For one key example, let's look at spy planes. The U-2 (pictured above) first flew in 1955, and was America's chief high-altitude spy plane for the entire Cold War. Giant film test patterns, made to calibrate the spy plane's original film cameras, still dot the American landscape, artifacts of time before satellite surveillance. U-2s cruise at 70,000 feet, and can do so for well over 10 hours. The chief constraint on a U-2 isn't the plane itself, but the need for an onboard pilot, who must be awake and seated for the entirety of that exhausting flight.