Get the fuel
Fuel is important, but Gravitar only allows you to carry 25,000 fuel units
-- sure it would be great to be able to carry an UNLIMITED SUPPLY,
but hey, you're flying around in space, stealing fuel from different planets -- you only get so much fuel, and that's it.
Fuel cells = 2,500 fuel units,
Use the the Tractor/Shield control button to collect fuel cells.
The tractor beam must reach to the surface of planet to beam fuel up to ship.
It does not matter which angle the ship is pointed, to tractor the fuel.
In fact you can be flying along low, near the landscape, hit the tractor button and keep zooming along. Most times though it makes sense to keep control of the ship, hover near (above or below, depending on the situation) the fuel and then get it.
Use of the Tractor/Shield control button will decrease the fuel supply, so it's important to use the button when you are getting the fuel or protecting yourself from getting shot by the red alien bunkers.
Beginners Advice from Ray Mueller (Gravitar World Champion)
"...you can go to the easy planets, steal their fuel without destroying all the turrets, then attempt a harder planet. This should give you plenty of extra fuel if that is what you need. When you then destroy one planet, all the fuel cells and turrets get replenished on the remaining planets, so you have even more fuel available to you."
Running out of fuel usually isn't a problem until you reach a level in the game that you cannot surpass (obviously) as you become more skilled, the challenges will be in the invisible landscapes. It took me more than a year of playing to get through the entire POSITIVE GRAVITY / INVISIBLE LANDSCAPE UNIVERSE.
When to get the fuel:
I usually try and get the fuel in an area AFTER clearing the red bunkers in that area (in the numbered order). So in the example below, I would destroy bunkers 6 and 7, then get fuel F3. Exit the cavern then destroy bunker 8 and get fuel F4. In other words, if there are bunkers in the immediate area, destroy them, then get the fuel.
Also the fuel cells and bunkers can sometimes represent an important landmark (or guidepost, if you will) -- a method of finding your way around the screen -- ** This is especially important in the invisible landscape planets. **
Solar System 2 - Planet 3 (North planet)
The final fuel cell F4 -- represents a guide to the entrance/exit of the lower cavern of the landscape (draw an imaginary vertical line from the right edge of the F4 fuel cell straight down) where you must enter to destroy bunker 6 and 7. In the invisible landscape, if you had destroyed F4 before this point, you will have to maneuver in and out of the lower cavern without that important landmark.
Lastly, here's an important point about the fuel locations:
Mission Complete - I destroyed all the bunkers, what if I have fuel cells left?