Thanks for trying it out. It's a regular inverse square law, no tricks. The numbers (masses, distance) determine the final acceleration but not the actual trend of the curve.

I've become too familiar with it over testing to notice unintuitive behaviour, but I think I understand what feels off: in real world units, the gradual region you describe is very wide, and feels linear. This would make for very boring gameplay (imagine spending minutes to reach the planet). You need to keep the playable area [radiusForce0, radiusForceMax] small. So you will either map that small [r0, rmax] into real world [F0, Fmax], which means the force will be almost constant across, or "compress" the [F0, Fmax] curve so that you can fit both [zero outer space gravity, strong surface gravity] into that [r0, rmax].

That's what happens here, I probably tweaked the values for the second case. It's kind of an accelerated version of reality and the margins feel very tight, and you have to "buy into" that reality.

For example, Master difficulty in Mission 1 may seem impossible, but if you try to be gentle and find a balanced orbit, you can complete it with miniscule fluctuations in distance and minimal input.

Just rambling though, I never really actually designed or balanced the game.

maybe some kind of indicator to show which level of acceleration/speed is best would help.

the unintuitive behavior is that it is very difficult to find that balance. if i am to slow i crash into the planet, if i am to fast i leave orbit with no chance to get back in time. being gentle always results in being to slow. in other words there is no gentle way to reach the balance. and if i don't know where that balance is, i don't know what to aim for.

you may argue that finding that balance is the goal of the game, but then i guess the game is not for me. i lose interest if i have to try 10 times and still can't figure out how to do it right.