Like the Marie Celeste, the Leonora Christine is a storied vessel, at least among science fiction readers. In his 1967 story “To Outlive Eternity,” expanded into the novel Tau Zero in 1970, Poul Anderson described the starship Leonora Christine’s stunning journey as, unable to shut down its runaway engines, it moved ever closer to the speed of light. Just how a real Leonora Christine might cope with the stresses of a ramjet’s flight into the interstellar deep is the subject of Al Jackson’s latest, which draws on memories not only of Robert Bussard, who invented the interstellar ramscoop concept, but a young scientist named John Ford Fishback.
by A. A. Jackson
Project Pluto – a program to develop nuclear-powered ramjet engines – must have been on Robert Bussard’s mind one morning at breakfast at Los Alamos. Bussard was a project scientist-engineer on the nuclear thermal rocket program Rover — Bussard and his coauthor DeLauer have the two definitive monographs on nuclear propulsion [1,2]. He said many times that the idea of the hydrogen scooping fusion ramjet came to him that morning. This was sometime in 1958 or 1959 and the SLAM (Supersonic Low Altitude Missile) would have been well known to him. SLAM was an nuclear ramjet, a fearsome thing, sometimes called the Flying Crowbar. Finding a solution to the mass ratio problem for interstellar flight was also something on Bussard’s mind. Thus was born the Interstellar Ramjet, published in 1960 .
Image: Al Jackson delivering a plenary talk at the recent Tennessee Valley Interstellar Workshop. Credit: Joey O’Loughlin.
Most here at Centauri Dreams know that the interstellar ramjet scoops hydrogen from the interstellar medium and uses this as both a fuel and energy source by way of fusion reactor. The sun does proton fusion using gravity as the agent of confinement and compressional heating. However, doing fusion in a ‘non-gravitational’ magnetic fusion reactor makes the process very difficult [3,4]. That is, the proton and Deuterium burning is quite severe to realize on a ‘small scale’. Dan Whitmire attacked this problem by proposing the use of a carbon catalyst using the CNO cycle . The CNO cycle is about 9 orders of magnitude faster than proton-proton fusion. It would still require temperatures and number densities way beyond any technology known at this time.
Bussard noted a number of problems such as losses from bremsstrahlung and synchrotron radiation. He also noted scooping with a material scoop would create a problem with erosion, hinting that magnetic fields might be used, and noting that drag would have to be accounted for.
About 8 years after Bussard’s paper, an undergraduate at MIT, John Ford Fishback, took up the problems Bussard had mentioned. He wrote this up for his Bachelor’s thesis under the supervision of Philip Morrison. The thesis was published in Astronautica Acta  in 1969.
Image: Physicist Robert W. Bussard.
Fishback did three remarkable things in his only journal paper: finding an expression for the ‘scoop’ magnetic field, computing the stress on the magnetic scoop sources, and working out the equations of motion of the ramjet with radiation losses. These calculations were done using a special relativistic formulation.
Fishback’s most important finding is noticing that when capturing ionized hydrogen to funnel into the fusion reactor, there is a large momentum flow of the interstellar medium which must be balanced by the scooping and confining magnetic fields. Using very general arguments, Fishback showed that sources (magnetic coils and their support) of the magnetic field determine an upper limit on how fast a ramjet can travel. The convenient measure of starship speed is the Lorentz factor
where v is the starship velocity and c the speed of light. It comes from the physical properties of the field sources, in particular the shear stress.
At the time, Fishback modeled the upper limit using diamond, because of its shear stress properties, and found that one could only accelerate until the Lorentz factor reaches about 2000 [5,6]. Tony Martin expanded on Fishback’s study [6, 7] in 1971, correcting some numbers and elaborating on Fishback’s modeling. Since that time, Graphene has been discovered and it has a shear stress that allows a limiting Lorentz factor of about 6000. This in turn implies a range of over 6000 light years when under 1 g acceleration. It does not mean the final range is 6000 light years, but one must travel at a reduced acceleration and then constant speed, which means a longer ship proper time.
This is bad news for the Leonora Christine of Poul Anderson’s Tau Zero .The range can probably be pushed to 10,000 light years, but accelerating at 1 g for 50 years would bust the Lenora Christine’s coils! That is, unless some magic material is found to take the stress loading at a Lorentz factor 1019, there is no way to circumnavigate the universe. And with the new accelerating universe, the story of Tau Zero becomes still more complicated.
Image: The interstellar ramscoop as envisioned by artist Adrian Mann.
What became of John Ford Fishback? I went to a lecture in California at Stanford in 1979 by Phillip Morrison. After the lecture I asked Morrison what had happened to Fishback. Morrison sadly told me that Fishback had gone to the University of California at Berkeley to work on his doctorate, but had committed suicide.
1. Bussard, R. W.; DeLauer, R.D. (1958). Nuclear Rocket Propulsion. McGraw-Hill.
2. Bussard, R.W.; DeLauer, R. D. (1965). Fundamentals of Nuclear Flight. McGraw-Hill.
3. Bussard, R.W., “Galactic Matter and Interstellar Flight”, Acta Astronatica, VI, pp. 179-195, 1960.
4. Whitmire, Daniel P. , “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”, Acta Astronautica, 2 (5-6): 497–509, 1975.
5. Fishback, J. F., “Relativistic interstellar spaceflight,” Astronautica Acta, 15 25–35, 1969.
6. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta, 16, 353-357 , 1971.
7. Anthony R. Martin, “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta, 18, 1-10 , 1973
8. Anderson, Poul (2006), Tau Zero, Gollancz. ISBN 1407239139.
Table 1. Cut-Offs and Range for Ramjet accelerating at 1g. Interstellar medium 1/cm-3 using the p-p fusion reaction.
Addendum: While he was working on this article and corresponding with me, Al shared the story with Greg Benford, who had further thoughts on John Ford Fishback, as below:
Good article. I can add a touch: I was interested in this, after Edward Teller pointed out to me Fishback’s 1969 paper in Astronaut. Acta. I discussed it with Teller and did some calculations (just exploratory, never published). The idea seemed extreme but enlivened my discussions of the paper with Poul Anderson, who lived in Orinda near my Walnut Creek home and whom I saw often.
Someone told me Fishback was at Berkeley and I called him, agreed to meet. I had a one-day-per-week agreement with the Livermore Lab, where I was just turning from being a postdoc for Teller into a staff physicist — I spent Wednesdays at the Lab in Berkeley. So I met him at an Indian restaurant–a rail-thin smoker, nervous, ascerbic. “I wanted to show that we could reach the stars, really do it, with the right engineering,” he said, approximately. His anxiety was clear, but not its cause.
I found him an odd duck but was shocked when a bit later I heard he had killed himself.
When I mentioned it to Poul, he found it contrasting that a man who wanted the stars would cut off his own personal hopes. We often discussed Tau Zero, Poul once remarking that he wished he had taken more time to polish and expand the novel, since it already looked as though it might be the most remembered of his works–and indeed, seems so. He said he had written it in a few months and needed the money–its 1967 serialization in Galaxy helped, but it was tough going as a full-time pro writer then. Plus he had a word limit on the hardcover.
Poul used his Nordic background in the novel, as he liked to do. From Wikipedia:
“Incidental to the main themes is the political situation on the Earth from which the protagonists set out: a future where the nations of the world entrusted Sweden with overseeing disarmament and found themselves living under the rule of the Swedish Empire. This sub-theme reflects the great interest which Anderson, an American of Danish origin, took in Scandinavian history and culture. In later parts of the book, characters compare their desperate situation to that of semi-mythical characters of Scandinavian legend, with the relevant poetry occasionally quoted.”