Fusion spaceships are a radioactive explosion generator with a tiny pressurized box of air on the other end. There are other kinds of propulsion systems in RttS, but this is among the fastest available. They're lucky that deuterium and Helium-3 are both quite cheap due to gas giant mining.
This chapter only has one log page, so next up is reader questions. Submit your questions for the characters at the usual Google form. The reader questions are going to be releasing every Tuesday until February 17th during the winter hiatus, read more about that here.
Transcript
LOG 8: Z-Pinch Fusion
The Runaway is a rapid transit vessel (colloquially, a "torchship"), designed to transport cargo and passengers as quickly as possible on intrasystem trips between wormholes. During the majority of a journey, the vessel acceleration is between 8 m/s² and 11 m/s², the comfort range of gravity for most sophonts. While this shaves days off of voyages and reduces crew time spent in microgravity, it requires a great deal of firepower. The Runaway's comparatively tiny habitat sits on top of a massive pair of fuel tanks and z-pinch fusion engines. As plasma fuel is injected into the fusion chamber, the engine’s capacitor banks deliver thousands of electrical pulses per second, creating magnetic fields strong enough to initiate nuclear fusion. This magnetic field “pinches” the plasma along the engine's z-axis into a blinding blue jet of super-heated gases, pushing the ship to its cruising acceleration.
Engine Schematic:
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fuel tanks
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deuterium
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capacitor banks
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shadow shield
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outer electrode
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inner electrode
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plasma
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electric current
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The magnetic field follows the direction of the electrical current.
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magnetic nozzle
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helium, hydrogen, and neutrons
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FUSION
Image: Bip grumpily holds up a sign in front of one of the deuterium fuel lines with “3He” written on it.
Bip: Ideally, I'd have one tank of ³He and one tank of deuterium, that would be more efficient than reacting doot with itself...
Talita: Beggars can't be choosers. Also, fusion engines aren't the only propulsion here!
Image: She points at a small multi-nozzle rocket on the side of the Runaway’s habitat.
Talita: On both ends of the vessel are rings of reaction control systems, aka RCS. These smaller chemical rockets are used for fine maneuvering and approaching dock, since fusion engines are illegal to operate too close to habitation.
While fusion engines have less radioactive exhaust than fission engines (which use a process similar to a nuclear bomb for propulsion), they still emit deadly neutron radiation when operating. To protect the inhabitants from being fried by their own engine emissions, there is something called a "shadow shield" between them and the thrusters. This is a block of material that absorbs the thruster radiation and creates a cone of protective shadow around the body of the ship.
Image: A diagram of the Runaway shows shape of the shadow cone, and the size of the habitable area of the ship (green) relative to the rest of the vessel. The main habitat is a four story structure under a 12th of the total length of the ship, with a short cargo bay hallway in the upper section of the spine. Most of the length of the vessel is occupied by fuel tanks, shielding, and the engines.
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Habitat (green)
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cargo bay (green)
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RCS (purple)
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shadow shield (dark blue)
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radiators (red)
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Neutron Radiation Zone (white with skulls and crossbones)
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Radiation Shadow Cone (light blue)
This stellar flamethrower also creates a ton of waste heat, which in the insulating vacuum of space, dissipates very slowly. In order to prevent heat from traveling up the spine of the ship and turning the habitat capsule into a multi-story oven, heat from the propulsion system is diverted to the radiator wings. The radiators pump waste heat away from the engine and spine of the ship, giving it more time and space to dissipate, and can glow red-hot while the engine burns. Their triangular shape allows them hide within the shadow cone during burns, and extend to cool faster and provide maintenance access when the ship is coasting or docked.
95 thoughts on “Runaway to the Stars: Page 238”
Tea
How quickly do the radiation emissions break down? If two torchships passes each other, would they irradiate each other’s crew compartments?
✧ Twilight
id imagine similar to planes, ships are required/expected to give eachother a wide berth. past this, if a torchship were to pass through another’s emissions, it wouldnt do much due to the size of the zone & the speed at which the ship would likely be travelling.
additionally, id imagine as a standard all ships would have radiation detection, protection and measuring systems, for the ship and crew to respond to on similarly standard regimens.
Logograms
Neutrons don’t break down (at least not on useful timescales), but neutron radiation drops off with the square of the distance like any other form of radiation, so as long as they don’t get too close, the dose delivered should be small. Distance is the best shielding.
https://en.wikipedia.org/wiki/Inverse-square_law
Jason Myers
This is fascinating and amazing and so cool but I’m so sorry the first thing I saw due to dyslexia was ‘Radiation Shadow Clone’ and I was just. Oh no. Naruto time-
HAL9000
i am very much a handwavium kind of scifi writer, but this page has given me an idea for a fun ship design that is just an improbably large triangle with a single fusion engine slapped on the pointy side. like a star destroyer but backwards. i love reading about all this stuff its so fun seeing people get really into worldbuilding with constraints and shit. the projectrho link in the comments has also dropped me into an excellent rabbit hole
JoB
> an improbably large triangle with a single fusion engine slapped on the pointy side
“Captain, we’re half a point off course. She’s yawing to port.”
“Who’s doing lookout on the bow?”
“Ensign Walker, Sir.”
“That skinny guy? Ugh. OK, we’ll have to have him move all the way over to starboard to try and balance her again!”
F657
Where do the capacitors get the energy from? Do you need a second reactor to power it?
JoB
Whether it’s a second reactor or the same, with all the systems of the Runaway – Bips mainframes in particular – up and running while the torch drive is cold out, there is some power being generated independent of the z-pinch fusion process.
(But seeing that the mainframes’ power lines run the entire length of the spine, I’d guess that both habitat and torch get their electricity from a single generator system.)
hmantegazzi
I guess they could divert some heat from the radiators to power a good old boiling water closed circuit generator…
Claire
Does RtTS have a forum? I feel like a RtTS forum would do well.
IncompleteMachine
it absolutely would, or a discord.
Theoneandonlyvoid
There is a official Rtts discord
longnose_gar
do you have the link?
Jay Eaton
The Discord is Patreon linked because I have moderated for a large public Discord server before and never want to again. It can be accessed by subscribing for 2USD or more on Patreon and linking your Discord to Patreon so the bot can add you.
thisfox
I don’t want to hear fans voices, I want to see their ideas and fanart. A forum would rock.
Sabella
I could listen to Talita explaining rocket science all day.
Lezzlebit
i bet someone could trick me into reading a rocket science research paper if they just put all the text in a giant speech bubble coming from Talita
JoB
I’m still wondering about one thing, though:
If the Runaway has to turn off the torch drive 1 million km or so from the space station it wants to dock at, and must reduce speed beforehand to the point that the small omnidirectional RCSes can manage the rest of the deceleration, doesn’t that lurching leg threaten to eat up a large part of the time advantage you bought by having the torch drive in the first place?
If you want to build a fast courier ship, why not have a safe-to-operate-in-populated-areas drive, with mid-range oomph beyond the “maneuvering thrusters” but pointed in only one direction (the one you need to make deceleration happen), to minimize your delay in the equivalent of the horse latitudes as well?
Scolopendra
Not really. I should do the math to prove it, but the standoff distance is probably less than a million km (that’s about, what, 3x the distance from the Earth to the Moon, the radiation hazard will get inverse-squared to nothing pretty quickly, and the exhaust can’t be *that* collimated) and having the excess delta-v to make 1g-ish constant accel economical means you can easily pull tricks like coming in ‘ahead’ of your destination so you get ‘free’ deceleration from its gravity well. I’d have to pull the old Atomic Rocket nomograms out but drive technologies also tend to be pretty heavily quantized; the “mid oomph” drives probably either don’t have enough oomph, are less practical, or are even more hazardous in one way or another.
If the idea is to have a second drive system for close-in affairs, that increases the complexity of your ship and its structure. It might need entirely different fuels and different tanks, or it might need complex piping to move shared fuels around, or it might require more structural scar weight if it’s on the end opposite the fusion torch (and you’d have to turn and burn anyway at some point because presumably you’d have to use that drive to *leave* as well). The ‘leaving’ part is what I’m curious about; maybe fusion torch ships never get close to anything and use lighters to close the distance as shuttles or maybe tugs bring them in and disposables/accelerators sling them out.
JoB
> It might need entirely different fuels and different tanks, or […]
(Sidenote: Given the necessity of a Whipple shield to ward against stray chunks of matter, I doubt the wisdom of having all of the type X fuel in just one huge tank, anyway. At least when we’re talking about craft that haul a “manned” mission all over whatever planet system.)
Leaving the cost relation of D vs. H aside, the Runaway already carries that and a stash of O for the life support systems …
> coming in ‘ahead’ of your destination so you
> get ‘free’ deceleration from its gravity well.
Please note that the Runaway (and most other interstellar vessels, hence Dirtball having a number of small ferries to carry personnel up’n’down) is not built to make planetfall, and needs to dock at some sort of space station (of probably negligible mass). Sure she could make celestial body swingbys (though the point of the torchdrive is to make such “tricks” and the time they take unnecessary), but I’m not sure those would take you anywhere near the orbital speeds (and directions) a station would necessarily travel at …
(Celestial bodies with substantial atmosphere would be rather safe from the effects of the torch drive, anyway, so one could possible do away with the stations and go the “clear the orbit, have ship torch its way into it, then send up the ferries again” way.)
I was thinking about having the “mid” drive’s nozzle peek out of the Whipple shield and fire in the opposite direction, to disentangle it from the torch assembly (and its radiation) and simultaneously take advantage of the spine axis being built to sustain the forces, but I have to admit that that would still require some structural strength added to the habitat section so as to be able to “carry the weight” of the other end.
> maybe tugs bring them in and disposables/accelerators sling them out.
I know that, for operation at that close range, Jay has already been mulling a couple concepts … (wish I could find those sketches again offhand)
Scolopendra
This is one of those things where it does really get dependent on CONOPS. A drive system one doesn’t generally use except to slow down *sometimes* is a *lot* of scar mass for the real burner to push and it may be more mass-time-fuel-cost efficient to do lighters for ship-to-shore. I recognize this kind of torchship isn’t supposed to make planetfall (just *look* at her) so she’s dependent on *some* arrival/departure infrastructure; the question is what that is and how nicely that plays with this (and alternate) drive schemes. It’s a classic hard s-f issue: whatever makes your ships go at a narratively interesting speed is also generally dangerous as hell to everything around it and what it takes to speed up is generally what it takes to slow down. I imagine the RCS are probably LH2/LOX burners akin to SMEs (plenty of fuel and oxidizer handy as you point out) so maybe they put out a lot more oomph than we would necessarily imagine.
It really comes down to the delta-v vs. time vs. mass budget at the littoral phases of operation.
Scolopendra
(also, just to clarify: my remark about coming in ahead of a gravitating body to slow oneself down is a form of gravity assist maneuver unrelated to planetfall; it’s basically the reverse of a slingshot. I wasn’t implying anything like aerobraking.)
JoB
> > maybe tugs bring them in and disposables/accelerators sling them out.
> I know that, for operation at that close range, Jay has already been mulling
> a couple concepts … (wish I could find those sketches again offhand)
Found ’em.
Claire
The economic cost of operating those safe-to-operate-in-populated-areas drives (in terms of mass stored) is simply far too costly to make long-range travel economically feasible. You could do it, occasionally, but for widespread use? Too expensive. So, they use those as little as possible and switch to the much cheaper per km system as soon as they can.
john
Million km standoff seems excessive, a few hundred might be enough for decent radiation safety, if you’re careful about which way the main beam is pointed. Maybe gradually reduce power during the approach, rather than an abrupt cutoff? Station can also afford to have a lot more shielding, on all sides, since it’s not going anywhere in a hurry. Difference between a hundred km and a million km only amounts to needing two more tenth-value thicknesses.
Match speed, close remaining distance at a leisurely 10 m/s… five or six hours from “we’re here” to “docked,” or from “disconnect” to “highway speed,” seems like it could be tolerable for a travel context where orbital mechanics often requires planning months in advance.
Claire
Maybe so, but im thinking about how astronomers ballpark within a couple of orders of magnitude…
Teod
Calling deuterium “doot” is such a cool little piece of worldbuilding. Of course a commonly used but needlessly long word would get shortened.
Greenjoe
What is the shadow shield made out of? Lead?
1d4-nadg
Probably not, lead is effective against X-rays but not so much against neutrons (there are many types of radioactivity). From the top of my head I’d say boron or lithium are good candidates, and I think I’ve read somewhere plastics with high hydrogen contents such as polyethylene were quite good too. And they are far less heavy than lead too, win win!
In modern science, water is also often suggested, as it can double as life support, but here it seems unlikely considering how hot that shield would get.
IncompleteMachine
its probably a layered thing; a thin disk of something very solid like iron or a lithium alloy and then an outer shell of polyethylene. id rather have multiple layers between the squishy people with their easily cooked DNA and the nuclear hellflame they’re riding on.
JoB
> here it seems unlikely considering how hot that shield would get.
On the other hand, (liquid) water being part of the shield would be a prime way to get heat out of it and over to the radiators.
(We’re definitely not building a Habakkuk here. 😉 )
Chakat Firepaw
The shadow shield will most likely be tungsten to block the high-energy photons and something like lithium hydride for the particle radiation, (mostly neutrons). A ship’s fuel and water tankage will also be positioned to give extra shielding, including against ambient particle radiation.
40
Handwaveium, of course!
Sci
I’ve been loving the story so far- glad Talita’s finally gotten her hug(s)!
(That said, just wanted to flag a minor typo- in both the transcript and the image, in the bottom paragraph, “Their triangular shape allows them hide” is missing a “to” before “hide”, I think? Just figured you’d rather know)
Anyways, excited to see where this goes, though… my “things are gonna get worse before they get better” senses are tingling…
AsTheJayFlies
this diagram (which i had early access to as a patreon patron) was very very helpful in my effort to create the runaway in spore 🙂
also god i love talitas trunk smile every time she gets to infodump abt mechanics. shes so happy to explain how ship go vroom vroom 🙂
wingedhorror
What an [almost] unbearably sexy page. I love this stuff*. Phew! :wipes brow:
*I accidentally typo-ed that as “stiff” which I nearly left in because it made me laugh.
Solidago
doot doot
Arcstone
So it’s actually really dangerous for ships to be to close to each other? Whats roughly a safe distance?
IncompleteMachine
That going to depend significantly on multiple factors ranging from the size of the ships to the thickness and type of their radiation shielding. Id need a lot of numbers before I could crunch you a minimum safe distance for the Runaway under acceleration but realistically most neutrons decay away after about 15s. Deuterium fusion generally produces neutrons in the 14 MeV range so about 52,000 kms. That means that the neutrons will not decay away untill they are about 780,000 km from the Runaway, however that shouldn’t be taken as the radius of serious danger, as the radiation will disperse over that distance very quickly. Water would be the go-to radiation shielding against fast Neutrons (at least these days) and planetary atmospheres are thick enough to stop this kind of thing dead so most ships, planets and stations are fine (so long as you don’t light it up at point blank) but if you’re out and about in a spacesuit you don’t want to be working when the torchships fly by.
Torch engines (regardless of how they work) are sufficiently dangerous that they actually could be used as quite powerful ship-to-ship weapons. Id need numbers before I could estimate how powerful a drive-beam the Runaway produces, but it would be enough to present a real threat even to an armoured gunship assuming that Jay is sticking (roughly) to IRL materials science rules.
Arcstone
I just love this community and the super detailed responses you get. 😄
Thanks man.
Nick Alcock
Fifteen seconds? The half-life of free neutrons is 878.45s: about fifteen *minutes*. (Also, they decay into protons, and while on average those protons will be moving a little more slowly than the neutron was, they’ll still be moving bloody fast. Good thing ships need shielding against those anyway, given that they’re also produced in copious quantities by, well, stars.)
IncompleteMachine
nope, my bad, you’re right, it is fifteen minuets, so the distance is significantly higher but I have to stress that the falloff by dispersion would be much more significant to how intense a dose you get than the half-life.
Peter Jensen
The inverse square law will likely make the radiation safe(-ish) at much closer distance than it would take for the neutrons to decay. But to calculate the safe distance, you’d need to know the energy production of the engine and the neutron flux that results from it.
IncompleteMachine
we have an acceleration, but i dont know if Jay has given us anything about the mass of the ship, which would give us a way to work out what the actual energy being output by the engine and we could probably estimate from there.
JoB
> i dont know if Jay has given us anything about the mass of the ship
120 kt (unladen).
Peter Jensen
Most of the energy would presumably be in the shaped and directed thrusting particle beam (the “torch”), which you absolutely don’t want to be in the path of, for a *considerable* distance (think “The Kzinti Lesson”), while the neutron radiation is incidental to the process and largely wasted for propulsion purposes (though presumably what little hits the shadow shield would add some impulse). Not sure anyone would be able to estimate what fraction of the desired output energy takes the form of neutrons.
Joanne
Is there a reason the shadow shield couldn’t wrap around more of the fusion assembly, to reduce the arc of the deadly radiation zone? Expense, mass, materials? Some kind of lensing/focusing effect that’d make the remaining arc super-ultra-mega deadly instead of merely deadly?
JoB
Truly penetrating radiation must be shielded with mass and all realistic considerations of torchships I’ve seen pointed to the living quarter’s shielding as constituting the majority of the ship’s mass. Make the shields considerably bigger than they absolutely need to be (still with the same thickness required to get radiation to safe levels) and their mass threatens to overwhelm your insanely powerful engine’s propulsion capabilities.
Having that said, the old adage of “you cannot fathom how big (and empty) space is” applies here, too. For most of the trip by far, there won’t even be a single dead chunk of rock in your killbox, much less anything that seriously needs to be shielded.
IncompleteMachine
Mass, definitely mass. Torch drives are rockets and the rocket equation is an evil bitch. you basically need a ratio of about 3kg of propellant for every 1kg of actual mass you want to move (this assumes a mass-to-propellant ratio of about 4, which is kinda the tipping point between economical and not economical), so shaving as much off the top as possible is the best way to do it.
technically, the ‘better’ layout actually puts the engines at the front on a long tether and tows the payload like a sled; doing that lets you build an even smaller shield, but the Runaway’s design is much more manoeuvrable.
if you want more of this, go to Atomic Rockets; they have al this stuff charted down in great detail XD
Fogblitz
I’ll forever be enamored by the thought and care you put into worldbuilding. This is more science than I’d ever get into for a writing project, but you make it look so tempting!
Vinemaple
I always look at how close the red-hot radiators come to the solar panels, and I wonder about how that works… this seems as good a place as any to mention it.
Jay Eaton
Realistically if they got hotter than a dull red they’d melt. But glowy looks neat, so artistic license.
Vinemaple
“Glowy” would probably be as far as one could talk an excited Hollywood director or VFX team down to, so that seems fair!
john
Could excuse that by saying it’s showing near infrared rather than strictly human-visible.
JoB
> near infrared rather than strictly human-visible.
“Why the [bleeep] does this movie show all ship approaches in this so-called ‘bug-ferret-o-vision’ for no reason whatsoever?!?”
“You don’t want to see them coming at you alight in avians’ UV range instead!”
Ashran
Also, due to the Steffan Boltzmann law, a radiator that big would likely be not that much hotter (relatively) than the radiator for the life support, as the heat emitted scales to the fourth power of the temperature (E=s*T^4) with E being the radiant energy, s the surface area and T the temperature. Thus, the radiator size for a nuclear reactor can often be smaller than that needed for life support, especially for larger vessels.
7arty
I’m curious as to why the radiators need to hide in the radiation shadow cone too:
Neutrons imparting heat? (is that a thing? I would assume it is, but I’ve no idea)
Delicate electronics or some other delicate material in the radiators?
Allowing for maintenance access, mid-burn?
Been ping-ponging on that last one: First thought was thinking hanging outside an accelerating ship must be a horrible idea, before realizing the accelerations involved should make it similar to exterior cleaning on a skyscraper back on Earth. …Yup, totally just like a normal skyscraper, sitting above a bottomless pit, raging nuclear inferno right below us. Not a scary prospect at all.
Jay Eaton
Basically, high energy neutrons will gradually rot and irradiate the non-living materials of the ship. Anything getting shot with neutron radiation has to be replaced after a certain mileage. If you want more information than you’d ever need, read here.
IncompleteMachine
Atomic Rockets Mentioned. I have been Summoned.
you should get a seal of approval :3
7arty
More information than I’d ever need you say? Oh I love this.
Thanks!
john
Also risk of radiation reflecting / scattering off of any protruding bits, potentially increasing the dose on more delicate systems which would otherwise be safely behind the shadow shield – so that magnetic-nozzle rim having line-of-sight to the outer corners of the radiator makes me a little bit nervous.
BravoLimaPoppa
Thank you! I love stuff like this.
Logan
I love how happy Talita looks whenever she gets to explain stuff :3
Jay Eaton
She’s literally ☝️🤓 right now
McFrugal
The arrows pointing away from the direction of emission confuses me.
Jay Eaton
It’s the magnetic field, so the direction of the lines is more about polarity/electrical current flow than the direction the plasma mass is traveling. It is unintuitive though.
Adrienne
This log page is giving me both an appreciation for people who bother to do the research that makes space travel make sense in their universe and the people who say “You know what fuck all that, in my world the space ships are powered by magic crystals that work forever”
Jay Eaton
The space crystal people are wiser than me. My compulsion to know how everything works will be my downfall.
Redacted
Hear hear
Voidmew
Calling deuterium ‘door’ for short is so funny
JoB
Poor heavy peroxide (structural formula DOOD) still getting ignored outright in the bright future, I see 😀