The last time anyone other than a fighter pilot went supersonic was 20 years ago. Today, you can only visit a Concorde in a handful of museums. It's a relic, left behind by an industry that invested in efficiency rather than speed.Now, companies are investing billions of dollars in restoring commercial Mach-speed flight, but the path to supersonic travel isn't going to be easy. Business Insider's Daniel Allen looks at the key reasons that bringing back supersonic is almost impossible.Show more