We don't cheat at making g-force here, it's be big, spin, go fast, or nothing. The obtuse physics fact I chose to leave off this page is that regular gravity is also a kind of acceleration (1g = 9.81 m/s²). You are currently accelerating towards the center of the Earth. Thank goodness there's a crust in the way.
There will be character AMA responses after this chapter! Submit questions over here!
Transcript
LOG 7.1: Simulated Gravity
When you stand on a planet, its enormous mass attracts the smaller mass of your body towards its surface, while the surface resists the force gravity exerts on your body. Animals that evolve to cope with a specific gravitational force can experience health issues if they spend too much time in low-gravity or weightless conditions, namely tissue degeneration and a potentially lethal buildup of fluid pressure in their upper body. Modern gene therapy regimens for humans and bug ferrets can combat these side effects, but it's still recommended to exercise regularly and avoid extended stays in microgravity. Although the gravitational mass of a planet is impossible to replicate in miniature, there are two commonly used forms of "gravity" in spacecraft which exert force on the body in a similar way.
Image: A centaur, human, avian, and bug ferret are pulled towards the center of a spherical mass and pushed back from the center by its surface.
Centrifugal Gravity
The most energy efficient way to simulate gravity is by spinning an entire habitat structure, creating centrifugal force that pushes the occupants against the interior surface. If the habitat stops spinning, the interior space becomes weightless.
In transit vessels, retractable structures like dumbbells or inflatable toruses are favored. These allow the structure to have a wide enough diameter to avoid causing nausea and disorientation, while keeping the ship narrow enough for wormhole passage. In large habitats, cylinders and sphere shapes are favored for being the easiest to pressurize.
Image: A centaur, human, avian, and bug ferret stand inside the surface of a spinning hollow circle and are pulled outwards from the center of the circle and pushed back by the interior surface.
Accelerative Gravity
Common in rapid transport vessels, this form of "gravity" is created by the inertia of the passenger's bodies as the ship's acceleration changes. "Down" is always towards the ship's thruster, and the interior of the vessel is weightless unless its speed is actively increasing or decreasing.
These vessels are weightless while docked, and experience two periods of gravity per trip, once while accelerating towards the destination, and once while decelerating towards the destination. Their interior spaces are designed to adapt between microgravity and weighted conditions.
Image: A centaur, human, avian, and bug ferret stand inside an accelerating rocket and are pushed aft by the acceleration, and are pushed back by the floors of the rocket ship.
Brachistochrone Transfer
- constant acceleration towards midpoint: simulated gravity pulls in the opposite direction of transit
- flip-over: the ship pivots to direct the thruster in the direction of transit and experiences microgravity
- constant deceleration towards destination: simulated gravity pulls in the direction of transit
54 thoughts on “Runaway to the Stars: Page 211”
Effyis Biblos
Huh. So how big *is* a wormhole gate aperture? Large enough for the Runaway’s main hull (obviously); not large enough for a big rotational-gravity torus (but how big is big? What is the necessary size to keep it psychologically manageable? I have no clue.)
This
Also, while there’s almost certainly engineering limitations at play, did the Bug Ferrets start building them larger when larger spacefaring species started using them?
holly...!
it must take some VERY clever interior design to make a space that can adapt to two different directions of gravity plus weightlessness!
holly...!
wait, never mind, just saw that gravity always points toward the thruster
Phasma Felis
I’ve wondered if you could make a ship with both spin and thrust gravity. You’ve got a big spinning wheel, with each cabin mounting in a cylinder along the edge of the wheel, and the cylinders can swivel to accommodate different gravity vectors. When you’re coasting with the thrusters off the vector is radially outwards, perpendicular to the direction of travel; if you were to shut down the wheel and thrust, the vector would be opposite the direction of acceleration; when you’re spinning *and* thrusting, it’s somewhere in between. Spin rate changes as needed with thrust to maintain a comfortable, consistent gravity.
This would be very impractical and require a lot more unnecessary moving parts than you really want a spaceship to have, but I could imagine someone pitching it for maximum comfort on a luxury yacht, for example.
JoB
Hm. I’d say that the “maximum comfort” would still be absent near the wormhole gates (where you won’t fit through with a large spinwheel, and can’t fire the main thrusters without damaging the gate, other ships waiting in line, etc.), and if “keep 1 g of thrust except near gates and terminals” is economically viable for express couriers like the Runaway, it should be an acceptable option for any $$ Moneybags as well …
(Meanwhile, my thoughts go to whether it’d be a viable less-lethal anti-spinwheel-spaceship weapon if “missiles” were to dock/clamp/tether/ram at the spinwheel, floor the figurative pedal, and increase its rotational speed threefold …)
Vinemaple
I’ve been thinking about dumbbells that fold in for wormhole transit and acceleration in such a way that their internal “down” is towards the thruster. This would make the curved dumbbell floor annoying under thrust, but it would still minimize the time spent in microgravity. If the dumbbells have to retract anyway, I’d say one might as well design them this way.
Claire
You could… but the vectors would multiply, and result in a very weird conal base shape? And unless the flooring could change angle, you’d have to keep the thrust VERY precise or the effective gravity would fluctuate and be tilting everything around.
Basically, you’re turning a tilt-a-whirl into a living situation. Not recommended.
Gar G
Fascinating!^^
And this reminds me of something I always thought about with sci (*cough*StarTrek!*cough*), how even when all systems are down, including life support (supposedly), they somehow still manage to maintain artificial gravity, even in derelict vessels that have been abandoned ages ago, gravity always stays the same
I get the out-of-universe reason but in-universe it makes absolutely zero sense and it just irks me to no end
Vinemaple
Especially since we’ve discovered that slowly rotating the camera and having the actor lean in a random direction conveys microgravity perfectly well, for close-up shots.
Ploapgusset
Gravity is super weird and cool!! I can’t remember if it was stated whether or not the lighter gravity of Dirtball was sufficient to prevent issues with the sophonts’ health. (I’m guessing livestock is more adaptable and able to be modified)
Fun gravity fact!! If we really want to get in the weeds, the earth’s surface is actually accelerating you outward! The curved paths we take during to the warping of space time (gravity) are straight paths through space time (think of a straight path on the surface of a sphere and how it’s still curved due to its geometry). We are trying to follow that path (or geodesic) but eventually earth gets in the and pushes us instead. PBS spacetime has great videos on gravity and other physics topics if folks wanna learn more!
Sen
Mhm. Delicious explanation pages. I unironically love them. Dump that info into my brain!
Can’t wait to see the Runaway in action! Bip must feel like a bird with clipped wings, grounded on Dirtball.
JoB
More like a bird buried alive, considering that the Runaway was never meant to make planetfall (short of getting scrapped, like what Dirtball usually does to “visiting” spaceships) …
… though that’s still speculation, to an extent. We don’t have information whether Bip – or, for that matter, any memories they might still have from Nabi-Nabu and earlier – has always been a deep-space-ship AI, or may have spent time “grounded” like Calcery did. Hmmm, actually, I’ll throw in an AMA request to that effect …
DasGanon
If you want to see another “universe” that does the same thing, Battletech does this too! It’s super cool.
Cat
And “The Expanse” too!
Phasma Felis
One thing I always thought was neat about BattleTech’s spaceships: In a galaxy of brutal war, all combatants have mutually agreed to never target each other’s jumpships, because they’re fragile and defenseless and they’ve all lost the technology to make more. You wait ’til the armed and armored dropships undock and you attack those. If you attack the enemy’s jumpships, then they’ll attack *your* jumpships, and pretty soon there will be no more FTL and no more interstellar empires for anyone.
(At least in the Succession War era, shit gets weird in the later periods)
Vinemaple
Erma Felna, EDF also did this, back in the 1970s. Really blew my mind, to discover that an old funny-animal space opera was using minimal rubber tech, and trusting that the audience had osmotized enough from the Apollo program to keep up!
ryttyr
Wouldn’t the second method be very fuel inefficient from having the thrusters on basically the entire trip instead of accelerating to a certain speed and then use the lack of gravity in space carry you to your destination? I know that method would mean no gravity for the vessel, but it’s just such a huge waste of resources, you have to expend all that fuel to accelerate, only to then use just as much fuel again to decelerate.
Jay Eaton
Welcome to science fiction, we do silly things sometimes because it would be cool if they weren’t ridiculously impractical. This is something called a “torchship” and it is an excuse to have gravity on a fast-as-possible (without flattening the inhabitants) vessel that ignores the fact that acceleration to midpoint is a both a tremendous power demand and an inefficient use of fuel. In universe, fuel is pretty cheap, fusion is viable for propulsion, and the Runaway is a fancy sportscar of a ship designed for expensive “express” ferry trips and shipping. The majority of vessels do Hohmann transfers and use spin gravity.
TeacupAngel
Jay already commented the in-universe and writer’s reasons do to it, but to give an IRL example, think of air travel. One could also call it a huge waste of resources compared to just taking a train or a ship somewhere, but as fuel got cheaper and airliners more efficient, we eventually decided that waste of resources is worth it for the convenience. And as this page itself notes, space has the extra issue that time spent there can easily cause health problems. If paying more meant you could avoid spending a year in a tin can feeling your health getting worse and getting that large dose of space radiation, wouldn’t you do it?
Varyon
Heck, a “closer to home” example (although I think your aircraft vs watercraft example has a closer efficiency comparison to continuous-thrust vs Hohmann transfer) is that an automobile traveling 30 mph (~50 km/h) uses less fuel to travel a given distance than one traveling 60 mph (~95 km/h), yet just about everyone on the highway is going to opt for the latter because a) it gets you to your destination faster and b) the difference in fuel cost is typically worth taking less time (gas in my area is currently around $3/gallon and my car averages 50 miles to the gallon; going 100 miles at 60 mph takes 1 hour and 40 minutes and costs me around $6; going 100 miles at 30 mph would take 3 hours and 20 minutes – even if that somehow cost me nothing, I’d rather pay the $6 and get there in half the time). If you have drive technology that can manage thrust for the entire trip without needing a sizable fraction of your ship’s weight taken up by remass, and remass isn’t tremendously expensive, you might as well just thrust the whole way to get there faster. And torchships are a case where that typically applies. Now, I don’t know if any realistic drive can manage that for more than extremely-short trips, particularly if you’re looking at maintaining 1G or close to it, but superscience is a long-held tradition for science fiction (and torchships typically qualify as such, to my knowledge).
Stray Cat
From what I understand, because air friction isn’t a thing in space, theoretically you could accelerate to the speed you want to go at, then just cruise along with the thrusters off until you need to slow down and/or change direction. Minimising exposure to microgravity would explain why, say, a ship using accelerative gravity wouldn’t do that. You could probably do so in a ship using centrifugal gravity, though.
Again, this is from my admittedly limited understanding of space physics. Interesting stuff, either way.
Michi
If you can, read if you have the fuel to do it, the Brachistochrone trajectory is the fastest way to get from A to B. Because you accelerate from A to midpoint and then slow down from midpoint to B. Any coasting in the middle will make the journey longer. The pseudogravity is essentially a bonus in that.
Vinemaple
And ships like the Runaway can do that because in RttS, nuclear fusion power was developed in the year 2000. IRL, we’re still stuck with pointing, burning, and drifting.
Teod
Wait, RttS timeline diverged already? I thought it was supposed to be a plausible future from now.
Jay Eaton
RttS is absolutely not a plausible future, it’s a fantasy story with a thick coating of verisimilitude. My real life opinions on the viability of space colonization and humans meeting aliens are uh… pessimistic. As in, I don’t think the latter will ever happen because space is too big, and the former (if it happens before human extinction) will be on a timescale irrelevant to modern humanity. But it’s a fun setting to build for.
BullCityFats
The problem isn’t so much fuel as reaction mass. A ship can carry only so much, even if fusion power renders energy considerations moot. As Jay mentioned, accelerating near turnover is inefficient because you will be traveling for very little time at the increased speed. The fastest way to get there is to expel half your reaction mass as quickly as possible at the beginning of the trip, coast at this top speed, and then use the other half of your reaction mass as quickly as possible when you approach your destination. That way you’ll be spending the most time at your maximum velocity.
Pepper
As the ship’s *velocity* changes, surely? I don’t think we want a change in acceleration except at the start, midpoint, and end of the trip.
Apollo235
Noticed that too, looks like there was a draft where it was ‘as the ship experiences acceleration’ and another that was ‘as the ship’s velocity changes’ and they accidentally mixed the two
Jay Eaton
Oops. Fixed.
sugar-goat
Okay, “you’re currently accelerating towards the center of the earth, thank goodness there’s a crust in the way” is definitely gonna haunt me for the rest of my life
JoB
As it should. After all, that acceleration will kill you if you experience it from a starting point, however you got there, that is 10+ m above said solid crust. Dreams about this event (“falling”) are considered nightmares for a reason.
Not that trying the same with a gravity well that lacks a solid crust (Jupiter) would be that much more enjoyable …
Miles
Probably a dumb question but do the passengers in an Accelerative Gravity ship feel like they are getting heavier or lighter as the speed increases or decreases? Or is it too gradual to notice?
Michi
Not speed, acceleration is what creates the feeling of extra weight. So if you are going 5000 kph or 10 kph doesn’t matter if there is no acceleration, you’ll float about in your spacecraft.
I’m not sure how sensitive humans or other sophonts are to acceleration, but you can feel it in an elevator at the start or the end for example, while on a train leaving the station it usually doesn’t register. Then again, elevators and trains move in different direction relative to gravity. TLDR depends on how quick the mass of the propelled object changes. IIRC the Space Shuttle had to throttle down the engines towards the end to keep within tolerable limits of the astronauts, because with all the fuel gone the vessel got so light the engines thrust created higher and higer acceleration.
JoB
> if you are going 5000 kph or 10 kph doesn’t matter if there is no acceleration
For reference, if the 230 km/s (= 514495 mph) of our solar system orbiting the center of the Milky Way were to have an effect, everything not firmly attached to the planet’s crust would end up smooshed together and doing daily laps around Antarctica.
Including all the water in the oceans, which would likely shove everyone trying to stay swimming on top of it clean into orbit. Consider this your last call to take a “swimming in a space suit” course. 😛
Trees
After trying out different habitats across the galaxy, Red Human and Purple Avian find themselves roommates when they move onto a rocket ship together. One of them is messy and the other one doesn’t like mess! Hilarity, no doubt!
Vinemaple
Trees, your MIND
Eux
Jay, i just love your work. the way you design everything from Talita and centaurs to the Runaway to the infographics and design about science, tech, signage, aesthetic and biology driven sensibilities in the different sophonts’ worlds… its all 👏so👏good👏
Vinemaple
I didn’t know there was a word for a brachistochrone transfer! I’ve always just described it… I guess I shouldn’t be surprised, since we don’t use them IRL, and we use Hohmann transfers all the time.
I hope we get a page on fusion propulsion. I haven’t been able to figure out if the Runaway’s engines work like the fission-powered rocket engine (which blasts out hydrogen reaction mass, superheated by a nuclear reactor) but with a fusion reactor, or is a completely different system. Deuterium goes in, what comes out?
jdeltia
i think it actually might be a term jay made for the book, i can’t find too much about branchistochrone transfers… but there is “constant acceleration and that’s pretty much the same thing!
jdeltia
i think it actually might be a term jay made for the book, i can’t find too much about branchistochrone transfers… but there is “constant acceleration and that’s pretty much the same thing!
jdeltia
i think it MAY have been a term jay made for the book, i can’t find too much about branchistochrone transfers… but there is “constant acceleration and that’s pretty much the same thing!
jdeltia
oh crap i pressed the post comment button too many times because it kept not working, now there’s three copies of my comment
JoB
You might want to look it up without the extra “n” …
jdeltia
oh i know about the brachistochrone CURVE, i just couldn’t find anything about a “brachistochrone transfer” that’s done under constant acceleration. but the name still makes sense, brachistochrone means the fastest path and accelerating then decelerating is the fastest way to transfer between planets
jdeltia
so THAT’S what the “Read ahead on Patreon” button’s icon is, it’s the transfer the runaway does between planets!
whirly
oh! wow, good catch.
waterfan2
After trying to understand as much physics as I can, my understanding of gravity is that it’s …
a. Pretty useful.
b. Weird (do gravitons exist? Who knows!).
c. ‘A heartless bitch.’
JoB
d. soooo weak
Michi
e. Always [url=https://en.wikipedia.org/wiki/Big_Crunch]wins[/url], or [url=https://en.wikipedia.org/wiki/Big_Rip]not.[url]
Michi
Whelp, how do I do hyperlinks in here?
jdeltia
it’s in html (i just found this out too), you use an anchor tag. an anchor tag looks like this (substitute G with ): GaLG/aL. and then you put in an “attribute” between the greater than and less than of the first part that looks like href=”example.com”. the title goes between the two greaterthan-lessthan pairs. so in the end, you get this: Ga href=”example.com”Lsuper cool titleG/aL. you can also look up diagrams that show you how to type so this comment is somewhat redundant
jdeltia
*substitute G with greater than and L with less than, tried to put the actual symbols and it got html’d out
JoB
> Whelp, how do I do hyperlinks in here?
You type <a href=”https:en.wikipedia.org/wiki/Big_Crunch”>this</a> to get this.
JoB
*derp* …https://en… of course !-/