The problem which I see is:
What is an antigravitational system useful for aliens who fly in space where is naturally no gravitation.
We would need more something like an extra gravitation causing machine which could use itīs own weight and the weight of the planet where itīs from to cause a centrifugal effect.
So seen on from that point of view a cat/butter engine wouldnt be useful for a higher speed reaching vehicle. It would be even slower than a common vehicle
