Interstellar Propulsion in ‘3 Body Problem’

by Paul Gilster | Apr 5, 2024 | Advanced Rocketry | 58 comments

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.

Medusa: Deep Space via Nuclear Pulse

by Paul Gilster | Apr 17, 2024 | Advanced Rocketry | 20 comments

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 Pellets and the ‘Bussard Buzz Bomb’

by Paul Gilster | Apr 12, 2024 | Advanced Rocketry | 20 comments

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.