Editor: Ben sent this correction burn in to us via email on April 5, 2016. We didn't have time to read it on the show, but thought there was enough good info to make it available here.
Greetings, citizens, I hope you are getting enough oxygen! One tiny correction from this week's show: your guest described Enterprise as a 'mockup'. A note, it was actually intended from the beginning to be completed for use on orbit after the glide test program was complete. It (OV-101) was the first hull constructed that was intended for space but after it was done and being used in the landing tests, the engineers were able to make some changes to the design of the frame that would cut more weight off the orbiter. Each kilo removed from the orbiter was an extra kilo of cargo because the spacecraft is taken all the way up the hill so this was a big deal.
When they began planning the Enterprise's, ah, refit to meet this new specification, they determined that the changes to be made would require such a massive amount of work on this frame because of how far along it was that it would be cheaper to start somewhere else. To add insult to injury, even after the changes, Enterprise would still have less cargo capacity to orbit than the next spacecraft in the pipeline, Columbia.
So they needed to start, but where? The answer came from an unexpected direction. Because of where the state of computer modeling wasn't, Rockwell had built a pair of Static Test Articles for testing. Essentially space shuttle frames, these were instrumented and then squeezed, squished, pushed, and shaken for hours to try and predict how the real shuttles would behave during launch and landing. After the 1978 decision not to upgrade the Enterprise to full flight status, STA-99 became the focus. They determined that it could be more cheaply modified to flight status than the first shuttle because it was just a frame and so they did exactly that. STA-99 was saved from destruction at the hands of a giant hydraulic test rig and built-out to be a fully operational space shuttle. The name of that shuttle: Challenger.
When looking at the hull numbers of the fleet, you'll note that Enterprise is OV-101, Columbia was OV-102, Discovery/Atlantis/Endeavour were OV-103, OV-104, and OV-105 respectively, but Challenger held onto a tiny piece of its original history and flew to space as OV-99.
Anyways, ginormous wall of text for a tiny correction burn, but it's a subject I'm passionate about. The number of well meaning if wrong people who keep calling it a mockup or suggesting it was "never intended to fly to space" is frustrating and this is one of my little missions in life.