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Tuning Up the Interstellar Ramjet

Catching my eye in the latest Carnival of Space, hosted by Brian Wang at Next Big Future, is Adam Crowl’s write-up of a rethinking of an exotic ramjet technology. Robert Bussard put the interstellar ramjet into the public eye back in 1960 in a paper proposing that a starship moving fast enough would be able to use the hydrogen between the stars as a source of fuel, enabling a constant acceleration at one g. You’ll recognize the Bussard ramjet in Poul Anderson’s classic novel Tau Zero (originally published in Galaxy in 1967 as To Outlive Eternity).

The Problem with Slow Fusion

Anderson’s ‘Leonora Christine’ was a runaway starship, accelerating ever closer to lightspeed until she was punching through entire galaxies in times experienced by the crew as mere minutes. But we don’t have to get quite that extreme with the Bussard idea. It’s built around the premise of gathering fuel along the way so as to avoid the vast mass ratio problems of conventional rocketry. We can imagine an enormous magnetic scoop that might gather up this interstellar hydrogen, but the problem is how to burn it.


Image: The Bussard ramjet concept, as envisioned by the space artist Adrian Mann.

Adam’s discussion nails the problem: Bussard relied on proton to proton fusion, but the problem is that it’s too slow. Assuming we could gather the hydrogen in the first place, we face the fact that proton burning is a kind of reaction that works in stars (like our Sun) because of the sheer size of the stars themselves. Adam describes this well:

The reaction rate of proton-proton fusion at “low” (i.e. an achievable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.

Lighting the Fire: The CNO Alternative

Physicist Daniel Whitmire tackled this problem in a 1975 paper that proposed using hydrogen for fuel but exploiting a catalytic nuclear reaction chain instead of straight proton burning. The so-called CNO Bi-Cycle becomes dominant in sufficiently hot main sequence stars (usually those about 1.5 times the mass of the Sun), while the proton-proton chain is more significant in smaller stars. Here’s Adam’s description:

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.”


Getting the Vessel Up to Speed

Whitmire points out in his paper that using this method, the catalyst fuel, carried along with the spacecraft, is not depleted. Interstellar hydrogen, however, remains the ultimate source of the energy. We still run into the issue of the density of the interstellar medium, which is tricky business. Whitmire comments on the problem of gathering enough fuel:

…independent of other restrictions, nuclear powered ramjets may require nebular regions of space for take off if accelerations greater than 1g are desired. However, once the desired velocity is obtained in a nebula runway, it will be maintained and even increased as the ship moves into less dense regions of space… Nebulae are also required for stopping interstellar ramjets since deceleration requires as much energy as acceleration.

Image: Science fiction has worked often with the ramjet concept, as seen here in this Analog cover from 1978. Artwork by Rick Sternbach.

What do we do about getting up to speed? There being no nearby sufficiently dense nebula, we on planet Earth might find ourselves unable to light the fire. But Whitmire goes on to speculate that technological civilizations living within or near a dense nebula would have no such constraints, and even goes to far as to outline the possibility of searching for the signature of extraterrestrial ramjets that might be tracked while using a nebula ‘runway.’

The Ramjet as Brake

Bussard’s ramjet ideas, followed by Whitmire’s modifications, really did open up the idea of practical interstellar flight some fifty years ago, but recent studies have shown that the ramjet idea has a key obstacle. The ramscoop needed to collect all that hydrogen acts more effectively as a brake. Indeed, we’re now considering similar magnetic scoop ideas as possible braking systems for decelerating a probe into a destination solar system. Is the ramjet through, or are there ways around the braking problem, including other ways of hydrogen collection?

The paper is Whitmire, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica Vol. 2 (1975), pp. 497-509 (available online, and well worth reading). The original Bussard ramjet paper is “Galactic Matter and Interstellar Flight,” Acta Astronautica Vol. 6 (1960), pp. 179-194. Adam Crowl also points out that Gregory Benford’s starship ‘Lancer’ in Across the Sea of Suns used the CNO cycle, an apparent nod to Whitmire.

Comments on this entry are closed.

  • ljk June 8, 2009, 14:17

    Bussard’s original 1960 ramjet paper is online here:


    Even if it is a better brake than a starship engine, it is a wonderful idea just the same.

  • ljk June 8, 2009, 14:22

    This page has both the blueprint and a small version of the Bussard Ramjet
    seen in Carl Sagan’s Cosmos series and companion book, along with a rather
    distressing analysis of the issues in making the starship concept a reality:


    And this fellow wrote was is probably one of the few songs about the concept:


  • hb June 8, 2009, 16:44

    So there’s one more reason, why ETIs have not visited us: it’s simply more difficult to stop here, so why bother? There are plenty interesting places in the galaxy, where it’s easier to stop.

    Or maybe ETIs are more clever than we are, at the moment.

  • Frank Smith June 8, 2009, 17:01

    The nuclear ramjet is just too cool of a concept not to work. I’ve always felt that perhaps a “boost” stage (fusion pulsejet/laser push/frozen Deuterium pellet impact) would get the ram scoop up to some fraction of c. And then light the torch!

    I don’t think our understanding of technology is advanced enough to conclusively rule out the nuclear ramjet as some have claimed.

  • Simon June 8, 2009, 18:11

    Instead of collecting hydrogen why not manufacture it in a green algae farm?
    At present that technology is not mature but it would seem far less challenging than fusion. It could also be used in a nuclear electric rocket as some designs ( MPD etc) use hydrogen as propellant.

  • Robin Goodfellow June 8, 2009, 18:46

    Another, even more ambitious, idea would be to use strangelets (strange quark matter fragments) as the heart of a “nucleonic” fusion reactor. In theory, addition of baryonic matter to a strangelet could realize as much as a 5% efficiency mass-energy conversion factor, which is about an order of magnitude higher than fusion mass-energy conversion efficiency. Obtaining a strangelet of manageable size, if they exist, and properly containing and utilizing it in a reactor is left as an exercise for the reader.

  • Dave Moore June 8, 2009, 20:29

    My idea: lay a cloud trail of Deterium-Tritium nanoparticles in the direction you want the ramscoop ship to go and accelerate the ship along the trail. You could probably get by with a smaller scoop (less drag). And if the particles were accerated to their position on the trail by some sort of mass driver/particle gun, the could be moving in the direction the ship is going and there would be less loss of momentum scooping up the particles.

  • Ron S June 9, 2009, 0:17

    It isn’t too surprising that a proton/hydrogen scoop acts like a brake when you consider that, when moving at relativistic velocities, the kinetic energy of the particle can be much greater than its mass. Since the momentum of the particle is of opposite direction to the spacecraft, capturing the particle will slow the spacecraft in proportion to the particle’s momentum.

    With any fusion reaction only able to harvest a small fraction of the particle’s rest mass, even 100% propulsion efficiency can’t hope to achieve net forward acceleration. The same is true even with complete mass conversion to propulsion if the spacecraft is moving at a high relativistic velocity.

    We need some miraculous technology to extract the particle’s energy without slowing it down.

  • Adam June 9, 2009, 6:02

    Hi All

    Frank, I agree, the ramjet is too cool not to work. Trick is, we can’t fuse protons fast enough via mere compression – they need catalysts like carbon. Another two issues is storing energy from the incoming mass-stream to return to the exhaust, and retaining the carbon after fusion. Potentially they’re all tractable problems, unlike the fast-burning of protons, which is almost a contradiction in terms. The reason? Proton-proton fusion into deuterium is a WEAK force reaction, unlike the STRONG force reactions for just about every other fusion reaction. That’s why a neutrino gets spat out when protons fuse:

    p + p -> D + neutrino

    …most of the Sun’s energy actually comes from stage II, fusing deuterium together to make tritium or helium-3, then attaching another proton to them in turn to make helium-4. They’re all fast reactions – D+D reactions are so fast in the Sun’s core that deuterium’s half-life is microseconds, unlike the 8 billion year proton half-life.

    Simon, the problem is not making hydrogen, but carrying it while accelerating before you then use it. Rockets suffer from the disadvantage of the Tsiolkovsky Rocket-Equation and there’s nothing we can do about it. Ramjets aren’t rockets, most of the time anyway.

    Robin, as always you make a brilliant suggestion, but strangelets are rather nasty for so many reasons – worse than antimatter in some ways. And we’re none too sure if they can be made. A good thing perhaps?

  • Adam June 9, 2009, 6:13

    PS Larry, that ramjet analysis you linked to assumes D-D or D-He3 fusion NOT CNO catalysis. A case of arguing against a strawman I suspect. T.A. Heppenheimer did the first critique against Bussard’s original idea, followed up by studies by Alan Bond, who was arguing for the related RAIR concept – a ramscoop carrying fusion fuel to energise ramscooped propellant mass.

  • Pointless Geometry June 9, 2009, 9:51

    If you had narrow enough funnel at the neck of the collector head you would create a burble point of whatever material you’re scooping up, creating a reservoir of fuel. And if the fusion exhaust area is greater than that of the scoop/Magnetic Torus, wouldn’t that constitute an Alcubierre drive?

  • James M. Essig June 9, 2009, 10:27

    Hi Folks;

    The CNO fusion cycle energy generation mechanism for ISR space craft is interesting. I am beginning to wonder how many other potentially viable fusion reaction sequences are available for powering ISR space craft. Perhaps the NIF facility within the united states will lead to a more comprehensive understanding of 100 million K range plasma hydrodynamics, and who knows, we might yet discover new fusion pathways.

    Science fiction is familiar with the concept of a nuclear device that would involve exothermic proton fission wherein the binding energy of quarks would be released. Recall that protons and neutrons are each composed of different combinations of up and down quarks. The rest mass of the quarks is only on the order of 1 percent of the mass of these nucleons, the additional mass comprising the roughly 800 MeV/[C EXP 2] rest mass of protons and neutrons comming from the relativistic kinetic energy of the bound quarks as well as the gluon field binding energy acting between and on the quarks. I would love to see an ISR able the harness a reaction such as this. The accelerations of the ISR could well approach 10 Gs in such an instance.

  • jim moore June 12, 2009, 12:34

    I think there is a basic mistake, in the system.
    You are not scooping up hydrogen, you are scooping up protons, because protons are charged particles and hydrogen atoms are not, right?
    And if you have charged particles moving in a particular direction, you have by definition an electric current, right?
    And electric currents generate a magnetic fields, right?

    Has any one looked at the Ram Jet as an electromagnetic system?

    Is it possible to make an interstellar linear induction motor?
    First fire off the probe and get it moving, deploy the super conducting wire loops.
    Have a big bank of lasers orbiting the sun and pointing to the destination. Tune the lasers to the frequency that ionizes hydrogen creating an electrical “wire” that grows at the speed of light and connects our solar system to theirs.
    How much energy would it take to scoot down that “wire”?

  • John Cartwright. October 22, 2010, 11:46

    Tau Zero was a mind-blowing book. It was a journey into the new creation and was the best hard Sci-Fi book in my humble opinion. Keep up the good posts.

  • Dakota Ewing November 26, 2010, 14:24

    What if you used the interstellar gas just as reaction mass? you could carry He3 and use a pure he3 reaction to power somthing akin to a vasimr engine, exept that all of the reaction mass is not carried on the ship.