All about Gravity
For the purposes of this game, gravity is a constant force pulling you down at a given speed. This will affect both your trajectory and your velocity. If your trajectory is anything but straight up and down, gravity will alter your trajectory until it's straight down. If your trajectory is taking you "down", gravity will increase your velocity, until you reach terminal velocity. If your trajectory is taking you "up", gravity will subtract from your velocity until it is 0, or your trajectory is taking you "down".
Earth's gravity pulls you down at 9.81m/s^2. To gain any altitude, you need to supply enough thrust to be able to push more than your craft's mass straight up. Should be as simple as calculating the force of gravity pulling you down(9.8m/s^2 * mass?), and subtracting it from the force of the rocket upwards(Which will either be a static number or scale based on how long it's been on). Fairly simple for straight up and down.
The tricky part comes when you add in horizontal movement... I believe you'll only need four 'major' variables, though. If I'm correct, you'll need to track the rotation of your craft, the force provided by your rocket, your current heading, and your current velocity. From your current heading and velocity, assuming no intervention of the player firing of the rocket, you should be able to calculate where you would move in the next game tick. THIS should be sufficient, I think...
If the player /does/ fire the rocket, though, things get more interesting... I'm not sure of the math involved, but you'll be altering the current heading and velocity based on that math and the ship's rotation and rocket's thrust.
Still have a lot of reading to do to get a complete understanding of this, but it's getting closer...
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