An Alternative Take on Fusion Fuel

Let’s talk about fusion fuels in relation to the recent discussion of building a spacecraft engine. A direct fusion drive (DFD) system using magnetic mirror technologies is, as we saw last time, being investigated at the University of Maryland in its Centrifugal Mirror Fusion Experiment (CMFX), as an offshoot of the effort to produce fusion for terrestrial purposes. The initial concept being developed at CMFX is to introduce a radial electric field into the magnetic mirror system. This enhances centrifugal confinement of the plasma in a system using deuterium and tritium as fusion fuel.

Out of this we get power but not thrust. However, both UMD’s Jerry Carson and colleague Tom Bone told the Interstellar Research Group’s Montreal gathering that such a reactor coupled with a reservoir of warm plasma offers prospects for in-space propulsion. Alpha particles (these are helium nuclei produced in the fusion reaction) may stay in the reactor, further energizing the fuel, or they can move upstream, to be converted into electricity by a Standing Wave Direct Energy Converter (SWDEC). A third alternative: They may move downstream to mix with the warm plasma, producing thrust as the plasma expands within a magnetic nozzle.

Image: The fusion propulsion system as shown in Jerry Carson’s presentation at IRG Montreal. Thanks to Dr. Carson for passing along the slides.

We also know that fusion fuel options carry their own pluses and minuses. We can turn to deuterium/deuterium reactions (D/D) at the expense of neutron production, something we have to watch carefully if we are talking about powering up a manned spacecraft. The deuterium/tritium reaction (D/T) produces even more neutron flux, while deuterium/helium-3 (D/He3) loses most of the neutron output but demands helium-3 in abundances we only find off-planet. Tom Bone’s presentation at Montreal turned the discussion in a new direction. What about hydrogen and boron?

Here the nomenclature is p-11B, or proton-boron-11, where a hydrogen nucleus (p) collides with a boron-11 nucleus in a reaction that is aneutronic and produces three alpha particles. The downside is that this kind of fusion demands temperatures even higher than D/He3, a challenge to our current confinement and heating technologies. A second disadvantage is the production of bremsstrahlung radiation, which Bone told the Montreal audience was of the same magnitude as the charged particle production.

The German word ‘bremsen’ means ‘to brake,’ hence ‘bremsstrahlung’ means ‘braking radiation,’ a reference to the X-ray radiation produced by a charged particle when it is decelerated by its encounter with atomic nuclei. So p-11B becomes even more problematic as a fuel, given the fact that boron has five electrons, creating a fusion plasma that is a lively place indeed. Bone’s notion is to take this otherwise crippling drawback and turn it to our advantage by converting some of the bremsstrahlung radiation into usable electricity. To do this, it will be necessary to absorb the radiation to produce heat.

Bone’s work at UMD focuses on thermal energy conversion using what is called a thermionic energy converter (TEC), which can convert heat directly into electricity. He pointed out that TECs are a good choice for space applications because they offer low maintenance and low mass coupled with high levels of efficiency. TECs operate off the thermionic emission that occurs when an electron can escape a heated material, a process Bone likened to ‘boiling off’ the electron. An emitter and collector in the TEC thus absorb the heat from the bremsstrahlung radiation to produce electricity.

Image: A screenshot from Dr. Bone’s presentation in Montreal.

I don’t want to get any deeper in the weeds here and will send you to Bone’s presentation for the details on the possibilities in TEC design, including putting the TEC emitter and collector in tight proximity with the air pumped out between them (a ‘vacuum TEC’) and putting an ionized vapor between the two (a ‘vapor TEC’). But Bone is upfront about the preliminary nature of this work. The objective at this early stage is to create a basic analytical model for p-11b fuel in a propulsion system using TECs to convert radiation into electricity, with the accompanying calculations to balance power and efficiency and find the lowest bremsstrahlung production for a given power setting.

The scope of needed future work on this is large. What exactly is the best ratio of hydrogen to boron in this scenario, for one thing, and how can the electric and magnetic field levels needed to light this kind of fusion be reduced? “It’s not an easy engineering problem,” Bone added. “It’s certainly not a near-term challenge to solve.”

True enough, but it’s clear that we should be pushing into every aspect of fusion as we learn more about confining these reactions in an in-space engine. Experimenting with alternate fusion fuels has to be part of the process, work that will doubtless continue even as we push forward on the far more tractable issues of deuterium/tritium.

A Fusion Drive Using Centrifugal Mirror Technologies

I want to drop back to fusion propulsion at this point, as it bears upon the question of a Solar System-wide infrastructure that we looked at last time. We know that even chemical propulsion is sufficient to get to Mars, but clearly, reducing travel times is critical if for no other reason than crew health. That likely puts the nuclear thermal concept into play, as we have experience in the development of the technology as far back as NERVA (Nuclear Engine for Rocket Vehicle Application), and this fission-based method shows clear advantages over chemical means in terms of travel times.

It’s equally clear, though, that for missions deep into the Solar System and beyond, the high specific impulse (ISP) enabled by a theoretical direct fusion drive sets the standard we’d like to meet. In his presentation at the Interstellar Research Group’s Montreal symposium, Jerry Carson discussed the ongoing work at the University of Maryland on creating fusion conditions using deuterium/deuterium (D/D) and deuterium/tritium (D/T) fuel with centrifugal mirror confinement. D/T fusion will likely drive our first fusion engines, but its higher neutron flux will spotlight the advantages of helium-3 when the latter becomes widely available, as shielding the crew on a fusion-powered spacecraft will be a critical factor.

Image: The Centrifugal Mirror Fusion Experiment at the University of Maryland at Baltimore (principal investigator Carlos Romero-Talamás, University of Maryland, Baltimore County). The plan is to achieve fusion conditions (D/D) by 2025. Credit: UMD.

Let’s dig into the centrifugal mirror (CM) concept. The beauty of plasma is that it is electrically conductive, and hence manageable by magnetic and electric fields. Hall thrusters use plasma (though not fusion!), as do concepts like Ad Astra’s VASIMR (Variable Specific Impulse Magnetoplasma Rocket). In a centrifugal mirror, the notion is to confine, compress and heat the plasma as it is spun within a fusion chamber, as opposed to the perhaps more familiar compression methods of inertial fusion, or the magnetic field structures within tokamaks. Carson argues that the CM makes for a more compact reactor and greatly reduces radiation and momentum loss.

The Maryland work implements this effect using magnetic ‘mirrors’ to create the rapid spin that imposes radial and axial forces on the plasma, confining it into a ‘well’ where fusion can be attained. The fuel is bouncing back and forth along the lines of force between the two magnets, a method first explored in the 1950s, when research indicated that mirrors of this kind are leaky and cannot maintain the plasma long enough to ignite fusion. Carson said that it is the addition of an electric field via a central electrode in the UMD design that spins the ‘doughnut’ around its axis, so that the plasma is held in place both axially as well as radially. The basic diagram is below.

Image: Centrifugal mirror confinement of a high energy plasma. Credit: UMD.

The ongoing work at Maryland grows out of an experimental effort in the 2000s that has led to the current Centrifugal Mirror Fusion Experiment (CMFX). The latter is designed with terrestrial power generation in mind, so we are talking about adapting a power-generating technology into a spacecraft drive. To do that, we fire up a centrifugal mirror fusion reactor in tandem with warm plasma (likely a reservoir of hydrogen, though other gasses are possible), so that high-energy fusion products escape the reactor downstream and deposit their energy in the plasma, causing it to expand as it passes through a magnetic nozzle to produce thrust. The reactor also uses energy leaving the upstream mirror to continue its own operations.

A direct fusion drive of this kind could, Carson said, make the round trip to Mars in 3 months, and reach Saturn in less than three years, a sharp contrast to nuclear electric methods. Even nuclear thermal methods would take over a year to make the Mars mission. Looking further out, the Uranus Orbiter and Probe (UOP), which is being considered as a flagship mission for the upcoming Decadal Survey, would make for a 12 year journey using chemical propulsion and a gravitational assist at Jupiter, while DFD-CM in these specs could do a considerably larger mission to more distant Neptune in as little as 3 years. A second generation Interstellar Probe (50 years to the heliopause in the NASA concept) could reach 1000 AU in 30-35 years using DFD-CM.

We’re not quite through with the University of Maryland, because Carson’s colleague Tom Bone has been analyzing a unique way to take advantage of otherwise problematic bremsstrahlung radiation, which complicates foperations with various kinds of fusion fuels. I’ll run through that work in the next post. Turning this challenging radiation into usable energy is conceivably a possibility, but requires fuel other than the deuterium/tritium combination examined for the DFD-CM drive. Bone’s choice is intriguing, to say the least, but more about this next time.

Interstellar Path? Helicity’s Bid for In-Space Fusion

Be aware that the Interstellar Research Group has made the videos shot at its Montreal symposium available. I find this a marvelous resource, and hope I never get jaded with the availability of such materials. I can remember hunting desperately for background on talks being given at astronomical conferences I could not attend, and this was just 20 years ago. Now the growing abundance of video makes it possible for those of us who couldn’t be in Montreal to virtually attend the sessions. Nice work by the IRG video team!

There is plentiful material here for the interstellar minded, and I will be drawing on this resource in days ahead. But let’s start with fusion, because it’s a word that all too easily evokes a particular reaction in those of us who have been writing about the field for some time. Fusion has always seemed to be the flower about to bloom, even as decades of research have passed and the target of practical power generation hovers in the future. In terms of propulsion, I’ve long felt that if we had so much trouble igniting fusion on Earth, how much longer would it be before we could translate our knowledge into the tight dimensions of a drive for an interstellar spacecraft?

But if you’ll take a look at Helicity Space’s presentation at the Montreal symposium, you’ll learn about a tightly focused company that approaches fusion from a different direction. Rather than starting with fusion reactors designed to produce power on Earth, Helicity’s entire focus is building a fusion drive for space. NASA’s Alan Stern, of New Horizons fame but also a veteran of decades of space exploration, is senior technical advisor here, and as he puts it, “our goal is not to power up New York City but to push spacecraft.” Stern appeared at Montreal to introduce a panel including Stephane Lintner, the company’s CEO and chief scientist Setthivoine You.

Let’s assume for a moment that at some point in the coming years – we can hope sooner rather than later – we do develop in-space propulsion using fusion. The easiest approach is to contrast working fusion with the methods available to us today. New Horizons was, shortly after launch, the fastest spacecraft ever built; it crossed the orbit of the Moon in a scant 9 hours, but it took a decade to reach Pluto/Charon. We can use gravity assists to sling a payload to the Kuiper Belt, perhaps a craft like JHU/APL’s Interstellar Probe concept, but here we’re dependent on planetary geometry, which isn’t always cooperative, and doesn’t provide the kind of boost that fusion could.

Image: Setthivoine You, co-founder and chief scientist at Helicity Space.

I’m a great advocate of sails, both solar sails and so-called ‘lightsails’ driven by beamed energy, but if reaching a distant target fast is the goal, we’re constrained by the need for a large laser installation and the likelihood, in the near future anyway, of pushing sails that are tiny. New Horizons would take two centuries to reach the Sun’s gravitational lens, a target of JPL’s SGL mission, which will use tight perihelion passes of multiple craft to get up to speed. We continue to talk about decades of travel time, even within the Solar System. As for interstellar, Breakthrough Starshot’s tiny craft, perhaps sent as a swarm to Proxima Centauri, might reach the target in 20 years if all goes well, but data return is a huge problem, and the mission can only be a flyby.

Helicity boldly sketches a future in which we might combine Interstellar Probe and the SGL mission into a single operation using a craft capable of taking a 5 ton payload to the gravitational lens (roughly 600 AU) in a little over a decade. Pluto becomes reachable in some of the Helicity concepts in about a year, with a craft carrying far heavier payloads than New Horizons and abundant power for operations and communications. All this in craft that could be carried into space by existing vehicles like Falcon Heavy or the SLS. The vision reminds us of The Expanse – imagine 4 months to Mars carrying 450 tons of payload as one step along the way.

All of this is a science fictional vision, but the work coming out of Helicity’s labs is solid. It involves so-called peristaltic magnetic compression to compress plasma and raise energy density. The method relies on specific stable behaviors within the plasma, a type of ‘Taylor State’ – these describe plasma within strong magnetic fields – discovered by You. These confine the plasma, while magnetic reconnection heating preheats it in the first place. Setthivoine You describes all this in the video, noting that what Helicity is developing is a method that we can consider magneto-inertial confinement, one producing a plasma jet stabilized through shear flows within the plasma itself. The firm’s computer simulations, in collaboration with Los Alamos National Laboratories, show four jets merging in a double-helix fashion inside the magnetic nozzle. You’s slides, available in the video, are instructive and dramatic.

What emerges from these results is scalable performance, allowing a path forward that the firm hopes to test as early as 2026 in a demonstration flight in orbit, with the possibility of a functional spacecraft at TRL 9 within roughly a decade. The scalable design points to the possibility of compact, reusable spacecraft with the unique feature of continuous thrusting. Think of the Helicity system, Stern has suggested, as a kind of afterburner put onto an electric propulsion system, and one that can scale into a working fusion drive. The concept scales by adding more fusion jets, allowing higher and higher efficiency, until the engine eventually becomes self-sustaining – the energy required to sustain the thrust is less than what the fusion begins to generate.

Does Helicity’s technology have the inside track, or will a contender like the UK’s Pulsar Fusion get to in-space fusion first? The latter is working on a study with Princeton Satellite Systems to examine plasma characteristics in a fusion rocket engine. Helicity’s founders are appropriately cautious about their work and stress how much needs to be learned, but they are continuing to develop the concept in their prototypes. With its laboratory in Pasadena, the company is privately funded and maintains active collaborations with Caltech and Los Alamos as well as UMBC and Swarthmore. Grants from the Department of Energy and other sources have likewise contributed. Have a look at the Montreal video to get a sense of where this exciting research stands, and also a glimpse of the kind of future that in-space fusion propulsion could produce.

Ion Propulsion for the Nearby Interstellar Medium

When scientists began seriously looking at beaming concepts for interstellar missions, sails were the primary focus. The obvious advantage was that a large sail need carry no propellant. Here I’m thinking about the early work on laser beaming by Robert Forward, and shortly thereafter George Marx. Forward’s first published work on laser sails came during his tenure at Hughes Aircraft Company, having begun as an internal memo within the firm, and later appearing in Missiles and Rockets. Theodore Maiman was working on lasers at Hughes Research Laboratories back then, and the concept of wedding laser beaming with a sail fired Forward’s imagination.

The rest is history, and we’ve looked at many of Forward’s sail concepts over the years on Centauri Dreams. But notice how beaming weaves its way through the scientific literature on interstellar flight, later being applied in situations that wed it with technologies other than sails.

Thus Al Jackson and Daniel Whitmire, who in 1977 considered laser beaming in terms of Robert Bussard’s famous interstellar ramjet concept. A key problem, lighting proton-proton fusion at low speeds during early acceleration, could be solved by beaming energy to the departing craft by laser.

Image: Physicist A. A. Jackson, originator of the laser-powered ramjet concept.

In other words, a laser beam originating in the Solar System powers up reaction mass until the ramjet reaches about 0.14 c. The Bussard craft then switches over to full interstellar mode as it climbs toward relativistic velocities. Jackson and Whitmire would go on in the following year to confront the problem that a ramscoop produced enough drag to nullify the concept. A second design emerged, using a space-based laser to power up a starship that used no ramscoop but carried its own reaction mass onboard.

The beauty of the laser-powered rocket is that it can accelerate into the laser beam as well as away from it, since the beam provides energy but is not used to impart momentum, as in Forward’s thinking about sails. In the paper, huge lasers are involved, up to 10 kilometers in diameter, with a diffraction limited range of 500 AU.

But note this: As far back as 1967, John Bloomer had proposed using an external energy source on a departing spacecraft, but focusing the beam not on a departing fusion rocket but one carrying an electrical propulsion system bound for Alpha Centauri. So we have been considering electric propulsion wed with lasers as far back as the Apollo era.

Now we can swing our focus back around to the paper by Angelo Genovese and Nadim Maraqten that was presented at the recent IAC meeting in Paris. Here we are looking not at full-scale missions to another star, but the necessary precursors that we’ll want to fly in the relatively near-term to explore the interstellar medium just outside the Solar System. The problem is getting there in a reasonable amount of time.

As we saw in the last post, electric propulsion has a rich history, but taking it into deep space involves concepts that are, in comparison with laser sail proposals, largely unexplored. A brief moment of appreciation, though, for the ever prescient Konstantin Tsiolkovsky, who sometimes seems to have pondered almost every notion we discuss here a century ago. Genovese and Maraqten found this quote from 1922:

“We may have a case when, in addition to the energy of ejected material, we also have an influx of energy from the outside. This influx may be supplied from Earth during motion of the craft in the form of radiant energy of some wavelength.”

Tsiolkovsky wouldn’t have known about lasers, of course, but the gist of the case is here. Angelo Genovese took the laser-powered electric propulsion concept to Chattanooga in 2016 when the Interstellar Research Group (then called the Tennessee Valley Interstellar Workshop) met there for a symposium. Out of this talk emerged EPIC, the Electric Propulsion Interstellar Clipper, shown in the image below, which is Figure 9 in the current paper. Here we have a monochromatic PV collector working with incoming laser photons to convert needed electric power for 50,000 s ion thrusters.

Image: An imaginative look at laser electric propulsion for a near-term mission, in this case a journey to the hypothesized Planet 9. Credit: Angelo Genovese/Nembo Buldrini.

Do notice that by ‘interstellar’ we are referring to a mission to the nearby interstellar medium rather than a mission to another star. Stepping stones are important.

Genovese and Maraqten also note John Brophy’s work at NASA’s Innovative Advanced Concepts office that delves into what Brophy considers “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” Here Brophy works with a 2-kilometer diameter laser array beaming power across the Solar System to a 110 meter diameter photovoltaic array to feed an ion propulsion system with an ISP of 40,000 seconds. That gets a payload to 550 AU in a scant 13 years, an interesting distance as this is where gravitational lensing gets exploitable. Can we go faster and farther?

Image: John Brophy’s work at NIAC examines laser electric propulsion as a means of moving far beyond the heliosphere, all the way out to where the Sun’s gravitational lens begins to produce useful scientific results. Credit: John Brophy.

An advanced mission to 1000 AU emerged in a study Genovese performed for the Initiative for Interstellar Studies back in 2014. Here the author had considered nuclear methods for powering the craft, with reactor specific mass of 5 kg/kWe. Genovese’s calculations showed that such a craft could reach this distance in 35 years, moving at 150 km/s. This saddles us, of course, with the nuclear reactor needed for power aboard the spacecraft. In the current paper, he and co-author Maraqten ramp up the concept:

The TAU mission could greatly profit from the LEP concept. Instead of a huge nuclear reactor with a mass of 12.5 tons (1-MWe class with a specific mass of 12.5 kg/kWe), we could have a large monochromatic PV collector with 50% efficiency and a specific mass of just 1 kg/kWe… This allows us to use a more advanced ion propulsion system based on 50,000s ion thrusters. The much higher specific impulse allows a substantial reduction in propellant mass from 40 tons to 10 tons, leading to a TAU initial mass of just 23 tons instead of 62 tons. The final burnout speed is 240 km/s (50 AU/yr), 1000 AU are reached in just 25 years (Genovese, 2016 [26]).

In fact, the authors rank electric propulsion possibilities this way:

  • Present EP performance involves ISP in the range of 7000 s, which can deliver a fairly near-term 200 AU mission with a cruise time in the range of 25 years.
  • Advanced EP concepts with ISP of 28,000 s draw on an onboard nuclear reactor, and produce a mission to 1000 AU with a trip time of 35 years. The authors consider this ‘mid-term development.’
  • In terms of long-term possibilities, very advanced EP concepts with ISP of 40,000 s can be powered by a 400 MW space laser array, giving us a 1000 AU mission with a trip time of 25 years.

So here we have a way to cluster technologies in the service of an interstellar precursor mission that operates well within the lifetime of the scientists and engineers who are working on the project. I mention this latter fact because it always comes up in discussions, although I don’t really see why. Many of the team currently working on Breakthrough Starshot, for example, would not see the launch of the first probes toward a target like Proxima Centauri even if the most optimistic scenarios for the project were realized. We don’t do these things for our ourselves. We do them for the future.

The Maraqten & Genovese paper is “Advanced Electric Propulsion Concepts for Fast Missions to the Outer Solar System and Beyond,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022 (available here). The laser rocket paper is Jackson and Whitmire, “Laser Powered Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 31 (1978), pp.335-337. The Bloomer paper is “The Alpha Centauri Probe,” in Proceedings of the 17th International Astronautical Congress (Propulsion and Re-entry), Gordon and Breach. Philadelphia (1967), pp. 225-232.


Ion Propulsion: The Stuhlinger Factor

How helpful can electric propulsion become as we plan missions into the local interstellar medium? We can think about this in terms of the Voyager probes, which remain our only operational craft beyond the heliosphere. Voyager 1 moved beyond the heliopause in 2012, which means 35 years between launch and heliosphere exit. But as Nadim Maraqten (Universität Stuttgart) noted in a presentation at the recent International Astronautical Congress, reaching truly unperturbed interstellar space involves getting to 200 AU. We’d like to move faster than Voyager, but how?

Working with Angelo Genovese (Initiative for Interstellar Studies), Maraqten offers up a useful analysis of electric propulsion, calling it one of the most promising existing propulsion technologies, along with various sail concepts. In fact, the two modes have been coupled in some recent studies, about which more as we proceed. The authors believe that the specific impulse of an EP spacecraft must exceed 5000 seconds to make interstellar precursor missions viable in a timeframe of 25-30 years, acknowledging that this ramps up the power needed to reach the desired delta-v.

Electric propulsion is a method of ionizing a propellant and subsequently accelerating it via electric or magnetic fields or a combination of the two. The promise of these technologies is great, for we can achieve higher exhaust velocities by far with electric methods than through any form of conventional chemical propulsion. We’ve seen that promise fulfilled in missions like DAWN, which in 2015 became the first spacecraft to orbit two destinations beyond Earth, having reached Ceres after previously exploring Vesta. We can use electric methods to reduce propellant mass or achieve, over time, higher velocities. [Addendum: Thanks to several readers who noticed that I had reversed the order of Vesta and Ceres in the DAWN mission above. I’ve fixed the mistake.]

Image: 6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory

Unlike chemical propulsion, electric concepts have a relatively recent history, having appeared in Robert Goddard’s famous notebooks as early as 1906. In fact, Goddard’s 1917 patent shows us the first example of an electrostatic ion accelerator useful for propulsion, even if he worked at a time when our understanding of ions was incomplete, so that he considered the problem as one of moving electrons instead. Konstantin Tsiolkovsky had also conceived the idea and wrote about it in 1911, this from the man who produced the Tsiolkovsky rocket equation in 1903 (although Robert Goddard would independently derive it in 1912, and so would Hermann Oberth about a decade later).

As Maraqten and Genovese point out, Hermann Oberth wound up devoting an entire chapter (and indeed, the final one) of his 1929 book Wege zur Raumschiffahrt (Ways to Spaceflight) to what he describes as an ‘electric spaceship.’ That caught the attention of Wernher von Braun, and via him Ernst Stuhlinger, who conceived of using these methods rather than chemical propulsion to make von Braun’s idea of an expedition to Mars a reality. It had been von Braun’s idea to use chemical propulsion with a nitric acid/hydrazine propellant, as depicted in a famous series on space exploration that ran in Collier’s from 1952-1954.

But Stuhlinger thought he could bring the mass of the spacecraft down by two-thirds while expelling ions and electrons to achieve far higher exhaust velocity. It was he who introduced the idea of nuclear-electric propulsion, by replacing a power system based on solar energy with a nuclear reactor, thus moving us from SEP (Solar Electric Propulsion) to NEP (Nuclear Electric Propulsion). Let me quote Maraqten and Genovese on this:

Stuhlinger immersed himself in electric propulsion theory, and in 1954 he presented a paper at the 5th International Astronautical Congress in Vienna entitled, “Possibilities of Electrical Space Ship Propulsion”, where he conceived the first Mars expedition using solar-electric propulsion [4]. The spacecraft design he proposed, which he nicknamed the “Sun Ship”, had a cluster of 2000 ion thrusters using caesium or rubidium as propellant. He calculated that the total mass of the “Sun Ship” would be just 280 tons instead of the 820 tons necessary for a chemical-propulsion spaceship for the same Mars mission. In 1955 he published: “Electrical Propulsion System for Space Ships with Nuclear Source” in the Journal of Astronautics, where he replaced the solar-electric power system with a nuclear reactor (Nuclear Electric Propulsion – NEP). In 1964 Stuhlinger published the first systematic analysis of electric propulsion systems: “Ion Propulsion for Space Flight” [3], while the physics of electric propulsion thrusters was first described comprehensively in a book by Robert Jahn in 1968 [5].

In 1957, the Walt Disney television program ‘Mars and Beyond’ (shown in the series ‘Tomorrowland’) featured the fleet of ten nuclear-electric powered spacecraft that Stuhlinger envisioned for the journey. As you can see in the image below, this is an unusual design, a vehicle that became known as an ‘umbrella ship.’ I’ve quoted him before on this, but let me run the passage again. It’s from Stuhlinger’s 1955 paper “Electrical Propulsion System for Space Ships with Nuclear Power Source”:

A propulsion system for space ships is described which produces thrust by expelling ions and electrons instead of combustion gases. Equations are derived from the optimum mass ratio, power, and driving voltage of a ship with given payload, travel time, and initial acceleration. A nuclear reactor provides the primary power for a turbo-electric generator; the electric power then accelerates the ions. Cesium is the best propellant available because of its high atomic mass and its low ionization energy. A space ship with 150 tons payload and an initial acceleration of 0.67 x 10-4 G, traveling to Mars and back in a total travel time of about 2 years, would have a takeoff mass of 730 tons.

Image: Ernst Stuhlinger’s Umbrella Ship, built around ion propulsion. Notice the size of the radiator, which disperses heat from the reactor at the end of the boom. The source for this concept was a Stuhlinger paper called “Electrical Propulsion System for Space Ships with Nuclear Power Source,” which ran in the Journal of the Astronautical Sciences 2, no. Pt. 1 in 1955, pp. 149-152. Credit: Winchell Chung.

While I’ve only talked about Stuhlinger’s work on electric propulsion here, his contribution to space sciences was extensive, ranging from a staging system crucial to Explorer 1 (this involved his pushing a button at the precise time required, hence his nickname as ‘the man with the golden finger’), to his work as director of the Marshall Space Flight Center Science Laboratory, which involved an active role in plans for lunar exploration.

For his contributions to electric propulsion, the Electric Rocket Propulsion Society renamed its award for outstanding achievement as the Stuhlinger Medal after his death. In terms of his visibility to the public, those interested in space advocacy will know about his letter to Sister Mary Jucunda, a nun based in Zambia, which laid out to a profound skeptic the rationale for pursuing missions to far destinations at a time of global crisis.

Image: In the above photo, taken at the Walt Disney Studios in California, Wernher von Braun (right) and Ernst Stuhlinger are shown discussing the technology behind nuclear-electric spaceships designed to undertake the mission to the planet Mars. As a part of the Disney ‘Tomorrowland’ series on the exploration of space, the nuclear-electric vehicles were shown in the program “Mars and Beyond,” which first aired in December 1957. Credit: NASA MSFC.

In the next post, I want to look at the deep space applications that Maraqten and Genovese considered in their IAC presentation.

For more details on Stuhlinger’s Mars ship, see Adam Crowl’s Stuhlinger Mars Ship Paper, and the followup I wrote in these pages back in 2015, Ernst Stuhlinger: Ion Propulsion to Mars. The Maraqten & Genovese paper is “Advanced Electric Propulsion Concepts for Fast Missions to the Outer Solar System and Beyond,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022 (available here). Ernst Stuhlinger’s paper on nuclear-electric propulsion is “Electrical Propulsion System for Space Ships with Nuclear Source,” appearing in the Journal of Astronautics Vol. 2, June 1955, p. 149, and available in manuscript form here. For more background on electric propulsion, see Choueiri, E., Y., “A Critical History of Electric Propulsion: The First 50 Years (1906-1956),” Journal of Propulsion and Power, vol. 20, pp. 193–203, 2004.


Getting There Quickly: The Nuclear Option

Adam Crowl has been appearing on Centauri Dreams for almost as long as the site has been in existence, a welcome addition given his polymathic interests and ability to cut to the heart of any issue. His long-term interest in interstellar propulsion has recently been piqued by the Jet Propulsion Laboratory’s work on a mission to the Sun’s gravitational lens region. JPL is homing in on multiple sailcraft with close solar passes to expedite the cruise time, leading Adam to run through the options to illustrate the issues involved in so dramatic a mission. Today he looks at the pros and cons of nuclear propulsion, asking whether it could be used to shorten the trip dramatically. Beamed sail and laser-powered ion drive possibilities are slated for future posts. With each of these, if we want to get out past 550 AU as quickly as possible, the devil is in the details. To keep up with Adam’s work, keep an eye on Crowlspace.

by Adam Crowl

The Solar Gravitational Lens amplifies signals from distant stars and galaxies immensely, thanks to the slight distortion of space-time caused by the Sun’s mass-energy. Basically the Sun becomes an immense spherical lens, amplifying incoming light by focussing it hundreds of Astronomical Units (AU) away. Depending on the light frequency, the Sun’s surrounding plasma in its Corona can cause interference, so the minimum distance varies. For optical frequencies it can be ~600 AU at a minimum and light is usefully focussed out to ~1,000 AU.

One AU is traveled in 1 Julian Year (365.25 days) at a speed of 4.74 km/s. Thus to travel 100 AU in 1 year needs a speed of 474 km/s, which is much faster than the 16.65 km/s that probes have been launched away from the Earth. If a Solar Sail propulsion system could be deployed close to the Sun and have a Lifting Factor (the ratio of Light-Pressure to Weight of Solar Sail vehicle) greater than 1, then such a mission could be launched easily. However, at present, we don’t have super-reflective gossamer light materials that could usefully lift a payload against solar gravity.

Carbon nanotube mesh has been studied in such a context, as has aerographite, but both are yet to be created in large enough areas to carry large payloads. The ratio of the push of sunlight, for a perfect reflector, to the gravity of the Sun means an areal mass density of 1.53 grams per square metre gives a Lifting Factor of 1. A Sail with such an LF will hover when pointing face on at the Sun. If a Solar Sail LF is less than 1, then it can be angled and used to speed up or slow down the Sail relative to its initial orbital vector, but the available trajectories are then slow spirals – not fast enough to reach the Gravity Lens in a useful time.

Image: A logarithmic look at where we’d like to go. Credit: NASA.

Absent super-light Solar Sails, what are the options? Modern day rockets can’t reach 474 km/s without some radical improvements. Multi-grid Ion Drives can achieve exhaust velocities of the right scale, but no power source yet available can supply the energy required. The reason why leads into the next couple of options so it’s worth exploring. For deep space missions the only working option for high-power is a nuclear fission reactor, since we’re yet to build a working nuclear fusion reactor.

When a rocket’s thrust is limited by the power supply’s mass, then there’s a minimum power & minimum travel time trajectory with a specific acceleration/deceleration profile – it accelerates 1/3 the time, then cruises at constant speed 1/3 the time, then brakes 1/3 the time. The minimum Specific Power (Power per kilogram) is:

P/M = (27/4)*S2*T-3

…where P/M is Power/Mass, S is displacement (distance traveled) and T is the total mission time to travel the displacement S. In units of AU and Years, the P/M becomes:

P/M = 4.8*S2*T-3 W/kg

However while the Average Speed is 474 km/s for a 6 year mission to 600 AU, the acceleration/deceleration must be accounted for. The Cruise Speed is thus 3/2 times higher, so the total Delta-Vee is 3 times the Average Speed. The optimal mass-ratio for the rocket is about 4.41, so the required Effective Exhaust Velocity is a bit over twice the Average Speed – in this case 958 km/s. As a result the energy efficiency is 0.323, meaning the required Specific Power for a rocket is:

P/M = 14.9*S2*T-3 W/kg

For a mission to 600 AU in 6 years a Specific Power of 24,850 W/kg is needed. But this is the ideal Jet-Power – the kinetic energy that actually goes into the forward thrust of the vehicle. Assuming the power source is 40% (40% drive and 10% payload) of the vehicle’s empty mass and the efficiency of the higher-powered multi-grid ion-drive is 80%, then the power source must produce 77,600 W/kg of power. Every power source produces waste heat. For a fission power supply, the waste heat can only be expelled by a radiator. Thermodynamic efficiency is defined as the difference in temperature between the heat-source (reactor) and the heat-sink (radiator), divided by the temperature of the heat source:

Thermal Efficiency = (Tsource – Tsink) / Tsource

For a reactor with a radiator in space, the mass of that radiator is (usually) minimised when the efficiency is 25 % – so to maximise the Power/Mass ratio the reactor has to be really HOT. The heat of the reactor is carried away into a heat exchanger and then travels through the radiator to dump the waste heat to space. To minimise mass and moving parts so called Heat-Pipes can be used, which are conductive channels of certain alloys.

Another option, which may prove highly effective given clever reactor designs, is to use high performance thermophotovoltaic (TPV) cells to convert high temperature thermal emissions directly into electrical power. High performance TPV’s have hit 40% efficiency at over 2,000 degrees C, which would also maximise the P/M ratio of the whole power system.

Pure Uranium-235, if perfectly fissioned (a Burn-Up Fraction of 1), releases 88 trillion joules (88 TJ) per kilogram. A jet-power of 24,850 W/kg sustained for 4 years is a total power output of 3.1 TJ/kg. Operating the Solar Lens Telescope payload won’t require such power levels, so we’ll assume it’s negligible fraction of the total output – a much lower power setting. So our fuel needs to be *at least* 3.6% Uranium-235. But there’s multipliers which increase the fraction required – not all the vehicle will be U-235.

First, the power-supply mass fraction and the ion-drive efficiency – a multiplier of 1/0.32. Therefore the fuel must be 11.1% U-235.

Second, there’s the thermodynamic efficiency. To minimise the radiator area (thus mass) required, it’s set at 25%. Therefore the U-235 is 45.6% of the power system mass. The Specific Power needed for the whole system is thus 310,625 W per kilogram.

The final limitation I haven’t mentioned until now – the thermophysical properties of Uranium itself. Typically Uranium is in the form of Uranium Dioxide, which is 88% uranium by mass. When heated every material goes up in temperature by absorbing (or producing internally) a certain amount of heat – the so called Heat Capacity. The total amount of heat stored in a given amount of material is called the Enthalpy, but what matters to extracting heat from a mass of fissioning Uranium is the difference in Enthalpy between a Higher and a Lower temperature.

Considering the whole of the reactor core and the radiator as a single unit, the Lower temperature will be the radiator temperature. The Higher will be the Core where it physically contacts the heat exchanger/radiator. Thanks to the Thermal efficiency relation we know that if the radiator is at 2,000 K, then the Core must be at least ~2,670 K. The Enthalpy difference is 339 kilojoules per kilogram of Uranium Oxide core. Extracting that heat difference every second maintains the temperature difference between the Source and the Sink to make Work (useful power) and that means a bare minimum of 91.6% of the specific mass of the whole power system must be very hot fissioning Uranium Dioxide core. Even if the Core is at melting point – about 3120 K – then the Enthalpy difference is 348 KJ/kg – 89.3% of the Power System is Core.

The trend is obvious. The power supply ends up being almost all fissioning Uranium, which is obviously absurd.

To conclude: A fission powered mission to 600 AU will take longer than 6 years. As the Power required is proportional to the inverse cube of the mission time, the total energy required is proportional to the inverse square of the mission time. So a mission time of 12 years means the fraction of U-235 burn-up comes down to a more achievable 22.9% of the power supply’s total mass. A reactor core is more than just fissioning metal oxide. Small reactors have been designed with fuel fractions of 10%, but this is without radiators. A 5% core mass puts the system in range of a 24 year mission time, but that’s approaching near term Solar Sail performance.