This is a really cool idea, but I had a lot of trouble actually getting the slingshot to work - I experimented with different planet sizes and angles, but the ship would inevitably fall into the planet and die. I think a slingshot tutorial would have helped me (e.g. something like "set the planet to this size and launch at this angle"), but maybe I'm just too small brain. I did get decently far without really using the planets much but gave up around the area with the up arrow into a few asteroids.
The general mechanics of fuel and "rewind currency" were neat and I appreciated the green line indicating the direction/speed. It was fun to play around with the movement, but I did have a lot of trouble getting the physics to do what I wanted.