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Crafting the Bussard Ramjet

The Bussard ramjet is an idea whose attractions do not fade, especially given stunning science fiction treatments like Poul Anderson’s novel Tau Zero. Not long ago I heard from Peter Schattschneider, a physicist and writer who has been exploring the Bussard concept in a soon to be published novel. In the article below, Dr. Schattschneider explains the complications involved in designing a realistic ramjet for his novel, with an interesting nod to a follow-up piece I’ll publish as soon as it is available on the work of John Ford Fishback, whose ideas on magnetic field configurations we have discussed in these pages before.

The author is professor emeritus in solid state physics at Technische Universität Wien, but he has also worked for a private engineering company as well as the French CNRS, and has been director of the Vienna University Service Center for Electron Microscopy. With more than 300 research articles in peer-reviewed journals and several monographs on electron-matter interaction, Dr. Schattschneider’s current research focuses on electron vortex beams, which are exotic probes for solid state spectroscopy. He tells me that his interest in physics emerged from an early fascination with science fiction, leading to the publication of several SF novels in German and many short stories in SF anthologies, some of them translated into English and French. As we see below, so-called ‘hard’ science fiction, scrupulously faithful to physics, demands attention to detail while pushing into fruitful speculation about future discovery.

by Peter Schattschneider

When the news about the BLC1 signal from Proxima Centauri came in, I was just finishing a scientific novel about an expedition to our neighbour star. Good news, I thought – the hype would spur interest in space travel. Disappointment set in immediately: Should the signal turn out to be real, this kind of science fiction would land in the dustbin.

Image: Peter Schattschneider. Credit & copyright: Klaus Ranger Fotografie.

The space ship in the novel is a Bussard ramjet. Collecting interstellar hydrogen with some kind of electrostatic or magnetic funnel that would operate like a giant vacuum cleaner is a great idea promoted by Robert W. Bussard in 1960 [1]. Interstellar protons (and some other stuff) enter the funnel at the ship‘s speed without further ado. Fusion to helium will not pose a problem in a century or so (ITER is almost working), conversion of the energy gain into thrust would work as in existing thrusters, and there you go!

Some order-of-magnitude calculations show that it isn‘t as simple as that. But more on that later. Let us first look at the more mundane problems occuring on a journey to our neighbour. The values given below were taken from my upcoming The EXODUS Incident [2], calculated for a ship mass of 1500 tons, an efficiency of 85% of the fusion energy going into thrust, an interstellar medium of density 1 hydrogen atom/cm3, completely ionized by means of electron strippers.

On the Way

Like existing ramjets the Bussard ramjet is an assisted take-off engine. In order to harvest fuel it needs a take-off speed, here 42 km/s, the escape velocity from the solar system. The faster a Bussard ramjet goes, the higher is the thrust, which means that one cannot assume a constant acceleration but must solve the dynamic rocket equation. The following table shows acceleration, speed and duration of the journey for different scoop radii.

At the midway point, the thrust is inverted to slow the ship down for arrival. To achieve an acceleration of the order of 1 g (as for instance in Poul Anderson’s celebrated novel Tau Zero [3]), the fusion drive must produce a thrust of 18 million Newton, about half the thrust of the Saturn-V. That doesn’t seem tremendous, but a short calculation reveals that one needs a scoop radius of about 3500 km to harvest enough fuel because the density of the interstellar medium is so low. Realizing magnetic or electric fields of this dimension is hardly imaginable, even for an advanced technology.

A perhaps more realistic funnel entrance of 200 km results in a time of flight of almost 500 years. Such a scenario would call for a generation starship. I thought that an acceleration of 0.1 g was perhaps a good compromise, avoiding both technical and social fantasizing. It stipulates a scoop radius of 1000 km, still enormous, but let us play the “what-if“ game: The journey would last 17.3 years, quite reasonable with future cryo-hibernation. The acceleration increases slowly, reaching a maximum of 0.1 g after 4 years. Interestingly, after that the acceleration decreases, although the speed and therefore the proton influx increases. This is because the relativistic mass of the ship increases with speed.

Fusion Drive

It has been pointed out by several authors that the “standard“ operation of a fusion reactor, burning Deuterium 2D into Helium 3He cannot work because the amount of 2D in interstellar space is too low. The proton-proton burning that would render p+p → 2D for the 2D → 3He reaction is 24 orders of magnitude (!) slower.

The interstellar ramjet seemed impossible until in 1975 Daniel Whitmire [4] proposed the Bethe-Weizsäcker or CNO cycle that operates in hot stars. Here, carbon, nitrogen and oxygen serve as catalysts. The reaction is fast enough for thrust production. The drawback is that it needs a very high core temperature of the plasma of several hundred million Kelvin. Reaction kinetics, cross sections and other gadgets stipulate a plasma volume of at least 6000 m3 which makes a spherical chamber of 11 m radius (for design aficionados a torus or – who knows? – a linear chamber of the same order of magnitude).

At this point, it should be noted that the results shown above were obtained without taking account of many limiting conditions (radiation losses, efficiency of the fusion process, drag, etc.) The numerical values are at best accurate to the first decimal. They should be understood as optimistic estimates, and not as input for the engineer.

Waste Heat

Radioactive high-energy by-products of the fusion process are blocked by a massive wall between the engine and the habitable section, made up of heavy elements. This is not the biggest problem because we already handle it in the experimental ITER design. The main problem is waste heat. The reactor produces 0.3 million GW. Assuming an efficiency of 85% going into thrust, the waste energy is still 47,000 GW in the form of neutrinos, high energy particles and thermal radiation. The habitable section should be at a considerable distance from the engine in order not to roast the crew. An optimistic estimate renders a distance of about 800 m, with several stacks of cooling fins in between. The surface temperature of the sternside hull would be at a comfortable 20-60 degrees Celsius. Without the shields, the hull would receive waste heat at a rate of 6 GW/m2, 5 million times more than the solar constant on earth.

Radiation shielding

An important aspect of the Bussard ramjet design is shielding from cosmic rays. At the maximum speed of 60% of light speed, interstellar hydrogen hits the bow with a kinetic energy of 200 MeV, dangerous for the crew. A.C. Clarke has proposed a protecting ice sheet at the bow of a starship in his novel The Songs of Distant Earth [5]. A similar solution is also known from modern proton cancer therapy. The penetration depth of such protons in tissue (or water, for that matter) is 26 cm. So it suffices to put a 26 cm thick water tank at the bow.

Artificial gravity

It is known that long periods of zero gravity are disastrous to the human body. It is therefore advised to have the ship rotate in order to create artificial gravity. In such an environment there are unusual phenomena, e.g. a different barometric height equation, or atmospheric turbulence caused by the Coriolis forces. Throwing an object in a rotating space ship has surprising consequences, exemplified in Fig. 1. Funny speculations about exquisite sporting activities are allowed.

Fig. 1: Freely falling objects in a rotating cylinder, thrown in different directions with the same starting speed. In this example, drawn from my novel, the cylinder has a radius of 45 m, rotating such that the artificial gravity on the inner hull is 0.3 g. The object is thrown with 40 km/h in different directions. Seen by an observer at rest, the cylinder rotates counterclockwise.


The central question for scooping hydrogen is this: Which electric or magnetic field configuration allows us to collect a sufficient amount of interstellar hydrogen? There are solutions for manipulating charged particles: colliders use magnetic quadrupoles to keep the beam on track. The symmetry of the problem stipulates a cylindrical field configuration, such as ring coils or round electrostatic or magnetic lenses which are routinely used in electron microscopy. Such lenses are annular ferromagnetic yokes with a round bore hole of the order of a millimeter. They focus an incoming electron beam from a diameter of some microns to a nanometer spot.

Scaling the numbers up, one could dream of collecting incoming protons over tens of kilometers into a spot of less than 10 meters, good enough as input to a fusion chamber. This task is a formidable technological challenge. Anyway, it is prohibitive by the mere question of mass. Apart from that, one is still far away from the needed scoop radius of 1000 km.

The next best idea relates to the earth’s magnetic dipole field. It is known that charged particles follow the field lines over long distances, for instance causing aurora phenomena close to earth’s magnetic poles. So it seems that a simple ring coil producing a magnetic dipole is a promising device. Let’s have a closer look at the physics. In a magnetic field, charged particles obey the Lorentz force. Calculating the paths of the interstellar protons is then a simple matter of plugging the field into the force equation. The result for a dipole field is shown in Fig. 2.

Fig. 2: Some trajectories of protons starting at z=2R in the magnetic field of a ring coil of radius R that sits at the origin. Magnetic field lines (light blue) converge towards the loop hole. Only a small part of the protons would pass through the ring (red lines), spiralling down according to cyclotron gyration. The rest is deflected (black lines).

An important fact is seen here: the scoop radius is smaller than the coil radius. It turns out that it diminishes further when the starting point of the protons is set at higher z values. This starting point is defined where the coil field is as low as the galactic magnetic field (~1 nT). Taking a maximum field of a few Tesla at the origin and the 1/(z/R)3 decay of the dipole field, where R is the coil radius (10 m in the example), the charged particles begin to sense the scooping field at a distance of 10 km. The scoop radius at this distance is a ridiculously small – 2 cm. All particles outside this radius are deflected, producing drag.

That said, loop coils are hopelessly inefficient for hydrogen scooping, but they are ideal braking devices for future deep space probes, and interestingly they may also serve as protection shields against cosmic radiation. On Proxima b, strong flares of the star create particle showers, largely protons of 10 to 50 MeV energy. A loop coil protects the crew as shown in Fig. 3.

Fig.3: Blue: Magnetic field lines from a horizontal superconducting current loop of radius R=30 cm. Red lines are radial trajectories of stellar flare protons of 10 MeV energy approaching from top. The loop and the mechanical protection plate (a 3 cm thick water reservoir colored in blue) are at z=0. It absorbs the few central impinging particles. The fast cyclotron motion of the protons creates a plasma aureole above the protective plate, drawn as a blue-green ring right above the coil. The field at the coil center is 6 Tesla, and 20 milliTesla at ground level.

After all this paraphernalia the central question remains: Can a sufficient amount of hydrogen be harvested? From the above it seems that magnetic dipole fields, or even a superposition of several dipole fields, cannot do the job. Surprisingly, this is not quite true. For it turns out that an arcane article from 1969 by a certain John Ford Fishback [6] gives us hope, but this is another story and will be narrated at a later time.


1. Robert W. Bussard: Galactic Matter and Interstellar Flight. Astronautica Acta 6 (1960), 1-14.

2. P. Schattschneider: The EXODUS Incident – A Scientific Novel. Springer Nature, Science and Fiction Series. May 2021, DOI: 10.1007/978-3-030-70019-5.

3. Poul Anderson: Tau Zero (1970).

4. Daniel P. Whitmire: Relativistic Spaceflight and the Catalytic Nuclear Ramjet. Acta Astronautica 2 (1975), 497-509.

5. Arthur C. Clarke: Songs of distant Earth (1986).

6. John F. Fishback: Relativistic Interstellar Space Flight. Astronautica Acta 15 (1969), 25-35.


Comments on this entry are closed.

  • Al Jackson February 19, 2021, 13:04

    Besides Fishback’s paper (which still seems unjustly not well known) are two important companion papers by Tony Martin making a correction and clarification of Fishback’s results.
    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 Acta18, 1-10 , 1973

    The issue of the scoop field has never been addressed in depth. Fishback showed what the conditions were for field configuration and the conditions for acceleration or deceleration drag. This needs further study a simple dipole may not be the proper source field.

    The technology for the Interstellar Ramjet is Kardashev scale especially the reactor conditions but Fishback showed there was probably no fundamental physics making the concept unobtainable.
    (I did write on this subject here last year.)

  • Mike Serfas February 19, 2021, 13:36

    There ought to be a way to deal with that waste heat and make it a more important part of the drive … neutrino pair production! The mass of the electron neutrino is unknown, but I see a figure that maybe two of the three neutrinos weigh more than 8 x 10E−3 eV. The Planck constant is 4.14 x 10-15 eV s, so 8 x 10−3eV = 2 x 1012 Hz – 2 THz, substantially lower than the energy of a thermal infrared photon (20 THz x h).

    So I want to take a thermal photon that doesn’t cost me anything, and convert it into a pair of neutrinos. That uses 4 THz x h of the energy to make a pair of neutrinos, and you should be able to put in about 3 kB T of thermal energy to the six degrees of freedom. Of course, you’d like them to go roughly backwards – the long and short of the relativistic momentum equation is they provide very slightly less momentum than a photon drive. But with a ramjet as hot as yours looking to dump heat, you should be able to make all three flavors of very hot neutrinos and emit them simultaneously with your regular photon emissions.

    My daydream for the catalyst would be a halo nucleus – something with two or three “clumps” held together tenuously at the edge of the strong force. It should have at least two subnuclear particles, such as a main nucleus and an alpha, that contain protons, so it can absorb EM. Then … somehow … it needs to produce a pair of neutrinos using this energy. Ideally this input and output of energy would stabilize this unlikely configuration and keep it from crashing together or splitting apart for … seconds. Who knows? :) While I’m dreaming, I’ll suppose the neutrinos go more along one axis of that, allowing you to aim the thrust. You’d use an external EM frequency as per NMR to keep the nuclei and hence the thrust going in one direction.

    Is that remotely, sci-fi grade sensible, or am I missing obvious physical impossibilities?

  • Brett February 19, 2021, 13:57

    Pretty cool, but wouldn’t the CNO-Cycle Fusion lose too much power to Brehmsstrahlung? That’s not a problem in stars because they’re big and dense enough to re-absorb it, but it would be a big issue with a much smaller spacecraft.

    It might be easier to run the Protium hydrogen you pull out of space with the scoop through a nuclear fission reactor, so the Protium can be bombarded with neutrons and converted to Deuterium and Tritium.

    • Al Jackson February 19, 2021, 15:00

      Frishback calculates Brehmsstrahlung and Synchotron radiation losses in his paper and the conditions under which acceleration is possible.

    • Bruce D Mayfield February 19, 2021, 16:20

      Good idea, except for the cost in mass of the neutron source.

  • Michael February 19, 2021, 15:46

    Just wondering if an of axial rotation would be better, the magnetic field coil rotates over itself effectively pulling more hydrogen in and shoving it down the throat into the reaction area, a sort of supercharger ramjet.

    • Jens Fridthjof Knudsen February 19, 2021, 16:55

      A magnetic scoop field that ‘swallows’ hydrogen? A field compressor, or field supercharger?

      • Michael February 20, 2021, 3:42

        If you imagine the ring rolling around itself inwards that would cause the magnetic field to become stronger to the oncoming hydrogen and helium extending its size.

  • charlie February 19, 2021, 16:37

    “Scaling the numbers up, one could dream of collecting incoming protons over tens of kilometers into a spot of less than 10 meters, good enough as input to a fusion chamber. This task is a formidable technological challenge. Anyway, it is prohibitive by the mere question of mass. Apart from that, one is still far away from the needed scoop radius of 1000 km.”

    “… a scoop radius of 1000 km…” Hmmm … that got the old brain cells to working. It got me thinking, what about an idea where instead of using electric and magnetic fields to funnel your protons down into the mall of the fusion reactor one instead uses the relatively recent discovered phenomena in which scientists have shown it is possible to manipulate objects with the use of some type of asymmetry in the nature of light beams (i.e. in this case I would think lasers) ??
    My understanding of this (which is relatively poor) has something to do with the fact that there is some type of asymmetry in the way light interacts with the material body that can either produce an attraction or a repulsion on the given body.

    Popular science writers equated to the tried-and-true storylines from the TV series “Star Trek” in which a tractor beam is applied to an object to draw it into the spacecraft – however, unlike that this phenomenon is real and perhaps the use of laser beams in some kind of fashion might be used to ‘draw in’ protons into your fusion device. Any thoughts here from the author of the article ?

  • charlie February 19, 2021, 16:48

    I just wanted to simply add an addendum to what I had said previously, and that is that the laser array which would produce the asymmetry in attractions on the protons in the interstellar medium would be those lasers that would be mounted on the ship itself proper. In looking at Wikipedia there was a mention of a laser which provided external power from Earth to the ship but it was castigated because of the fact that it would lose energy due to the extreme distances involved in reaching the interstellar vehicle. My idea suggest that the laser itself should be placed upon the vessel in question…

  • Jens Fridthjof Knudsen February 19, 2021, 16:48

    Would it be possible to bring along a small(?) amount of ‘additive’ to the ‘scoop fuel’ that would make it more ‘processable’ regarding a suitable fusion process?

    • Al Jackson February 19, 2021, 17:50

      There are ideas for ‘augmented’ ramjet operation.
      I like using antimatter
      A. A. Jackson
      Journal of the British Interplanetary Society, Vol. 33, pp. 117-120, 1980

      • Jens Fridthjof Knudsen February 24, 2021, 15:20

        I’ll have a look at it.

      • Jens Fridthjof Knudsen May 1, 2021, 12:45

        I received the issue last night. It is an old article, and it is brief, but it has numbers in it!! :)

  • torque_xtr February 19, 2021, 18:12

    The whole idea of ramjets is to use incoming flow without decelerating it in the vessel’s frame. An air-breathing scramjet can do this because of stoichiometry. The mass of fuel is much less than the needed mass of air, so it is possible for fuel first to mix with the air without decelerating it much, and then to convert it’s chemical energy to kinetic, compensating for deceleration and resulting in exhaust that is going backwards in the rest frame.

    But CNO cycle is a multi-stage process, and it is catalytic. Every proton must go through at least several interactions with catalyst nuclei, and with on-board catalyst, that is impossible without bringing protons to the co-moving frame of spacecraft. This ultimately limits the velocity of ramjet with on-board catalyst to about the velocity of alpha particles resulting from fusion, a fraction of c.

    The only way around is to use CNO nuclei present in the interstellar medium, but this would require extreme catalytic efficiency and ridiculously long combustion chamber. I dare not to calculate it directly but got a feeling that the numbers are monstrous even if it could be compacted into a coil.

    • Andrei February 20, 2021, 11:56

      You do address the most difficult problem there, while the collected gas arrive at the speed the spacecraft travel, it has to be slowed down for fusion process. (Or as you say having one impractical long chamber for the process.) In both cases there will be drag, which might offset the entire gain from gathering gas for fusion in this manner.

      • torque_xtr February 20, 2021, 17:05

        On the other hand, if we abandon the idea of complete hydrogen burning, but the non-deceleration requirement is well satisfied, we can omit the slow stages and still extract some energy from interstellar medium. Just adding a proton to each heavier-than-helium nucleus in the ISM is somewhat exotermic. I.e. p + 12C -> 13C + 1.95 MeV (in gamma). But these are fast strong-interaction-mediated reactions. The specific energy is low, probably in low tens of kilovolts per every nuclon in the ISM, but here the magic of ramjets comes into play. The specific energy does not matter as long as exhaust moves backwards in the rest frame.

        More, triple-alpha process can be rather fast. If it could be harnessed in Bussard ramjets, then additional tens of keV is added to specific energy per nuclon, because helium is abundant. And maybe it even could be used in pure inertial confinement mode, when a part of the incoming flow inertia and energy is used to raise pressure and temperature (by using axisymmetrical convergent flow), but this energy is not radiated away because of very high reaction rate. The reaction zone needs to be optically thick for gammas, which still implies colossal size of the ramscoop, but it looks like small window of hope.

    • Mike Serfas February 25, 2021, 5:08

      There could be a way around that. The protons and catalyst need to keep moving, but who says they need to move in a straight line? The motion of the universe, relative to the ship, makes it a linear accelerator, from which positively charged nuclei are fed into a sort of synchrotron. That is to say, the ship modifies the magnetic field in a small region of space to be perpendicular to its flight so that the particles loop around many times. This costs no energy beyond that to maintain the field, and the particles don’t need to be decelerated. True, each nucleus will loop around with a different radius like in a mass spectrometer, but maybe there is a way to exploit that to make the progress of the fusion cycle more orderly. At the end, the stream of waste nuclei merely needs to be directed away from the others and out of the ship.

  • djlactin February 20, 2021, 7:59

    “in a time of flight of almost 500 years… let us play the “what-if“ game: The journey would last 17.3 years,…” What is the destination?

    • Gary Wilson February 20, 2021, 14:55

      Dr. Schattschneider mentions the destination in a sort of off-hand way early in his article. The destination is Proxima Centauri. A great article by the way.

  • torque_xtr February 20, 2021, 9:30

    Although I now imagine ramjet with circulation chamber, where incoming fuel is fed into a solenoid. It circulates there with the speed equal to ramjet rest-frame velocity until reaction is completed, heats up and then is allowed to expand through magnetic nozzle, giving it some rest-frame backwards motion. Maybe even in a continuous manner, with feeding through a straight-section break with reduced magnetic field, and expanding through another one. The compression results from centrifugal force balanced by magnetic field pressure. Still requires tremendous magnetic fields and n*tau, and it has to overcome radiation problem at long retention times. All proton fusion undergoes proton-to-neutron conversion at some stage, which is an intrinsically slow weak interaction process, although it is accelerated to some extent inside neutron-deficient nuclei. But even the fastest CNO cycle branch, HCNO-III, has a limiting stage of 17 seconds, and takes place at conditions close to surface flashes on white dwarfs. If we have no way to compress fuel to trillions of megabars, and magnetic pressure is limited to the highest possible strength of structural materials, then basic CNO cycles are the only hope, requiring multiple minutes-retention times.

  • Gregory Benford February 20, 2021, 14:30

    I suggest using the proton + Boron reaction, yields 3 alpha particles, few neutrons.

    • Andrei February 22, 2021, 13:20

      Boron – proton fusion is indeed relatively free of neutrons. But then the craft will have to carry quite large fuel tanks with Boron as only minute amounts is found in space . It do also require quite higher ion energies than the kinds of fusion usually proposed. It is attractive and would simplify several aspects of any interstellar spacecraft that is designed to carry it’s own fuel. The energy yield is somewhat less for the mass of fuel used – if I remember correctly.

  • Henry Cordova February 20, 2021, 15:43

    Discussions of the Bussard Ramjet, Fusion Propulsion, indeed, even Fusion thermoelectric generation illustrate yet another potential obstacle to future technologies. Just because something occurs in nature and violates no known physical law does not necessarily mean it can be harnessed by some clever application of future engineering. It may simply be impossible to generate a controlled fusion reaction with a net energy surplus. Yes, fusion occurs in the stars, and we can duplicate the process in a thermonuclear weapon, but it may simply not be possible to build a fusion starship drive or even a fusion steam plant, for that matter. Fission-sparked fusion is demonstrable, and we know it can happen at high temperature, pressure and density conditions in stellar cores. But that doesn’t mean it is something we can rely on on to power our future dreams.

    We don’t know for sure, of course. We never can tell what clever new technologies may be developed tomorrow. But the point is, we can’t COUNT on them being developed. Our record over the last few centuries has give us the confidence (or hubris?) we can make anything happen as long as it does not violate natural law. Maybe not.

    Just as there are limits to natural law, there may also be limits to engineering possibility, which is, after all, determined by natural law. And yes, I know dire warnings like mine have been shown in the past to be premature, if not overly downright short-sighted. But that is no guarantee we can do anything we want…in a century or two when the the technology finally catches up to the science.

    • Ron S. February 21, 2021, 20:30

      I think you’d strike closer to the mark by suggesting that we may never harness the reactions economically. With lots of time and future progress we can probably do it, but if it isn’t economical there’s no good reason to do it. Of course our descendants may decide to redefine economics so who knows.

      • henry cordova February 21, 2021, 22:46

        Interstellar travel is worth the price, at least for me. Even if I can’t afford a ticket for myself.

        But then again, I would have loved to fly on the Concorde, too. And I’m still waiting for those flying cars and jet packs they promised us.

        I think the one alternative future none of us seems willing to consider is that a century from now, people will be saying; “Back in the early 21st century we were exploring the solar system. Now we can’t see why we even bothered. There’s nothing out there but ice, slag and hard radiation.”

        As our society continues to be more and more dominated by purely commercial concerns, it is becoming harder and harder to imagine a future as enthusiastic about exploration and discovery as we are in the present.

        • Alex Tolley February 22, 2021, 17:35

          I would have loved to fly on the Concorde, too

          In the mid-1980s, BA was flying Concorde on very short round trips out of Manchester Airport, UK (I think) for £400 (if memory serves). The trip was a subsonic flight over the Irish Sea and back. Not having any inkling that Concorde was going to be abandoned 20 years later, I regret not buying a ticket at the time just to experience flying on that airplane. I recall seeing a grounded Concorde at Kennedy airport 10 years later, looking rather forlorn on a wet and raining afternoon. She was a magnificent experiment that politics and ticket costs eventually sunk, although supersonic passenger planes are being developed for the super-rich as status symbols. I recall the British government in the 1960s canceling almost all aerospace development to save money, but keeping the Anglo-French Concorde as a hoped-for profit-making enterprise, much as they hoped the Comet would have been.

          • Henry Cordova February 23, 2021, 1:47

            There are space groupies, and then there are airplane groupies. The supersonic transport was a technical tour de force, but not a sound business proposition. It was a solution looking for a problem. After all, how many people really need to get to Tokyo in 6 hours instead of 10? There may a niche for an SST in a research or military mission, but certainly not as a business enterprise, at least, not one that would justify the enormous development and operating costs. But groupies will be groupies; there are still ongoing fantasies about HYPERsonic transports–presumably so Mick Jagger can get to New York even faster.

            Concepts like fusion power and Bussard ramjets may simply be engineering impossibilities, though, and that is a different bird altogether. And by “impossibilities” I don’t mean they require new laws of physics or the discovery of unobtanium, but that it may simply be impossible to practically control and manage those naturally occurring physical processes with human-produced artifacts or human-accessible materials, which are subject to very real physical limitations.

            The prize (in this case, limitless clean energy or relativistic interstellar travel) may be worth pursuing, but we should be prepared that it may not be actually achievable. We should be ready for failure, and perhaps hope some collateral discovery renders the whole question moot.

            • Alex Tolley February 23, 2021, 12:39

              The supersonic transport was a technical tour de force, but not a sound business proposition. It was a solution looking for a problem. After all, how many people really need to get to Tokyo in 6 hours instead of 10? There may a niche for an SST in research or military mission, but certainly not as a business enterprise, at least, not one that would justify the enormous development and operating costs.

              I don’t agree with your statements. Concorde was expensive to fly, but there is little question that it was primarily pitched at businesspeople crossing teh Atlantic both ways within a working day. Whether the already high ticket prices were sufficient may have been compromised by the infrequent flight schedule. The Concorde was flying at about the same time wide-body jets like the 747 with low ticket prices and slower airspeeds than earlier commercial jets were catering to the burgeoning tourist travel. Concorde also faced the sonic boom issue that resulted in banning its flights at supersonic speeds over land, crippling its possible use. The Boeing SST, a much larger SS transport was on the drawing boards before it was scrapped as it became apparent that widebody jets were the future.

              Yes, people do want to travel as fast as possible. Sub-orbital rocket flights have been suggested since the V2 era. SpaceX’ Starship is also posited for very long-distance flights for just this reason. Although as Clarke remarked, the problem with such craft is that the toilets are unreachable for 1/2 the flight and unusable for the other 1/2. Whether the fast sub-orbital hop is worth the hours of travel to and from the flight pads and the endless security is another issue. Virgin Galactic has thrill ride flights booked even without the spaceplane going anywhere (if it even leaves the ground).

              Do SS airplanes have a customer niche? All the airframe companies with SS craft in development think so. Wealth inequality has driven private and corporate jet ownership. Flight costs for the super-wealthy are not really an issue. Bragging rights and status are likely more important.

              Lastly, the market for wide-body jets is now in decline. The peak was the commercial failure of the Airbus 380 series. The pandemic seems to have accelerated the trend. Airlines have started to abandon the hub-and-spoke route model in favor of going back to point-to-point, using more frequent flights of smaller aircraft. A SS airplane that does not create an audible sonic boom on the ground could well gain a niche in that market after proving itself viable for private and corporate use. The military has wanted rapid force projection too, as you note.

              There are “groupies” for also sorts of artifacts, especially for cars, planes, and now spaceships. Even Picard and Data exhibited this in First Contact as Picard lovingly touched the Phoenix, the first-ever warp drive ship built by Zephram Cochran. Car and plane museums abound, and there are frequent meets for car and even plane owners. Near me is an SR71 on display. (If only they had an X-15 too. *sigh*).

            • Ron S. February 23, 2021, 13:46

              The speed of a SST can be misleading. For the objective of getting from A to B as rapidly as possible it is an inferior solution. There are two problems. First, a lot of the travel time is spent in queues and the filling/emptying the plane of passengers and baggage. Second, as mentioned, you have to wait for the next available flight.

              The better solution is corporate and private jets. No queuing and few passengers. I did this type of flying during my career and it is astounding how fast and convenient travel can be when those problems disappear. Delays, as they are, are whether the plane is fueled, the regulatory paperwork done and, at the busiest airports, being assigned slots for take off and for the estimated arrival time.

              Of course I wasn’t paying the bills! But if time is all important and money is available this mode of travel is far superior. There are small firms that will take you anywhere on their small fleets on short notice if you have no plane of your own. SST is an expensive toy.

              • Alex Tolley February 23, 2021, 14:41

                So why is a corporate/private SS jet just an expensive toy? (Or are you just saying that a commercial passenger SST compared to a subsonic commercial airliner is an expensive toy?)

                As all the SSTs being designed today are for the corporate/private market, doesn’t that imply that there is a potential market for this form of transport?

                • Ron S. February 23, 2021, 16:05

                  I thought the discussion was about a commercial SST for scheduled air travel market. Private SST? Maybe. It seems more of a status symbol than a necessity.

                  It reminds me of software developers who used to come to me with great proposals for improving transaction rates by 10%. Meh. Talk to me when you can do 50%.

                  • Alex Tolley February 23, 2021, 20:48

                    Let’s talk about SSTs. In particular a suborbital craft like Starship. A flight in a conventional airliner across the Pacific takes about 16 hours (LAX -> HKG). A suborbital hop should be less than an hour. Being conservative, adding in extra time to get to and from a dedicated launch platform, say 2 hours.
                    Given fixed overheads, with fuel as teh variable cost, you could fly the same vehicle 8x as frequently as a commercial jet. Skimming off the more lucrative travelers where time is more important, this should be a very profitable business, despite the higher fuel costs.

                    While I don’t think hypersonic suborbital passenger spaceplanes will become regular airliners, the point is that IF a high flight frequency can be maintained, then the high speed is economically desirable. [It is the same reasoning that makes airships hopelessly uneconomic compared to airplanes, restricting them to niche uses.] Supersonic planes flying at 2-3x subsonic speeds may, or may not, have sufficient economic advantage. IDK. What is clear is that there do not seem to be any such planes on the drawing boards today. If anything, flying more slowly but with higher fuel efficiency seems to be the direction the major airframe manufacturers are headed. A 10x speed advantage might tip the business case, and suborbital ballistic flight might be the solution, especially if conventional airport runways could be used.

              • Henry cordova February 24, 2021, 0:16

                An excellent point. What makes flying so aggravating is not the time you spend in the air, it’s the time you spend at the airport and on the runway. Getting there, parking, finding transport at your destination, standing in line, it never seems to end. Hurry up and wait, and $20 Cuba Libres in the lounge. Once you’re boarded, you can sleep, read, look out the window, relax with a highball, get some work done, even talk to your fellow passengers! Rather than spending a fortune developing exotic aircraft and burning fuel extravagantly to get there a few hours faster, this is where airlines should be working to make the flying experience easier. But tech groupies need their gadgets, don’t they?

                If you need objective proof, look at the actual history of supersonic air travel. We had it for years. Safety and environmental issues aside, the SST lost money. That’s why there aren’t any Concordes any more. The market just wasn’t there.

                But that’s incidental. The point we’re really discussing here is that just because something CAN be done doesn’t necessarily mean its a good idea.

  • Mark Thomason February 20, 2021, 19:13

    This mechanism requires a long thin object, for which a length of 1 km would be on the small side.

    Its scoop and propulsion system would need to be turned off when passing through a local density of matter, like a solar system. Designed to scoop atoms, it would not be likely to handle well the various sized chunks to be found orditing a star.

    Perhaps it is mere random chance, but this does describe our limited observations of Oumuamua passing out of our solar system.

    Perhaps we could look for signs of such an engine turning off from where it approached unseen, or igniting out where it disappeared from direct observation.

  • Alex Tolley February 21, 2021, 2:00

    Would Clarke’s Asymptotic Drive (Imperial Earth) work to create the energy needed? In the novel, a microscopic black hole is used to release the energy from the hydrogen fuel to create the intensely hot, energetic plasma reaction mass.

    If that worked, then rather than bringing along all the fuel, use the scoops to supply the H2. Clarke wrote the story before it was understood that black holes evaporate as they emit Hawking radiation.

    So I imagine a starship that has a micro black hole, a buffer of hydrogen, and a ram scoop to collect most of the fuel/propellant. The buffer is to ensure that the BH does not explosively evaporate when the drive is turned off but is primarily used to even out the flow rate of the scooped H2 as the ship passes through heterogeneous ISM density. Interstellar liners would have larger BHs with very slow evaporation rates but live with the mass penalty reducing performance. Military ships OTOH would have the least massive BHs that required very delicate control but gained in performance. (Although as with Heinlein’s story about fission reactor accidents (“Blowups Happen”), the occasional explosion of a military vessel might happen due to loss of control of the BH).

    If this works, we transfer the difficulty of managing a fusion reaction by compressing and/heating the hydrogen to stellar conditions, to creating or finding BHs suitable for the drive. A charged BH would probably need to be used to allow it to be anchored in the drive tube. If nothing else, it might offer another approach to liberate the energy of the H2 in the ISM.

    Let us not forget that the ram drive does need to be propelled up to a velocity that allows it to work. It doesn’t work from a standing start. For that, the ship will need an onboard supply of H2, which means that it will be subject to the rocket equation constraints until it gets up to ram velocity. If Jeff Greason’s Q-Drive works, then it offers a more parsimonious solution to reach the needed velocity to start the ram engine. As it needs to generate huge magnetic fields to interact with the charged medium, is it possible to adapt them to the ram scoop configuration when needed?

    • Michael February 21, 2021, 9:43

      Alex I believe the charge of a black hole is internal to it, we can’t feel it’s effect directly. However moving the small BH is easier than thought, firing particles including light, very high frequency, into it imparts a momentum to it, it can be controlled that way.

      • Alex Tolley February 21, 2021, 23:46

        This paper suggests that magnetic fields are external to the BH. However, BH physics is well beyond my understanding.

        Magnetic fields around black holes

      • Ron S. February 22, 2021, 10:42

        The charge is seen externally. However, the amount of charge imbalance (say, lots of electrons) to impart an observable, let alone effective charge is truly enormous.

        Moving a small BH is not so easy! I dare you to focus a laser on a BH that is far smaller than the wavelength of the photons. Particles also will sail right past it since it’s such a small target. It can pass through ordinary solid matter with little effect, and can grow only very very slowly.

        • Michael February 22, 2021, 16:20

          If this was the case BH’s which spin at high fractions of the speed of light would create huge magnetic fields which we do not see. Also if I was in the hole, a large low tidal one, I could arrange the electric field to communicate to outside the hole. That magnetic field is remnants of the plasma that fell in and gets wrapped around the hole but must disappear as well. I have a thought experiment in which we have two orbiting BH’s, I strip hydrogen and put the all the electrons in one hole and all the protons in the other, I wonder what would happen, eventually the electric charge should over power even gravity as it’s magnitudes stronger than gravity. The frequency of light would be very very high to be absorbed but then do we have to. The small BH will radiate energy, perhaps a reflector system which pushes some of the energy back in could work to control it or even the tiny electron could do the job.

          • Ron S. February 23, 2021, 10:13

            The electric field is uniform and therefore static. Even a spinning BH (and all real BH do spin) have no magnetic field. Any magnetic effects come from matter that approaches the ergosphere (such as the accretion disk) and gets spun by the radial rotation gradient of spacetime itself.

            It is possible to “communicate” from within the ergosphere but not from within the event horizon of a spinning BH. There are theoretical methods of extracting energy from the angular momentum by piercing the ergosphere with exotic structures.

            Yes, the EM fundamental force is far stronger than gravitation. But, as I said, it’ll take a lot of charge to make it felt in a real BH, which is at minimum of stellar mass. I would be easier to do with micro-BH if such can ever be constructed. There is no known physical path to doing so from where we currently are, although it is possible some were made in the initial stage of the big bang.

            You can’t stuff the matter and radiation back into the BH. See my previous comment. If we could do this we could make a micro-BH. Confining energy and matter in this fashion is beyond extremely difficult. So difficult that (as I said above) they may have been impossible to form during the big bang.

            • Michael February 23, 2021, 14:58

              An excess of electrons could hold up the sun’s mass against its gravity, and it would weigh less than a gram. Two electrons at 1 nm would over power a neutron stars gravity by over a million times.

    • ljk February 22, 2021, 11:54

      I have not read the novel Imperial Earth in a long while. Did Clarke say whether they made the mini black hole that powered the spaceship’s drive or somehow found it somewhere in space?

      I am going to assume the former unless mini black holes are freely roaming the Sol system and the rest of the Milky Way galaxy, which I think would be quite the travel hazard.

      Mini black holes were trendy enough at the time that they were given a front seat as a potential explanation for the Tunguska Event of 1908 in a 1974 cover issue of The New York Times Sunday Magazine.

      Some tech talk on the Asymptotic Drive from the novel:


      A very interesting and eye-opening critique of the novel, which makes me think I need to read it again, as my knowledge and perceptions on life have certain changed since circa 1976:


      Side note:

      Imperial Earth is where I first learned that the American flag planted on the Sea of Tranquility by Apollo 11 in 1969 was knocked over by the exhaust blast of the Lunar Module ascent stage when the astronauts took off for the return home.

      Some of the folks in this version of 2276 were debating whether or not to stand Old Glory back up or leave it in the lunar dust where history and fate had placed it.

      I can see an actual discussion on this matter happening some day, probably much sooner than 2276.

    • Adam February 23, 2021, 4:32

      Alex, Clarke’s Asymptotic drive was – oddly enough – pre-Hawking in its understanding of Black Holes. Its mass was only ~1,000 tons. Sadly, although the energy efficiency of mass falling into a Black Hole is about 5% (vs 0.7% for hydrogen fusion), the Eddington Limit means it chokes itself off at a luminosity of about 6.4 watts per kilogram of mass. Super-Eddington accretion *might* be arranged, but not to the power-to-mass ratio that Clarke’s version needs.

      Robert Freitas discusses using Hawking Radiation for propulsion in his “Xenology” (c.1978) book which is AFAIK the first mention. Charles Sheffield used spinning Black Holes for propulsion power batteries in his “McAndrews” tales (c. 1978), but only mentions Hawking radiation as a nuisance side-effect.

    • Jeff Greason February 24, 2021, 19:27

      No need for the q-drive in this application; Slough’s plasma magnet can get you up to ~700 km/s towards alpha/proxima Centauri, and that’s plenty fast for ramjet operation if the problem of a low parasitic drag, high area, low mass intake were to be solved.

      The intake is really the fundamental problem. If fusion could not be solved, use something else in a RAIR mode as others have suggested. But the intake has been a big problem to date.

  • Michael February 21, 2021, 14:54

    Could we have two scoops side by side or a centre pinched ring so there is two channels, both ends feed into the chamber at the bottom. We could bend the incoming hydrogen/Helium and cause it to collide at the centre of the reaction chamber. To get going we could use antimatter to boost it. Half bakery time…

  • Earth Pig February 22, 2021, 1:27

    Isn’t this the type of propulsion featured in the novel “Bowl of Heaven” Trilogy?

    • Adam February 23, 2021, 4:22

      In “Bowl of Heaven” the engine is a star, so absent the issues of a much smaller ramscoop starship.

    • Al Jackson February 23, 2021, 19:34

      The ‘stellar’ engine in Bowl of Heaven is a Shkadov thruster .

  • Brett P Bellmore February 22, 2021, 7:20

    The problem with the CNO cycle, is that, aside from the need to recycle carbon, it has more than one rate limiting step based on radioactive decay. The Nitrogen to Carbon decay has a half-life of ten minutes.

    This means you have to bring all the incoming gas to a stop, and keep it around for maybe a half hour, and then accelerate it back on its way, plus a bit for thrust.

    That might not be a theoretically fatal problem, but it probably is fatal from a practical engineering standpoint, because everything from end to end has to operate at near 100% efficiency for you to get net thrust.

  • Project Studio February 22, 2021, 19:15

    If we think about what kind of field we want first, a field that can funnel positive ions across a wide arc of space ahead of the craft it can help define the problem better.
    Rather than an electical loop generating an ineffective mangenetic field, why not an anular arrangement of many electromagnet coils, with poles in the forward and aft direction, arranged in a circle to funnel particles into the reaction chamber, thus mmimicing the magnetic lense mentioned in the article.
    Since the idea is to attract poistive ions from affar to some point directly in front of the scoop, why not incorporate an electrostatic negative charge (maintained by power from the fusion reaction) at some point ahead of the craft?
    Also, I always imagined that it was largely the concentration and magentic squeeze of the incoming stream of fast moving (in the craft’s reference frame) particles that triggered the fusion process. Perhaps a catalyst needs to be injected into the stream, but it would seem to be incongruous of the the notion of ‘ram jet’ to stop the stream in its tracks, so to speak, so perhaps the funnel and craft needs to be quite long.

  • Adam February 23, 2021, 7:06

    Hi Paul
    Pure proton reactions might take a different tack… Practical Proton-Proton Fusion. The writer suggests fusing a purely ionic plasma, with a steady supply of neutrons to make deuterium. Once the incoming ion stream hits the right energy to make neutrons, this seems a natural for a Bussard ramjet – if we can get the magnetic scoop working!

  • Tim Whitworth March 17, 2021, 14:08

    Relative to the ship, the ionised gas is approaching at a high speed – that’s the current. The desired motion is inwards towards the centre of the field (in line with where the physical ship is). So, the field should be at right angles to both of these – we want the field lines to run circularly around the axis of travel.

    That can’t be done with a permanent magnet, but I seem to remember that it can be done with an electromagnet? A donut shaped coil, probably wrapped around a steel donut core.

  • Michael Spencer July 9, 2021, 10:38

    As a non-scientist lurker here, I frequently require several re-readings to understand these pieces. One question that came to mind is this: how exactly does a chosen material “fail” with time? Is this a mechanical issue? Exposure to prolonged compression? What sort of picture should the non-scientist, with a fair amount of exposure to undergraduate scientific/ mathematic education, have in mind?