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Musings on Fusion and the Interstellar Ramjet

Proton-proton fusion produces 99 percent of the Sun’s energy, in a process that begins with two hydrogen nuclei and ends with one helium nucleus, releasing energy along the way. We’d love to exploit the fusion process to create energy for our own directed uses, which is what Robert Bussard was thinking about with his interstellar ramjet when he published the idea in 1960. Such a ship might deploy electromagnetic fields thousands of kilometers in diameter to scoop up atoms from the interstellar medium, using them as reaction mass for the fusion that would drive it.

Carl Sagan was a great enthusiast for the concept, and would describe it vividly in the book he wrote with Russian astronomer and astrophysicist Iosif S. Shklovskii. In Intelligent Life in the Universe (1966), the authors discuss a journey that takes advantage of time dilation, allowing a lightspeed-hugging starship powered by these methods to reach galactic center in a mere 21 years of ship-time; i.e., time as perceived by the crew, while of course tens of thousands of years are going by back on Earth. If you also hear echoes of Poul Anderson’s Tau Zero here, you’re exactly on target.

Shklovskii and Sagan assume proton-proton fusion as the reaction, as Bussard originally did, but Thomas Heppenheimer was able to show in 1978 that it would take more power to compress the protons gathered from the interstellar medium than the reaction would produce. Ramscoops are tricky, and this is just one of their problems — gathering interstellar materials is another, dependent as it is on the density of the gases where the starship travels. Drag is yet another issue, making interstellar ramjets a segue into magsail deceleration rather than starship-enabling speed, though it’s a segue I’ll follow up on another occasion.

But the fusion itself is still interesting. If Bussard assumed proton-proton, it wouldn’t be long before Daniel Whitmire was able to show that a different reaction could produce far more power. The Carbon Nitrogen Oxygen cycle (CNO cycle) came to mind this morning because of word that the team working on the Borexino experiment in the Laboratori Nazionali del Gran Sasso (Italy), which studies the Sun’s fusion reactions through the neutrinos it produces, has been able to identify the CNO cycle as a small component of the Sun’s production of energy.

Image: The Borexino research team has succeeded in detecting neutrinos from the sun’s second fusion process, the Carbon Nitrogen Oxygen cycle (CNO cycle) for the first time. Credit: Borexino Collaboration.

That’s interesting in itself and confirms work by Hans Bethe and Carl Friedrich von Weizsäcker from the 1930s, the first experimental confirmation of their independent investigations. But I cycle back to Bussard’s ramjet. The Carbon Nitrogen Oxygen cycle involves four hydrogen nuclei combining to form a helium nucleus using carbon, nitrogen and oxygen as catalysts and intermediate products in the reaction. Maybe ‘catalysts’ isn’t the right word — I was reminded by reading Adam Crowl’s thoughts on the matter some years back that we’re not talking about chemical catalysis and should perhaps refer to all this simply as ‘nuclear chemistry.’

What boggles the mind about the CNO cycle, which I’ve read is the dominant energy source in stars more than 1.3 times more massive than the Sun, is the degree of energy unlocked by it, far exceeding uncatalyzed proton/proton fusion. And it would take something highly energetic to work on Bussard’s ramscoop, for Whitmire’s 1975 paper showed that a proton-proton reactor built in the fashion originally suggested by Bussard would need a scoop 7,000 kilometers across to make the reaction work.

Isn’t that odd? You would think that a reaction that powers the Sun would be perfectly sufficient to drive the Bussard ramjet, but it turns out that the rate of proton-proton fusion is too low. Looking back through my materials on the problem, I find that the Sun produces less than 1 watt per cubic meter when averaged over its whole volume, which means that the energy produced in a light bulb filament is more powerful. Whitmire realized that the Sun’s vast energy output could occur because of its size. Making equally massive starships is out of the question.

It turns out that Whitmire and Centauri Dreams regular Al Jackson were friends at the University of Texas back in the 1970s, and I’ll remind you of Al’s reminiscence of Whitmire that can be found here — it was actually Al who introduced the Bussard ramscoop idea to Whitmire. Bussard would write to Whitmire that his 1975 paper offered a solution to the proton-proton fusion problem and would “become an enduring classic in this field.”

If you know your science fiction, you’ll recall that Greg Benford uses the CNO cycle in his 1984 novel Across the Sea of Suns, where he gives a poetic description of the process at work as perceived by his protagonist via the ultimate in futuristic telepresence:

He watches plumes of carbon nuclei striking the swarms of protons, wedding them to form the heavier hydrogen nuclei. The torrent swirls and screams at Nigel’s skin and in his sensors he sees and feels and tastes the lumpy, sluggish nitrogen as it finds a fresh incoming proton and with the fleshy smack of fusion the two stick, they hold, they wobble like raindrops — falling together — merging — ballooning into a new nucleus, heavier still: oxygen.

But the green pinpoints of oxygen are unstable. These fragile forms split instantly. Jets of new particles spew through the surrounding glow — neutrinos, ruddy photons of light, and slower, darker, there come the heavy daughters of the marriage: a swollen, burnt-gold cloud. A wobbling, heavier isotope of nitrogen….

Ahead he sees the violet points of nitrogen and hears them crack into carbon plus an alpha particle. So in the end the long cascade gives forth the carbon that catalyzed it, carbon that will begin again its life in the whistling blizzard of protons coming in from the forward maw of the ship.

And there you are: Carbon – Nitrogen – Oxygen in a cycle that makes starship fusion work. And all of this reminiscing suggested by the results of an experiment deep below the the Italian Gran Sasso massif which has turned up evidence for the CNO cycle within the Sun, a small but ongoing component of its output. If you want to read more on what turned up at Borexino, the paper is The Borexino Collaboration, “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun,” Nature 587 (2020), 577-582 (abstract). The Whitmire paper is “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica 2 (1975), pp. 497-509 (abstract).


Comments on this entry are closed.

  • Pr. Théodose December 1, 2020, 17:34

    A blog article was recently published on a possible way towards practical proton-proton fusion, using (p,n) reactions to convert hydrogen into deuterium, then deuterium into helium :

    • Mike Serfas December 2, 2020, 21:23

      Nice! That’s a beautifully written article that breaks the topic down into simple but very meaningful concepts. “The CNO cycle achieves higher performance than the pp-chain by deferring atomic number violation until after the fusion reaction is complete.” Contrast with the Wikipedia article on the topic (despite its use of multiple charts and diagrams) and you’ll see what I mean.

    • charlie December 3, 2020, 21:32

      An extremely, extremely interesting article ! I’m not sure it indicated who wrote the article and whether that person who wrote it was some type of specialist in this area or whether or not he was simply somebody who ‘copy and pasted’ some other information from another specialized area, but irrespective of that it did appear to have a considerable amount of thought put into it.

      It got me thinking after I’d finished the article that one way around this entire problem of changing protons ultimately into helium ash might be accomplished by using the magnetic scoop to pick up an extremely large amount of solid space matter in the form of dust and under the presumption that this dust would be composed to a large degree of oxygen bearing minerals it might be possible to sidestep the production ultimately of oxygen through the CNO synthesis route and go directly the use of the remaining oxygen as your input.

      If there is any carbon in the collection process it could be processed in a separate fuel stream. Also what would preclude a collection process which would produce a sidestream and accumulation of needed ready to use fuel which would be introduced into the main combustion stream and processed as needed ? I realize I’m not being as clear as I possibly could but what I’m suggesting is a separate stream to accumulate fuel and use as needed. Could this be a possibility?

      • Mike Serfas December 7, 2020, 6:23

        That Wikipedia article does have some uses – it shows regular oxygen (O-16), being very stable, releases only 0.6 MeV after fusing with a proton to make F-17 in the CNO-II variant, versus 12.1 MeV for N-15 becoming O-16. N-14, which may be encountered in space, yields 7.4 MeV when fused with a proton, but that is also the slowest step of the cycle. I think more effective recycling should deliver more energy by retaining the uncommon intermediates like N-15, given even small inputs of C-12 or N-14 to replace what might be lost.

  • charlie December 1, 2020, 18:47

    “So in the end the long cascade gives forth the carbon that catalyzed it, carbon that will begin again its life in the whistling blizzard of photons coming in from the forward maw of the ship.”
    I’m guessing that u meant in the “…whistling blizzard of photons …”, u meant in the “…whistling blizzard of protons …”

    • Paul Gilster December 1, 2020, 21:27

      Good catch. My typo, not Greg’s.

  • Gregory Benford December 1, 2020, 19:14

    GAD, how strange to run into a passage writ & forgot long ago!
    Amazed too you recall, Paul.

    • Paul Gilster December 1, 2020, 21:25

      Hey, I marked that copy up pretty well. The most poetic invocation of a detailed physics process I’ve ever seen.

      PS: It wasn’t all that long ago that I marked it. You’re forgetting that I was doing a re-reading of the entire Galactic Center series when we talked up at Goddard.

      • Gregory Benford December 2, 2020, 15:02

        Ah, yes–I never reread my books. Many writers seem to. I’d rather write the next to come…

  • Mike Serfas December 1, 2020, 19:20

    My daydreams will stick with the Gemino Drive. The idea: you just need to figure out a clever way to make normal matter interact with neutrinos… specifically, pairs of neutrino+antineutrino. Thermal energy, related to kT = 25 mEV at room temperature, will spread out into every degree of freedom available. The mass of all three flavors of neutrinos is known to be less than 120 mEV. So if one flavor has a much lower mass, you should be able to create the particle and its antiparticle out of thermal energy and use what’s left over to give them some momentum with the amount of thermal energy available in a room, and use the newly made particles as propellant. If the mass is higher you may need a heat source to raise temperature at the emitter, but you do anyway to keep the ship from freezing over. As I understand most of the existing neutrinos are at microwave background temperatures and not in equilibrium, so the reverse reaction should not be a major issue. This isn’t my field, but I haven’t seen a reason why this shouldn’t work…

  • Project Studio December 1, 2020, 19:35

    Always an engaging topic. Wondering if there is a threshold speed where the Bussard Ramjet becomes sufficiently scoop becomes efficient, or what impact the CNO catalyst might have on that threshold. It seems reasonable that there would be some threshold speed would increase the rate of particles entering the scoop — even more so considering time dialation effects. Or perhaps there is a threshold acceleration, for example the Unruh effect would theoretically cause a relatavistic increase in the density of virtual particles entering the scoop.

  • Zanstel December 2, 2020, 7:28

    Because we are talking about futuristic fusion technology, I guess that the best alternative beyond using tiny black holes is to use multiple fusion reactor close to the point where energy is absorbed instead of released.

    So, for example, CNO turns 1-H into He4. Triple alpha process turn He-4 into C-12…
    That should maximize the energy extraction, but focus into propulsion could be very difficult.

  • Michael Fidler December 2, 2020, 9:02

    Just a quick thought, according to Amir Siraj and Abraham Loeb “Interstellar Objects Outnumber Solar System Objects in the Oort Cloud” there may be a lot of small small objects in interstellar space just packed full of carbon and oxygen.


    What about some type of vortex system that could draw these objects in and separate them for use in the CNO fusion?

    • Zanstel December 2, 2020, 11:00

      The problem is that you can’t capture this objects while you travel at high speed.

      • Michael Fidler December 2, 2020, 19:42

        Well, that’s the problem since everything will be flying by at cosmic rays speed but a vortex could capture and pull it in to actually cause the fusion! As for the larger objects you need a giant sink garbage disposal to grind them up before entering the fusion drive. The big problem is getting up to near lightspeed for this to work, but like scramjets there are ways.

        • Michael Fidler December 4, 2020, 9:52

          Manipulating the wave structure… >>>

          New analysis shows a way to self-propel subatomic particles.


          “This manipulation of waves could be accomplished using specially engineered phase masks—similar to those used to create holograms, but at a much smaller scale. Once created, the particles “self-accelerate,” the researchers say, in a way that is indistinguishable from how they would behave if propelled by an electromagnetic field.”

          “The electron is gaining speed, getting faster and faster,” Kaminer says. “It looks impossible. You don’t expect physics to allow this to happen.”

          It turns out that this self-acceleration does not actually violate any physical laws—such as the conservation of momentum—because at the same time the particle is accelerating, it is also spreading out spatially in the opposite direction.

          “The electron’s wave packet is not just accelerating, it’s also expanding,” Kaminer says, “so there is some part of it that compensates. It’s referred to as the tail of the wave packet, and it will go backward, so the total momentum will be conserved. There is another part of the wave packet that is paying the price for the main part’s acceleration.”

          “It turns out, according to further analysis, that this self-acceleration produces effects that are associated with relativity theory: It is a variation on the dilation of time and contraction of space, effects predicted by Albert Einstein to take place when objects move close to the speed of light. “

          • Michael Fidler December 4, 2020, 11:25
          • Mike Serfas December 10, 2020, 20:59

            Wow! The Aharonov-Bohm effect came out in 1958 … but it seems like every week it is upending physics. And this week must be special! Unfortunately I did not find an ArXiv equivalent for https://www.nature.com/articles/nphys3196 … is it too corny to smile and wave back at Alexandra every time? But if I start reading about self-accelerated particles at relativistic speeds, and “an engineered wavepacket of a muon can have longer lifetime thanks to the time dilation we find here”, maybe it’s best to see peer review was passed. I don’t pretend to understand – it looks simple by the standard of this kind of physics but well beyond my ken. A new solution to the Dirac equation – that must be fundamental. Can you do muon-catalyzed fusion with a profit margin and solve global warming?

  • Robert December 2, 2020, 18:26

    Robert Bussard was one of the scientists in the Atomic Energy Commision that directed the U.S. fusion enterprise towards Tokamaks, a decision he came to regret later in life when he developed the Polywell concept for compact fusion. It’s a bit dated now but here is his 2006 talk at Google titled Should Google go Nuclear?


    • Al Jackson December 4, 2020, 17:32

      Gosh, I did not know that video of Bussard existed. He was a very interesting man. His PhD was from Princeton , I think he was involved with the initial fusion work there in the late 40s and early 1950s. Became an expert in nuclear propulsion at Los Alamos during his stay there in the 1950s. This was a year before he passed away.

  • Jens Fridthjof Knudsen December 2, 2020, 18:59

    Has there ever been an engineering study to look at how such a craft could be built?

  • John F Bramfeld December 3, 2020, 14:22

    I’ve been wondering for years why no one seems to be calculating what the exhaust of one of these ships might look like at various velocities relative to earth, and then looking for that signature. I’m guessing an exhaust aimed in the general direction of earth might be visible a long way off.