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My question isn't about whether constant acceleration can create artificial gravity, as I know that it can. I am thinking more about worldbuilding in the sense of a practical transportation system where that would occur.

I'm treating as a given that zero-gravity is not livable. I'm basing this off what our current science tells us that zero gravity is not workable for humans except as an endurance test. And life beyond plants and single-cell organisms is not going to be make it without at least a little gravity.

In my setting, humans are now essentially "trans-humans" having done genetic engineering hard to better live in space: bones and muscles grow freakishly without cues from gravity, cells can repair themselves from radiation and cosmic ray exposure, and there are small things like strong resilience to the coriolis effect. So humans and other engineered life are fine with at least low gravity.

I'm envisioning transportation in the solar system can be done "cheaply," i.e., by wheel or cylinder ships which accelerate hard, shortly, and then mostly drift while spinning, flung in a certain direction.

Then, more comfortable and rapid transportation is done via constant acceleration, powered by deuterium, H3, or Hand-Wavium if I must. We're only talking about intra-solar transportation here, [EDIT: intra-solar system, not through the sun] so nothing needs to be an outrageous amount of acceleration, anything where people experience fairly low, sub-luna gravity up to 1G.

My ships may look like a flying saucer, for instance, and go through an atmosphere flying horizontally, like a traditional saucer image. But once in space and accelerating, they are flipped in a way which would look like they're vertical, which would allow the people inside to have acceleration still giving them a sense of gravity letting them sense what is the overhead and what is the deck the same as when flying horizontally within gravity.

Here's my question about constant-acceleration gravity, though. You obviously have to calculate it in such a way where you'll flip over roughly halfway in your trip start expelling thrust in the opposite direction to slow down.

Obviously, once the ship stops accelerating in a certain direction, everything inside is weightless. My question is what happens with the g-forces when the ship flips over and starts accelerating (maybe at the same rate?) in the other direction to slow down on the second half of the trip?

Would beginning to accelerate in the opposite direction with the same thrust, simply feel the same as the first part of the journey to the passengers, assuming they've flipped over? Or would the bodies inside experience the extreme discomfort like when I'm in a passenger train which is decelerating rapidly to slow down to come into a station? (A sensation I would not like to experience for more than a few minutes.) Or something else?

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    $\begingroup$ As long as the acceleration is constant you can flip as many time as you want, other times just have paper bags ready ;D $\endgroup$ – user6760 Jul 26 '20 at 9:07
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    $\begingroup$ This may seem trivial. But the phrase "intra-solar transportation" means "transportation inside the Sun". What you're talking about is interplanetary transportation or transportation between planets or over interplanetary distances (which are distances inside the solar system). $\endgroup$ – a4android Jul 26 '20 at 9:07
  • $\begingroup$ Total recall the modern movie, with the thing called "the fall" through the earth? $\endgroup$ – Trioxidane Jul 26 '20 at 18:08
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    $\begingroup$ Off topic, but as you get into the details like trip time, remember that constant boost travel is FAST. See Heinlein's 1950 essay "Where To?" in his book "Expanded Universe". At opposition, Earth to Mars and back at 1 G is under 5 days. At 0.1 G it's still under 2 weeks. And Pluto, 50 times more distant, takes only 7 times longer to get to than Mars. $\endgroup$ – MTA Jul 26 '20 at 23:22
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    $\begingroup$ Regarding your saucer ships, it is highly unlikely in any remotely realistic setting that space ships would be expected to do anything more than take off and land in atmosphere. And if your civilization is very developed in space, it's likely that interplanetary ships would never land at all. They would dock with space stations and then small shuttles or space elevators would handle atmospheric travel. Designing a ship that could fly in atmo AND do interplanetary travel would result in far, far more expensive ship than an interplanetary ship that was designed never to enter a gravity well. $\endgroup$ – Harabeck Jul 27 '20 at 16:53
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Turnover point doesn't need a period of zero-g or any kind of noticeable effect on the passengers. Simply don't stop thrusting.

That is, when you get to the halfway point of the trajectory you start slowly turning the ship around while keeping your thrust at normal level. If you do your turn slowly enough, your passengers won't notice a thing, say over a period of 5 minutes. You do thrust sideways for a couple minutes, pushing you 'off course', but you can easily account for that in your original trajectory plan. At the end of the turn your ship has turned 180 degrees and is now slowing down.

This way your passengers constantly experience the comfortable 'gravity' from the thrust without having to endure a period of zero-g. And done slow enough the turn is unnoticeable to humans.

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    $\begingroup$ P.S. You can practice this manoeuvre here! astrds.com $\endgroup$ – chasly - supports Monica Jul 26 '20 at 17:05
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    $\begingroup$ Science fiction writer Robert Heinlein called this a "skew-flip". I always thought the "skew" part implied a helicoid rotation that would impart no net sideways thrust. $\endgroup$ – John Doty Jul 26 '20 at 18:30
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    $\begingroup$ @chasly-reinstateMonica Can you? It seems to me that this game uses much too large of a frictional force to get the feel for how this would work in interplanetary travel; there's a low terminal velocity in this game so you can't keep accelerating very much. $\endgroup$ – Eliza Wilson Jul 26 '20 at 19:11
  • $\begingroup$ @JohnDoty: The easy way to get zero net sideways thrust is to turn very slightly more than 180° during the initial flip and then wait for the slightly off-axis thrust to put you back on your original trajectory before turning exactly retrograde. No need for any fancy helicoid rotations or anything. $\endgroup$ – Ilmari Karonen Jul 27 '20 at 14:47
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    $\begingroup$ Yes, you wouldn't need anywhere close to the 1G thrust to torque the ship enough to spin the ship 180 degrees over 5 minutes. That would average to less than 1/10 the speed of a second hand on a clock. And even if it pushed you to a full (9.8m/s2*(60s*5))=2940m/s of sideways velocity and 1/2*(9.8m/s*(5s*60)^2)=441000m off course, you could compensate for that in less than an hour with some off-axis thrust. Flipping the ship in 30s at the speed of a second hand (6 degrees/sec) wouldn't add more than 294m/s and 4410m worth of course corrections to be compensated for. $\endgroup$ – Dave X Jul 27 '20 at 17:54
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They should be fine.

Slowing down in a train is more uncomfortable than speeding up because the acceleration is directed outward through your chest and face, pushing them ahead of you, rather than pushing you back into your seat, which people can generally tolerate a little better. If your seats rotated around 180 degrees before the train slowed down, though, you'd get the more tolerable pushed-back sensation in both cases.

If the ship is lined up with its thrust vector in the same way while speeding up and slowing down (living quarters are stacked vertically over the engine), though, the acceleration that passengers observe will be down towards the floor in both cases, so they should feel no different on either leg of the journey.

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  • $\begingroup$ That's what I thought. But then I also realized I was assuming a lot. It is fascinating to me that it's even theoretically possible. For that matter, when standing in the vestibule of a train while it's slowing down I would often turn backwards and the nausea would be alleviated, but that's also a very specific experience on Earth with different forces going on. $\endgroup$ – dozTK421 Jul 26 '20 at 14:01
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    $\begingroup$ And laying prone is even better still $\endgroup$ – Steve V. Jul 26 '20 at 20:05
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    $\begingroup$ @parasoup, the accelerations and decelerations you describe are amusingly called "eyeballs in" and "eyeballs out" in technical documents. $\endgroup$ – Vorbis Jul 27 '20 at 9:23
  • $\begingroup$ The question is about the transition between the legs of the journey, which isn't really addressed here. Are you suggesting that all the living quarters rotate floor-to-ceiling so that acceleration is always toward the floor, similar to a train seat that spins so that acceleration is always toward the back? That sounds a bit more complicated than spinning in a chair. $\endgroup$ – Nuclear Hoagie Jul 27 '20 at 15:28
  • $\begingroup$ Maybe it's different elsewhere, but I'm used to trains having half the seats facing forwards and half backwards. From the description it's clear enough you mean facing forward, but maybe a car or bus is a better analogy. $\endgroup$ – Mark Jul 27 '20 at 15:49
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It should be noted that (unlike a train) a ship can reverse the direction of thrust relative to its destination, without significantly changing the direction or amount of relative thrust (/acceleration/gravity) experienced by it's occupants. The ship merely needs to slowly swing it's tail around while continuing to thrust at 1G. This is slighty less efficient as you need make a slight bend/course correction in your path, but that's it.

The only discrepancy would be the centripetal forces experienced by the swing, which could be minimized both by rotating more slowly and/or by swinging in a wider arc. This could definitely be reduced to the point where passengers wouldn't even notice it.

In short, there's no need for your passengers to experience any disruption of their apparent gravity (until you dock or land, that's a different story).

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You don't shut down for the flip and you don't adjust the orbit other than in a tiny amount around the time of flip. I first read about this in Heinlein as a torchship skew turn.

Basically, think of it as an exercise in symmetry - within a short segment of the trajectory, you have to over-rotate after turning, enough to compensate for the pertubation of orbit by starting to turn with the drive on.

So, depending on the duration of the turn, relative to total trajectory, and power of the drive, the shape of the skew is not entirely symmetrical. The second half not only directly compensates for the original skew shape but is affected by how your total trajectory has been changed.

Naively, for example, say for gravitationally-warped reasons, your trajectory represents a continuous curve in two dimensions. The turn would be performed at right-angles to that curve so as to disturb it the least, so looking in one view the curve would have an undisturbed smooth shape. Viewed as a "plan view" from 90 degrees, the straight line of the trajectory would have a little wiggle in it representing the skew turn.

(I have both worked as a programmer on 3D CAD for a few years and done house planning so I tend to think in 2D "views" as projections).

Skew turns are mentioned in passing, without explanation of how the turn works from this site

Gotcha

I just thought of one thing that might be an issue depending on drive type - if your engine is blasting out radioactive particles you're now flying the ship (backwards) into that exhaust.

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  • $\begingroup$ Good point. Nice thing about fictional worldbuilding is I can easily fix this. $\endgroup$ – dozTK421 Jul 28 '20 at 19:58
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Lack of gravity is only a problem for the human body if has to be endured for an extended period of time. If your ships have high power drives then presumably their maneuvering thrusters should be fairly efficient as well. So no problems winding down the main engines just long enough to flip. The whole process would at worst only take a couple of hours to perform. Your greatest danger would be passengers who or newbie crew members needing a barf bag.

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  • $\begingroup$ While I treat gravity as necessary, I also realize that needing to spend time in zero or micro-gravity is as necessary to living in this world as much as a modern sailor has to endure occasionally getting wet. You might enjoy it a lot at times, but it will also put a heavy toll on your health the more submerged you get. $\endgroup$ – dozTK421 Jul 28 '20 at 20:04
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Your passengers would experience less and less gravity, weightlessness, and then more and more gravity as the spacecraft stopped accelerating, flipped over, and then decelerated. It would be somewhat like a ride on the Vomit Comet, albeit for a much longer duration. Here's a video describing the experience. It wouldn't be deadly by any means, but there's a chance of people getting motion sickness in the transition. Once you've flipped over, the g-forces would feel exactly the same as they did on your way to the planet. You'll be able to walk around and feel a firm surface underfoot.

I'm actually more concerned about the transition to atmospheric flight. You would want to be careful about transitioning from moving vertically (from the perspective of your passengers) to horizontally. It's doable, but if you're not careful you could end up in a situation where you are trying to enter the atmosphere sideways while your passengers aren't yet experiencing the planet's full gravity. So they would go from feeling the same sensation that we have on Earth (gravity pulling firmly downward) to the sensation of less gravity and moving sideways. Again, not a dealbreaker but you should keep those air sickness bags handy.

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    $\begingroup$ In a long, intra-solar flight, the turnover would be perhaps a ritual thing where people either strap in or party for a bit in zero-G, (as it's a thing they rarely experience.) The transfer to atmospheric flight will be fixed with much fictional worldbuilding. As I would imagine is realistic, only rare vessels do both, as most space transports, shuttles, and atmospheric aircraft will be built for those purposes. I'm also incorporating planetary tethers/elevators for easing getting on/off planet in the universe as well. No beaming. $\endgroup$ – dozTK421 Jul 26 '20 at 14:07
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    $\begingroup$ BTW, to escape a gravity well, you must accelerate at >1G, where 'G' in this case is the gravity well you want to climb out of... otherwise you'll just hover in midair. OTOH if you can afford a continuous burn at that rate, ISTM like you ought to be able to transition from orbit to geostationary above some point while continuously reducing your altitude at a modest rate. The catch is you'll probably be thrusting sideways, which might mean the ship is tilted, but it would be a pretty smooth landing. $\endgroup$ – Matthew Jul 27 '20 at 14:32

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