I have a layman's (re: not very good) understanding of nuclear physics, and it's pretty cool to me, but I'm quite bad at math. I'm working on a sci-fi book (classic, I know) and just finished a draft that's been sent in to the editing office. I'm trying to keep it as realistic as possible, aside from semi-plausible to glaring violations of the laws of physics such as FTL arrays, which there's really now way around. I was hoping someone could take a look at a design I came up with for a fusion engine, and give me pointers on how it can serve the story. What I need is the following: 1: ability to achieve several hours at up to 15 Gs of acceleration (2 to 2.5 being cruise, and 15 being flank speed on the most complex vessels) 2: A pulse-style travel, where the ship accelerates at incredible speeds for 15 mins to a few hours, and then stops burning for about the same amount of time. 3: for the method to make it roughly possible for a ship to make it from about earth to saturn in a month or slightly shorter or less time. If you can help me out, I'll definitely credit you.

Here's what I have so far:

The most important part of a voidship, barring the hull and RCS thrusters is a fusion reactor. Fusion reactors, being a technology having been in use for most of recorded history (approx 400 years, though for orphaned human civilizations, fission was used for early space exploration of their foster systems.)

It is such a ubiquitous technology, in fact, that there is a sometimes-verging-on-the-unseemly distinction between peoples that have discovered and utilized fusion (The ‘civilized’ peoples) and those who did not, (the ‘primitives’ who were limited to fission, rocket or other less powerful reactions.) Fusion powers nearly everything, from hydroponics, orbital stations, centrifuges, cities, warships and terraforming initiatives. While it is relatively easy to keep a land or void-installation-bound fusion reactor running without worrying about overheating due to the lack of size constraints and thrust-to-mass ratios, ships are closed, mobile environments and much smaller by definition, and as such, have a lot more moving parts.

Every ship has at least one fusion reactor. These reactors smash the atomic nuclei of light elements together to form heavier elements, and in doing so, create a tremendous amount of energy. The primary fuel for fusion reactors is referred to as Fuel Rods, shielded cylinders approximately the size of two human fingers, containing a deuterium payload, and a shielded tritium-containing ‘cap’ (Helium-3 for more modern models). These are loaded into a reactor’s Core Feed, in a vessel’s powerplant module. The core feed is a simple robotic assemblage that feeds the fuel rods into the magnetically-shielded fusion reactor chamber on an as-needed basis.

The initial reaction is a standard fissile reaction. This differs between low-yield (for ships that can travel within a planetary atmosphere, computers are hardwired to prevent going above low yield to prevent fallout from any potential catastrophic meltdown) and high yield, used outside of an atmosphere. Most ships of civilian models are in fact hardwired to be fully incapable of starting a fusion reaction within atmosphere, instead using electromagnetic trans-orbital launch rails, space elevators, EM trans-orbital launch tubes (being fired out of a rail cannon, more or less) or standard hydrogen rocket boosters to achieve enough Delta-V to break orbit. The only exceptions to this are to organizations with special dispensations, (Contracted explorers, official paramilitaries, governmental entities, corporations with dispensation, etc) This does mean that breaking orbit takes more time for standard civilian models. The ‘high yield’ fissile reaction that begins the fusion reaction is equivalent to a low-intensity thermonuclear bomb detonated (either within the reaction chamber or approx 300 meters behind the ship's sheilded aft, with magnetic bottle fields projected behind the vessel to direct the force), with additional amplification and shielding measures on a by-design basis to increase efficiency. This creates a chain reaction within the reactor region, beginning the fusion reaction once the fuel rod is activated. As the fuel rod's contents are emptied into the chamber, the tritium cap is used as the chamber’s initial source of tritium (As it is bred and reused within the chamber once initialized so long as the reaction is maintained.) The other tritium caps on the remaining fusion rods will be removed and sequestered in a separate stockpile magazine by the core feed before they are fed, if the reaction is still ongoing when a new rod is required. At this point, an electric current is run through the core and the deuterium-tritium is ejected into the core’s plasma bed, releasing high-energy neutrons and helium atoms that then collide with each other at variable efficiency. These particles can pass through the Core Shielding magnetic field and impact the lithium walls making up the interior blanket in the reaction chamber, where they are absorbed and converted into energy. The intensity of the reaction is controlled from most ship’s bridge or cockpit, via the main reactor, which will usually be running in the background when a ship is not ‘at burn’ (at any force-speed above 0.0002 newtons), though at an incredibly low intensity, as it powers the ship’s subfusion ion drives, life support, hydrogen-oxygen electrolysis (air scrubbers), active sensors, and all other onboard systems. When thrust is needed, however, the fusion drive’s dumbfire computer (distinct from the quantum-computer bio-cybernetic cogitator brains in use in more complex robotics) increases the strength of the reaction in question, until it is powered down, the superconducting magnets (which must be kept at a temperature near or below absolute zero at all times to provide shielding) are too overheated by the fusion reaction, the cooling or shielding systems are stressed past the safety point, the fuel rods in the feed are exhausted, the reaction chamber is over-stressed, there is otherwise any danger of a meltdown or if the crew is suffering adverse effects from acceleration to the point that their bio-monitors alert the ship’s main cogitator brain (this last can be tweaked or shut off, however. This is not recommended.) This means that usually, a ship ‘thrusts’ for anywhere between fifteen minutes to three, five or even six hours on a ‘high burn’ (usually around 2 to 2.6 gravities of acceleration) depending on the complexity and hardiness of the reactor, before it must significantly reduce the amount of fuel being fed to the reaction chamber lest it risk a meltdown, damage the ship’s thrust nozzles or the structural integrity of the reactor. The most common reasons to shut down the reactor is heat buildup and simple over-irradiation. The ultra-dense plasma and collisions create dangerous buildups of radiation, some of which is infused into the Tritium to the point of making it lethally radioactive (to the point where physical touch can kill within 5 minutes). Beyond this, the Eiden Effect, (known to the current-day humans of Earth as Brehmsstahlrung Effect) means that massive amounts of this radiation saps energy from the plasma, preventing fusion from occurring. Though Radiation Sinks (speculative technology) catch most of this early on, (being capable of working for longer without over-irradiating depending on a reactor core’s sophistication) the radiation still builds up over time in the reactor, and, at some point, the reactor must be cycled and flush this radiation to allow it to cool down, and for the Aftgard assemblage (more on this more speculative technology later) to be recharged and flushed of radiation & heat buildup. At this point, (usually a heating or radiation issue issue as mentioned) the reaction must be brought down significantly in intensity or shut down entirely for the reactor core & aftgard to cycle. When a reactor core and it’s allied components cycle, this usually takes about as much time as it recently spent burning (accelerating). At this point, the tritium must be ejected into the Containment Module, a magnetically and physically shielded module within the ship, where it is allowed to expend the most active and lethal portion of it’s radioactive half life as irradiated hydrogen molecules, which is in relevance to the amount of time spent fueling the reactor, with modern hyper-treated tritium. Meanwhile, the Radiation Traps must be flushed and swapped out from the core during this period as well, usually directly into space in increments. This lets off immense amounts of heat, among other things, and can be done safely due to the extremely short half-life of the Radiation Traps in question. (This being said, the tritium and hydrogen in the containment chamber remains lethally radioactive for several years after being used, and should under no circumstances be flushed into space. Rather, the containment chamber should be dumped in a secure location at a service station every few months once it has built up. The radioactive material will then be sequestered and brought to a Core Dump facility on an uninhabited planetary or dwarf-planetary mass). During this time, the interior of the core must also be cycled, with the now-depleted lithium being replaced, and time must be spent to allow the weakest physical components such as the interior drive naccales & the physical components of the magnetic shielding to cool down, while other portions of the reactor, notably the superconductive magnets, are submerged in coolant. (Liquid hydrogen at -400 C) During cycling, a ship is often having a force of approx 0.0002 newtons enacted on it via the Subfusion Ion drives (known as passive drives, powered by stored power or low-yeild fusion) to maintain very low acceleration, low enough that it is functionally in zero-gravity, with unsecured floating items moving about 1 meter aft every 5000 seconds. Finally, the Aftgard must be recharged and flushed of radiation & heat as well. Aftgards are a fundamental part of how modern fusion reactions can safely take place for long periods. Since the thrust is achieved via accelerating the particles that make up the thrustpower to a fraction of lightspeed, and these particles are highly irradiated and being fired out of the thruster like bullets from a gun to fly all around a given star system hitting who knows what, this makes both the potential for disaster and the potential for misuse extreme. The aftgard’s Radiological Net jettisons some of the excess radiation being spewed from the thruster laterally from the vessel. The aftgard’s primary function however is to allow the ship to cool down,. Fusion reaction is incredibly powerful and intensely hot (averaging around 100 million degrees kelvin), and many, many things already need to be cooled down. Most fusion cores are infused with ultra-concentrated ultra-efficiency Hafnium-Carbide-Tantium-Carbide superalloys that can withstand intense heat up to the tens of millions of degrees and use magnetic shielding to keep much of it contained. Even so, a hundred million degrees of heat will literally destroy a ship in trilliseconds. As such a critical part of the Aftgard assemblage is it’s Gardian Dampers, Neutron-Kinetic Sinks, the Electromagnetic Discharge Module & the Hotplate.

Gardian Dampers are short, replaceable pylons along the exteriors of thrust nozzles that are part of the critical Aftgard waste-heat absorption schema. They absorb a small portion of waste heat directly out of the Hafnium-carbide-Tantium-carbide superalloy thrust nozzles, which are the most vulnerable part of the vessel, being the closest to the thermonuclear explosion that is the reaction.

Neutron-Kinetic Sinks are another critical part of the Fusion Core, and are essential for stopping ships from simply vaporizing themselves. Fractal-built nanomatrixes (essentially small plates with massive surface area) of supercooled and super-absorbent material bleed off and absorb excess heat over the course of a burn. Depending on the level of sophistication in one’s reactor, they can keep going for up to five hours of solid 2.5 G burn, four hours of 6-G burn, or 2 hours of 12 G burn before breaking down and needing replacement, a process which takes usually around four hours, as the nanomatrixes making up the structure must be stripped off of by a reactor’s internal robotics and then replacements fitted on to shielded portions of the reactor. It takes longer if the reactor is still running, to prevent radiological contamination or overheating of the rest of the ship. Notably, the materials used in making reactors begin to suffer adverse effects from ongoing high-power reactions after anywhere between that which is required for .5 Gs of thrust to 5 Gs of thrust. As such, for maximum travel-time efficiency, most ships utilize 1.8 to 2.5 Gs during travel time, mostly focusing on higher-G levels of thrust, to squeeze as much energy as they can out of the reaction.

This makes for space travel to be a relatively bumpy process of several burns and cool-down cycles over each ship’s interior day-night cycle (20-30 hours)

Electromagnetic Discharge modules store the built up electrostatic charge from the sheer power of the fusion reaction. These, located mostly on a ship’s lower hull, can hold awesome amounts of power for up to one and a half years , in the most extreme cases, before they need to be discharged. Most reactor mechanics would recommend a discharge at least every two months however, as if they reach capacity, the electrostatic charge will arc into the hull and fuse the ship and everyone on it into torched carbon. Discharges can be made in the wild, or at stations, so long as a small-planetary-mass grounding agent is available. This can range from large asteroids to the atmospheres of gas giants.

Hotplates are another part of the critical heat sinks that a ship must use. Hotplates are complex super-alloy plates that siphon off heat from reactors and store it in ‘subatomic worlds’ that simply sink background heat into a small object (approx the size of a dinner table) with the fractal surface area of nearly 10,000 squre km, drawing in heat and storing it, either when a ship is undergoing Silent Running (Retaining heat to avoid showing up on thermal sensors) when one is exceeding heat parameters but needs to continue burning, or when one needs to bleed off heat extremely quickly. The Hotplate will draw off immense amounts of heat from the reactor, and then be submerged in a tank of hyperfrigid liquid hydrogen coolant (approx -400c)

Overall, this creates a fusion reactor that, while not perfect, and not utterly energy efficient, at around 50-75% wastage, is still capable of powering modern ships across solar systems at relatively quick speeds, with only short times spent cooling down. If these were used in the Sol (earth) system, for example, one could travel from Earth to Saturn in about two months at the longest, with faster ships making the journey in 20-36 days.

Right now, I'm caught between wanting the fusion reaction to take place within a reactor core in the ship, or about 300m behind the ship, with the force being captured in magnetic fields to be used as power.

Please point out any problems, like what I should focus less on to make the speculative more believable, and what I should be more specific in explaining, as well as if/how this could work, assuming some small leaps of scientific logic.

  • 1
    $\begingroup$ A nuclear fusion (or fission, for that matter) reactor is a device for producing heat. They do not produce "force" of any kind; they produce heat. On Earth, where we have this heatsinks like the ocean or the atmosphere, we use the heat to boil water to make steam to turn turbines. That's all you need to know, and all that readers would be expected to know. Now you have this great source of heat aboard the ship: what do you do with the heat? Heat by itself won't move the ship... This is the only interesting part, and sadly this is the part which is missing from the spectacular technobabble. $\endgroup$
    – AlexP
    Dec 21, 2019 at 22:01
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    $\begingroup$ Thanks Alex, you're right. Now that I look at it, I get as far as the lithium absorbtion, and sort of drop it off there. I figured that it's mostly going to be generated with supersurface turbines (Small turbines with vast cumulative surface areas). That said, the force I was referring to was the reaction in pushing the ship from expelling the particles from the thruster, which is what I was under the impression as being how fusion drives were supposed to work, more or less. (If I'm wrong, please correct me) What would you say is the best way to generate energy from heat produced by fusion? $\endgroup$
    – Nepty
    Dec 21, 2019 at 23:40
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    $\begingroup$ I'm a pro at scaling wall but this is too much... $\endgroup$
    – user6760
    Dec 22, 2019 at 3:26
  • 6
    $\begingroup$ I'm with user6760 on this one... holy wall of text, batman. Brevity is always appreciated. $\endgroup$ Dec 22, 2019 at 12:27
  • 2
    $\begingroup$ You lost me at "-400 C". $\endgroup$
    – void_ptr
    Dec 22, 2019 at 21:42

2 Answers 2


The biggest problem with this wall of text is you really don't seem to understand the process of fusion very well, if at all. Fusion is the merging of lighter elements to create heavier ones (i.e. Hydrogen to Helium), so a lot of the technobabble about reactor cores, melt downs etc. really has no bearing on the process - you seem to have it confused with nuclear fission.

As a story element, the first thing that needs to be addressed is "why" do the readers need to know all the technical details about how the fusion reactor works? Is this an important plot element? No one writes about the process an internal combustion engine goes through when a character gets behind the wheel of a car, unless the engine has been sabotaged for some reason (even then, the description might only go as far as a cut wire of hose, or a handwave about the engine chip being hacked).

If the engine details are important somehow, then you may have to do a bit of research on how fusion actually happens and how various sorts of fusion engines are supposed to work. Most current fusion engine concepts revolve around ICF (Inertially confined Fusion) or MTF (Magnetized Target Fusion), with some exotica like "gun" fusion on the side. I'll leave it for you to do the research on these terms, but the ever helpful "Atomic Rockets" website and "Tough SF" have some good information. This link to the Tough SF's analysis of the "Epstein Drive" from "the Expanse" series of novels and shows should provide a fairly detailed description of a plausible drive.

So the real issue that you need to address is "why" is it important for the characters and readers to understand how the fusion drive works, and "why" it is a plot point? If the story could be told without this information, then why would you need to know this sort of detail? As a world builder, you might need to be aware of this so you don't write yourself into a corner, or make illogical assumptions in the story (Accelerating at more than 3 G is going to be very hard on human characters, unless you make other assumptions, like they are packed in an oxygenated fluid and all their internal "void" spaces are filled with this fluid as well...), which lead to other complications that eventually interfere with the reader's suspension of disbelief and enjoyment of the story.

Story first should be the rule to follow.

  • $\begingroup$ The story itself is fully written and in for the publisher, I'm just the type of writer who prefers to over-world-build compared to under-worldbuild, so right now I'm overhauling the technology. The technobabble is there to pull buzzwords out of my hat and establish a better sense of the technology I'm working with. Unfortunately I have a math-related learning disability which makes physics kind of hard, as cool as it is. I mostly want to be able to understand the process of fusion in as simplistic terms as possible so that I can meet the criteria of 1: Pulsed torch engines 2: Periodic 0-G $\endgroup$
    – Nepty
    Dec 21, 2019 at 23:31
  • $\begingroup$ Also, I read the ToughSF's peice on Fusion, and it seems like a lot of complex equations, is there a more or less 'more word-based' way of describing the process? The big thing I'm struggling with is a process that creates enough energy for crossing vast distances in a very short amount of time (like say a month to 2 months Earth-Saturn), while not being as fuel efficient or burning as long as the Epstein Drive. As for the insane G-forces, I do use some intentionally-vague unobtanium super-G-suits, body-internal crash gel and cybernetics to explain characters surviving that sort of force. $\endgroup$
    – Nepty
    Dec 21, 2019 at 23:36
  • $\begingroup$ If I were to sum it up, I'm more or less trying to figure out how to get significantly more energy than nuclear fission, (which I don't want to overuse due to the probability of people just turning ships into nukes willy-nilly) without having a 'super efficient' method like the Esptein drive. I'd like there to be periodic times where it would have to either cool down and stop accelerating, or else have to dump irradiated material, and am trying to figure out an explanation for this sort of activity. Got any advice on that? $\endgroup$
    – Nepty
    Dec 21, 2019 at 23:45
  • $\begingroup$ @Nepty: Your problem is not energy, it's momentum. And in order to change the momentum of the ship you need to push something in the opposite direction, because momentum is a conserved quantity. $\endgroup$
    – AlexP
    Dec 21, 2019 at 23:56
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    $\begingroup$ If you have already written and submitted the manuscript, then I would expect that the editor might be asking many of the same questions I did, and likely using the "red pencil" to excise a lot of stuff. As far as world building goes, about 90% should never reach the page, but just be there to inform you so you don't write yourself into a corner. Imagine having an elaborate astronomical calendar in your story. Does anyone care how it works? You just need to know so when the character says "Capricorn 15 at noon, or in your calendar..." you don't say August 15. $\endgroup$
    – Thucydides
    Dec 22, 2019 at 5:04

Short, non-technical answer: Thucydides has the right idea here. Conserve detail. It almost certainly doesn't matter to the progress of your story, and it mostly gives you ways in which you can be wrong. You've got fusion reactors and fusion rockets, using D-T or D-3He fusion. Great! Stick a fork in it, its done.

Longer, rambling nitpicky and slightly more technical answer:

(note, not all issues will be addressed here because you've already given us one terrifying wall of text, so I don't want to inflict another on everyone but it is still gonna be a bit sprawling)

and a shielded tritium-containing ‘cap’ (Helium-3 for more modern models)

Both D-T and D-3He are "fusion" and so might be considered to be basically the same, but its a bit like saying that both a Boeing 747 and a Boeing X51 are both basically the same because they're aircraft with airbreathing combustion engines.

The temperatures and pressures involved are quite different, and the reaction products are very different so the ways in which you extract energy or develop thrust with them will likewise be very, very different. A reactor or rocket that runs on one fuel may well be entirely incapable of running on the other.

tritium [...] is bred and reused within the chamber once initialized so long as the reaction is maintained.

Dubious. I doubt you'd get a high enough density of lithium in the place where the reaction is actually taking place to produce more tritium. You'll be slurping the tritium out of the lithium blanket, separating unwelcome stuff mixed with it (like 3He, which a D-T reactor isn't going to make much use of) and then pumping it back in later.

(on a related note, why is every fuel rod capped with tritium, if generally all you need is a small amount to spark off the reaction to start with? just keep it separate. It is a radiation hazard, unlike deuterium, so you should probably be treating it quite differently)

The ultra-dense plasma and collisions create dangerous buildups of radiation, some of which is infused into the Tritium to the point of making it lethally radioactive

You can't make tritium more radioactive. You can transmute it into something else, but then it isn't tritium anymore. It'll either decay naturally into 3He, or it'll fuse with some deuterium to make 4He and a neutron. That's it.

What you do have to worry about is the material surrounding the reaction chamber, which will suffer from a high degree of neutron activation and so will end up radioactive once the reactor has been running long enough.

the tritium and hydrogen in the containment chamber remains lethally radioactive for several years after being used, and should under no circumstances be flushed into space

Other than "no, it won't be", you should note that a) space is Really Big and b) is already full of pretty hazardous radiation. Even if you vent colossal amounts of radioactive gas into space, it'll expand extremely rapidly to the point where it is far too diffuse to be a threat (or probably even detectable without specialist equipment) and then it will be blown away by the solar wind, no doubt.

Also note that if your fusion reactor is doing its job, you won't find a whole lot of any kind of hydrogen in the "spent" fusion plasma, because it'll all have been fused into helium. Helium doesn't have any long-lived radioactive isotopes at all.

Liquid hydrogen at -400 C

Bzzzzt! The coldest you'll be getting anything is about -273.15°C, because that's absolute zero. You won't be going below that unless you're firmly in the realms of science fantasy, and if you were, you wouldn't be worrying about your fusion reactor designs, would you?

edit: did you really mean -400°F here? That works out to a much more sensible 33.15K, though it is above the boiling point of hydrogen...

Beyond this, the Eiden Effect, (known to the current-day humans of Earth as Brehmsstahlrung Effect) means that massive amounts of this radiation saps energy from the plasma, preventing fusion from occurring.

Bremmstrahlung increases as plasma temperature rises. D-T is a relatively cool fusion plasma, and D-3He whilst an order of magnitude hotter isn't nearly as inconvenient as other fusion recipes. The amount of energy lost as x-rays is 8% and 20% of the reaction yield, respectively. For D-T fusion, the x-ray emission is small beans compared to the huge amounts of fast neutrons you have to deal with.

thrust is achieved via accelerating the particles that make up the thrustpower to a fraction of lightspeed, and these particles are highly irradiated and being fired out of the thruster like bullets from a gun to fly all around a given star system hitting who knows what

Remember that at the centre of any solar system is a fusion reactor that is hugely, vastly, mindbogglingly more powerful than anything on your ship. Standing next to the nozzle of your nuclear rocket might be hazardous, but you don't have to go far away before its effect will be about as threatening as a fart in a hurricane. Again, remember that space is Really Big.

I'd also say that ion drives accelerate their reaction mass, fusion rockets direct it via a magnetic nozzle. Those helium ions come out of the reaction already going a moderate fraction of lightspeed (a few percent) and you really don't have enough energy to make them go faster (and if you did, you wouldn't need a fusion rocket, you'd be using some other kind of plasma or ion rocket instead).

Notably, the materials used in making reactors begin to suffer adverse effects from ongoing high-power reactions after anywhere between that which is required for .5 Gs of thrust to 5 Gs of thrust. As such, for maximum travel-time efficiency, most ships utilize 1.8 to 2.5 Gs during travel time, mostly focusing on higher-G levels of thrust, to squeeze as much energy as they can out of the reaction.

Lets say your ship weighs a thousand tonnes, and is using a D-T rocket to provide a 1G acceleration. That means a thrust of about 10MN.

If you want to make it to Saturn in 60 days, you'll need a delta-V of ~270km/s. With a sane mass-ratio of 3, that means an exhaust velocity of ~245km/s. An engine with this exhaust velocity developing that thrust has a power of ~1.2 terawatts. A D-T fusion reaction spits out about 21% of its energy as charged particles, and those are the things that will mostly be heating your reaction mass. Lets say you capture about 10% of the energy released as x-rays and neutrons in the reaction mass, too. That means your reactor needs to be developing at least 3.9TW, giving your ship an awe-inspiring power density of 3.9 megawatts per kilo. Meaning that your rocket has to have a power density at least four times higher than that, because you have to take into account the fuel, the passengers, the cargo, the heatsinks, the superstructure of the ship and so on.

I'm not gonna tell you that's flat out impossible... after all, you already have handwavium in the form of your hotplates, so anything goes, but those numbers are frankly unreal. High thrusts and high exhaust velocities just don't go together in the real world... basically, they can't. Either slow things down, or use offboard beam-powered propulsion (magbeam, sailbeam, laser sail, magnetic sail, mag-orion, the list goes on) if you want things to stay sensible.


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