Add an option (I'll say it again for hardcore players, an OPTION) to have unlimited fuel on Sandbox mode.
I love to fool around in sandbox games. I also used to do that on SimplePhysics and Santa's Engineer. I'm still learning how to properly achieve landings but in the meantime i want to have some fun building crazy rockets on sandbox. Unlimited fuel would help greatly, especially on bigger planets.
And, again, this would be an OPTION. A lot of people whines about these things even when they're optional. If you don't want it, don't use it.
Youc an do this with modding.
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How about a special fuel tank that never runs out of fuel that is only available in Sandbox mode and is clearly marked as such so that if screenshots are taken, it will be obvious you are using the 'magic' fuel tank.