The Interstellar Ramjet at 60

The interstellar ramjet conceived by Robert Bussard may have launched more physics careers than any other propulsion concept. Numerous scientists over the years have told me how captivated they were with Poul Anderson’s treatment of the idea in his novel Tau Zero. Al Jackson takes a look at Bussard’s concept in today’s essay, referencing its subsequent treatment in the literature and adding a few anecdotes about Bussard himself. The original paper was submitted on February 1, 1960 to Astronautica Acta, then edited by Theodore von Kármán (a ‘tough judge,’ Al notes) and published later that spring. Although the ramjet faces numerous engineering issues, its ability to resolve the mass-ratio problem in interstellar flight makes it certain to receive continued scrutiny.

by A. A. Jackson

Writers of science fiction prose noticed the difference between interplanetary flight and interstellar flight earlier than anyone. Various fictional methods of faster-than-light (FTL) were invented in the 1930s, John Campbell even inventing the term ‘warp drive’. Asimov’s Galactic Empire is only facilitated by FTL ‘jump-drives’. Slower than light interstellar travel made an appearance in Goddard and Tsiolkovsky’s writings in the form of ‘generation ships’, usually called ‘worldships’ now.

As far as I know, the first engineer to look at the very basic physics — quantitative calculations — of relativistic interstellar flight was Robert Esnault-Pelterie; he made relativistic calculations before 1920 that were published in his book L’Astronautique (1930). The first derivation of the relativistic rocket equation occurs in Esnault-Pelterie’s writings. This was long before Ackeret (J. Ackeret, “Zur Theorie der Raketen,” Helvetica Physica Acta 19, p.103, 1946). The classical mass ratio rocket equation of Tsiolkovsky showed the difficulty of space travel. The relativistic rocket equation showed that interstellar flight was even more difficult.

Eugen Sänger, who had been interested in interstellar flight in the 1930s, addressed the interstellar mass ratio problem in 1953 with a paper on photon rockets, “Zur Theorie der Photonenraketen” (Vortrag auf dem 4. Internationalen Astronautischen Kongreß in Zürich 1953). Sänger, more than almost anyone before him, studied the hard physics of antimatter rockets and relativistic rocket mechanics. Using the most energetic energy source, antimatter, would require tons of it in a conventional rocket. There was sore need of a better method.

Bussard

Robert W Bussard was a rangy man who looked like he walked the halls of power. I had dinner with him at a San Francisco section of the American Institute of Aeronautics and Astronautics meeting in 1997. We had invited Poul Anderson, author of Tau Zero; Anderson and Bussard had never met. Over dinner Bussard told me he started working on nuclear propulsion at Los Alamos in 1955, and that he and R. DeLauer wrote the first monograph on atomic powered rockets in 1959 [1]. He also said he had been looking at work at Lawrence Radiation Laboratory in 1959.

Bussard told me he had always been interested in interstellar flight. One day at breakfast at Los Alamos he got a tortilla rolled up with scrambled egg in it. That cylinder made him think of a fusion ram starship! I have to wonder if that story is true, for had he been looking at Livermore’s lab papers he probably saw Project Pluto, the nuclear powered atmospheric ramjet.

Bussard sat down in 1959 and wrote the paper “Galactic matter and interstellar flight,” published in Astronautica Acta in 1960. This paper is thoroughly technical; Bussard summarizes Ackeret, Sänger and Les Shepherd’s studies of interstellar flight [2]. Sänger had shown that even using antimatter one still had a mass ratio problem with a conventional rocket. Bussard then presents an amazing new concept that solved the mass ratio problem [3]. He notes that one can scoop interstellar hydrogen and fuse it to produce a propulsion system.

The treatment is rigorously special relativistic; using conservation of energy and momentum he derives the equations of motion of an interstellar ramjet. He accounts for the energy production and propulsion efficiency of the vehicle in general terms. He uses the most energetic fusion mechanism, the proton-proton fusion reaction which converts .0071 of the rest mass of collected protons to energy. Bussard derives the property that the ramjet will need to be boosted to an initial speed.

Image: Robert Bussard in 1959 with his Astronutica Acta issue.

Bussard discusses the engineering physics problems; the difficulty of using the p-p chain is enormous. He notes that interstellar hydrogen can be unevenly distributed, there being rich and rarefied regions. He gives a simplified model for scooping and sometimes it is missed that he mentions magnetic fields as a ‘collector’. Bussard also notes both radiation losses and radiation hazards during the operation of the ramjet.

Sagan

The Bussard Ramjet got a boost in 1963 when Carl Sagan noted that there was a solution to the mass ratio problem for interstellar flight [4]. Sagan summarized this paper in Intelligent Life in the Universe in 1966 [5], probably the best popularization of the Ramjet. Sagan also noted that ships accelerating at one gravity could circumnavigate the universe, ship proper time, in about 50 years. He references Sänger in the paper version [4] and the calculation of the mechanics of a 1g starship. As far as I know, the 1957 paper of Sänger [6] is the first exposition of a constant acceleration starship and the consequences of time dilation when extreme interstellar distances are traveled. Bussard mentioned, very briefly, a magnetic field as a scoop, but Sagan describes such a collector in a more elaborated though qualitative way.

Fishback

John Ford Fishback published his MIT bachelor’s thesis in Astronautica Acta in 1969 [7]; this was supervised by Philip Morrison. Morrison and Cocconi were the fathers of radio SETI. Morrison seems to have taken an interest in Sagan’s mention of Bussard’s ramjet — I’m not sure if it was Morrison or Fishback who suggested the study. The paper is a remarkable marshalling of electrodynamics, charged particle motion, plasma physics, the physics of materials and special relativity.

Fishback constructs a model for the magnetic scoop field taking into account the fraction of hydrogen ingested and reflected. Using conservation laws, he derives the most detailed equations of motion accounting for mass and radiation losses that had been published anywhere. In the scooping process, Fishback examines the statistical distribution of gas in the galaxy and derives a relativistic expression for ship proper acceleration with ‘drag’. An important consequence, expressed for the first time, is the mechanical stress on the scoop field magnets. He derived an upper limit on the maximum Lorentz factor that can be obtained as a ramjet accelerates at 1 g for a long time due to stress on the source of the scoop field.

[For more on Fishback, see Al’s John Ford Fishback and the Leonora Christine from 2016, with further thoughts by Greg Benford.]

Image: John Ford Fishback in 1967 and first page of his paper in Astronautica Acta. Sadly, Fishback would take his own life in 1970 at the age of 23.

Martin

In 1971 [8] and 1973 [9] Tony Martin reviewed Fishback’s paper, making useful clarifying observations. Martin provides details of calculation that Fishback leaves to the reader on the relation of the fraction of particles that are magnetically confined to the reactor intake as a function of the confining field and the starship’s speed. In his second paper, Martin corrects a numerical error by Fishback showing that the cutoff speed due to the stress properties of the magnetic source is 10 times larger than was calculated. Martin also gives a nice calculation of the size of the magnetic scoop field. Fishback and Martin’s papers account for the ‘drag’ due to reflected particles; this result seems unknown to later critics of the ramjet.

Whitmire

I met Dan Whitmire in 1973, when we were both working on doctorates in physics at the University of Texas at Austin. Dan and I were talking about interstellar flight one day and I showed him Bussard’s paper. Dan was in the nuclear physics group at Texas and took an immediate interest in the problem with proton-proton fusion as had been pointed out by Bussard and Martin. Then he came up with an ingenious solution: Carry carbon on board the starship and use it as a catalyst to implement the CNO fusion cycle [10]. The CNO process is 1018 times faster than the PP chain at the fusion reactor temperatures under consideration. This reduces the fusion reactor size to 10s (and more) of meters in dimension. Since carbon cycles in the process, in theory one would only need to carry a small amount; however it is not clear how under dynamic conditions one would recover all the catalysis needed.

Later Developments

The above are the core studies of the interstellar ramjet. Hybrid methods occurred to several researchers. Alan Bond [11] proposed a vehicle that carried a separate energy source yet scooped-up interstellar hydrogen not as fuel but simply as reaction mass, this is known as the augmented interstellar ramjet. Conley Powell [12] presented a refined analysis of this system. The author [13] presented a study using antimatter added to the scooped reaction mass for propulsion as an augmented method. Relevant to the augmented ramjet is antimatter combined with matter for propulsion as studied by Forward and Kammash [14, 15].

T. A. Heppenheimer published a paper in the Journal of the British Interplanetary Society [16] noting the problems with the p-p chain for fusion without citing Dan Whitmire’s solution. Heppenheimer notes radiation losses but does not cite Whitmire and Fishback, who addressed the problems of bremsstrahlung and synchrotron radiation in the reactor and the scoop field.

Matloff and Fennelly [17] have interesting papers on charged particle scooping with superconducting coils. Cassenti looked at several modifications and aspects of the ramjet [18].

Recently Semay and Silvestre-Brac [21, 22] re-derived the equations of motion of the interstellar ramjet, first done by Bussard and Fishback. They find some new extensions with solutions of the relativistic equations for distance and time.

Dan Whitmire and the author [23] removed the fusion reactor by taking the energy source out of the ship and placing it in the Solar System. If one scoops hydrogen but energizes it with a laser system it is possible to make a ramjet that is smaller and less massive. Such a system probably has a limited range similar to laser pushed sails.

An excellent survey of interstellar ramjets and hybrid ram systems can be found in the books by Mallove and Matloff [24] and a recent monograph by Matloff [25], see these books and the references listed in them. See also Ian Crawford’s paper [26].

The Interstellar Ramjet in Science Fiction

It seems the Bussard Ramjet first appeared in a Larry Niven short story called “The Warriors” (1966). Later Niven used the Ramjet in his other fiction, inventing, I think, the term Ram Scoop. However I think the best known use of the Ramjet is Poul Anderson’s Tau Zero [26]. The core story in Tau Zero is not the Interstellar Ramjet but the constant acceleration circumnavigate-the-universe calculation first done by Eugen Sänger.

My guess is that Anderson only saw Carl Sagan’s exposition on this in Intelligent Life in the Universe. The Greek letter ‘Tau’ was introduced by Hermann Minkowski in 1908; it is the time measured by the travelers in the starship Leonora Christine, while the time measured by people back on earth is t. Special relativistic time dilation leads to (ship time)/(Earth Time) going to almost zero. Accelerate at one g for 50 years and one covers a distance of about 93 billion light years that is roughly the size of the universe.

Image: What would become Tau Zero first appeared in shortened form as “To Outlive Eternity” in the pages of Galaxy in June, 1967.

The Bussard Ramjet Leonora Christine sets out for Beta Virginis, approximately 36 light years away. A mid-trip mishap robs the ship of its ability to slow down. Repairs are impossible unless they shut down the ramjet, but if the crew did that, they would instantly be exposed to lethal radiation. There’s no choice but to keep accelerating and hope that the ship will eventually encounter a region in the intergalactic depths with a sufficiently hard vacuum so that the ramjet could be safely shut down. They do find such a region and repair the ship.

Anderson then introduces the mother of all twists. The Leonora Christine has accelerated for so long that the crew discover relative to the universe a cosmological amount of time has elapsed. The universe is not ‘open’ but fits the re-collapse model, it is going for the big crunch. I know of no other science fiction novel with more extreme problem solving that this hard SF story.

Anderson’s cosmology for Tau Zero seems to come totally from George Gamow [28]. Gamow and his students did pioneering work on early time cosmology, an elaboration of earlier work done by Georges Lemaître. When Poul Anderson wrote the novel, he may have been aware that Big Bang cosmology had evolved beyond Gamow’s models …. However, having his starship eventually orbit the ‘Cosmic Egg’ or Ylem was a solution to the crew’s problem. Alas, even in Gamow’s cosmology the ‘Ylem’ is the universe, so no way to ‘orbit’ it. Poetic license for the sake of a Ripping Yarn! (An intersecting exercise is to see what the trajectory of the Leonora Christine’s plot problem is in current accelerating universe cosmology.)

After Niven and Anderson, the Bussard Ramjet became common currency in science fiction, although it has faded somewhat in recent times. Recently a fusion ramjet, SunSeeker, appears as an integral part of the Bowl of Heaven series by Greg Benford and Larry Niven [29].

Final Thoughts

There seems to be a thread of pessimism about the Bussard Ramjet centered around drag on the ramjet due to interaction with the scoop field. This is an issue that Fishback deals with in his analysis; he shows one cannot just use a dipole magnetic field. A more complex collector field is needed. Fishback and Martin do show there is a fundamental physics limitation. Even using the strongest material theoretically possible, there is an upper limit to a mission Lorentz factor, probably equal to 10,000. Above this one will bust the scoop coil due to magnetic stress. The cosmological peril of the Leonora Christine depicted in Tau Zero is not physically possible.

The main show stopper for the ramjet is the engineering. There is no way with foreseeable technology to build all the components of an interstellar ram scoop starship. Several aspects should be revisited. (1) The source of the magnetic scoop field, Fishback [7] derived one, Cassenti elaborated another [20]; (2) the fusion reactor — the aneutronic fusion concept is direct conversion of fusion to energy [30]; (3) hybrid systems, especially laser-boosted ramjets.

Since basic physics does not rule a ramjet out, it is possible that an advanced civilization might build one. Freeman Dyson [31] pointed out many times that what we could not do might be done by some advanced civilization as long as the fundamental physics allows it. An interesting consequence of this is that interstellar ramjets may have been built and might have observable properties. Doppler-boosted waste heat from such ships might be observable. Plowing into HII regions in the galaxy, a starship’s magnetic scoop field might produce a bow-shock which could be observable. Isolated objects in this galaxy with Lorentz factors in the thousands would be unusual and if they are accelerating even more unusual.

The idea of picking up your fuel along the way in your journey across interstellar space may be the optimal solution to the mass ratio problem in interstellar flight. The interstellar ramjet warrants more technical study.

Appendix

Because Robert Bussard sketched a ramjet with a physical ‘funnel’ …all the many illustrations I have seen since seem to have some kind of ‘cow catcher’ on the front. Though it is reasonable that such a structure is the source of an electromagnetic device, I think it more likely that the ‘scoop’ field will be produced by a magnetic configuration that directs the incoming stream into the mouth of the reactor without any extra funnel-like forward structure. Here is a rough schematic done for me by artist Doug Potter. There is a ‘bulb’ representing the magnetic source field (maybe the parabolic magnetic field calculated by Fishback), a reactor section and an exhaust. Not a very elegant representation of the ramjet but a suggested configuration.

References

1. Bussard, R. W., and R. D. DeLauer. Nuclear Rocket Propulsion, McGraw-Hill, New York, 1958

2. L. R. Shepherd, “Interstellar Flight,” Journal of the British Interplanetary Society, 11, 4, July 1952

3. R.W. Bussard, “Galactic matter and interstellar flight,” Astronautica Acta 6 (1960) 179–195

4. C. Sagan, “Direct contact among galactic civilizations by relativistic interstellar spaceflight,” Planet. Space Sci. 11 (1963) 485–498

5. Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House

6. Sänger, E., “Zur Flugmechanik der Photonenraketen.” Astronautica Acta 3 (1957), S. 89-99

7. Fishback J F, “Relativistic interstellar spaceflight,” Astronautica Acta 15 25–35, 1969

8. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta, 16, 353-357 , 1971

9. Anthony R. Martin; “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta 18, 1-10 , 1973

10. Whitmire, Daniel P., “Relativistic Spaceflight and the Catalytic Nuclear Ramjet” Acta Astronautica 2 (5-6): 497–509, 1975

11. Bond, Alan, “An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 27, p.674,1974

12. Powell, Conley, “Flight Dynamics of the Ram-Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 28, p.553, 1975

13. Jackson, A. A., “Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, v33, 117, 1980.

14. R.L. Forward, “Antimatter Propulsion”, Journal of the British Interplanetary Society, 35, pp. 391–395, 1982

15. Kammash, T., and Galbraith, D. L., “Antimatter-Driven-Fusion Propulsion for Solar System Exploration,” Journal of Propulsion and Power, Vol. 8, No. 3, 1992, pp. 644 – 649

16. Heppenheimer, T.A. (1978). “On the Infeasibility of Interstellar Ramjets”. Journal of the British Interplanetary Society 31: 222

17. Matloff, G.L., and A.J. Fennelly, “A Superconducting Ion Scoop and Its Application to Interstellar Flight”, Journal of the British Interplanetary Society, Vol. 27, pp. 663-673, 1974

18. Matloff, G.L., and A.J. Fennelly, “Interstellar Applications and Limitations of Several Electrostatic/Electromagnetic Ion Collection Techniques”, Journal of the British Interplanetary Society, Vol. 30, pp. 213-222, 1980

19. Matloff, G.L., and A.J. Fennelly , B. N , “Design Considerations for the Interstellar Ramjet,” 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2008

20. Cassenti, B. N , “The Interstellar Ramjet,” 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2004

21. Claude Semay and Bernard Silvestre-Brac, “The equation of motion of an interstellar Bussard ramjet,” European Journal of Physics 26(1):75, 2004

22. Claude Semay and Bernard Silvestre-Brac, “Equation of motion of an interstellar Bussard ramjet with radiation loss,” Acta Astronautica 61(10):817-822, 2007

23. Whitmire, D. and Jackson, A, “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30pp. 223-226, 1977

24. Mallove, E. F., and G.L. Matloff, The Starflight Handbook, Wiley, New York, 1989

25. Matloff, G., Deep-Space Probes, Praxis Publishing, Chichester, UK, 2000

26. Ian A Crawford, “Direct Exoplanet Investigation Using Interstellar Space Probes.” In Handbook of Exoplanets Springer 2017

27. Anderson, Poul. Tau Zero. New York: Lancer Books (1970)

28. George Gamow, The Creation of the Universe (1952)

29. Benford, G. and Niven, L., Bowl of Heaven series, Macmillan.

30. Benford, G., Private communication.

31. Dyson, F. J., “The search for extraterrestrial technology,” in Marshak, R.E. (ed), Perspectives in Modern Physics, Interscience Publishers, New York, pp. 641–655