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.
“…fusion runway ramjet would know exactly the trajectory to take to collect the needed fuel pellets as it accelerates.”
That is extremely unlikely! Sufficiently precise trajectory prediction and tracking for both the fuel pellet and the spacecraft over the necessary distances is well beyond the conceivably feasible. Course correction to pick up the pellet would minimally require a beacon on the pellet that can be received from a large distance. That would increase pellet mass by at least 2 orders of magnitude.
Was the engineering to achieve this outcome dealt with in the papers? I haven’t read them.
Neither paper gets into the engineering for creating ‘smart pellets’ with incorporated beacon, though I think both assume this would be necessary.
Yeah that is advanced technology. No one has ever looked at the runway placement technical problem in detail that I know of.
Then fusion propulsion could be 50 or even 100 years off, so advanced technology will exist.
If the micro sails are very reflective they can be seen and perhaps powered up by the passing spacecraft to move it.
This is another reason why I like the Orion nuclear propulsion concept. The fuel is right there; we do not have to chase after it and hope for the best. The “fuel” is also something that we have had in reality since 1945.
https://www.projectrho.com/public_html/rocket/realdesigns2.php#id–Project_Orion
Well at least since 1952. Not sure if anyone proposed fission bombs, could have. Its true , bona fided fusion propulsion has existed since the 1950s. (Fusion propulsion of the ‘ordinary kind’ depends making fusion with fuel in a confined environment work. Seems ICF has been achieved and maybe Tokamak-like soon. But seems to me it’s a long long long ways to putting those systems on a spaceship.)
The ‘seeded runway’ will still depend on the ICF? (-like) process,or something like it working. I mean even the take-the-pellets-with-you ICF fusion method is , to me, a distant horizon, quite distant.
One idea occurs. Seed the ‘runway’ with small robot ships , placed there with sails?, with GN&C systems to keep all the trajectory details taken care of, full of seeds which a placed with the right timing , or something like that.
@LJK
Orion still suffers from the limitations of the rocket equation. The fusion-powered Enzmann starships have vast hydrogen tanks because of this.
I would go with the Enzmann ships myself more, except for the whole fusion thing. Actually, I will go with any realistic plan for interstellar travel.
https://www.icarusinterstellar.org/wp-content/uploads/2012/05/BISEnzmann-Talk-2011kflong.pdf
https://projectrho.com/public_html/rocket/slowerlight2.php#id–Go_Slow–Sublight_Starships–Enzmann_Starship
https://www.physicsforums.com/threads/revisiting-the-enzmann-starship-design-a-sixty-year-perspective.1060324/
I also have an issue with the propulsion method of the Breakthrough Starship concept: In addition to all the technical problems that need to be overcome with a giant powerful laser, I can also see major funding issues and the possibility of it being turned into a weapon against humanity. Tell me I am wrong.
@LJK
And the nuclear bomb[lets] on Orion are not already potential weapons with known capabilities? Imagine the hijacking of those bombs from construction to the Orion ship itself. Security is needed at every stage, let alone any accidents resulting in fissile material being released. A fusion starship is far less of a problem. Yes, the phased laser arrays are a potential weapon, but it is a bulky device and an easy target. If placed anywhere off-planet to allow it to be targeted at points on Earth, it could be hit by Earth-based laser arrays, missiles, military weapon satellites parked close by, etc, etc. It would be a great James Bond villain’s tool (c.f. Die Another Day (2002)) but while impressive, easy to destroy or rendered inoperative.
James Bond villains only steal nuclear bombs as a last resort. It’s such a cliche.
Now giant freaking laser beams, on the other hand…
Could fusion pellets have a simple corner-cube reflector design? The idea would be that the ship would send forth laser pulses that the corner cubes would reflect back to it, thus telling the ship where they are. No beacon needed.
“Space is big. Really big…”
You are attempting to illuminate a tiny target (that corner reflector) from far far away, perhaps no less than 1 million km, with a laser that will have spread to several km diameter. The return path loss is huge. Worse, there is very likely to be a very bright black body light source directly behind the target, namely the star at the destination stellar system, that will push the SNR well into the negative. I see little hope for your idea to work.
They put a reflector on the moon and it was able to be seen, now a phase array laser would do much better at finding them. Perhaps then use a particle beam to guide it back into range. Also if it is fast enough it can use the ISM as a deflector medium to knock it back on course.
Wasn’t there a post some years ago about a fusion ramscoop targeted towards the sun to scoop up the denser solar wind? I don’t recall if there was some other starter power, but I can imagine the same phased laser arrays for beamed sails and asteroid deflection being used to power the Ramscoop initially.
I don’t recall that one, but it’s possible I was writing about Al Jackson’s idea back then and just forgot about it. I’ll check the archives.
Idk the effective range of the lasers, but I thought this was relatively short unless the ship can tolerate the lower energy that can be extracted from the diverging beam.
Extracting the fusion energy from H/D/T pellets is nice, but currently difficult even under the best conditions. What about using the laser to power electromagnetic accelerators to use the “pellets” as reaction mass? Greason’s Q-Drive, if it works, could similarly be powered by laser.
One issue to overcome for propulsion that scoops up slower-moving particles is drag. The drag is useful for magsails and electric sails moving more slowly than the particle source such as the solar wind, but detrimental when moving faster than the medium. Particle beams accelerated to very high velocities can be used for such sails, so why not use them instead of the solar and ISM medium?
I can imagine collimating and accelerating the solar wind to create a particle beam[s] that create the power for sails and even fusion drives, where any collection drag is useful. A CME would make for a very useful source for such beams.
Could a gravity lens generate the power to push particles emitted from the star into high-velocity, reasonably higher-density beams useful for interstellar travel networks? Close double stars would generate a thin torus of light (from a white dwarf) and particles (from the main sequence star) as the stars orbit each other, sweeping across space. The dense beam would allow ships or probes to travel between stars where the beams align. If the orbital period of the WD was 1000 years, the beam would sweep past any target with this periodicity. A “lighthouse beacon” and particle energy source to aid sails and ramscoop-propelled vehicles.
The post on pinch fusion which used the model of the particle beams emitted by black holes suggests a beam source for interstellar and even intergalactic travel using the beam to directly push a vehicle, or provide the medium for a fusion drive from both behind and in front of the vehicle.
We have often commented on the small size of a WD. This reduces the needed mass to focus its energy using mirrors and other devices. The beam could propel a sail, like the Bob Forward approach, or push the dust from the destroyed planets out as a particle beam that would create a runway that could be used as the fuel for a fusion rocket.
If our ISM probes detected denser particle beams within the ISM, that might be a possible technosignature.
Hi Paul
Good discussion. One advantage of pellets that are launched via fields is they can be caught via fields and decelerated, with the recovered energy added back into the reaction-mass stream. This reduces drag considerably.
I’ve just started the 2nd book of the 3 Body Problem which had been on my reading list for a while – but, jumped my list when I saw the Netflix version came out (but, I won’t watch until I read the books). I love the timely real-world analysis of the concept I’m in the process of reading about.
Great to see you, Frank. I had read only the first novel until just recently and I’ve found all three books stuffed with intriguing ideas. I’m still wondering where the propulsion concept of the Netflix series came from, because I can’t find this specific mode mentioned in the books. Let me know if you see the hybrid sail/runway concept anywhere. I’ll discuss a real-world study of something akin to this next week here on the site.
Great post and wonderful responses! One thought I had is that the Netflix version of “The Three Body Problem” complicates the technology required to intercept an in-coming fleet from Centauri moving at about 0.01c. Rather than using advanced tech such as the Ramjet Runway, a much more effective technique is to shoot low-velocity B-B pellets towards the enemy fleet. A B-B striking a spacecraft at a relative velocity of 0.01c packs an enormous wallop!
@Greg
I can’t recall, but wasn’t the use of clouds of matter one of the Earth’s defenses against the Trisolarians, albeit to act as a signal to other races, not to destroy the ships.
The BB attack could be stopped by the use of huge ice shields as used by the starship Magellan, in Clarke’s “The Songs of Distant Earth”.
Have you calculated the velocity at which a DT pellet of the largest radius compatible with self-heating from T decay will compress sufficiently on impact with a material of density ~7 to initiate fusion . A yield curve would help in analying the matter further
During the Star Wars days in the 80s I directed a laboratory program called GEDI (JEDI was taken) that used ten stages of sequentially fired plasma jets to accelerate a projectile to several km/s. The limiting factor is that the projectile eventually moves as fast as the plasma jets.
The main problem with the interstellar ramjet idea is what Paul Gilster has commented: one would need to get enough fuel for it to work. There is not enough fuel in interstellar space due to the low density of hydrogen. Flying through Nebula like stellar nurseries or planetary nebula might work. To be efficient, the ramjet would need to move at relativistic speeds. It could use a strong magnetic field to guide the relativistic interstellar hydrogen atoms into an efficient fusion engine. I get the idea that to get enough fuel one would have to move through it quickly. I don’t like the idea of needing to refuel. Technically the ramjet still has a specific impulse and therefore is not a propellant free drive and therefore limited to sub light speed and limited to how fast the particles are expelled from the engine and engine efficiency.
We still have to build the fusion propulsion engine. An efficient one would be able to fuse the hydrogen into helium and then the helium back into hydrogen. A efficient self sustaining reaction would allow for electric power to be drawn from it through DEC. Not any fuel would be lost. The electric current could be used to make a strong, magnetic field for a space warp or propellant less drive.
Geoffrey:
This is a perpetual motion machine. So is this intended to be *sarcasm*?
Oof! This could be a lot more difficult than imagined, but the incorporation of lasers is interesting. Remember, magsails can be a useful braking mechanism. If it’s used to exploit interstellar medium for fuel (which already provides a braking mechanism), it might be able to do both.
https://www.researchgate.net/publication/359203763_The_Interstellar_Ramjet_Engineering_Nightmare
Interstellar magnetic fields can apparently be in the microgauss to milligauss range. (open access: https://www.annualreviews.org/content/journals/10.1146/annurev-astro-091916-055221 ) Is it conceivable with foreseeable technology to build e.g. large well-oriented superconducting rings capable of using this for meaningful propulsion?
You could potentially use something like the combined particle/EM beam discussed some time ago, to be the fusion runway. You know, where the particle density acts like an optical fiber keeping the light focused, the light centers the particles due to dipole interactions, and the difference in speed between the two stabilizes the beam against instabilities by shear? You’d just have to have the craft approach the beamer from behind, and yank it out of the way at the last minute.
In principle matter beams can have much higher pointing accuracy and focus than EM beams, due to the ultra-short ‘wavelength’ of the particles. So containing the runway to high precision isn’t impossible. Your main concern would actually be pointing stability, as the beam ‘waving around’ even a little would result in impossible course corrections to follow it at distance.
But the matter/EM beam concept stabilizes pointing, too.
Making progress in the controlled fusion arena, even in small steps:
https://www.space.com/nuclear-fusion-reactor-south-korea-runs-48-seconds
Nuclear fusion reactor in South Korea runs at 100 million degrees C for a record-breaking 48 seconds.
By Ben Turner published 2 days ago
The experimental fusion reactor sustained temperatures of 180 million degrees Fahrenheit for a record-breaking 48 seconds.
For information, these guns were in the bunker of the city of Mimoyecque (pronounced “mi-mo-yek”) in northern France. There are huge concrete sites, very impressive because of their dimensions, still in place, which have become museums classified as historical monuments. Not far there is the bunker of Eperlecque (“a-per-lek) which was reserved for the V2. There is still one of these weapons on our sites here but I did not know the pump cannon or to my knowledge there is no piece left. these museums make the link with space exploration…
https://en.wikipedia.org/wiki/Fortress_of_Mimoyecques
https://en.wikipedia.org/wiki/La_Coupole (Eperlecque)
https://lacoupole-france.com/en/
Fred
Thanks for this! Fascinating. I must visit when next in France.
Indeed, it is necessary to make the visit because the mass of concrete – technique that the Germans mastered perfectly – is really impressive in terms of architecture. We feel very small in front of these fortresses and a V2. In addition there is a special atmosphere reminiscent of prehistoric caves, it is quite strange. For the record, the assembly of the V1 were made just near my home in careers. There are a few left on their launch pads that have been restored and preserved. For the history of the pump gun, the Germans would have been inspired by French plans and huge rail guns (“big Berta”) that bombed Paris during the WWI but the French would have been inspired by an idea of two Americans at the end of the 19th century. Finally it’s quite funny to see that Mélies had already imagined the trip to the moon…in a cannon:)
https://www.battlefieldsww2.com/st-leu-d-esserent.html
https://famousfrenchfilms.wordpress.com/2010/01/20/le-voyage-dans-la-lune-de-georges-melies-1902/
Fred
Scientists build breakthrough nuclear fusion device with household magnets
Story by Rizwan Choudhury • 2w
A team of scientists has made a major breakthrough in fusion energy technology. They’ve built the first-of-its-kind fusion experiment using permanent magnets, a surprisingly simple technique that could potentially slash the cost of future fusion power plants.
The team, based at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), pioneered a new design for a type of fusion machine called a stellarator.
Full article here:
https://www.msn.com/en-us/news/technology/scientists-build-breakthrough-nuclear-fusion-device-with-household-magnets/
To quote:
But what’s a Stellarator in the first place?
Stellarators use intricate magnetic fields to confine plasma, the superheated state of matter needed to fuel the fusion reactions that power the sun and stars. If harnessed on Earth, fusion could offer an abundant source of clean energy.
Stellarators and tokamaks are both devices designed to contain the incredibly hot plasma needed for nuclear fusion.
The key difference lies in how they create the magnetic field that keeps the plasma in place. Tokamaks use a strong current flowing through the plasma itself, along with external coils, while stellarators rely solely on complex, twisted coils to shape the field. This makes stellarators inherently more stable than tokamaks, and suitable for continuous operation.
However, tokamaks are currently better at keeping the plasma hot. Scientists hope to use stellarators as power plants in the future, potentially replicating the fusion process that occurs within stars like our Sun.