Spaceships That Fly Like Spaceships

Science fiction movies and TV shows consistently get many things wrong about flying a spaceship. Some of these errors may be intentional for ease of filming or to make things more visually appealing or intuitive for the Earth-bound audience, while some may be from ignorance.  The main issue I want to discuss here is that spaceships in movies and television fly like airplanes rather than spaceships.  This is true for practically every movie and TV show I have ever seen depicting space flight, which makes it all the more striking that one show finally got it basically correct.

First of all, let's address what most shows get wrong.  There is absolutely no reason for a spaceship to have wings unless it sometimes flies in the atmosphere of a planet.  When does so, it can and should fly like an airplane.  But in space it should still fly like a spaceship.  That means that it needs maneuvering thrusters to control its attitude along all three axes of rotation (roll, pitch, and yaw) as well as a powerful main engine.  There is no reason for a spaceship to bank when it turns.  There is nothing in space for the wings to push against.  The only thing that a spaceship can do to change its velocity (which consists of both speed and direction) is to fire its main engine or rely on the gravitational pull of a planet or star.  Very small changes in velocity can be accomplished with thrusters alone (when docking, for example), but large changes require the main engine.  The thrusters are primarily for attitude control to point the main engine in the right direction for firing.

So how could a real spaceship execute something like a 90 degree turn?  There would hardly ever be a reason for such a maneuver.  Trajectories are carefully planned because adjustments are very costly.  Furthermore, it would be difficult to define what a 90 degree turn in space is.  It would need to be relative to something.  Everything is in motion: every star, every planet, every asteroid, and every comet.  If they were not in motion, gravity would pull them all together.  There is no such thing as absolute rest (something every Star Trek series got wrong with their "full stop" commands).

So ignoring these difficulties for a moment, lets execute a 90 degree turn.  We will ignore gravity (which we can never do in the real universe) and execute a 90 degree turn in a hypothetical gravity-free coordinate system.  Lets suppose we are traveling at 20,000 mph in the y direction.  In order to turn so that we are now traveling at 20,000 mph in the x direction, what do we do?  We cannot simply bank and turn using our current momentum.  We have to completely stop our momentum in the y direction and then get it going in the x direction.  We can do this with a combination of maneuvers or with a single maneuver.  With a single maneuver, we would use thrusters to point our spaceship so its nose is pointed towards the positive x and negative y direction at a 45 degree angle (towards quadrant IV in a Cartesian coordinate system).  We would then fire for the appropriate amount of time.  We could calculate the firing length ahead of time using vectors.  Alternately, we can point our ship backwards relative to our direction of travel and fire our engine until our velocity is 0.  Then we can point our nose in then new direction and fire our engine until our new velocity in that direction is 20,000 mph.  Either way we do it would consume exactly the same amount of fuel because two directions of motion are independent from a physics standpoint.  It is also exactly the amount of fuel required to completely reverse our direction and go back where we came at the same speed.

Real spaceships fly in 3 dimensions similar to the way the spaceship flies in 2 dimensions in the classic arcade game, Asteroids.  The only thing the Asteroids spaceship gets wrong is the inertia of rotation.  In a real spaceship you fire thrusters to start rotating and then you have to fire again to stop it.  Just like our simple example, the Asteroids game ignores gravity.  In the real universe we cannot ignore gravity because it affects every trajectory.  When a spaceship leaves planetary orbit, it is still in orbit around a star.  When it leaves a solar system, it is still in orbit about the center of the galaxy.  Galaxies and galaxy clusters mutually orbit each other also.  Our spaceship is part of this complex system of orbits within orbits and nothing is at rest.  We can usually just take into account the most dominant gravitational body, but the point is that we are always orbiting something, even if our orbit is hyperbolic.  Orbital dynamics can be very non-intuitive, but that is a subject for another time.

So which show gets it right?  It is not always exactly right, but very often the small fighters in Battlestar Galactica fly almost like real spaceships.  They use thrusters to control their attitude and they do not always point in their direction of travel.  They tumble without changing their velocity.  Sometimes you can see them adjusting their attitude and then firing their main engine to maneuver.  The trajectories and maneuvers are much more like those of a real spaceship than any other show I have seen.  Above all, they do not turn by simply banking and continuing to basically fly forward.  They do have wings, but that is because they are capable of flying in a planetary atmosphere.  I am sure that this was no accident.  Undoubtedly, they had good advisors and chose to try for greater realism, at least in this one area.