How To Build A Flying Saucer
Summary: Building a viable flying saucer is not that hard. The irony is it can't work in space.
A classic flying saucer can be built by human hands and human ingenuity. The principles behind it are over a century old for some cases, and even older for others. Despite this, the resulting craft possessed most of the traits of a classic flying saucer. Instead of a military aircraft, it would be better for a hobbyist for a low-budget science fiction film using practical effects.
The vacuum balloon is a concept dating from 1670, where it was hypothesized that a vessel of vacuum would be lighter than air, flammable hydrogen, or expensive helium. The core of the craft was a functional vacuum balloon, although the craft could easily substitute a sack of helium or hydrogen in the same role. A composite material, made of boron ceramics and a lair of aluminum, was strong enough to withstand atmospheric pressure and keep a reliable vacuum within. The 1 m-2 m sphere was large, but necessary to keep the rest of the craft aloft.
The air displaced by the craft gave it an ever-upwards buoyancy, tempered by another feature. Just as a submarine could descend and ascend by releasing or taking in water ballast, the craft could do the same with air. If it needed to ascend rapidly, it merely needed to rapidly expel air from internal tanks. The reverse would be even easier, as internal cavities could simply be exposed to the ambient atmosphere.
Non-mechanical propulsion is quite possible, despite how outlandish it seems. For over a century, hobbyists have built ionocraft, also known as lifters. These craft use a high current, generated between an exposed wire and aluminum foil, to ionize the air into ozone. While silent, the toxic ozone limits its use indoors. Similarly, most lifters are tethered to their high-voltage power supplies, and only a few grams in weight, so they can only lift their own weight. Mounting such a device on the aircraft would enable it to navigate without need for mechanical propellers. If necessary, jets or mechanical propellers could be built inside the craft.
This craft concept, which I call the Invader, could operate up to the edge of space. It floats by means of air buoyancy, so it couldn't work in vacuum. Despite this, the craft would have its advantages. It would be very silent, and it would be able to ascend very fast. Such a craft would be largely unsuitable for military applications, save perhaps surveillance, which is better conducted by satellites, drones, and aircraft. Silent landing and takeoff would be useful for small cargo delivery, though. Reaching the edge of space and launching microsatellites from a detachable rocket, in the manner of a "rocket-balloon" or rockoon, also becomes much cheaper.
The system can be designed for a human pilot, autonomous system, or drone configurations. The unique airframe potentially represents a far more stable aerial platform than static, fixed wing aircraft, conventional lighter than air designs, and rotary-winged copters. The vertical takeoff and landing make it suitable for smaller areas, and the lack of noise (and greater safety) means it could be deployed near populated areas. The weight of the power supply would be the biggest drawbacks, although a simple electro-mechanical propeller system could be used for lateral movement during testing. Therefore, it is with great irony that a "classic" flying saucer, with a disc shaped frame around a spherical center, is feasible only in atmosphere.