The Dassault Balzac V
The Dassault Balzac V was an easily accepted concept. It was also another "vertical takeoff" plane that eventually became a fad in aviation. There's a reason why this was a fad, and it was because it was quickly understood as very dangerous. A young pair of pilots died in the initial flight test. The second test was met with the same response and it nearly killed the pilot. Thankfully, he managed to eject himself.