Centauri Dreams
Imagining and Planning Interstellar Exploration
Re-Thinking The Antimatter Rocket
Once when reading Boswell’s monumental life of the 18th Century writer and conversationalist Samuel Johnson, I commented to a friend how surprised I had been to discover that Johnson didn’t spend much time reading in his later years. “He didn’t need a lot of time,” replied my friend, a classics professor. “He tore the heart out of books.” That phrase stuck with me over the years and re-surfaced when I started working with Adam Crowl. More than anyone I know, Adam can get to the heart of a scientific paper and explain its pros and cons while someone like myself is still working through the introduction. And because of his fine work with Project Icarus, I thought Adam would be just the person to explain the latest thinking about a classic concept that Friedwardt Winterberg would like to take to the next level.
by Adam Crowl
In Jules Verne’s From the Earth to the Moon, the bold Frenchman Michel Ardan, in his first speech to the Baltimore Gun Club, when discussing travelling to the Moon via a cannon-shell, makes the following statement…
Well, the projectile is the vehicle of the future, and the planets themselves are nothing else! Now some of you, gentlemen, may imagine that the velocity we propose to impart to it is extravagant. It is nothing of the kind. All the stars exceed it in rapidity, and the earth herself is at this moment carrying us round the sun at three times as rapid a rate… Is it not evident, then, I ask you, that there will some day appear velocities far greater than these, of which light or electricity will probably be the mechanical agent?
Rockets replaced cannon-shells as the preferred means of interplanetary travel in the early 20th Century, thanks to the work of Tsiolkovksy, Goddard, Oberth and Noordung. They took up Verne’s insight and developed Ardan’s hand-waving further. Applying electricity to rocket motion resulted in the Ion Rocket, and applying light, the Photon Rocket. However the first rocket scientist to propose an engineering solution to how light might be directly harnessed to rocket propulsion, rather than just pushing solar-sails, was Eugen Sänger [1].
Antimatter and the Photon Rocket
Sänger’s discussion of photon rockets showed clearly how difficult it would be – every newton of thrust would require 300 megawatts of photon energy released. Any vehicle generating photons by conventional means would be confined to painfully low accelerations, thus Sänger proposed using matter-antimatter reactions, specifically the mutual annihilation of electrons and positrons, with the resulting gamma-rays (each 0.511 MeV) being reflected by an electron-gas. Unfortunately the electron-gas mirror would need a ridiculously high density, seen only in white-dwarf stars.
The next stage for the matter-antimatter photon rocket saw the work of Robert Forward [2], and more recently Robert Frisbee [3], who applied more modern knowledge of particle physics to the task. Instead of instant and total annihilation of proton-antiproton mixtures, resulting in an explosion of pure high-energy gamma-rays in all directions, the reactions instead produce for a brief time charged fragments of protons, dubbed pions, which can be directed via a magnetic field. According to theoretical analyses by Giovanni Vulpetti [4], in the 1980s, and more recently by Shawn Westmoreland [5], the theoretical top performance of a pion rocket is a specific impulse equivalent to 0.58c. However the pion rocket isn’t strictly a pure photon rocket and suffers from the inefficiency of magnetic nozzles. Simulations by John Callas [6] at JPL, in the late 1980s suggested an effective exhaust velocity of ~1/3 the speed of light could be achieved.
The other difficulty of matter-antimatter propulsion, as graphically illustrated by Frisbee’s work, is the extreme difficulty of storing antimatter. The old concept of storing it as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter. What it also means is immensely long and thin spacecraft designs. Frisbee’s conceptual designs are literally the size of planets, thousands of kilometres long, but merely metres wide. This minimises the gamma-radiation exposure of heat-sensitive components and maximises the exposure of radiators to the cosmic heat-sink. To achieve 0.5c, using known materials, results in vehicles massing millions of tonnes [3].
Harvesting the Fire
Friedwardt Winterberg’s recent preprint [7] suggests a different concept, with the promise of near total annihilation and near perfect collimation of a pure gamma-ray exhaust. Poul Anderson described such a vehicle’s operation in fiction in his Harvest the Fire (1995), describing an advanced matter-antimatter rocket – the exhaust was so efficiently directed that it was invisible for thousands of kilometres before finally appearing as a trail of scattered energy. So what is Winterberg proposing?
We’ve encountered Winterberg’s work before [8] in Centauri Dreams in his designs for deuterium fusion rockets, and his new work is an outgrowth of his work on the magnetic collapse of ions into incredibly dense states. Using the technique he describes, high compression of fusion plasma can be achieved, but in the case of a matter-antimatter ambiplasma (a plasma that is an even mix of the two) the result is even more spectacular.
Essentially what Winterberg describes is generating a very high electron-positron current in the ambiplasma, while leaving the protons-antiprotons with a low energy. This high current generates a magnetic field that constricts rapidly, a so-called pinch discharge, but because it is a matter-antimatter mix it can collapse to a much denser state. Near nuclear densities can be achieved, assuming near-term technical advancements to currents of 170 kA and electron-positron energies of 1 GeV. This causes intensely rapid annihilation that crowds the annihilating particles into one particular reaction pathway, directly into gamma-rays, pushing them to form a gamma-ray laser. By constricting the annihilating particles into this state a very coherent and directional beam of gamma-rays is produced, the back-reaction of which pushes against the annihilation chamber’s magnetic fields, providing thrust.
Figure 1. [from Ref.7] Gamma-ray Laser
The figure above depicts the processes involved – the magnetic field of the ambiplasma (from the electron-positron current) squeezes a linear atom of protons-antiprotons which begin annihilating, stimulating more annihilation, all in one direction from the annihilation being triggered at one end of the discharge. Thanks to the very confined channel created by the magnetic pinch, the laser beam produced has very limited spread. Intense magnetic-field recoil is created by the firing of the gamma-beam, with a pulsed field-strength of 34 tesla. The recoil force can thus be transferred back to the vehicle by the right choice of conductor surrounding the reaction chamber.
Winterberg ends his paper with an anecdote about Edward Teller, one of the many fathers of the H-bomb, who was of the opinion that photon rockets would eventually be possible – in “500 years” which equates to “impossible” in the minds of the short-sighted. Certainly making antimatter efficiently will be a Herculean task, as the energy requirements are immense. Storing it is equally “impossible”. However, as Winterberg notes, there might be a quicker pathway to confinement.
Over the last decade researchers at the University of Gothenburg, led by Leif Holmlid, have been studying exotic states of deuterium. In the past two years they have reported [9] an ultra-dense state, which has also been independently computed [10] to form inside low-mass brown-dwarf stars. This exotic quantum liquid is one million times denser than liquid deuterium and apparently a superconducting superfluid at room-temperature. Only minute amounts have been made and studied so far, but such a material could be able to sustain intense magnetic fields, up to 100,000 tesla. If it can be manufactured in large amounts, and is stable in intense magnetic fields, then the problem of magnetic confinement of anti-hydrogen at friendlier temperatures becomes more tractable.
To quote Winterberg [7], paraphrasing Teller…
Therefore, if nature is kind to us, the goal for a relativistic photon rocket might be closer than the 500 years prophesized by Teller.
References
[1] E. Sänger, 4th International Astronautical Congress, Zürich, Switzerland 3-8 August 1953.
[2] R.L. Forward, “Antiproton Annihilation Propulsion”, AFRPL TR-85-034, (1985)
[3] G. Vulpetti, “Maximum terminal velocity of relativistic rocket,” Acta Astronautica, Vol. 12, No. 2, 1985, pp. 81-90.
[4] S. Westmoreland, “A note on relativistic rocketry,” Acta Astronautica, Volume 67, Issues 9-10, November-December 2010, pp. 1248 – 1251.
[5] J.L. Callas,”The Application of Monte Carlo Modeling to Matter-Antimatter Annihilation Propulsion Concepts,” JPL Internal Document D-6830, October 1, 1989.
[6] R. H. Frisbee, 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Huntsville, AL, July 2003, AIAA-2003-4676
[7] F. Winterberg, “Matter-Antimatter GeV Gamma Ray Laser Rocket Propulsion”, 2011 (preprint).
[8] F. Winterberg, “Advanced Deuterium Fusion Rocket Propulsion For Manned Deep Space Missions”, JBIS Vol.62 No 11/12 (2009).
[9] P.U. Andersson and L. Holmlid, “Superfluid ultra-dense deuterium D(-1) at room temperature”. Phys. Lett. A 375 (2011) 1344-1347. doi:10.1016/j.physleta.2011.01.035.
[10] L.Berezhiani, G.Gabadadze and D.Pirtskhalava, “Field Theory for a Deuteron Quantum Liquid”, JHEP 1004, 122 (2010). Preprint available.
Correction re WISE
Last week I reported on information from a source on the WISE mission that no new red dwarfs had yet been discovered out to a distance of 10 light years. This past weekend I received an email from my source apologizing for mis-typing. He had meant to say no brown dwarfs — not red dwarfs — out to a distance of 10 light years. And as I mentioned with the earlier post, the data analysis continues and there may be surprises yet to come.
A nearby brown dwarf is something I’ve been writing about here for some time, pondering its implications and wondering whether one might actually turn up that was closer than the Alpha Centauri stars. So the news is that no brown dwarfs matching the description have yet turned up, but the hunt continues.
Interstellar (Precursor) Mission & Vehicle Design
by Marc Millis
Tau Zero’s first graduate student project has been completed. Berkeley Davis, a 2nd Lt. at the United States Air Force Institute of Technology, Dayton Ohio, completed his Masters thesis on a deep space probe to perform Claudio Maccone’s gravitational lens mission (FOCAL). For those unfamiliar with FOCAL, it is a mission to utilize the gravitational lens effect that begins at approximately 550 AU from the Sun, one that in the view of Maccone will offer huge magnifications for the study of targets like the Cosmic Microwave Background. For more, see the Centauri Dreams archives.
IMPETUS
Maccone, Deep Space Flight and Communications: Exploiting The Sun as a Gravitational Lens (Springer, 2009).
MISSION/VEHICLE STUDY
Davis, Berkeley. R. (2012) Gravitational Lens: The Space Probe Design (Thesis), AFIT/GA/ENY/12-M06, Air Force Institute of Technology.
To provide a realistic baseline on what is possible, the student was asked to constrain his design to commercially off-the-shelf technology. The mission involves taking a 12 m diameter radio telescope beyond 550 Astronomical Units (AU), continuing outward thereafter, to examine the gravitational lensing of our own Sun. A secondary mission, which occurs before that point, is to measure the magnetic fields, particles, and dust while traveling through our Solar System and the transition through the edge of our Solar System (the termination shock, the heliosheath, heliopause) and into true interstellar space.
Image: Beyond 550 AU, we can start to take advantage of the Sun’s gravitational lens, which may allow astrophysical observations of a quality beyond anything we can do today. Credit: Adrian Mann.
In short, it is found that this mission could be performed with EXISTING technology for roughly $3B-$5B (FY 2011 dollars), and that it would take roughly 34 years to reach the edge of our Solar System, and roughly 110 years to reach its primary mission point of 550 AU, and continuing thereafter for almost 80 years of data taking until the spacecraft reaches about 1000 AU, where it will have likely exceeded its 2-century life-time projection.
Using those goals and constraints, the student designed a two-stage vehicle that is delivered into orbit by a “Delta IV-H/Star48/Star37” launcher. The 1st stage, which has a 22 kW solar array, uses four “NEXT” ion thrusters to propel the vehicle from Earth orbit to Jupiter while thrusting almost constantly in a spiral trajectory for 17 years.
At Jupiter that boost stage is jettisoned, and the main stage completes a Jupiter gravity assist. The main stage also includes four “NEXT” ion thrusters powered by 20 Radioisotope Thermal Generator (RTGs) which have roughly 4.4 kW at this point in the mission. The spacecraft thrusts continuously for roughly another 17 years until the propellant runs out at roughly 90 AU. At this point its velocity is 6.7 AU/yr which is almost twice the speed of Voyager (3.6 AU/yr).
For the next 20 or so years it coasts through the border between our Solar System and true interstellar space, taking data for the secondary mission. Then after about 55 more years, it reaches 550 AU, the closest point where gravitational lensing would ideally begin. By this point its velocity has slowed to 6.2 AU/yr. It takes another 12 years to reach 625 AU, which is the closest predicted for observable signals at the focal point. The spacecraft will continue coasting outward for the next 60 years and will be able to continue taking data (observations of our Sun’s gravitational lensing) until the spacecraft passes 1000 AU roughly 180 years after launch. Provisional estimates of the number of cycles of the attitude control system, processor, etc, suggest that this vehicle might function for 2-centuries.
The thesis also made the following recommendations:
– Since spacecraft power is the most limiting technological factor at this time, that should be the focus of next-step research for interstellar missions.
– To complete such missions with RTGs, the production of Plutonium-238 would have to fully resume.
– Mission durations are longer than practical for on-Earth life tests, so novel testing techniques will need to be created to ensure the spacecraft will still be functioning by the time it reaches its interstellar mission location.
– Note: Mission uses about 10% of total annual Xenon production – Xenon which will not return to Earth.
This study was merely a first-cut at examining these possibilities, and these findings should not be considered the last work on this specific topic.
| 1. Stelio Montebugnoli | 10. Giovanni Vulpetti |
| 2. Ed Belbruno | 11. Maria Sarasso |
| 3. Jean Heidmann | 12. Rinaldo Bertone |
| 4. Jorg Strobl | 13. Franco Palutan |
| 5. Gregory Matloff | 14. Vittorio Banfi |
| 6. Ettore Antona | 15. Mario Pasta |
| 7. Constance Bangs | 16. Federico Bedarida |
| 8. Renato Pannunzio | 17. Luciano Santoro |
| 9. Sigfrido Leschiutta | 18. Claudio Maccone |
EXTRA INFORMATION
Boost stage
4 Ion Thrusters = 225 kg (615 W – 7.2 kW ea)
Xe tank = 309 kg
Xe load = 2996 kg
Solar Power 900 kg, 22 kW
Thrusts from LEO to Jupiter
Jettisoned at Jupiter
Main Stage
4 Ion Thrusters = 225 kg (615 W – 7.2 kW ea)
Xe tank = 217 kg
Xe load = 1888 kg
RTG Power: 20 General Purpose Heat Source Radioisotope Thermal Generators
Each 58 kg, 246W initially, with half-life of 90 yrs
12 Attitude control thrusters, 0.8 kg ea
Attitude Control Propellant Tanks = 12 kg
Attitude Control Propellant = 151 kg
Science Payload, 51kg, 40W
• 12 m radio telescope (doubles as communication High Gain Antenna)
• Magnetometers
• Particle detectors
• Dust detection
Thrusts from Jupiter to 200 AU (runs out of propellant)
Coasts from 200 AU outward
100 kBit/sec @1000AU
ESO: Habitable Red Dwarf Planets Abundant
Red dwarfs are all over the news thanks to an announcement by the European Southern Observatory. Results from a new HARPS study show that tens of billions of planets not much larger than Earth are to be expected in the habitable zones around this class of star. The finding reinforces the growing interest in M-class stars and becomes especially interesting when you realize that faint red stars like this make up as much as 80 percent of the stars in the Milky Way. That leaves plenty of room for astrobiology, depending on factors we need to discuss below.
Do we suddenly have a close destination for a potential interstellar probe? Well, Barnard’s Star has always been in the running for an early mission because of its relative proximity to us at 5.94 light years. But we still have no word on planets there (despite a much publicized but soon discredited set of observations from a 1969 paper). Proxima Centauri is available at 4.2 light years, but we have yet to learn whether it has planets. And as far as anything closer, a source on the WISE team passes along the information that no new red dwarfs have been discovered, as of yet, within 10 light years, though of course the WISE results are still under heavy analysis.
Image: This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone. Credit: ESO/L. Calçada.
But back to the ESO announcement, which focuses on results obtained with the HARPS spectrograph at La Silla. Let me quote Xavier Bonfils (Institut de Planétologie et d’Astrophysique de Grenoble) directly on this:
“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet. Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”
What the work comes down to is a survey of 102 M-class stars studied over a period of six years, in which nine super-Earths with masses up to ten times that of Earth were found. 460 hours of observing time went into the mix, with 1965 radial velocity measurements made between 2003 and 2009. Interestingly, more massive planets like Jupiter and Saturn turn out to be rare around such stars, with fewer than 12 percent of them expected in M-dwarf systems. From all this, the team thinks that there should be about 100 super-Earth planets in the habitable zones of stars within about 30 light years of the Sun.
We can point to interesting worlds like Gliese 667Cc — discovered in the HARPS survey — as a promising preview of what is out there. This is a planet within a triple star system that is about four times the mass of the Earth and orbits close to the center of the habitable zone. But even assuming an abundance of super-Earths in conditions allowing liquid water on the surface, we still have the old M-class problems to contend with. A habitable world around such a small star needs to orbit close to it, leading to the potential for tidal lock and creating climate conditions that may not favor life. We also know that red dwarfs are frequently flare stars, creating unique evolutionary pressures on any life that does manage to emerge.
Xavier Delfosse (IPAG, Grenoble), another member of the team working the HARPS data, is lead author on one of two papers examining the results that have recently become available. The outlook on tidal locking is troubling but perhaps not a show-stopper for astrobiology, at least not if our own Solar System is any indication. Although a habitable M-dwarf planet is likely to be captured into a spin-orbit resonance, it will not necessarily be forced into synchronous rotation. From the paper (I’ve omitted internal references for brevity — the paper citations are below):
The ?nal equilibrium rotation of a tidally in?uenced planet depends on both its orbital eccentricity and the density of its atmosphere… Mercury, for instance, has been captured into the 3:2, rather than 1:1, spin-orbit resonance…, and Venus has altogether escaped capture into a resonance because thermal atmospheric tides counteract its interior tides… Whatever the ?nal spin-orbit ratio, the tidal forces will in?uence the night and day succession, and therefore the climate. As discussed above however, energy redistribution by an atmosphere at least as dense as that of the Earth is e?cient… and will prevent glaciation and atmospheric collapse on the night side.
And what about stellar flare activity? M-dwarfs are more active than G-class stars like the Sun, with the result that a planet in the habitable zone of a young M-dwarf takes a huge hit from X-ray and ultraviolet radiation, a period of irradiation 10 times longer than the approximately 100 million years that the Solar System dealt with similar activity on our star. How planetary atmospheres evolve under such conditions, and whether they can actually be stripped away by coronal mass ejections, are issues we haven’t as yet resolved. Digging into the Delfosse paper I find several points worth noting on the matter:
- We don’t have a good read on just how frequent coronal mass ejections from M-dwarfs are, and how intense they tend to be. Right now these questions need more investigation, though the authors believe the frequency may be less than some earlier studies have indicated.
- A strong magnetosphere can help to shield a planet that would otherwise be imperiled.
- The atmospheric chemistry and composition may be key, and there is one study that shows that around active M-dwarfs with an atmosphere consisting mostly of CO2, the atmosphere remains stable despite nearby flare and CME activity.
The paper summarizes the issue this way:
These di?erences imply that a planet in the habitable zone of an M dwarf is unlikely to be a twin of the Earth. Habitability however is not restricted to Earth twins, and Barnes et al. (2010) conclude that “no known phenomenon completely precludes the habitability of terrestrial planets orbiting cool stars.” A massive telluric planet, like Gl667Cc (M2.sin i = 4.25 M?), most likely has a massive planetary core, and as a consequence a stronger dynamo and a more active volcanism. Both factors help protect against atmospheric escape, and super-Earths may perhaps be better candidates for habitability around M dwarfs than true Earth-mass planets.
So there we are: Tens of billions of rocky planets in the habitable zones of red dwarfs, and perhaps 100 relatively near to the Sun, according to the estimates of these researchers. What we need to do now is increase the red dwarf planet inventory with future instruments made to order — state of the art near-infrared spectrographs that, in the authors’ estimate, should be able to identify between 50 and 100 planets in the habitable zones of M-dwarfs. That should be enough, even with a 2-3% transit probability, to find at least one transiting habitable world.
The Delfosse et al. paper is “The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C,” submitted to Astronomy & Astrophysics (abstract). The Bonfils paper is Bonfils et al., “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample,” submitted to Astronomy & Astrophysics (full text). The ESO news release is also available.
An Interstellar Reminiscence
by A. A. Jackson
Although it was probably science fiction that got Al Jackson into interstellar flight, he remembers discovering the work of Eugen Sänger back around 1960 and becoming energized to seek out the few scientific papers on relativistic rocket designs that were then available. With a firm background in engineering, he turned to physics in 1975, receiving a PhD from the University of Texas at Austin, a natural move for a man who had worked for NASA during the heyday of Apollo as astronaut trainer on the Lunar Module Simulator. Going through Al’s papers is a fascinating exercise in its own right, but I was reminded because of our recent articles on Robert Bussard’s ramjet ideas that Al had worked with Daniel Whitmire.
Bussard spoke about fusing protons in his ramscoop engine, but subsequent analysis showed that the power needed to compress protons to fusion densities far outweighed the power that would be produced. It was Daniel Whitmire who developed the ‘catalytic ramjet’ idea we looked at yesterday, in which the starship draws its power through the much more energetic CNO cycle, a catalytic cycle that is the dominant energy source in stars more than 1.3 times the mass of the Sun. Thus the ‘catalytic ramjet’ was born, and Al would go on to collaborate with Whitmire on a laser ramjet in a paper the two published in 1977. I asked Al if he had any reminiscences of Whitmire and this period and he was kind enough to send the following.
I was looking at Bussard’s original paper the other day and I noticed he does mention without elaborating too much the idea of using a magnetic scoop and the attendant problems with synchrotron and bremsstrahlung radiation. He just did not do a quantitative calculation of these things. Seems to me it was Sagan who gave a long discussion, in print, of using a mag scoop, though that was not quantitative either, so I think Bussard and Sagan originated that idea.
Rather than making any technical comments, let me tell you a story. When I was at the University of Texas from 1970 to 1975 I used to go to the physics library a lot. I am a real library haunter… I noticed there was another grad student there as much as me, which got me curious. He noticed me too. Well I introduced myself to the guy, who turned out to be Dan Whitmire. We became boon friends, have been now for, lord, nearly 40 years. (Have not really seen much of Dan in the last 10 years).
Dan was in nuclear physics, at the nuclear studies center there, working on his dissertation, but he was interested in all sorts of things, especially relativity. So we got together a lot, schmoozing at lunch, families got together for picnics on weekends, saw a lot of him. He was not much interested in science fiction, which got me into interstellar flight…. He did not know much about the subject of starflight, but when I
brought it up, he really thought it was a neat problem.
We talked about fusion and fission propulsion and he noticed the mass ratio problem. I told him a Robert Bussard had solved that problem, and gave him a copy of Bussard’s paper. Dan read it very carefully, noticed the problem with the smallness of the proton-proton reaction (in fact that was kind of Dan’s dissertation work, the main work on that had been done by Willy Fowler — Dan worked on some variant on it, so Dan really knew the physics).
He told me he had a solution for the proton-proton problem. It took Dan a month or so of calculating, but one weekend he wrote up the paper, showed it to me and asked where to send it. I thought the same place Bussard had published his, Astronautica Acta (which , for some reason, later swapped its name around to Acta Astronautica). I could have suggested JBIS but AA seemed more to the point. I kind of understood that paper better then than I do now.
A funny story. Dan’s adviser let him use his secretary to type up the manuscript, thinking it was an interesting exercise. Dan sent it to Astronautica Acta, where there was some minor revision and they accepted and published it. Dan did not get much reaction… but about a few months later Dan got a bill for page charges. Dan had thought publication was free! He showed this to his supervisor, who had no money for extracurricular papers so he told Dan to throw it away. Dan never heard anything about it, but he never published there again.
Dan graduated the same year as me and went to Southwest Louisiana in Lafayette (I think it’s called U of Louisiana now, it was and still is a pretty large school). In 1976 or so Dan got a call from Bob Bussard saying he would be in Lafayette and would like to meet him. Bob really loved Dan’s paper and they had a great day of it, wish I had been there.
Bob and Dan corresponded for many years. Enclosed is one of Bussard’s early letters that Dan copied to me. (Dan had some idea about detecting XT civilizations by the radioactive waste they dumped into their local star). You can kind of see what Bob thought about interstellar flight in the letter, he was really interested but never wrote another ‘really technical’ paper on it. (I can’t find my letter from Bob, but that did not say much except that he would accept our invitation to come to an AIAA meeting in San Francisco).
A few clips from Bussard’s letter to Whitmire (dated November 19, 1976) follow. We learn, for example, that Bussard became a champion of the catalytic ramjet idea that so significantly extended his concept:
Of course, your paper on the catalyzed ISR [interstellar ramjet] is a milestone contribution, as it offers a solution to the dominant n2 (? V) problem of the p,p chain invoked (hypothetically) in my own 1960-ish note on the ISR. Your paper will become an enduring classic in this field.
We also learn that Bussard, at least as of late 1976, was not at all taken with many of the ramjet’s alternatives, especially those with a laser component:
Not so, I fear, for the proliferating array of alternative interstellar propulsion means. From experience and intuition (no physics) I am suspect of all schemes which solve the problem by putting it somewhere else, e.g., in the external-laser-driven ships, whether of photon-reflective, scoop-collecting, or on-board-fuel-mass variety. The problems of the driving laser(s) seem to me so overwhelming as to make those of the ISR, which is (in principle) self-contained, seem tractable.
And finally, Bussard’s thoughts on Whitmire’s interest in detecting extraterrestrial civilizations through unusual stellar signatures, a form of SETI we’ve described in these pages as ‘interstellar archaeology’:
No, I have not yet written the book in my mind on this topic (implications of diffusion theory) – sheer procrastination – but may get to it in 1977. Meanwhile, I still think it would be fascinating to do the ‘observables’ problem for the ISR galactic community, and seek NASA funds to go and look for signatures.
Thanks to Al Jackson for his reminiscences and the above letter. It’s fascinating that both Bussard and Whitmire were already thinking (in the 1970s!) along the lines of those who today would like to look for the signatures of advanced civilizations as revealed in astro-engineering or other large scale modifications to their environment. We’ve written a good deal here about people like Milan ?irkovi? and Richard Carrigan, who have urged hunting, among other things, for interesting infrared signatures that might reveal Dyson spheres. But the possibilities are numerous (see Eternal Monuments Among the Stars for more, or search the archives). What would a galactic community look like if interstellar ramjets were in use? Where would we look, and using what tools, to observe their signatures?
Catalyzed Fusion: Tuning Up the Ramjet
Long-time Centauri Dreams readers have learned to tolerate my eccentricities (or, at least, they’re kind enough not to dwell on them). One of them is my love of poking around in old books related to space travel, which is how Benjamin Field’s A Narrative of the Travels and Adventures of Paul Aermont Among the Planets (1873) recently caught my eye. I don’t know much about Field other than that he chose to produce this tale of interplanetary wanderings under a pseudonym, but what’s fun about his tale is that after his journeys to Jupiter, Saturn, Mars and Venus are over, Field’s protagonist returns to Earth to find that the planet is fully fifty years older, though he himself has aged hardly at all.
Time dilation, the reader might say, but of course Field wouldn’t have known anything about special relativity. It’s fun to consider, though, how an idea that in 1873 would have been simple fantasy — that someone might travel at high speed and age at a different rate than those he left behind — became in 1905 the logical outgrowth of a breakthrough theory and, by the 1960s, a publicly accepted concept. As we saw yesterday, the propagation of Robert Bussard’s interstellar ramjet ideas, first through scientific papers, then through accounts of popular science and eventually science fiction (think Poul Anderson, or Larry Niven), fueled the public imagination. We’d had relativistic effects in science fiction before (as far back as 1931’s Out Around Rigel, by Robert H. Wilson), but the ramjet gave the idea its vehicle.
Image: The Bussard ramjet ushered in an era of public enthusiasm for relativistic flight concepts, along with a host of unresolved issues. Credit: Adrian Mann.
I haven’t mentioned the name of British researcher Anthony Martin in this context yet, but Martin, a major player in the Project Daedalus work in the 1970s, examined Bussard’s ideas early in that decade and saw the possibilities of the ramscoop for deceleration. He was hardly alone in going to work on the many problems of the ramjet. Bussard spoke about fusing protons in his ramscoop engine, but subsequent analysis showed that the power needed to compress protons to fusion densities far outweighed the power that would be produced — Thomas Heppenheimer went on to demonstrate this in a 1978 paper. But three years before this, Daniel Whitmire had studied how a ramjet could draw its power through the much more energetic CNO bi-cycle (carbon-nitrogen-oxygen), a catalytic cycle that seems to be the dominant energy source in stars more than 1.3 times the mass of the Sun. Thus the ‘catalytic ramjet’ was born.
Adam Crowl describes the CNO bi-cycle this way in a recent post:
Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry”.
Whitmire was a friend and colleague of Al Jackson, a physicist (and these days a Centauri Dreams regular) who collaborated with Whitmire on still another ramjet idea we’ll be discussing soon, a ramjet/laser hybrid. Tomorrow I’ll be running a reminiscence by Al about Whitmire’s work on the catalytic ramjet, along with some thoughts from a letter Bussard himself wrote to Whitmire. For today, though, we focus on catalytic fusion, in which 12C or 20Ne (isotopes of carbon and neon respectively) are the fusion catalysts. Whitmire worked through reaction sequences for each and found that given sufficient temperatures, the reaction rates were far higher than for uncatalyzed proton/proton fusion (in fact, 1018 to 1019 times higher).
The problem with Bussard’s original idea is that the rate of proton-proton fusion is low. Consider that the Sun generates something less than 1 watt per cubic meter as averaged over its whole volume, considerably weaker than the energy production taking place in a light bulb filament. We get huge energy output because the Sun is vast, occupying a volume more than one million times that of Earth. The need for something more energetic aboard the relatively limited confines of the Bussard starship was clear. Either that or the ship would grow huge: Whitmire calculated that if a working proton-proton reactor were built around Bussard’s ideas, it would have to be on the order of 7000 kilometers across, making for one gigantic vehicle! As Whitmire wrote:
This problem was recognized in Bussard’s original work, but no viable alternative to the PPI [proton-proton] chain has yet been suggested. Here we show that the problem of the slow PPI rate can be resolved in principle by exploiting a proton burning catalytic cycle similar to the well known CNO BiCycle occurring in sufficiently hot main sequence stars. The catalyst “fuel” can be taken along since it is not depleted, but the ultimate source of energy is the interstellar hydrogen. The slowest links in the catalytic chains will be found to be 1018 – 1019 times faster than the PPI rate at an ion temperature of 86 keV and number density of 5 x 1019 cm-3.
We should note that Bussard was impressed with Whitmire’s work and saw catalytic fusion as the solution to the proton-proton problem, a point he makes in the letter I’ll be quoting tomorrow. Even so (and Gregory Matloff makes this point in The Starflight Handbook), these reactions are about a million times slower than the deuterium-deuterium reaction, so ramjet fusion remains more than a little problematic.
There are numerous issues with the Bussard ramjet, and Whitmire would go on to look at how interstellar ions could be collected through the combination of electric and magnetic fields, with the ramscoop’s efficiency enhanced by firing laser beams to ionize hydrogen atoms ahead of the vehicle (Matloff and A.J. Fennelly were working on forward-firing lasers for ramjets at more or less the same time). We’ve seen as well how Robert Zubrin and Dana Andrews analyzed the ramjet and found its scoop actually created more drag than thrust, but there are those who continue to study it, believing with Whitmire that “…it would be premature to discount the fusion ramjet as a potentially viable means of relativistic interstellar space?ight, especially for technological civilizations within or sufficiently close to nebular regions of the galaxy.”
The paper is Whitmire, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica 2 (1975), pp. 497-509.
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