Hot-air balloons have been around for centuries. They're certified aircraft for those looking to take the slow and scenic route, but owning one can set you back considerably — 30,000 euros for a smaller balloon and 150,000 euros for a larger one.
X-planes are a class of experimental aircraft designed to test and demonstrate new innovations in flight—some were created for NASA, others for DARPA or the U.S. military. The first X-plane, X-1, was flown by Chuck Yeager in the 1940s and was the first craft to fly faster than the speed of sound. Over the decades,…