Avro engineers found that routing exhaust over the lower hemisphere of a saucer forms a cushion beneath the craft, enabling it to hover. However, they overestimated the cushion’s height and power.
Some airplanes, such as the C-17 Globemaster III, use the Coanda effect to route exhaust across the tops of their wings to speed airflow, increasing lift when planes are flying at low speeds.
Some aircraft can wiggle their jet engine nozzles to help steer. In certain designs, the entire nozzle can swivel to point in a new direction for even more dramatic maneuverability.
America’s saucer was designed to use the same propulsion system to hover or fly. Air was sucked into an intake and shunted to the rim of the craft; shutters directed the exhaust downward for hovering, or to the side for moving laterally.
Missiles can change direction by adjusting their nozzles; the F-22 Raptor‘s engine diverts its exhaust to allow sharp turns. The F-35B Lightning II, made to operate from carrier decks, points its exhaust nozzle downward to hover.
A curved shape is well-suited to aircraft that fly across a wide range of speeds. At high speeds, the shape dissipates the heat of air friction. At lower speeds, it generates lots of lift but little drag.
Avro sold the U.S. on the idea of a supersonic aircraft that could take off vertically and intercept bombers. A disc would provide predictable aerodynamics at high and low speeds. And it could fly in any orientation.
Round shapes are used on the bottoms of space capsules and the Dream Chaser space plane (right). Blunt fuselages are ideal for reentry craft that go from hypersonic speeds to a halt.