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LIQUID HYDROGEN AS A PROPULSION FUEL,1945-1959
Appendix A-3
Hydrogen Technology, 1900-1945
[265] In addition to the considerations of liquid hydrogen for rockets by Tsiolkovskiy, Goddard, and Oberth, other concurrent activities contributed to hydrogen technology. The largest and best known of these were the development and operation of the large dirigibles in Germany, Italy, Great Britain, and the United States from 1900 to 1937. Much has been written about these giants of the sky that need not be repeated here.1 Thousands of passengers were carried safely in hydrogen-filled dirigibles, yet there were many accidents, which finally killed the giants. The most spectacular-and final-accident involved the Hindenburg, filled with 200000 cubic meters of hydrogen, which burst into flames at Lakehurst, New Jersey, on 6 May 1937, killing 13 passengers and 22 members of its crew as well as one ground crewman.2 When the ease with which hydrogen-air mixtures can be ignited is examined, the wonder is that there were not more accidents. For example, the Germans found in 1912-1913 that faint, finger-length flames of hydrogen-air mixtures could be produced merely by rubbing rubberized surfaces together, and the same fabric generated static electricity if torn. One modern safety manual points out that the minimum spark ignition energy to ignite a hydrogen-air mixture at atmospheric pressure is 0.000019 joule.3 If that means nothing to you, the manual warns against certain actions that can generate static electricity to ignite hydrogen in test areas: combing the hair, wearing clothes made of nylon or other synthetics or wool, and allowing furred animals in the area. The legacy of dirigible operations, where millions of cubic meters of hydrogen were generated, stored, transferred, and flown, is not so much the safety procedures, but fears of using hydrogen that the accidents instilled in the minds of so many people.
One aspect of dirigible activity not so well known was attempts to use hydrogen as a fuel. Dirigibles had to vent buoyancy gas and Paul Haenlein obtained a U.S. patent in 1872 to use that otherwise wasted gas in the dirigible's engines. Haenlein, however, used coal gas and apparently did not get around to using hydrogen.4 Using hydrogen in dirigible engines surfaced again after World War I in Italy, Great Britain, Germany, and the United States. In 1920, two British investigators estimated that dirigible range could be increased 20 percent by burning the hydrogen usually vented. They found that an engine could operate on hydrogen as an additive or on hydrogen alone, but in the latter case there was a tendency to knock.5 Similar results were found elsewhere but the idea never gained widespread use. Experiments in Germany and the United States on using hydrogen in diesels met with some success in 1935, but by then the use of hydrogen in dirigibles was close to the end.6
[266] In addition to dirigible developments, another great stimulus to the development of hydrogen technology during the first four decades of' the twentieth century was scientific investigation. Unlike dirigible applications, which were centered on gaseous hydrogen, the scientific investigations that advanced hydrogen technology were primarily concerned with low temperature phenomena.
Hydrogen Technology from Science, 1900-1940
Progress in developing equipment for liquefying gases during the last decade of the nineteenth century was matched by equally impressive gains during the first decade of the twentieth. Carl von Linde's air liquefaction equipment, capable of liquefying 8 liters per hour, was exhibited in Paris in 1900 and purchased by the College of France.7 In 1902, a process for separating oxygen from air, developed by Georges Claude, was in commercial operation in France. Two years later the British Oxygen Company exhibited a hydrogen liquefier, designed by James Dewar, at the Louisiana Purchase Exhibition in St. Louis. The National Bureau of Standards purchased it for $2400 for low-temperature thermometry.8 By 1905, Linde liquefaction plants were operating in both Germany and France, and in 1907 the Linde Air Products Company began operations in the United States.
In 1906 interest in low temperature phenomena was stimulated when a German chemist, Walther Nernst, postulated the third law of thermodynamics-that the total and free energies become equal as absolute zero is approached. Heike Kamerlingh Onnes, founder of the cryogenic laboratory at Leyden in 1894, reached 4.2 K when he first liquefied helium in 1908. By evaporating helium, scientists were soon able to reach within one kelvin of absolute zero.
In 1924-1926, a new era began in theoretical physics-wave (quantum) mechanics. It was introduced by a 32-year-old scientist, Louis Victor de Broglie, in his doctoral thesis. He postulated a relationship between the velocity or momentum of electrons and wave lengths of radiation. His work stimulated many other physicists, and among those who carried the theoretical work further were Clinton Davisson, George Thomson, Erwin Schrodinger, and Weiner Heisenberg. All won Nobel prizes in physics for their contributions.
Heisenberg, a 24-year-old German physicist, believed that the theory should include only observable elements. His new wave mechanics theory expressed wave length frequencies and intensities of radiation emitted by the atoms in matrix mathematics. He used his theory to postulate in 1926 that the hydrogen molecule existed in two forms, which subsequently were called orthohydrogen and parahydrogen. Heisenberg's 1932 Nobel prize in physics was awarded "for the creation of quantum mechanics, the application of which has, among other things, led to the discovery of the allotropic forms of hydrogen."9
In orthohydrogen, the two hydrogen nuclei in the molecule spin in the same direction; in parahydrogen, the two nuclei spin in opposite directions. The two molecules have different physical properties but their chemical properties are the same.
In 1927 a British physicist, D. M. Dennison, used Heisenberg's postulate to make one of his own. Earlier observations of the specific heat of hydrogen had indicated an anomaly; the rotational specific heat decreased with time and temperature. Dennison [267] postulated that this was caused by the two kinds of hydrogen not being in equilibrium at the lower temperature. In 1928, William Giauque and Herrick L. Johnston at the University of California at Berkeley attempted to test Dennison's postulate by keeping a sample of hydrogen at a low temperature for six months, but the observed changes were so small that their experiment was inconclusive. The following year another team of investigators, K. F. Bonhoeffer and P. Harteck, used a catalyst to obtain equilibrium at low temperature and obtained almost pure parahydrogen. They showed the differences between orthohydrogen and parahydrogen in terms of specific heat and thermal conductivity of the gases.10
At room temperature and above, ordinary hydrogen is 75 percent orthohydrogen and 25 percent parahydrogen. At 77.4 K (temperature of liquid nitrogen used for cooling) the hydrogen mixture at equilibrium is 52 percent orthohydrogen and 48 percent parahydrogen. At the boiling point of liquid hydrogen, 20.3 K, the equilibrium composition is 99.8 percent parahydrogen.* When gaseous hydrogen is liquefied, it will slowly and spontaneously seek equilibrium, with orthohydrogen changing to parahydrogen. At 20.3 K, the conversion releases more heat (532 joules per gram) than is required to vaporize the liquid (453 joules per gram), so that liquefied normal hydrogen evaporates completely on conversion to parahydrogen-even in a perfectly insulated container-a situation Dewar did not foresee. The vaporization loss during the conversion at 20.3 K amounts to about 1 percent of the stored liquid hydrogen per hour, a loss much too high to be tolerated in practical applications.11
Another line of scientific investigation that led to new information about hydrogen and provided a powerful stimulus for developing liquid hydrogen technology began at the University of California at Berkeley with the research of Gilbert Lewis and William Glauque in testing the validity of the third law of thermodynamics. In 1926 Giauque devised a method for attaining very low temperatures by an adiabatic demagnetization technique. It was now possible to get within a few thousandths of a degree of absolute zero. At these temperatures, thermal motion of atoms almost ceases and Giauque was able to measure energy changes associated with the transition in the states of the atoms. In 1929, Giauque and an associate, Herrick L. Johnston, published the results of a discovery that set in motion a train of events leading to the discovery of heavy hydrogen in 1931. In studying the spectrum of oxygen, they found that in addition to atoms of atomic mass 16, there were others with masses of 17 and 18. The three types of oxygen atoms existed in the atmosphere in the proportions of 3150:1:5, respectively, and gave an average atomic mass for oxygen of 16.0035.12 This startled physicists and chemists, for the whole scale of atomic mass was based on oxygen with an atomic mass of 16.0; now the base and all masses related to it had to be changed. Giauque was awarded the 1949 Nobel chemistry prize for this and other contributions to low temperature physics.
[268] Prior to the Giauque-Johnston discovery, Francis Aston developed a highly accurate (1 :20 000) spectrographic measurement technique and investigated a number of elements, including hydrogen. He measured hydrogen's mass as 1.00778, based on an oxygen mass of 16.0, which compared well with chemical determinations of hydrogen's mass of 1.00777.13 The Giauque-Johnston change in oxygen's mass meant a greater difference between the spectropic and chemical measurements of hydrogen's mass. In 1931, R. T. Birge and D. H. Menzel concluded this difference to be too great for experimental error and postulated that among the hydrogen atoms of atomic mass 1 must be some of atomic mass 21 in the proportion of about 1 in 4500.14 This was an exciting challenge to physicists and chemists and the race began to determine whether the Birge-Menzel postulate was correct.
The winner of the race was Harold Urey, who had studied at the University of California and was influenced by the work of Lewis and Glauque. Urey first had to concentrate the isotope to identify it. He calculated that the difference in vapor pressures would provide the means for concentrating deuterium by distillation of solid hydrogen at the triple point. He postulated that the same differences in vapor pressure might also apply to the liquid state. He turned to the National Bureau of Standards where F. G. Brickwedde agreed to help. Brickwedde evaporated 4000 cubic centimeters of liquid hydrogen near the triple point, ending up with only one cubic centimeter. In the fall of 1931, Urey and his assistant, G. M. Murphey, placed Brickwedde's sample in a spectrograph and established the presence of deuterium, beyond all doubt.15
Urey won the 1934 Nobel chemistry prize for his achievement. Eight months after Urey's discovery, E. W. Washburn discovered that hydrogen and deuterium could be separated by electrolysis. When water is electrolyzed and hydrogen gas escapes, the residual water contains a greater proportion of deuterium oxide (heavy water). This discovery led Norway to undertake large-scale production of heavy water at a hydroelectric plant at Rjukan. Since heavy water is a good moderator for atomic reactors, the Allies raided the Norwegian plant during World War II to prevent Germany's obtaining a supply of the isotope. Deuterium can also be concentrated by a diffusion process.16
In 1935, the third hydrogen isotope, tritium, was prepared by Lord Rutherford, Marcus Oliphant, and Paul Harteck by bombardment of cleuterophosphoric acid with fast deuterons.17
In summary, the scientific interest in low temperature phenomena provided a powerful driving force for advancing the technology of liquid hydrogen. The spontaneous conversion of orthohydrogen to parahydrogen, the release of enough heat in the conversion process to vaporize the liquid hydrogen, and the use of a catalyst to speed the conversion process were discoveries essential to later developments of technology for the storage and transportation of liquid hydrogen in quantity.
Rocket Experiments with Liquid Oxygen and Liquid Hydrogen, 1937-1940
The first to experiment with a low temperature liquefied gas in a rocket was Robert Goddard, who began using liquid oxygen in 1921. By 1923, Goddard had successfully operated a gasoline-liquid oxygen rocket, incorporating pumps for both, on a test [269] stand. Three years later, on 16 March 1926, Goddard launched the world's first liquidfueled rocket at Auburn, Massachusetts.
The first to profit from Goddard's experience were the Germans during the 1930s. The German A-4 (V-2) using alcohol-liquid oxygen was the first practical application of a liquid-fueled rocket and the first to be mass produced. The V-2 established beyond all doubt the practicality of using a low temperature liquefied gas as a rocket propellant.
With all the German experience with gaseous hydrogen in dirigible operations, plus their experience with liquid oxygen for rockets, it was inevitable that they would consider liquid hydrogen for rockets. They did, but according to Wernher von Braun, the experience was brief and the results not very satisfactory.
In 1932, Walter Dornberger, a Germany army officer, organized a small rocket research station on the artillery proving grounds at Kummersdorf.18 Among the engineers brought there were von Braun, Walter Riedel, and Walter Thiel. By 1936, the Kummersdorf group had the basic concept for the A series of rockets, and Dornberger started construction of a new rocket station at Peenemûnde the same year. In April 1937, von Braun left Kummersdorf to become the technical director at the new station.19 Thiel stayed at Kummersdorf and continued research on novel injection methods, more effective cooling, and higher combustion chamber pressures using alcohol-liquid oxygen as propellants for experimental rocket engines. Thiel also tried other propellant combinations including gasoline-liquid oxygen, methane-liquid oxygen, hydrazine-nitric acid, liquid hydrogen-liquid oxygen, and liquid hydrogenliquid oxygen-fluorine mixtures. The experiments covered combustion characteristics, cooling, and general handling aspects of the fuels and oxidizers. The small rocket engine (less than 200 newtons, or 44 lb thrust) could be regeneratively cooled with one or both propellants or by water in a separate system. Von Braun observed an experiment with liquid hydrogen:
As to Thiel's liquid hydrogen tests with this set-up, I remember seeing liquefied (outside) air dripping from the supercold liquid hydrogen line. In discussing liquid hydrogen's potential, Thiel fully endorsed Oberth's earlier optimism, but pointed out that tightness of plumbing connections was a critical problem and the ever-present explosion hazard caused by accumulation of leaked-out hydrogen gas in an unvented structural pocket would require extreme care in the design of a liquid hydrogen-powered rocket or rocket stage.20
Von Braun remembered the hydrogen experiments as occurring between 1937 and 1940. The exploratory work was not followed up as the Germans concentrated on developing rockets using alcohol-liquid oxygen.
* Sources differ as to the boiling point of liquid hydrogen at 1 atm with some quoting 20.3 K and others 20.4 K. Some of the confusion comes from the fact that liquid hydrogen can be "normal" hydrogen (75% ortho, 25% Para), "equilibrium" hydrogen (21%, ortho, 79%) or parahydrogen (99.8% para). Two National Bureau of Standards authors, Richard B. Steward and Hans M. Roder in chap. II, "Properties of Normal and Para Hydrogen," in Technology and Uses of Liquid Hydrogen ,ed. R. B. Scott, w.H. Denton, and C.M. Nichols (New York: Macmillan, 1964), p. 380, give the boiling point at 1 atm for normal hydrogen as 20.390 K and for parahydrogen. 20.268, citing the work of Woolley. Scott, and Brickwedde for the former and Roder, Diller, and Weber for the latter.
More info to confuse you....
Mostly concurring with the above answers: what you describe is what happens when an atom absorbs a photon. In fact it's only approximately what happens. Changing the orbit of an electron about a nucleus changes the position/velocity/wavefunction of both the electron and the nucleus, and a transition in one electron changes all the electron-electron interactions so that the wavefunctions of the "spectator" electrons all change too. But those changes are all small, since the electron whose orbit changes has a mass thousands of times smaller than the mass of the nucleus, and since the electron whose orbit changes exerts only a small part of the force on any of the spectator electrons. For most transitions the approximation that only one electron orbit changes is good enough to be useful.
You have a suggestion above that nuclear magnetic resonance is that happens when a nucleus absorbs a photon. I would disagree: nmr is also a process where photons are absorbed and emitted by the entire atom, as a collective object. There the transition in the atom is, in the same sort of approximation, a change in the angles between the spin of the nucleus, the spins of the outermost electrons, and the planes of the electrons' orbits. NMR transitions use low energy (audio- or radio-frequency) photons because the changes involve the magnetic fields of the subatomic particles, whose interactions carry less energy than those involving the electric fields. But because of the way that different sorts of spin mix in quantum mechanics, you don't get to say "aha! I've flipped the spin of a nucleus!" unless you add some extra experimental cleverness.
If you're comfortable with the idea of an electron jumping orbits, there is a useful model for thinking about what happens when a nucleus does absorb a photon. You might remember that the reason for talking about "inner" and "outer" electrons is that electrons obey the exclusion principle. A given electron orbit can only hold two electrons, one with each spin; once the orbit is full, any extra electrons have to go into a higher, less tightly bound orbit. Well, protons and neutrons in the nucleus also obey the exclusion principle, and so their orbits within the nucleus also pile on top of each other in the same way as the orbits of the electrons pile onto the atom. The approximation that protons and neutrons (together, nucleons) are independent particles which can jump between well-defined paths is not so good as the approximation that atomic electrons are independent particles jumping between their well-defined orbitals, but it's good enough to expand into a quantitatively useful statement of what happens when a nucleus absorbs a gamma ray.
Finally there's the possibility of a photon with so much energy that it excites a nucleon directly, instead of just rearranging the nucleons inside of a nucleus. Some other folks have already alluded to this above: the first excited state of the nucleon is a particle called the delta, Δ. You might expect based on the explanations above that the Δ could be described as a proton with one of its three quarks in some sort of an excited orbit. It turns out this is not a good approximation: there's not any "shell model" approximation for the quarks in a nucleon that gives quantitatively useful results. It turns out that it is quantitatively useful to describe a Δ as an unstable "atom" made of a nucleon orbited by a pi meson. So a reasonable hand-waving model of the Δ resonance is that a photon creates a quark-antiquark pair in the vicinity of a proton, over the (brief!) lifetime of the Δ particle the four quarks and one antiquark arrange themselves into a nucleon and a pion, and then the pion falls off.
posted by fantabulous timewaster at 7:14 PM on May 10, 2010
Several things can happen. First, if the energy of the photon is too low, the photon could just scatter off of the proton - this is especially true if it is a "free" proton, which is just an ionized hydrogen atom; however, there isn't a huge amount of H sitting around - it is the most metallic element, and as such, quickly acquires an electron and bonds with something else (often another hydrogen atom) to make a molecule.
Given this, in a theoretical case where you have a free proton, the will just scatter off of the much larger (and more energetic) proton in most cases (if the energy of the electron isn't sufficiently high). Keep in mind, however, that a proton is a hadron, and as such, is made up of 3 quarks - it is not a fundamental particle, like the electron. Given enough energy, you could produce (a very improbable, due to the energy required and the number of vertices in the Feynman diagram) an interaction such that the photon scatters off an electron (or a virtual one), and the electron is energetic enough to produce deep inelastic scattering, which was first used to prove the existence of quarks. You *could* - although it is very unlikely - have the scattered electron hit the proton and produce a neutron and an anti-electron neutrino. A similar process, known as inverse beta decay, occurs *within* some radioactive atoms, but here the proton isn't free, and it "swallows" one of it's own electrons.
For photons interacting with protons in a nucleus, it is possible to change the nucleus's total spin state (which has to do with adding up all the spins in the nucleus using nasty Clebsch-Gordan coefficients and Lie Algebra and is in no way as simple as adding two ups to a down and getting an up spin). But in this case, the photon isn't interacting with a single proton, but rather a collection of protons and neutrons in the nucleus to change its overall energy state. In this respect, photon-nucleus scattering is a little similar to photon/atomic electron scattering in that there is a discrete number of energy levels in the nucleus (none in between), and a photon, if it has the EXACT energy required, can cause a transition from one energy level to another. In most cases, there is no definitive answer to your question, because as with all quantum mechanics, it is probabilistic in nature and you can't know what will happen until the interaction occurs. For the most part, protons do not often interact with protons individually.
i like that.