Since we are all humans (well, maybe), who have been raised on planet Earth (again, maybe), we pretty much understand how things work in an atmosphere. We end up thinking about a wide variety of things with an atmosphere in mind. Take airplanes for instance.
Airplanes fly by generating lift with their wings. There is an air pressure difference between the top of the wing and the bottom of the wing that causes an upward pressure difference, giving the plane lift. When this lift is larger than gravity, the airplane goes up. When it is less than gravity, it goes down. When it exactly balances gravity, it flies level.
Since an airplane typically has two engines, one on the left and one on the right, you may think that an airplane can turn by adjusting the engine speeds. Like, in order to turn right, the left engine thrusts a bit harder and the airplane is rotated to the right. Any who has flown on an airplane knows that this is not the case. As you probably are well aware, an airplane turns by tilting one way or the other: it banks to make a turn. This banking, or tilting, points the lift vector from being perfectly vertical to being directed in one direction or the other. The airplane literally uses air pressure to change directions! This force then causes an acceleration that is perpendicular to the direction of motion (centripetal acceleration). Interestingly, tilting the airplane reduces the effective lift, so the plane will start to descend if it doesn’t speed up to compensate for this. Take a look at the diagram below.
If an airplane wants to turn a 90° turn, it tilts in the direction that it wants to go, causing a force (and acceleration) that points towards the center of the turn (centripetal acceleration). It continues like this until it is done with the turn, then stops tilting.
It would be incredibly difficult for airplanes to not fly like this. Pretty much by definition, they use the atmosphere to help with flight: Air-Plane.
In space, this is not the case at all, since there is little to no atmosphere. This means that spaceships (in space) can’t fly like airplanes. They don’t use air pressure to give them lift, since there is no air. They use thrusters to move from one place to another.
This means that spaceships don’t have the atmosphere to allow them to make banking maneuvers. So, how do they make turns if they can’t bank? Well, ships that fly in space have thrusters that cause an acceleration in the direction that they want to move. If a spaceship wants to move to the right, it has to thrust towards the left, which will accelerate it to the right. If the ship wants to reverse course, it doesn’t make a big circle, it just turns around and starts accelerating in the opposite direction that it is moving. This would look like it is slowing down, eventually stopping completely, then moving in the other direction, getting faster and faster all of the time.
Spaceships don’t have to turn in giant circles like airplanes have to. Airplanes can’t simply turn around like a spaceship, since the air puts a huge amount of pressure on the forward facing side of the airplane. It would be very bad if you simply rotated a 747 by 180° while it was still moving at 600 miles per hour. Very bad. Like, wings being ripped off and fuselage breaking in half type of bad. Seriously, don’t do it.
But, in space, there is (essentially) no atmospheric pressure to rip the wings off of the ship or break the fuselage in half. So, space ships can orient themselves in whatever direction they want when they are not thrusting.
The Millennium Falcon, on the other hand, banks all of the time. Take a look at this youtube video of just the Falcon flying. It is constantly banking. In an atmosphere, this way of flying totally makes sense (except it is not shaped like a wing at all, so I don’t understand the lift, but we will set that aside.) And, the Millennium Falcon does fly in the atmosphere, so maybe that is why it is portrayed as constantly banking.
Since ships like the Millennium Falcon have one large thruster, and not thrusters on each side of the ship, they have to rotate so it is facing the way that it wants to go, then thrust, thrust, thrust. No banking! Imagine, in reality, what it would look like for the Millennium Falcon to turn to the right. It would be going in one direction (not even having to use thrust, since you don’t have to thrust to move in space – just to change velocity!). Then it would have to rotate around facing perpendicularly to the velocity, then thrust, continuously rotating, so it always thrusts towards the center of the turn. When it is done turning, it can turn in whichever direction it wants to, but it would probably face the direction that it is moving. Take a look at the figure.
If the Millennium Falcon wanted to reverse directions, it would simply flip over so that it was facing in the opposite direction, and would thrust, slowing down, and then accelerating in the opposite direction. There would be no back flips or anything like that, since backflips are another form of maneuvering in an atmosphere.
One very cool thing about spaceships is that they don’t have to face in the direction that they are moving (if they are moving at a constant speed and want to continue moving in that speed). This is very convenient if you want to shoot another ship – you can simply rotate your ship around and shoot in whatever direction you want. Battlestar Galactica did a really fantastic job of portraying this. If you watch this video at around 7:30, you can see the ships moving in one direction while shooting in a different direction, then rotating around to thrust in another direction to change their velocity. Pretty nice! This is how ships should really move in space!
Ok. I love the Millennium Falcon. Who doesn’t? But ships in the Star Wars universe really don’t follow the laws of physics. I have often wondered why. I think that back in 1975, George Lucas didn’t really understand the physics, and movies that were made back then didn’t really care as much about the physics. Now though, people do care more, and they have started to take the pains to get it closer to correct. But, I have to imagine that George Lucas (or Disney) had to make a decision about whether he (they) should change the physics to be more realistic, thereby changing some of the fundamental things about the universe. It was most likely a hard choice. I probably would have given in to the dark side and changed it. I am weak.