A disk or cylinder could be a very efficient shape for a spacecraft if aliens from planets with different gravities want to travel together. Flying saucer sightings frequently involve something rotating. If this is real centrifugal power would have a similar effect to gravity but the force would be different at different distances from the centre. Therefore different aliens that evolved on different planets or moons with different gravities could all have homes somewhere in a flying saucer where the simulated gravity fits them. If a flying saucer picked up humans somewhere in the craft would be an area that simulates the gravity of the Earth. What about flying saucers with a hub in the centre? Perhaps that’s imaginary. Perhaps the hub is for some alien machinery that we don’t understand. A