Many thanks for the detailed explanation. This does involve a factor you seem to have forgotten (to be honest, I didn't realize what it was either until I tested it in game myself
). Looking more closely at your screenshots, notice that in the first two you are overweight, but once you remove the second engine you are not and your speed increases significantly. Predicted energy costs are calculated for a per-turn basis, and you're obviously flying, so once you start to fly faster you'll be moving further each turn, which consumes even
more energy, hence the sudden negative rate.
Glad you're enjoying the game and keep up the good work--I noticed you doing well on the high scores