Every now and then I run into a paper that opens up an entirely new perspective on basic aspects of space exploration. When I say ‘new’ I mean new to me, as in the case of today’s paper, the relevant work has been ongoing ever since we began lofting payloads into space. But an aspect of our explorations that hadn’t occurred to me was the obvious question of how we coordinate time between Earth’s surface and craft as distant as Voyager, or moving as close to massive objects as Cassini. We are in the realm of ‘time transformations,’ and they’re critical to the operation of our probes.
Somehow considering all this in an interstellar sense was always much easier for me. After all, if we get to the point where we can push a payload up to relativistic speeds, the phenomenon of time dilation is well known and entertainingly depicted in science fiction all the way back to the 1930s. But I remember reading a paper from Roman Kezerashvili (New York City College of Technology) that analyzed the relativistic effects of a close solar pass upon a spacecraft, the so-called ‘sundiver’ maneuver. Kezerashvili and colleague Justin Vazquez-Poritz showed that without calculating the effects of General Relativity induced by the Sun’s mass at perihelion, the craft’s course could be seriously inaccurate, its destination even missed entirely. Let me quote this:
…we consider a number of general relativistic effects on the escape trajectories of solar sails. For missions as far as 2,550 AU, these effects can deflect a sail by as much as one million kilometers. We distinguish between the effects of spacetime curvature and special relativistic kinematic effects. We also find that frame dragging due to the slow rotation of the Sun can deflect a solar sail by more than one thousand kilometers.
Clearly, what seem like tiny effects get magnified as we examine their consequences on spacecraft moving under differing conditions of velocity and gravity. The measurement of time is a key aspect of this. And even the tiniest adjustments are critical if we are to build communication networks that operate accurately even in so close an environment as that between the Earth and the Moon. Thus the occasion for this musing, a paper from the Jet Propulsion Laboratory’s Slava Turyshev and colleagues that discusses how the effects of gravity and motion can be understood between the Earth and the network of assets we’re building around the Moon and on its surface. Exploration in this space will depend upon synchronizing our tools.
The Turyshev paper puts it this way:
As our lunar presence expands, the challenge of synchronizing an extensive network of assets on the moon and in cis-lunar space with Earth-based systems intensifies. To address this, one needs to establish a common system time for all lunar assets. This system would account for the relativistic effects that impact time measurement due to different gravitational and motion conditions, ensuring precise and efficient operations across cislunar space.
And in fact a recent memorandum from the White House Cislunar Technology Strategy Interagency Working Group was released on April 2 of this year noting the “policy to establish time standards at and around celestial bodies other than Earth to advance the National Cislunar S&T [Science and Technology] Strategy.” So here is a significant aspect of our growth into a cislunar culture that is growing organically out of our current explorations, and will be critical as we expand deeper into the system. One day we may go interstellar, but we won’t do it with a Solar System-wide infrastructure.
As an avid space buff, I should have been aware of this all along, especially since gravitational time dilation is easily demonstrated. A clock on the lunar surface, for example, runs a bit faster than a clock on Earth. Because time runs slower closer to a massive object, our GPS satellites have to deal with this effect all the time. Clearly, any spacecraft moving away from Earth experiences time in ways that vary according to its velocity and the gravitational fields it encounters during the course of its mission. These effects, no matter how minute, have to be plugged into operational software adjusting for the variable passage of time.
So moving from time and space coordinates in one inertial frame (the Earth’s surface), we need to reckon with their manifestation in another inertial frame, that aboard a spacecraft, to make clocks synchronize accurately and hence enable essential navigation, not to mention communications and scientific measurements. The necessary equations to handle this task are known in the trade as ‘relativistic time transformations,’ and it’s critical to have a reference system like the Solar System Barycentric coordinate frame (SSB) that is built around the center of mass of the Solar System itself. This allows accurate trajectory calculations for space navigation.
Image: The complexity of establishing reference systems for communications and data return is suggested by movements in the Solar System’s barycenter itself, shown here in a file depicting its own motion. Credit: Larry McNish / via Wikimedia Commons.
As you would guess, the SSB coordinate frame has been around for some time, becoming formalized as we began sending spacecraft to other planetary targets. It was a critical part of mission planning for the early Pioneer probes. Synchronization with resources on Earth occurs when data from a spacecraft are time-stamped using SSB time so that they can be converted into Earth-based time systems. Supervising all this is an international organization called the International Earth Rotation and Reference Systems Service (IERS), which maintains time and reference systems, with its central bureau hosted by the Paris Observatory.
‘Systems’ is in the plural not just because we have an Earth-based time and a Solar System Barycentric coordinate frame, but also because there are other time scales. We use Coordinated Universal Time as a global standard, and a familiar one. But there are others. There is, for example, an International Atomic Time (TAI – from the French ‘Temps Atomique International’), a standard that is based on averaging atomic clocks around the world. There is also a Terrestrial Time (TT), which adds to TAI a scale reflecting time on the surface of the Earth without the effect of Earth’s rotation.
But we can’t stop there. Universal Time (UT) adjusts for location, affected by the longitude and the polar motion of the Earth, both of which have relevance to celestial navigation and astronomical observations. Barycentric Dynamical Time (TDB, from ‘Temps Dynamique Barycentrique’) accounts for gravitational time dilation effects, while Barycentric Coordinate Time (TCB, from ‘Temps Coordonné Barycentrique’) is centered, as mentioned before, on the Solar System’s barycenter but excluding gravitational time dilation effects near Earth’s orbit. All of these transformations aim to account for relativistic and gravitational effects to keep observations consistent.
These time transformations (i.e., the equations necessary for accounting for these differing and crucial effects) have been a part of our space explorations for a long time, but they hover beneath the surface and don’t usually make it into the news. But consider the complications of a mission like New Horizons, moving into the outer Solar System and needing to account not only for the effects of that motion but the gravitational time dilation effects of an encounter not only with Pluto but the not insignificant mass of Charon, all of this coordinated in such a way that data returning to Earth can be precisely understood and referenced according to Earth’s clocks.
The Turyshev paper focuses on the transformations between Barycentric Dynamical Time and time on the surface of the Moon, and the needed expressions to synchronize Terrestrial Time with Lunar Time (TL). We’re going to be building a Solar System-wide infrastructure one of these days, an effort that is already underway with the gradual push into cislunar space that will demand these kinds of adjustments. These relativistic corrections will be needed to work in this environment with complete coordination between Earth’s surface, the surface of the Moon, and the Solar System’s barycenter.
The paper produces a new Luni-centric Coordinate Reference System (LCRS). We are talking about a lunar presence involving numerous landers and rovers in addition to orbiting craft. It is the common time reference that ensures accurate timing between all these vehicles and also allows autonomous systems to function while maintaining communication and data transmission. Moreover, the LCRS is needed for navigation:
An LCRS is vital for precise navigation on the Moon. Unlike Earth, the lunar surface presents unique challenges, including irregular terrain and the absence of a global magnetic field. A dedicated reference system allows for precise positioning and movement of landers and rovers, ensuring they can target and reach specific, safe landing sites. This is particularly important for resource utilization, such as locating and extracting water ice from the lunar poles, which requires high positional accuracy.
Precise location information through these time and position transformations will be, clearly, a necessary step wherever we go in the Solar System, and a vital part of shaping the activity that will build that system-wide infrastructure so necessary if we are to seriously consider future probes into the Oort Cloud and to other stars. Turyshev and team refer to all this as the establishment of a ‘geospatial context’ within which the placement of instruments can be optimized, but the work also becomes vital for everything from the creation of bases to necessary navigational tools. For the immediate future, we are firming up the steps that will give us a foothold on the Moon.
The paper is Turyshev et al., “Time transformation between the solar system barycenter and the surfaces of the Earth and Moon,” now available as a preprint. If you want to really dig into time transformations, the IERS Conventions document is available online. The Roman Kezerashvili paper cited above is R. Ya. Kezerashvili and J. F. Vazques-Poritz, “Escape Trajectories of Solar Sails and General Relativity,” Physics Letters B Volume 681, Issue 5 (16 November 2009), pp. 387-390 (abstract).
The propulsion technology the human characters conceive in the Netflix version of Liu Cixin’s novel The Three Body Problem clearly has roots in the ideas we’ve been kicking around lately. I should clarify that I’m talking about the American version of the novel, which Netflix titles ‘3 Body Problem,’ and not the Chinese 30-part series, which is also becoming available. In the last two posts, I’ve gone through various runway concepts, in which a spacecraft is driven forward by nuclear explosions along its route of flight. We’ve also looked at pellet options, where macroscopic pellets are fired to a departing starship to impart momentum and/or to serve as fusion fuel.
All this gets us around the problem of carrying propellant, and thus offers real benefits in terms of payload capabilities. Even so, it was startling to hear the name Stanislaw Ulam come up on a streaming TV series. Somebody was doing their homework, as Freeman Dyson liked to say. Ulam’s name will always be associated with nuclear pulse propulsion (along with the Monte Carlo method of computation and many other key developments in nuclear physics). It was in 1955 that he and Cornelius Everett performed the first full mathematical treatment of what would become Orion, but the concept goes back as far as Ulam’s initial Los Alamos calculations in 1947.
Image: Physicist and mathematician Stanislaw Ulam. Credit: Los Alamos National Laboratory.
Set off a nuclear charge behind a pusher plate and the craft attached to that plate moves forward. Set off enough devices and you begin to move at speeds unmatched by any other propulsion method, so the deep space concepts that Freeman Dyson, Ted Taylor and team discussed began to seem practicable, including human missions to distant targets like Enceladus. Dyson pushed the concept into the interstellar realm and envisioned an Orion variant reaching Alpha Centauri in just over a century. So detonating devices is a natural if you’re a writer looking for ways to take current technology to deep space in a hurry, as the characters in ‘3 Body Problem’ are.
Johndale Solem’s name didn’t pop up on ‘3 Body Problem,’ but his work is part of the lineage of the interstellar solution proposed there. Solem was familiar with Ted Cotter’s work at Los Alamos, which in the 1970s had explored ways of doing nuclear pulse propulsion without the pusher plate and huge shock absorbers that would be needed for the Orion design. Freeman Dyson explored the concept as well – both he and Cotter were thinking in terms of steel cables unreeled from a spacecraft as it spun on its axis. Dyson would liken the operation to “the arms of a giant squid,” as cables with flattened plates at each end would serve to absorb the momentum of the explosions set off behind the vehicle. Familiar with this work, Johndale Solem took the next step.
Image: Physicist Johndale Solem in 2014. Credit: Wikimedia Commons.
Solem worked at Los Alamos from 1969 to 2000, along the way authoring numerous scientific and technical papers. In the early 1990s, he discussed the design he called Medusa, noting in an internal report that his spacecraft would look something like a jellyfish as it moved through space. He had no interest in Orion’s pusher plate because examining the idea, he saw only problems. For one thing, you couldn’t build a pusher plate big enough to absorb anything more than a fraction of the momentum from the bombs being detonated behind the spacecraft. To protect the crew, the plate and shock absorbers had to be so massive as to degrade performance even more.
The solution: Replace the pusher plate with a sail deployed ahead of the vehicle. The nuclear detonations are now performed between the sail and the spacecraft, driving the vehicle forward. The sail would receive a much greater degree of momentum, and it would be equipped with tethers made so long and elastic that the acceleration would be smoothed out. I quoted Solem some years back on using a servo winch in the vehicle which would operate in combination with the tethers. Let’s look at that again, from the Los Alamos report:
When the explosive is detonated, a motorgenerator powered winch will pay out line to the spinnaker at a rate programmed to provide a constant acceleration of the space capsule. The motorgenerator will provide electrical power during this phase of the cycle, which will be conveniently stored. After the space capsule has reached the same speed as the spinnaker, the motorgenerator will draw in the line, again at a rate programmed to provide a constant acceleration of the space capsule. The acceleration during the draw-in phase will be less than during the pay-out phase, which will give a net electrical energy gain. The gain will provide power for ancillary equipment in the space capsule…
This is hard to visualize, so let’s look at it in two different ways. First, here is a diagram of the basic concept:
Image: Medusa in operation. Here we see the design 1) At the moment of bomb explosion; 2) As the explosion pulse reaches the parachute canopy; 3) Effect on the canopy, accelerating it away from the explosion, with the spacecraft playing out the main tether with its winch, braking as it extends, and accelerating the vehicle; 4) The tether being winched back in. Imagine all this in action and the jellyfish reference becomes clear. Credit: George William Herbert/Wikimedia.
Second, a video that Al Jackson pointed out to me, made by artist and CGI expert Nick Stevens, shows what Medusa would look like in flight. I recall Solem’s words when I watch this:
One can visualize the motion of this spacecraft by comparing it to a jellyfish. The repeated explosions will cause the canopy to pulsate, ripple, and throb. The tethers will be stretching and relaxing. The concept needed a name: its dynamics suggested MEDUSA.
Bear in mind as you watch, though, that Solem’s Los Alamos report speaks of a 500-meter canopy that would be spin-deployed along with 10,000 tethers. The biggest stress that suggested itself to readers when we’ve discussed Medusa in the past is in the tethers themselves, which is why Solem made them as long as he did. Even so, I became rather enthralled with Medusa early when I first encountered the idea, an interest reinforced by Greg Matloff’s statement (in Deep Space Probes): “Although much analysis remains to be carried out, the Medusa concept might allow great reduction in the mass of a nuclear-pulse starprobe.” With Dyson having given up on Orion, Medusa seemed a way to reinvigorate nuclear pulse propulsion, although to be sure, Dyson’s chief objection to Orion when I talked with him about it was the sheer impracticality of the concept, an issue which surely would apply to Medusa as well.
Like so much in the Netflix ‘3 Body Problem,’ the visuals of the bomb runway sequence are well crafted. In fact, I find Liu Cixin’s trilogy so stuffed with interesting ideas that my recent re-reading of The Three Body Problem and subsequent immersion in the following two novels have me wanting to explore his other work. I haven’t yet attempted the Chinese series, which is longer and presents the daunting prospect of dealing with a now familiar set of plot elements with wholly different actors. I’ll need to dip into it as Netflix ponders a second season for the American series.
Anyway, notice the interesting fact that what you have as a propulsion method on ‘3 Body Problem’ is essentially Medusa adapted to a nuclear bomb runway, with the sail-driven craft intercepting a series of nuclear weapons. As each explodes, the spacecraft is pushed to higher and higher velocities. I’m curious to know how the Chinese series handles this aspect of the story, and also curious about who introduced this propulsion concept, which I still haven’t located in the novels. I’m not aware of a fusion runway combined with a sail anywhere in the interstellar literature. Nice touch!
The Los Alamos report I refer to above is Solem’s “Some New Ideas for Nuclear Explosive Spacecraft Propulsion,” LA-12189-MS, October 1991 (available online). Solem also wrote up the Medusa concept in “Medusa: Nuclear Explosive Propulsion for Interplanetary Travel,” JBIS Vol. 46, No. 1 (1993), pp. 21-26. Two other JBIS papers also come into play for specific mission applications: “The Moon and the Medusa: Use of Lunar Assets in Nuclear-Pulse Propelled Space Travel,” JBIS Vol. 53 (2000), pp. 362-370 and “Deflection and Disruption of Asteroids on Collision Course with Earth,” JBIS Vol. 53 (2000), pp. 180-196. To my knowledge, Freeman Dyson’s ‘The Bolo and the Squid,’ a 1958 memo at Los Alamos treating these concepts, remains classified.
Fusion runways remind me of the propulsion methods using pellets that have been suggested over the years in the literature. Before the runway concept emerged, the idea of firing pellets at a departing spacecraft was developed by Clifford Singer. Aware of the limitations of chemical propulsion, Singer first studied charged particle beams but quickly realized that the spread of the beam as it travels bedevils the concept. A stream of macro-pellets, each several grams in size, would offer a better collimated ‘beam’ that would vaporize to create a hot plasma thrust when it reaches the spacecraft.
Even a macro-pellet stream does ‘bloom’ over time – i.e., it loses its tight coherency because of collisions with interstellar dust grains – but Singer was able to show through papers in The Journal of the British Interplanetary Society that particles over one gram in weight would be sufficiently massive to minimize this. In any case, collimation could also be ensured by electromagnetic fields sustained by facilities along the route that would measure the particles’ trajectory and adjust it.
Image: Clifford Singer, whose work on pellet propulsion in the late 1970s has led to interesting hybrid concepts involving on-board intelligence and autonomy. Credit: University of Illinois.
Well, it was a big concept. Not only did Singer figure out that it would take a series of these ‘facilities’ spaced 340 AU apart to keep the beam tight (he saw them as being deployed by the spacecraft itself as it accelerated), but it would also take an accelerator 105 kilometers long somewhere in the outer Solar System. That sounds crazy, but pushing concepts forward often means working out what the physics will allow and thus putting the problem into sharper definition. I’ve mentioned before in these pages that we have such a particle accelerator in the form of Jupiter’s magnetic field, which is fully 20,000 times stronger than Earth’s.
We don’t have to build Jupiter, and Mason Peck (Cornell University) has explored how we could use its magnetic field to accelerate thousands of ‘sprites’ – chip-sized spacecraft – to thousands of kilometers per second. Greg Matloff has always said how easy it is to overlook interstellar concepts that are ‘obvious’ once suggested, but it takes that first person to suggest them. Going from Singer’s pellets to Peck’s sprites is a natural progression. Sometimes nature steps in where engineering flinches.
The Singer concept is germane here because the question of fusion runways depends in part upon whether we can lead our departing starship along so precise a trajectory that it will intercept the fuel pellets placed along its route. Gerald Nordley would expand upon Singer’s ideas to produce a particle stream enlivened with artificial intelligence, allowing course correction and ‘awareness’ at the pellet level. Now we have a pellet that is in a sense both propellant and payload, highlighting the options that miniaturization and the growth of AI have provided the interstellar theorist.
Image: Pushing pellets to a starship, where the resulting plasma is mirrored as thrust. Nordley talks about nanotech-enabled pellets in the shape of snowflakes capable of carrying their own sensors and thrusters, tiny craft that can home in on the starship’s beacon. Problems with beam collimation thus vanish and there is no need for spacecraft maneuvering to stay under power. Credit: Gerald Nordley.
Jordin Kare’s contributions in this realm were striking. A physicist and aerospace engineer, Kare spent years at Lawrence Livermore National Laboratory working on early laser propulsion concepts and, in the 1980s, laser-launch to orbit, which caught the attention of scientists working in the Strategic Defense Initiative. He would go on to become a spacecraft design consultant whose work for the NASA Institute for Advanced Concepts (as it was then called) analyzed laser sail concepts and the best methods for launching such sails using various laser array designs.
Kare saw ‘smart pellets’ in a different light than previous researchers, thinking that the way to accelerate a sail was to miniaturize it and bring it up to a percentage of c close to the beamer. This notion reminds me of the Breakthrough Starshot sail concept, where the meter-class sails are blasted up to 20 percent of lightspeed within minutes by a vast laser array. But Kare would have nothing to do with meter-class sails. His notion was to make the sails tiny, craft them out of artificial diamond (he drew this idea from Robert Forward) and use them not as payload but as propulsion. His ‘SailBeam’, then, is a stream of sails that, like Singer’s pellets, would be vaporized for propulsion as they arrived at a departing interstellar probe.
Kare was a character, to put it mildly. Brilliant at what he studied, he was also a poet well known for his ‘filksongs,’ the science fiction fandom name for SF-inspired poetry, which he would perform at conventions. His sense of humor was as infectious as his optimism. Thus his DIHYAN, a space launch concept involving reusable rockets (if he could only see SpaceX’s boosters returning after launch!). DIHYAN, in typical Kare fashion, stood for “Do I Have Your Attention Now?” Kare’s role in the consideration of macro-scale matter sent for propulsion is secure in the interstellar literature.
And by the way, when I write about Kare, I’m always the recipient of email from well-meaning people who tell me that I’ve misspelled his name. But no, ‘Jordin’ is correct.
We need to talk about SailBeam at greater length one day soon. Kare saw it as “the most engineering-practical way to get up to a tenth of the speed of light.” It makes sense that a mind so charged with ideas should also come up with a fusion runway that drew on his SailBeam thinking. Following on to the work of Al Jackson, Daniel Whitmire and Greg Matloff, Kare saw that if you could place pellets of deuterium and tritium carefully enough, a vehicle initially moving at several hundreds of kilometers per second would begin encountering them with enough velocity to fire up its engines. He presented the idea at a Jet Propulsion Laboratory workshop in the late 1980s.
We’re talking about an unusual craft, and it’s one that will resonate not only with Johndale Solem’s Medusa, which we’ll examine in the next post, but also with the design shown in the Netflix version of Liu Cixin’s The Three Body Problem. This was not the sleek design familiar from cinema starships but a vehicle shaped more or less like a doughnut, although a cylindrical design was also possible. Each craft would have its own fusion pellet supply, dropping a pellet into the central ‘hole’ as one of the fusion runway pellets was about to be encountered. Kare worked out a runway that would produce fusion explosions at the rate of thirty per second.
Like Gerald Nordley, Kare worried about accuracy, because each of the runway pellets has to make a precise encounter with the pellet offered up by the starship. When I interviewed him many years back, he told me that he envisioned laser pulses guiding ‘smart’ pellets. Figure that you can extract 500 kilometers per second from a close solar pass to get the spacecraft moving outward at sufficient velocity (a very optimistic assumption, relying on materials technologies that are beyond our grasp at the moment, among other things), and you have the fusion runway ahead of you.
Initial velocity is problematic. Kare believed the vehicle would need to be moving at several hundreds of kilometers per second to attain sufficient velocity to begin firing up its main engines as it encountered the runway of fusion pellets. Geoff Landis would tell me he thought the figure was far too low to achieve deuterium/tritium ignition. But if it can be attained, Kare’s calculations produced velocities of 30,000 kilometers per second, fully one-tenth the speed of light. The fusion runway would extend about half a light day in length, and the track would run from near Earth to beyond Pluto’s orbit.
And there you have the Bussard Buzz Bomb, as Kare styled it. The reference is of course to the German V-1, which made a buzzing, staccato sound as it moved through English skies that those who heard it would come to dread, because when the sound stopped, you never knew where it would fall. You can’t hear anything in space, but if you could, Kare told me, his starship design would sound much like the V-1, hence the name.
In my next post, I’ll be talking about Johndale Solem’s Medusa design, which uses nuclear pulse propulsion in combination with a sail in startling ways. Medusa didn’t rely on a fusion runway, but the coupling of this technology with a runway is what started our discussion. The Netflix ‘3 Body Problem’ raised more than a few eyebrows. I’m not the only one surprised to see the wedding of nuclear pulse propulsion, sails and runways in a single design.
Clifford Singer’s key paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107-115. He followed this up with “Questions Concerning Pellet-Stream Propulsion,” JBIS 34 (1981), pp. 117-119. Gerald Nordley’s “Interstellar Probes Propelled by Self-steering Momentum Transfer Particles” (IAA-01-IAA.4.1.05, 52nd International Astronautical Congress, Toulouse, France, 1-5 Oct 2001) offers his take on self-guided pellets. Jordin Kare’s report on SailBeam concepts is “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” Final Report, NIAC Research Grant #07600-070, revised February 15, 2002 and available here. You might also enjoy my SailBeam: A Conversation with Jordin Kare.
Let’s talk about how to get a spacecraft moving without onboard propellant. As noted last week, this is apropos of the design shown in the Netflix streaming video take on Liu Cixin’s novels, which the network titles ‘3 Body Problem.’ There, a kind of ‘runway’ is conceived, one made up of nuclear weapons that go off in sequence to propel a sail and its payload. The plan is to attain 0.012 c and reach an oncoming fleet that is headed to Earth but will not arrive for another four centuries.
This is an intriguing notion, and one with echoes in the interstellar literature. Because Johndale Solem mixed sails and nuclear weapons in a design called ‘Medusa’ that he described in a Los Alamos report back in 1991, although its roots go back decades earlier, as I’ll discuss in an upcoming article. Mixing sails, nuclear weapons and a fusion runway is an unusual take, a hybrid concept that caught my eye immediately, as it did that of Al Jackson, who alluded to runways in a paper in the 1970s. I’ve just become aware of a Greg Matloff paper from 1979 on runways as well.
So let’s start with the runway concept and in subsequent posts, I will be looking at how Medusa evolved and consider whether the hybrid concept of ‘3 Body Problem’ is worth pursuing. Jackson’s paper, written with Daniel Whitmire, is one we’ve considered before in these pages. The concept is to power up a starship by a laser but use reaction mass gathered from the interstellar medium, collecting the latter with a ramscoop. Here the model draws on Robert Bussard’s ramjet notions, originally published in 1960 and more or less immortalized in Poul Anderson’s novel Tau Zero. Jackson and Whitmire’s version was one of several variants on Bussard’s original concept and offered a number of performance benefits.
Image: The interstellar ramjet, as envisioned by British artist Adrian Mann. Variants have appeared in the literature to get around the drag issue induced by the ramscoop design. A fusion runway seeds fuel along a track that the craft follows as it accelerates.
You’ll notice that this is also a hybrid concept, combining ramjet capabilities with laser beaming. Lasers had already been considered for beaming a terrestrial or Solar System-based laser at the departing craft, which could deploy a lightsail to draw momentum from the incoming photons. Jackson and Whitmire found the latter method inefficient. Their solution was to beam the laser at a ramjet that would use reaction mass obtained from a Bussard-style magnetic ram scoop. The ramjet uses the laser beam as a source of energy but, unlike the sail, not as a source of momentum.
Jackson and Whitmire were a potent team, and this is one of their best papers. These methods could be used to reach 0.14 c, allowing the vehicle to switch into full ramjet mode at that point. And because the laser is a source of energy rather than momentum, it can also be used as a means to decelerate on the return trip. For our purposes today, I turn to the last part of the paper, which outlines other starship concepts that grow out of the laser beaming analysis. Here is the relevant passage:
Another possibility would be to artificially make a fusion ramjet runway. Micron-size frozen deuterium pellets could be accelerated electrostatically or electromagnetically beginning several years prior to take-off at which time a fusion ramjet with a relatively modest scoop cross section (perhaps a physical structure) would begin acceleration.
So we have a spacecraft that collects its fuel along the way. As opposed to the ‘pure’ ramjet, which scoops up interstellar material and is dependent on the medium through which it moves, this fusion runway ramjet would know exactly the trajectory to take to collect the needed fuel pellets as it accelerates. Bear in mind the original Bussard ramjet problem of having to reach a certain percentage of lightspeed before being able to ignite its fusion engine. Problem solved.
In recent correspondence, Jackson pointed out that the idea harkens back to the German Vergeltungswaffe 3 (“Vengeance Weapon 3”), which was a gun originally designed to bombard London but only saw use against Allied targets in Luxembourg in 1945 (the bunkers at the Pas-de-Calais were destroyed by bombing raids). Multiple solid-fuel rocket boosters were ignited by the gases pushing the projectile as it moved in staged fashion through the barrel. The French Army had considered plans for such a staged cannon as far back as 1918, and the idea dates to the 19th Century.
Image: The prototype V-3 cannon at Laatzig, Germany (now Zalesie, Poland) in 1942. Credit: Bundesarchiv, Bild 146-1981-147-30A / CC-BY-SA 3.0.
Greg Matloff picked up on the Jackson and Whitmire paper in a 1979 paper in The Journal of the British Interplanetary Society which he was kind enough to pass along to me. The Jackson/Whitmire fusion runway would, he believed, improve ramjet performance and alleviate aerodynamic drag, which is a problem that sharply reduces a Bussard vehicle’s acceleration. He considered in the paper a fusion fuel released as fuel pellets moving in the direction of the destination star, with the ramjet moving up from behind to capture and fuse the pellets. In one scenario, tanker craft would be launched over a 50 year period to produce a runway 0.1 light years long.
Matloff as well as Jackson and Whitmire considered other variations on the interstellar ramjet idea, and I want to just mention these before moving on. From the Matloff paper:
As Whitmire and Jackson have mentioned, the performance of a ramjet might be of interest just above the photosphere of the Sun, n a high-energy, high-particle environment. More prosaically, a ramscoop could be utilized near the Earth to collect fusion fuel from the solar wind over a few decades. Then, if the fuel is utilized to power a ram-augmented interstellar rocket (RAIR), such an approach might be competitive in any discussion of the difficulties and merits of the various ramjet derivatives.
A Sun-skimming ramjet is one I had never seen discussed until I read Jackson and Whitmire. It would make for a lively hard SF tale, that’s for sure. Given the problems with ramjet drag that have been wrestled with in the subsequent literature, it’s worth considering Matloff’s idea of solar wind fuel collection at much lower speeds in the inner system. In any case, the fusion runway notion offers one way to collect a known supply of fuel over the length of a runway that would launch an interstellar craft.
When I wrote my Centauri Dreams book early in the century, I was unaware of both the papers we’ve looked at today, and focused on the only runway concept that was then known to me, the so-called ‘Bussard Buzz Bomb’ of the free-thinking Jordin Kare. Kare is, alas, no longer with us, but I enjoyed a long conversation with him on his runway concept, and I want to cover that in the next post before moving on to Johndale Solem’s Medusa.
The Jackson/Whitmire paper on laser-powered ramjets is “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30 (1977), 223-226. Al Jackson: A Laser Ramjet Reminiscence presents Al’s thoughts on this paper as written for this site. Greg Matloff’s paper on fusion runways is “The Interstellar Ramjet Acceleration Runway,” JBIS Vol. 32 (1979), 219-220. The ToughSF site offers a detailed explanation of runway concepts in Fusion Highways in Space.
You never know when a new interstellar propulsion concept is going to pop up. Some of us have been kicking around fusion runway ideas, motivated by Netflix’s streaming presentation of the Liu Cixin novel The Three Body Problem. There Earth is faced with invasion from an extraterrestrial civilization, but with centuries to solve the problem because it will take that long for the fleet to arrive. Faced with the need to get as much information as possible about the invaders, scientists desperately search for a way to get human technology up to 1.2 percent of lightspeed to intercept the fleet.
Image: 20 different examples of periodic solutions to the three body problem. Credit: Perosello/Wikimedia Commons. CC BY-SA 4.0.
So how would you do that with technology not much more advanced than today’s? The Netflix show’s solution is ingenious, though confusing for those who assume that the Netflix ‘3 Body Problem’ is based solely on the first of the Cixin novels. Actually it edges into the rest of the trilogy, which includes 2008’s The Dark Forest and 2010’s Death’s End. The whole sequence is known as Remembrance of Earth’s Past, and I had to dig into not just The Three Body Problem but The Dark Forest to find much discussion of any kind of propulsion.
Now we’re in a dark wood indeed. For in The Dark Forest (the title is an allusion to the Fermi paradox, usually linked with concerns over METI), the idea of a precursor scouting of the alien invasion fleet does not appear, nor does it appear in the first novel. What we do get is a lot of confusing discussion, such as this:
“If controlled nuclear fusion is achieved, spacecraft research will begin immediately. Doctor, you know about the two current research forks: media-propelled spacecraft and non-media radiation-drive spacecraft. Two opposing factions have formed around these two directions of research: the aerospace faction advocates research into media-propelled spacecraft, while the space force is pushing radiation-drive spacecraft… The fusion people and I are in favor of the radiation drive. For my part, I feel that it’s the only plan that enables interstellar cosmic voyages.”
The book’s many references to a ‘radiation drive’ seem to be referring to antimatter. What Cixin calls ‘media-propelled spacecraft’ is opaque to me, and I’d welcome reader comments on what it represents. Then there is a ‘curvature drive’ that appears in the final volume of the trilogy, but let’s leave that out of the discussion today. Perhaps it’s a kind of Alcubierre concept, but in any case I want to focus on fusion runways and sails for now, because the Netflix eight-part video presents the idea of sending a relatively small payload toward the invasion fleet using a form of nuclear pulse propulsion.
Here the presentation is accurate if rudimentary but the idea is fascinating. Because I don’t find this in the novels, I am wondering about where, along the route to production, the show acquired a technology made famous originally by Project Orion, with its sequence of nuclear explosions visualized as occurring behind a spacecraft’s huge shock absorbers. Wonderfully, the idea opens up to multiple interstellar propulsion ideas in the literature, including Johndale Solem’s Medusa concept and various fusion runway notions that emerged decades ago, one by my friend Al Jackson and Daniel Whitmire, another by Jordin Kare, who christened his concept the ‘Bussard buzz bomb.’
So we’ve got a lot to talk about. And out of the blue Adam Crowl wrote to remind me of something Martyn Fogg pointed out in 2017, when I wrote about Medusa then. Here’s Martyn’s comment:
Suppose these Solem sails were to have a small hole in their centre, they could be steered accurately, and that nuclear propulsion charges could be lined up perfectly in space, perhaps by laser guidance. Then one might imagine an ‘Interstellar Solem Sail Runway’ which would impart a jolt of pulse propulsion each time its sail overtook each charge, thereby accelerating the outgoing ship as a whole up to interstellar cruse velocity. The vessel would only need fuel to decelerate at the target system: a considerable reduction in the mass it would need to carry.
Talk about prescient! Because this is what shows up on the Netflix series.
I’m slammed for time this morning and have way too many ideas floating around as well as tabs open in various screens, so I’m going to break here and pick up this discussion next week, when I want to get into the details of fusion runways, and then I want to relate all this to Solem’s Medusa work by way of illustrating not only how ingenious all these ideas are, but how striking the design in the screen version of the Three Body Problem turns out to be. The designs we’ll be discussing are some of the most innovative that have come out of the interstellar effort thus far.
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.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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