**Correction** (I'd originally used a 4,000 mile radius. But one day on the throne, it occured to me that was globular thinking. 4,000 is the the radius of the globe. The plane's radius is more like 11,000 miles. That's the distance GE gave me when I measured from the North pole to the ice wall. I've revised my comment accordingly.) If the plane's radius is approximately 11,000 miles, I'd say the firmament, at the North Star zenith is likely 11,000 miles up. The height decreases as one nears Antarctica. Of course, that's assuming the dome sits perfectly atop our disc. It's Antarctic edge might be thousands of miles inland, making the dome even bigger. 73 miles is way too low. There's a weather balloon that touched 100 miles with a decent camera view. Even at that altitude, the horizon was a line. I think what people are seeing/hearing in that rocket that stops in it's tracks is the parachute deploying. The nose cone explosion that blows the shoot is like hitting the brakes, hence the popping sound associated. Firmament is probably out of our reach.

If you can get your hands on a 1958 Encyclopedia Britannica it talks about the firmament in the section on Antarctica. It describes the height of the firmament at a particular latitude and I believe it may even speculate on the overall height of the firmament. You have to get the 1958 encyclopedia because it was removed in the future versions of Encyclopedia Britannica. It's curious that it is very difficult to find that particular year.