Constant acceleration as artificial gravity. Would flipping over halfway still be comfortable?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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).

Answer 4

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.

Answer 5

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.

Answer 6

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.