Where is the gravity generated on a spaceship in super fast space travel?

Question

After reading the question: Is artificial gravity feasible in manned long-term space exploration?, I am having a hard time imagining what experiencing the "continuous acceleration" method of gravity generation would be like.

First, what direction on the ship would the gravity be generated in?

Imagine the following spaceship travelling from left to right:
         UP
BACK  #>-----O>  FRONT
        DOWN

DOWN:  (earth like) as depicted in most movies, like driving a car
BACK:  (makes most logical sense to me) like spinning a bucket of water or getting
       pulled back in a car from accelerating quickly
UP:    like walking on the ceiling
FRONT: opposite of back

Second, how long would you get the artificial gravity, and would it mess up half way through the flight?

This answer on that question states that you would travel at 1g until you reach the halfway point, then you turn retrograde and de-accelerate for the rest of the trip.

So would you have 1g throughout the entire trip? Including the retrograde phase?

In the retrograde phase, would the gravity be reversed? Causing you to have to walk on the ceiling for the remainder of the trip?

My assumption is that future spaceships that travel in this manner would essentially be an RV, standing on it's nose on the back of a truck. The truck would be travelling so fast that you would be able to stand upright in the RV. Then for the last part of the journey (retrograde), you would stand on the ceiling for a while until slowly loosing gravity (and while changing to retrograde you would be in zero g breifly).

As you can see I am having a little trouble flying on one of these spaceships in my head. Can anyone help?

Answer 1

The apparent direction of gravity depends on the direction in which the spaceship is accelerating.

Assuming a constant 1G boost (which is far beyond our current capabilities), "down" would be in the direction of the engines.

Such a ship would have to be designed with its decks perpendicular to the direction of flight. During the initial part of the journey, while the ship is accelerating away from Earth, Earth and the Sun would be "down" from the perspective of the crew, and the destination would be "up".

To arrive at the destination at low speed, you'd need to rotate the ship 180° halfway through the journey, and then accelerate away from the destination (i.e., decelerate) for the second half of the trip. During that part of the journey, Earth and the Sun would be "up", and the destination would be "down" from the perspective of the crew.

The change could be done by shutting off the engines, rotating the ship, and the restarting the engines. Or, if you want to maintain on-board gravity at all times, you can rotate the ship as it continues to thrust (which might require a bit of calculation to avoid messing up the trajectory).

Most spaceships we see in fiction (like, say, the USS Enterprise and the Battlestar Galactica) have on-board artificial gravity, and are designed more like modern surface ships, with "up" and "down" perpendicular to the direction of travel. A ship that gets its on-board gravity by constant thrust acceleration would have to be designed differently, with "up" and "down" parallel to the direction of travel -- or, more precisely, to the direction of acceleration.

If you're unable to accelerate at a constant 1G for years at a time, the other obvious way to generate on-board gravity is by rotating the ship. In that case, the decks would be cylindrical, and "up" would be toward the axis of rotation. We see this in the Discovery in "2001: A Space Odyssey". Most likely the axis of rotation would be parallel to the direction of travel, so the Earth and Sun would be "behind" you along the axis, and the destination would be "ahead" of you.

With current technology, we can accelerate a ship at 1G for no more than a few minutes. Rotational artificial gravity is probably feasible, but as far as I know it's never actually been tried. Artificial gravity by means other than accelerating or rotating the ship (as on Star Trek, Battlestar Galactica, and so forth) is still in the realm of magic technology beyond any current theories I'm aware of.

Answer 2

First of, it wouldn't be true gravity, but since we experience gravitational force same as a constant acceleration, there wouldn't be any apparent difference in its effect on anything you'd experience inside an accelerating spacecraft. You wouldn't experience constant speed as acceleration, no matter how fast your spaceship goes. It would have to be constant acceleration, otherwise you're inertial with your frame of reference (your spaceship) and you wouldn't feel any weight. The vector of this artificial gravity would be in the opposite direction to the constant acceleration of your spaceship, as per Newton's laws of motion and conservation of momentum, so in your graph that would be towards the back of the spaceship.

                                            

                                         ^{Rocket principles and Newton's third law (Source: NASA)}

The strength of this force, its uniformity and duration would be exactly equal and opposite to the acceleration achieved by your spaceship, its ability to keep it constant and for the duration you could sustain it. So these are already technical limitations of your spacecraft, like e.g. how much of propellants it could carry, engine performance, etc.

For deceleration, or any change in your acceleration vector, with any conventional means of propulsion at least (chemical rockets, ion thrusters, etc.), you would have to rotate your thrusters in the direction opposite to the one you want to apply force to, so as far as you're concerned, being inside the spaceship, your up/down orientation likely wouldn't change for long, if you rotated the whole spaceship to decelerate by applying thrust in the opposite vector. Deceleration is nothing else than a negative acceleration, so we have changed its vector by 180°, but have also rotated the spaceship for exact same amount.

In your common reference frame, your up and down would stay the same, even though you'd swap your zenith and nadir in an extended reference frame, looking at you in the spaceship from the outside. I.e., if you stood before with your feet towards the Earth you departed from, you'd now be facing it upside down on deceleration, with your feet towards your destination. For a moment though, while you rotate your spaceship (likely using smaller, side-mounted thrusters), you would experience zero gravity, i.e. no acceleration in relation to your spaceship, and your common, inertial frame of reference.

Hope that explains it well enough, and now, of course, an obligatory XKCD (barely relevant, just cute... :)