Assume you have an inhabited O'Neill or McKendree-style cylindrical habitat or ship designed to move between star systems—not unlike Rama. What are the major advantages and disadvantages of orientations A and B below?

A: propulsion along longest axis; B: perpendicular to longest axis

Yellow indicates direction of travel and and acceleration. Blue, rotation.

Notes: The length of the journey is irrelevant, and you should take it as given the structure can withstand the stresses associated with rotation, acceleration, etc. The pseudogravity generated by rotation (including any influence from propulsion) should fall within realistic tolerances of plausible Earthlike organisms—say 0.5–2.0 g—as the ship must remain habitable for the duration.

  • $\begingroup$ Do you actually have any examples of the "B" pattern? I don't think I've seen it anywhere. $\endgroup$ Commented Feb 19, 2021 at 11:33
  • $\begingroup$ @StarfishPrime: imagine a ferries wheel... Both the axle and the gondolas look like pattern B ;D $\endgroup$
    – user6760
    Commented Feb 19, 2021 at 12:01
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    $\begingroup$ @user6760 ferris wheels don't rotate to produce artificial gravity, and as such aren't really relevant to this situation. $\endgroup$ Commented Feb 19, 2021 at 12:40
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    $\begingroup$ B is best, of course. It provides a Carnival Ride. with gravity bouncing between (Spin+Thrust) and (Spin-Thrust). The added thrill of imminent structural failure cause by this imbalance, plus the greatly added exposure to obstacle impacts, makes for a really thrilling ride! After all, the poor engineers that had to design and install a propulsion system for that arrangement will be VERY stressed, and could use some diversion. $\endgroup$
    – PcMan
    Commented Feb 19, 2021 at 14:40
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    $\begingroup$ As a potential addition to this, a rotating habitat/ship is very likely to have a non-rotating shell surrounding it. This can provide structural support for the rotating section as well as allow heavy shielding without putting the stress of the shielding's weight on rotating section. $\endgroup$ Commented Feb 19, 2021 at 16:30

11 Answers 11


Option A is the safest long-term orientation

This is because it minimizes the surface area of the cylinder along the direction of travel. Assuming the cylinder is traveling at relativistic speeds i.e. some small % of the speed of light this will be an important consideration because the vessel will almost certainly encounter gas molecules and dust particles etc if not larger objects during its journey.

And the kinetic momentum of those objects will be equal to to their mass (however tiny) multiplied by the velocity of the ship at the time of impact (which is huge).

So if the ship is traveling at say .1 C on a journey to Alpha Centauri and the trip takes about 50 years then for most of that time it will be being bombarded by high energy impacts and will probably need shielding of some kind. I say probably because as far as I am aware we don't have good data yet on the density of the interstellar medium in our local neighborhood.

Point is the shield can be much smaller if it just has to cover one or both ends depending on whether it 'flips' at the deceleration point or possibly has engines at both ends.

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    $\begingroup$ We have a reasonable grasp on the density of the ISM around us. The details are missing, but they're less important here. A starship travelling at .1c doesn';t have to worry too much. One travelling at more like .5c very much does, and wouldn't be able to flip over half way, either. $\endgroup$ Commented Feb 19, 2021 at 11:28
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    $\begingroup$ Thanks, I knew work was being done but not how far it had gone. Actually I was concerned more about larger objects than gas molecules and other nano scale objects, particularly millimeter and sub millimeter impacts. $\endgroup$
    – Mon
    Commented Feb 20, 2021 at 23:11
  • $\begingroup$ Does B have any advantages over A? I already expected (knew) A was better overall for the reason you list, but the question isn't about which is best. $\endgroup$
    – rek
    Commented Mar 9, 2021 at 15:54
  • $\begingroup$ PART 1: Not that I can see. Assuming a more or less conventional (hard SF) form of propulsion like a fusion drive acceleration forces would be manageable under both options (but still easier in option (A). This because high accelerations are not needed & indeed that would be inadvisable generation mission. But the thing is while accelerating passengers will experience different force effects. $\endgroup$
    – Mon
    Commented Mar 10, 2021 at 1:19
  • $\begingroup$ PART 2: In option (A) with the drive at one end passengers will experience a slight continuous acceleration force (gravity) towards the (rear) end of the cylinder where the drive is attached. At the same time the level centrifugal force force (artificial gravity) will be constant all along the main access. $\endgroup$
    – Mon
    Commented Mar 10, 2021 at 1:26

Option A has better engine mounting, and less wear and tear while thrusting.

As another answer has already pointed out A will take impacts onto a smaller surface area - that's a good thing, but there's one other thing to consider - where the engines go and how they are fired for acceleration and deceleration.

Option A you can have an engine at either end. While accelerating or decelerating the engine can be firing constantly, and gravity will remain down and everyone will have an equal force pushing them slightly sideways.

If the ship is designed for constant acceleration, you could build your cylinder interior with a saw-tooth floor such that rotation + acceleration forces always equal the felt "down".

Option A also allows for a single engine - at the mid point of your journey you just need to stop rotation, rotate 180, start rotation, and then fire the original engine again.

Option B, (barring some larger structure in addition to the cylinder) the engine has to be on the curved outside of the cylinder, and can only fire in pulses when the rotation lines up with the direction of travel. You'll only need one engine for accelerating and decelerating, but you're going to have to run it in few second bursts which will not be great for longevity. Also citizens will experience varying forces depending on where they are - they could be jolted up / down, or side to side, which would drive them mad as it would feel like every 90 seconds they'd get hit in the gut. This will probably also loosen every bolt on the ship and trip everyone walking over.

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    $\begingroup$ "Option B, (barring some larger structure in addition to the cylinder) the engine has to be on the curved outside of the cylinder, and can only fire in pulses when the rotation lines up with the direction of travel." You could also mount engines on rotating brackets mounted on either end of the rocket. $\endgroup$
    – nick012000
    Commented Feb 21, 2021 at 9:03
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    $\begingroup$ Stopping rotation would be a major disaster, as anything held to the ground mainly by (pseudo)gravity will lift - water, earth, pavement, people, animals - will start to drift through the air. You'd probably just reduce rotation, so the structural stress on the cylinder and the Coriolis forces inside each don't create something really unpleasant. $\endgroup$
    – toolforger
    Commented Feb 21, 2021 at 9:32
  • $\begingroup$ @nick012000 that creates far more problems, since you now need some for of gigantic bearing that can withstand the forces, adding a bunch of extra failure points, what's worse you don't need one set you need a set for each engine, and you had better hope you can keep them al synched. $\endgroup$
    – John
    Commented Feb 22, 2021 at 15:57

Option B is utterly terrible in almost every regard. It's harder to build, it's less structurally sound, the gyroscopic effects the drum creates will make it into an absolute nightmare to control, and the gravity will begin to fluctuate while you are under acceleration, which will give everybody inside seasickness. The increased frontal crossection is almost an afterthought at that point.

In comparison, option A devoid of nearly all of these issues. The vector of simulated gravity will diverge under the acceleration, but it will do this uniformly across the entirety of the cylinder, making the inside feel more like a conical valley with the bottom towards the engines rather than just a valley, and it's far easier to make this type of layout more structurally sound (It's easier to build a mile-high rotating tower than a mile-long rotating bridge section).


Layout A can be Hinged to Compensate for Acceleration

While there are a lot of answers so far that say that A is best for things like drag and wair-and-tair, all of those things can be hand-waved away depending on the technology in use, but since this question is about the creation of artificial gravity using a rotating habitat ring, there is one advantage of A that you can not handwave into layout B, and that is the ability to compensate for acceleration.

Layout A can be made to turn your cylinder segments outward during the acceleration portion of your journey so that you retain a consistent downward appartant G at all times. Layout B can not do this; so, acceleration would cause drastic cyclical changes in your appartant G; so, accelerating would feel like the floor is bobbing up and down like a ship on rough seas.

In the below example, your ship is accelerating at 0.6g and the rotation of your cylinder is slowed to 80% of normal and opened up to ~36.9°. This will create a vector sum of 1.0g straight down from the cylinder segments giving you consistent Earth like gravity despite your acceleration... at least, ideally speaking.

enter image description here

As pointed out in comments, there would be a different apparent gravity closer to the hing than at the tips; so, you might really get 1.0g in the middle, 0.5g near the hing and 1.5g at the tips. This still falls into the OP's acceptable range, and more importantly prevents nausea from skewed or bobbing gravity... but is still not really ideal. You could solve this though if your ship can accelerate at 1g, because then you would not need to spin your habitats at all during acceleration, only once you reach a cruising speed.

An even better layout would be Option C

By using a shorter cylinder with a greater radius, you can minimize the apparent gravity variation between the bow and stern of the cylinder when hinging it at sub 1G accelerations, but the advantages do not stop there. Narrow cylinders create a much sharper gravity gradient than a wider one which would make your feet feel heavier than your head. Moving through a more narrow rotating cylinder can also cause vertigo since things in certain directions that feel straight will look more curved and vice versa. Moving in a narrower cylinder also means that you will feel more lighter or heavier as you walk depending on if you are moving with or against the rotation. So, on many levels, a wider cylinder helps gravity, and your environment feel more normal.

enter image description here

  • $\begingroup$ You don't get a constant 1g everywhere when you open up configuration A, since the centripetal acceleration depends on distance from the axis of rotation. People closer to the axis at the aft end of the ship will feel greater gravity than those further from the axis at the fore end. A sawtooth pattern with angled floors but near-constant radius would fix this issue, as mentioned in another answer. $\endgroup$ Commented Feb 19, 2021 at 16:00
  • $\begingroup$ @NuclearHoagie Good point about the radius, but a sawtooth pattern only works if your ship is under consistent acceleration for the whole journey, I've modified the answer accordingly. $\endgroup$
    – Nosajimiki
    Commented Feb 19, 2021 at 16:19
  • $\begingroup$ I'd think you get greater apparent gravity at the tips rather than hinge -- If you set rotations for 1G at the tips, the apparent G would taper down linearly to zero as you approached the axis. (a=r*w^2, a=accel, r=radius, w=rads/sec) $\endgroup$
    – Dave X
    Commented Feb 19, 2021 at 18:23
  • $\begingroup$ If you don't do the hinging, with 1g thrust and 1g spin, subjectively it would feel/work as though you were in an odd 1:1 slope, valley at 1.41g -- down is 45 degrees to the floor. If you hinge the terrain internally, it would become like terraces. $\endgroup$
    – Dave X
    Commented Feb 19, 2021 at 18:32
  • $\begingroup$ @DaveX nice points, indeed, but for smaller accelerations, 0.1g as an example, it can be passable, and the difference is a second-order effect. with 1g however, it is clear u do not rotate the thing at all, just open it, reorient the segments. and we then have a clear B case. $\endgroup$
    – MolbOrg
    Commented Feb 21, 2021 at 0:28

option B has some advantages over A, so as A has some advantages over B.

impact reduction

as mentioned in another answer frontal projection in the A case is smaller so the number of impacts can be lower than in case B. However if some significant impact happens, something sizeable, like a mass of let's say square meter of the hull, depending on speeds there may be different effects and in case of A it may go through the whole hull, axial-wise, while in B case it may affect only a section of the cylinder, still may be disastrous effects, but a portion of construction to survive can be bigger with the same impact.

But really depends on how protection and detection are done and other factors, if done right then generally A case is preferred.


For acceleration cases, B may be better or may not, depends on tech and other factors, but general tendency holds - accelerating force can be applied to structural elements more evenly(if we have some line of nozzles on the side) while accelerating in A orientation it will have the most stresses at the bottom(engine/acceleration nozzle) which will put restrictions of accelerating rate for given length/mass of construction but lesser forces on top.

That difference can be mitigated by design and technological means, to some extent, but the B case is more freer in terms of length of construction.

Addressing that rotation pulsing regime of work of those engines - they do not have to rotate with the hull at all, so as it may be a perfectly fine mode of operation for some thermonuclear engine, or even be the only mode of operation it works - so depends on tech heavily, but generally is not a problem.

apparent gravity

it may be more convenient to have changes in "gravity" once some minutes than the constant inclination of that force. Constant inclination can affect tree growth, if acceleration time is long enough for that, and it may have some weird effects when there is no acceleration.

However, changing force periodically over a long period of time may have some unpredictable health effects, so with a slight advantage A wins here, but if the problem is known to have no bad side effects on humans convenience of straight-looking trees may win.


B will have more drag aka deacceleration forces against the interstellar medium, that's a fundamental one.

All in all - A is preferable, most likely, but the difference isn't huge


it is an offtopic part of the answer, but it seems that my statements brought more misunderstanding even for the smart part of the community, than I have expected it to be.

Hence I kinda forced to put some context and perspective and dive a little bit into space habs design considerations. Not extensively, but it just a big topic by itself.

Structural stress by acceleration

  • what I mention in the main body of the answer seems not apparent, so let try to clarify what's it meant

let's have some rudimentary picture drawing of A and B cases, left is B case, right is A case

enter image description here

in A case we have a typical rocket problem - internal pressure, thin walls, elongation in the direction of acceleration. The most compression force the structure experience is at the bottom where the weight created by acceleration is connected to the acceleration force created by the engine nozzle. or we can say it is a typical skyscraper problem - the bottom has to be stronger as it keeps the weight of everything above.

Case B has a different situation, it indeed is more dynamic, but is a separate question('ll address that too). The hull is loaded by stretching forces - it is internal atmosphere pressure, it is weight created by the rotation of that volume - all the soil, all the materials used so as humans try to escape and fly away, by ripping hull apart and one of the things holding it together is the tensile strength of the hull. But tearing apart I mean stretching hull construction until it breaks - direction and orientation of stress forces are important here.

engine working in the B case works against those created forces which would like to rip the hull apart. So in some sense(not so simple) but there is the reduction of those forces along the line of acceleration (on both sides of the hull next to the engine and on the opposite side). However, of course, it has a price and we have deformation and other changes including stress forces - in general, the situation gets less symmetrical.

here we have to touch 101 of interstellar with no FTL

  • really guys, u have to put less magic in your questions, because u lose touch with reality, robbing yourself of the context and intricate details which as classics say is greater than a typical imagination.

Let's say we have a thermonuclear engine, exhaust 0.1c, target speed 0.9c. Applying rocket equation and ignoring relativistic effects which aren't that big here we get a picture:

  • fuel to construction ratio is 22000:1 (deacceleration done by magic, not included)
  • with 8.5m/ss we reach the target speed in about a year, so less than about 25% of our total travel time for the closest possible target.

if the target speed 0.1c(i know many like that number) then:

  • fuel to construction ratio is 2.7:1 (deacceleration done by magic, not included)
  • acceleration at 25% travel time - 0.1 m/ss, travel time close to 40 years to the closes target.

So accelerations of 0.01-1g cover pretty much every target speed which is practical.

And surprise surprise 0.01g is enough for all of those who do not see any practicality in interstellar speeds above 0.1c

Structural stress by acceleration, strikes back again

in case of A, and acceleration 0.01g the length of a cylinder is limited(if we take modern skyscraper as a somewhat lazy reference) by 100km, and it scales linearly, so 0.1g it is 10km, 1g it is around 1km

in the case of B, it is more tricky, but the first thing to notice that even 1g acceleration, forces from the engine comparable to the forces which already act on the hull structure and sort of (if we smear average it over the hull, which may or may not be the case by design, later about that) does not exceed them, and somewhat even counteracts them in some nonsymmetrical way.

So if your diameter of habitat is 20km as an example, u still have the ways to make all that work under 1g, in the respect to the structural integrity of the construction. As 1g is kinda too much for A and B. Sure where is the will are the ways, but it all depends on the maturity of technologies and we have the most basic situation here.

And the length of your 20-100km diameter habitat is not limited at all.

Second thing to notice is that 0.01g changes of apparent gravity barely even noticeable, it is like to drink a bottle of Kvas and increase your bodily water content by a 0.5-1L

yes, sure, 0.1g is more noticeable, but for closes targets, it does not reduce your travel time that much, if u keep the same target speed in goal, so it applicable not to all case

but what if u don't, 0.1g and >0.1c target speed

but is we stick to higher acceleration rates and willing to travel faster, mass ratio of fuel to construction:

  • 0.1c, 2.7:1, with deacceleration 7.3:1
  • 0.2c, 7.4:1, with deacceleration 55:1 <- close to your typical rocket
  • 0.3c, 20.1:1, with deacceleration 404:1
  • 0.4c, 54.6:1, with deacceleration 2980:1

here we use a fraction of light of speed as speed, but sure habitat as the space ship is good because u can use basically any speed even much lower than those, but then a difference between A and B is negligible or not existing, and one can use accelerations way bellow 0.01g.

So even with a 2.7(7.3) ratio, we have to think about fuel which we carry to propel the construction - where it is, how we carry it - is it inside of habitat or has it its own separate tank, how we combine all the stuff together, etc. And more so with other ratios, those questions have their solutions.

design, of a space habitat with the intention to accelerate it.

let's draw again:

enter image description here

  • Great picture from me, as usual, I'm great, MIT license forever, we live and die for it, no pasaran!

So in some sense, it is your typical A case, in a sense engine and fuel block aren't rotating. There is a gap between it and the rotating space habitat hull we have to close somehow.

  • a thing to notice that the engine fuel tank block isn't rotating not for the reasons of engine management problems wear tear and difficulty to pulsate it which are no problems at all(and no I won't dive in it here, we already way off-topic, ask your own separate question about how thermonuclear engines work). reasons are mostly about fuel containment structure, which is an important question considering that even in a low case scenario it 7 times more than the whole habitat structure, with a slight acceleration of the structure in which fuel (H, D, T, He3) are stored can be very lightweight, basically non existing as we can have those things as ice form (beyond Neptune it is okay for hydrogen to be ice) or a liquid with low vapor pressure.

  • another thing to notice, engine placement isn't optimal, but again there are more reasons and more questions, and more design considerations - as an example we have ideas about great materials which work superiorly under tension(cnt) and no great materials that do the same with compression(first can be converted into a second, but there are nuances).

So we have a gap to close to accelerate the cylinder, to conduct accelerating force to it and there are ways to do so.

your typical maglev solution is easier to imagine on that scale and with that particular design - but its way too energy-hungry, tech-savvy, and all that

An easier solution would be some air cushion between the fuel block and cylinder, and gas pressure acts on the external shell of a cylinder, evenly distribute the force on it. The principle lays in direction of air bearings.

  • again sure there are technical nuances to recapture that air as an obvious one and a lot more. They have their own solutions.

a bit more realistic design which considers few factors.

That oblivious gap problem emerges mostly because of reduction design, making it simpler than it actually may be.

if u still in the '70s and imagine cylindrical habitat like a plain rotating tin can - u do it wrong. it has to be a more complex structure, so as for benefits a different design may provide, so as a result of necessity to solve problems which are specific to such design.

enter image description here

one of the approaches is to have a multilayer structure, which advantages it may have:

  • alleviate stress on the internal hull where humans are, or remove it almost completely. the internal structure is not stressed means, in the first place higher safety of inhabitants and lesser concerns about the degradation of the structure. it allows having a deeper soil, where u need it as an example(for tree and forests as an example).
  • it also removes or eases constraints on the diameter of a structure so do take your 15km sky with you, which looks and feels like home.

Compensation is done by the pressure of gases which are in the volume where intermediate layers are placed. But we get friction of the hull and that surrounding air and if it gets supersonic it may be not the best thing to have. And for that, we have those intermediate layers, which also are shells. they are not necessarily massive or robust, something on a lower side, just to get the job done - which is to separate gas volumes and have certain, defined by design, differences in speeds of shells, keep it subsonic to whatever value is good. (layers are not needed, but it is a more advanced approach, or less)

How they don't rub each other or collide? yeah, imagine an airplane(this thing with wings) kind there are ways to keep those shells separate - more strictly it gets back to air- and hydrodynamic bearings.

Most of the structural load bears the last shell - external hull, which by the magic of that layered design can have zero(0) angular velocity. But at the same time, it is the most pressurized can and has to be the most strong out of all those layers. But what advantages it gives:

  • the part of the construction which holds things together, does not rotate and thus isn't affected by centrifugal effects, and its mass brings no additional requirements to the strength of the construction. Meaning as it does not rotate there are no additional forces we have to consider because of its mass.

    or we can formulate it differently - there is no limit on the mass of the external shell. it can be as massive as a planet, or km's tick - and more massive a bigger internal volume habitat it can hold together. So it can be as tick as needed.

    in a sense, it is the opposite situation and one closer to the reality than when we look at the guys who imagine them being able to burrow a hole in an asteroid and rotate it to make habitat from it and for some magical reason it does not fly apart by what is in essence greatly exceeding The Roche limit for the body. The shell does not rotate and it stays put.

  • another good thing about not rotating shell is the ability to put some loose layer on top of it, which won't fly away, but at the same time serves as a protective layer for all kinds of things, like laser rail gun projectiles ... uuups sorry, that's a different subject, like it protects against meteorites particles radiation, drunk rocket drivers, etc. And being in a loose state it does not mind that mich being bombarded periodically, maintenance is a low effort thing, etc.

    and in the case of accelerating space hab, it is where your engines press and where(maybe) your fuel is and the shielding from enormous fusion reaction happening next to u, which effects dwarfs any other effect like space radiation or radiation from the collision with interstellar medium - if we talk about 0.1g as an example, so honestly the same with 0.01g I guess. This can be solved but then it requires a different approach to your engine attachment.

    • I'm especially amused with the comments like at A case one end we will have shielding compartment etc - guys u are delusional, next to the end of cylinder unknown amounts of Kuskina mother happens per second, to propel the massive thing, and u think 10km apart from the place is a safe place? And if not, do u really think it much stable to accelerate the stick in A case(but that will be a separate matter). The way I drew it also isn't much better, but there is more flexibility, not necessarily by much, but still.

there are other advantages but enough about that for now? more space habitat question for wb, I guess.


Some mention stability and it is indeed an interesting question, and have to start from the very beginning - O'Neill cylinder is not stable, to begin with.

There 2 videos to illustrate:

Rotating Solid Bodies in Microgravity

Dancing T-handle in zero-g, HD

while all life happens at a pace habitat masses are moving inside of it, stresses change, minuscule bending happening, etc and even if it may be okay for a year or many years if we consider it over longer periods of time it is problem to be solved p reactive propulsion is one of the ways, multilayer structure another one out of many smart solutions we can come by.

To grasp instability of rotating rod it is not necessary to go to iss, it is a widely known problem in the machining of shafts, to which attention has to be paid.

So if someone imagines that pushing A case means more stability, it is not correct - a slight displacement on a different end for a structure that is rotating in one direction and being accelerated at the same time - is a first step for the whole construction to go haywire (it can compensate itself, but it depends on own frequencies and a lot of things) and if turning point conditions are missed, u can't do anything to save the situation - cutting down engine won't help, too late rotating forces already work on making it go out of control. u tilt engine - u do it even worse to the stick - the thing starting to wobble will be a best-case scenario.

A space habitat on the scale when it meaningful to call it a colony can't be considered a rigid body.

There is no other way but B (ed: not so fast boy)

Diving a little bit in some specifics of certain direction or specific design family of habitats - it is possible to see that for those there are no other ways but to be oriented in B orientation. And it does not matter how big the disadvantages or advantages of A are, it is the only way for that design to go - if we push the throttle a little.

it does not mean there aren't other designs and approaches which can't benefit from A, but I hope this clarifying section shows that there are designs that have their advantages to be built that way, on their own, fulfilling the space habitats function and which would prefer B case.

There is no other way but A (not so fast boy)

After extensive consultations with our space hab design engineer @John and him investigating papers on motion sickness (comment section for links and refs) it became apparent that even if B case may have some advantages from a structural perspective(so as problems), it has some disadvantages for human content of the space habitat. As a vector of apparent gravity will oscillate around a vertical axis, and it causing some unwanted consequences, like motion sickness, because the threshold is quite low, ~0.007 m/ss.

So people have to adapt, but it is not known which other consequences it may have to humans living 24.7.365 in such conditions. Have seen an infant living on a yacht in Atlantics, just recently on yt, seems doing fine enough, so as there are some historical tribes which live on floating means, so as there are people living half-year at sea, but still, it is a problem which has to be considered, especially on the scale millions or more of different people living in such conditions. 33% of children below age 12, are more sensitive to the problem, for lesser ages it can be as high as half of them. people adapt, but yeah the problem has to be investigated specifically before the B case is rated for humans.

  • Good news for treemans - they do not have to have wind for their branches to grow healthy and strong, maybe.

So B case has problems as well.

There is no other way but A or B

The full circle here and we back at more moderate statement A and B have their pros and cons. And it is a matter of design decisions - which problems do you choose to negate and which you choose to live with. either way, no matter the choice there will be a price to pay for the decisions.

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    $\begingroup$ Your advantages for B are negligible or nonexistent. It makes shielding harder (mostly), engine mounting harder, disposition of the internals less convenient when under thrust, potentially induces varying gravity, requires more internal structures and reinforcement. It is all round terrible. $\endgroup$ Commented Feb 19, 2021 at 11:30
  • $\begingroup$ @StarfishPrime if one uses own exhaust plume, shielding isn't harder, there are reason for that and I was quite certain had mention some of them here on wb, but looking at answers I have, it seems I did it elsewhere, same for certain aspects of designs of space habitats. So, i can see how it may not fit for the models you have/imagine, been there done that, but one of the points writing that answer was to challenge those a little bit. i was dead certain there was an q/a on wb at least about shielding problem for interstellar, if there is none fire a q, may I or s'one else add that information $\endgroup$
    – MolbOrg
    Commented Feb 19, 2021 at 18:34
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    $\begingroup$ B has much higher stress problem, because all engines must produce exactly the same thrust for exactly the same time or suffer serious stress issues. one misfire and your structure will tear itself apart. Worse acceleration will try to make the entire structure bend which will introduce all kinds of unpredictable thrust issues as the thrust vectors will actually fluctuate as the structure bends and recoils. $\endgroup$
    – John
    Commented Feb 20, 2021 at 17:14
  • $\begingroup$ Hi @John, added some clarification section, hope it addresses some of your concerns $\endgroup$
    – MolbOrg
    Commented Feb 20, 2021 at 23:09
  • $\begingroup$ yours as well, @StarfishPrime, hope it clarifies something, if not - let me know $\endgroup$
    – MolbOrg
    Commented Feb 20, 2021 at 23:10

Option C: it travels in whatever orientation it happened to be in already

This option will make the most sense for "short" trips (i.e. within a solar system) at low acceleration, i.e. much less than 1g. (Such a trip may of course still take many decades.)

In such a case it probably doesn't make sense to reorient the cylinder. It's an extremely large rotating object, and reorienting it means fighting against gyroscopic forces, and must be done extremely slowly to avoid generating weird Coriolis forces inside. On the other hand, if the thrust is low enough then you can probably use directional thrusters to accelerate it in any direction without reorienting the whole structure. The thrust needs to be low in order for the structure to take the load, so journeys will be long, but if there's a self-sufficient civilisation living inside it that probably doesn't matter all that much.

For travel between the stars you'd probably want to use as high an acceleration as possible. In that case you'd almost certainly want to build it so that it can take compressive loads along its axis, and reorient it to position 'A'.

  • $\begingroup$ The question specifies interstellar travel, but your point about slow transit within a system is noted. $\endgroup$
    – rek
    Commented Mar 9, 2021 at 16:17

During the transit between stars, the only difference is the view. As far as rotation of the ship is concerned, the travel is not going to make any difference whatever. It only changes what stars you will see out the windows, if you have any windows. Other than that you literally will not be able to tell the difference.

Keeping Rama in mind, in the book it did a gravity well maneuver near our sun. If such an operation was performed near a very dense object (neutron star for example) then tides might be important. You might need to orient the ship so the tides affect it in the direction causing the least problems. That will depend on internal arrangements.

The biggest deal is likely to be the direction of thrust. If the ship has to point "this way" to thrust "this way" (like a rocket typically has to) then you need to orient the ship that way when you want to maneuver. Only matters when you are actually thrusting.


If nuclear powered engines are used Option A is better, safer and easier because the engine can be at one end of the cylinder and the crew quarters and stations at the other end. With this option the crew would get less exposure to radiation from the engines than they would with Option B.

  • 1
    $\begingroup$ These cylinders are large enough that the crew is a fraction of the population. A McKendree has potentially as much internal surface area as Africa as I recall. $\endgroup$
    – rek
    Commented Mar 9, 2021 at 16:18

As in The Expanse, option A could be used like a tall building. When accelerating, this means that gravity is created in the correct direction. The ship can then be flipped and decelerate to move 'downwards' to the passengers and keep that same gravity.

The spinning would be a major problem for option B because you're accelerating in a changing direction from the perspective of the passengers - if they stay in one place, they are still being 'pushed' to the edges, while also being pushed opposite to the direction of acceleration, which would cause a variable force, not ideal.

  • $\begingroup$ I don't understand this answer. Are you proposing launching/landing the cylinder as tower on the surface of a planet? $\endgroup$
    – rek
    Commented Mar 9, 2021 at 16:19
  • $\begingroup$ I just meant that there are problems caused if the cylinder is accelerating, which are somewhat mitigated by having the direction of acceleration perpendicular to the rotation and could be very problematic in B depending on magnitude of acceleration. I didn't realise how big it would be when I mentioned the launching (regardless of where from)! $\endgroup$
    – Doot
    Commented Mar 9, 2021 at 16:56

Simply, during the journey there will be no difference whatever. How could there be?

If yourn is a planetary craft, designed to land - and perhaps take off again - then the orientation on approach could be crucial.

If it's a purely inter-planetary craft, designed for travel - ignoring launch or landing - please explain what difference orientation might make?

  • $\begingroup$ friend, u need to read what a O'Neill or McKendree-style cylindrical habitat is, u was a bit too rush with your answer, it not worth it. $\endgroup$
    – MolbOrg
    Commented Feb 21, 2021 at 3:33
  • $\begingroup$ @MolbOrg Friend, you need to read what "up, down left or right" mean in space… or just look at your own diagrams. Orientation is going to make a difference only externally, as for instance if you hang light sales on the cylinder. $\endgroup$ Commented Feb 22, 2021 at 12:17
  • $\begingroup$ orientation is going to make a big difference if it has engines or sails on it. at which point you have to deal with thrust an inertia. those are not just external forces. $\endgroup$
    – John
    Commented Feb 22, 2021 at 15:34
  • $\begingroup$ True, most have assumed that orientaion to velocity vector is the same as orientation to acceleration vector, which not necessarly the case. Without that assumpton question makes not that much sense. So as another assumption was that speed is big enough to make a diference in interaction with interstellar matter, which again not necessarly so, as op didn't stated that clear enough. So then combine all that in a statement, it will look better than current formulation which is of quality of a comment or rethorical question, which does not fare well on wb. $\endgroup$
    – MolbOrg
    Commented Feb 22, 2021 at 19:28
  • $\begingroup$ Also, would be intersting to look at space hab of named types to which landing is an option, so far that part does not fit op's premise - but maybe u forgot to mention something which would make sense out of it $\endgroup$
    – MolbOrg
    Commented Feb 22, 2021 at 19:31

Option B has advantages in radiation shielding/management.

When flying in orientation B, the angle can be selected so that the one end always points towards the Sun regardless of flight path. This end can then be covered with a thick radiation blocker which keeps the rest of the structure in it's "shadow". By doing this, the radiation shielding doesn't need to cover the entire circumstance of the tube and you can have solar panels or thermal management systems which are always exposed to the Sun. Furthermore, it would allow you to do closer flybys to stars and visit inner planets without melting.

Edit: Ya'll seem to think that I'm saying option B is better in every way. I'm not. I'm simply stating that there are potential advantages of option B, not that they outweigh those of A. Jeez.

  • $\begingroup$ The "designed to move between star systems" somewhat derails this answer. $\endgroup$
    – PcMan
    Commented Feb 20, 2021 at 10:47
  • 1
    $\begingroup$ @PcMan Airplanes don't need landing gear because they spend most of their time flying, right? $\endgroup$
    – Dragongeek
    Commented Feb 20, 2021 at 10:49
  • $\begingroup$ Most airplanes don't fly belly-first, either. $\endgroup$
    – PcMan
    Commented Feb 20, 2021 at 10:50
  • $\begingroup$ This answer seems to rely on a constant source of intense radiation perpendicular to the direction of travel, but I'm not sure why. That's hardly to be expected, and nothing in the question requires the ship pass close to a star. $\endgroup$
    – rek
    Commented Mar 9, 2021 at 16:23

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