While high-performance aircraft were being tested at places like Edwards Air Force Base and Naval Air Station Patunxet River, equally important work was being done at places such as Holloman Air Force Base.
Edwards and Patunxet River tested the flying machine. Holloman tested the men who would fly them. Among the work that was done at Holloman was the effects of high acceleration on the human body with Col. John Stapp one of the investigators. There was also Project Manhigh which took men to the edge of space to study the effects of cosmic rays on the body.
All of this was essential in order to put men into space. Among those who would participate were Maj. David Simons and Capt. Joe Kittinger.
"On the Threshold of Space" tells the latter part of that story and does so quite effectively, being quite matter-of-fact. It shows some of the work being done that would help make high-altitude flight and spaceflight itself not only possible but safe for the crews. This was cutting-edge research conducted even before Sputnik was launched and there were a lot of questions that had to be answered before anybody could be launched into space.
There is a side story in which the main character gets married to one of the civilian assistants, but, fortunately, it doesn't take much away from the plot.
I liked it better than I expected. If you're an aviation and/or space buff, this movie would be definitely worth watching. I came across this one quite accidentally and I'm glad I did.