Unlimited fuel:
Enter the weight and fuel section in the settings menu. Set the fuel load
to maximum. This will make the fuel tank full again. This can be repeated
as needed.
Get into space faster:
Pilot the X-15 and use the B-52 drop in Location\Special In-flight. Press
[Space] to be released and aim straight up. By the time you are out of
fuel, you will be in space.