency of 1, will incur a power supply mass penalty through the decrease of the amount of thrust per unit power. This mass penalty could easily overwhelm the mass savings, due to high exhaust velocity, indicated by Tsi- olkovksy’s rocket equation. Thrust with relativistic electron velocities is therefore most expensive from a power supply point of view. 22 This may not be directly evident but can easily be seen by using the definitions of thrust, mass flow rate and current density to write T = ˙ mu
= j q m i Au ex , (4) and invoking the Child’s law for space-charged limited current density (for an idealized 1-D electrostatic accelerator), j ∝
m i 1/2 V 3/2
d 2 , (5) then solving for the exit area, A, of the accelerator in terms of thrust and exhaust velocity (and not in terms of applied potential as more commonly done): A ∝
2 u 4 ex q m i 2 . (6) In practice d is limited by design constraints and the thrust and exhaust velocity are mission requirements. This leaves the area to scale with the square of the ion’s charge-to-mass ratio and emphasizes the benefits of heavier propellants. by one by him and other pioneers, but only over a time period extending over the next four decades. Goddard’s notebooks show that EP was a constant, if not a consuming, idea in his mind. Between 1906 and 1912 the evolution of his thoughts on that subject led him to appreciate the advantages of relying on the reaction of ions in an electrostatic accelerator, and the need for neutralizing the charged exhaust with a stream of oppositely charged particles. He explicitly stated the latter realization in the following quote from the March 9, 1907 notebook entry: If [negative] particles are shot off, the car will have an increasing [positive] charge un- til the potential is so great that [negative] particles cannot be shot off. Hence [posi- tive] particles must be emitted in a quantity equal to that of the [negative] particles. As in many instances 23 in the career of this ingenious and practical scientist, his ideas culminated, by 1917, in two inventions whose importance to the history of EP has been largely unrecognized. The first invention, whose patent application was filed in 1913 (granted in 1915), is a method for pro- ducing “electrically charged particles”[19] which re- lies on an applied magnetic field to confine electrons in a gas and thus greatly enhance the probability of their ionizing collisions with neutral molecules –much like it is done in the ionization chamber of modern electron bombardment ion thrusters and magnetron plasma sources. In 1917 Goddard, who by then had become an assistant professor of physics at Clark Uni- versity, filed another U.S. patent application titled “Method of and Means for Producing Electrified Jets of Gas”[20]. In that patent 24 , granted in 1920, God- dard presented three variants of apparatus, the first two of which are means of charging a stream of gas without having the stream affect the charging pro- cess. The third variant, however, is of direct rele- vance to our history as it is the world’s first docu- mented electrostatic ion accelerator intended for 23 There are 214 patents in Goddard’s name. 24 In an autobiographical article written in 1927 and pub- lished in 1959[17], Goddard stated that the experimental work which checked the conclusions set forth in that patent was car- ried out at Clark University by two students during 1916-1917.
CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 9 propulsion. Goddard, in his patent description of this particular variant of the invention, in fact mentioned propulsion as the main application 25 and stated, re- ferring first to the exhaust velocities of a chemical rocket from an earlier patent 26 :
yet been produced in any way with masses of gas of appreciably large magnitude, but are much less than are possible by the method herein described, for the reason that the potential of the container M , which pro- duces the high velocity, may be as high as desired. The schematic of that accelerator is shown in Fig. (3) whose caption describes the concept 27 . With the entry of the United States into World War I on April 6, 1917, Goddard offered his services to the Smithsonian for developing rockets for mili- tary applications. By this time his intermittent but visionary explorations of EP seem to have ceded to 25 Although the word “space” was not mentioned in the patent (instead RHG stated that the intended application was “jet propulsion”) there is little doubt that Goddard, who was well aware of the smallness of the reactive forces inherent in electrostatic acceleration, was thinking of spacecraft propul- sion as the ultimate application. (This assumption would best be ascertained by experimental measurements or more detailed description Goddard and his students may have made with an actual device, however we did not find any such documen- tation in the Goddard’s Archives at Clark University.) It is relevant to mention in this context that while Goddard of- ten wrote in his notebooks about the technical problems of space travel he rarely mentioned this ultimate application in official communications and confined his stated goals to the “reaching of high altitudes” for scientific studies. Later in his career he stated[21]: “I regard it as most unfortunate that the interplanetary aspect of rocket theory was seized upon and sensationalized. This has discouraged public confidence and in some cases has turned away serious support from the researches that need to be carried on into the fundamental problems of rocket and jet propulsion”. It is often said that Goddard never fully recovered from the humiliation of a 1920 New york Times editorial[22] in which his ideas on the use of rockets in the vacuum of space were severely ridiculed. 26 US Patent No. 1,102,653: “Rocket Apparatus”; Applica- tion filed October 9, 1912; Patent granted July 7, 1914. 27 A possible reason why this early electrostatic accelerator was overlooked as such is that the patent description deals with a number of aspects of “electrified jets of gas” only one of which is electrostatic acceleration. Figure 3: The the world’s first documented electro- static thruster. Schematic of Goddard’s third vari- ant of his 1917 invention as it appears in US patent #1,363,037 (granted in December 1920)[20]. The
propellant is injected through the tube labeled T 3 ; charge is added to the flow from the cathode filament F 1 which is placed in the wake of the stream and whose anode is a metallic plate at location P . The filament is powered by the power supply B 2 . The whole is enclosed in a metallic sphere M , which is “kept at a very high potential” using the power sup- ply B 1
that of the ions in the jet thus causing their repulsion away from the device at high velocities proportional to the applied potential[20].” his almost exclusive intellectual dedication to chemi- cal rocket launch vehicles. It is at this point in our story of EP that we must deal with the historically problematic role of Yuri V. Kondratyuk 28 (1897-1941). There is no doubt 28 There is presently no extensive biography, in English, of this most obscure of early thinkers on astronautics. The fol- lowing events of his life have become known through a recent biographical sketch[23]. His original name, Alexander Shargei, was changed to evade the authorities in the course of a woeful life. He landed in prison in Kiev while still in his mother’s womb. After demonstrating his intellectual brilliance at the gymnasium of his birthplace town of Poltava in the Ukraine he was forced to abort his engineering education in Kiev to com- mand a machine-gun platoon on the Transcaucasian Front dur- ing WWI. He then had a stint with the White Guard army, was CHOUEIRI: CRITICAL HISTORY OF EP (1906-1956) 10 that this relatively little known thinker deserves a place in the pantheon of astronautical visionaries for his bold, far-reaching and original ideas 29 , and that his name also deserves to be featured in EP’s early history. In a section under the heading “Concern- ing other Possible Reactive Devices” in a manuscript quaintly titled “To whomsoever will read in order to build” 30
31 , Kondratyuk, like Tsi- olkovsky and Goddard before him, wrote about EP in the context of cathode rays. Speaking of the high- velocity charged particles, however, he noted Their drawback is the tremendous energy required, and their velocity is greater than need be; the larger the velocity, the greater the amount of energy that we must expend to obtain the same reaction, . . . [9, p. 23]. The last sentence demonstrates that Kondratyuk was fully aware of Equation 3 and its practical implica- tions. That he fully appreciated the advantage of accelerating more massive particles is evidenced by almost killed by the Cheka while trying to escape to Poland, escaped to Siberia where he worked as a mechanic in Novosi- birsk then was caught and served three years in a labor camp before being released to work on wind turbines in Kharkov. He Download 329.88 Kb. Do'stlaringiz bilan baham: 
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