Inspired by this thread.
It’s certainly true that wings and jet engines cannot function once the air gets too thin. But would it be possible to build a commercial airliner that went as high as it could go, then activated rocket boosters to enter a low earth orbit? This would make transcontinental air travel incredibly fast. It would also, of course, require atmospheric reentry, which is not a simple thing. However, the space shuttle lands like an airplane after reentry. What engineering barriers stand in the way of building one that can take off like an airplane?