Alright,
So today I had launched the SimpleBeast to determine the minimum altitude possible to be definable as an orbit (meaning orbital decay can occur but it must be above the point where atmospheric parameters are measurable). I achieved orbit at 85km w/low eccentricity before dropping the orbit to 60.5km w/low eccentricity (eccentricity was 0).
I believed that in the region between 59-57km an orbit would attain too much decay to be able to sustain, and thus would quickly lose velocity and deorbit. I also believed that below 60km time acceleration to 10x would not be allowed.
The first thing that surprised me was that in the orbit I was in to start, the altitude of the Periapsis and Apoapsis began flickering down to lower altitudes over a course of 3 hours. Falling through 60km I was surprised to be able to continue using time acceleration to reorient the spacecraft which turned onwards toward prograde. Below 59km the time acceleration continued to work, that is until roughly 58.3. At that altitude temperature (-270C to be exact) and Mach number (mach 9.8) were measurable and immediately the spacecraft began to lose velocity and altitude.
After reentry and landing I ended the flight to find a time of 4:15:??. About three and a half hours in atmospheric slowdown. What I also found was a maximum possible orbital velocity in a circular orbit of 3455 m/s. With that velocity any program using a function of altitude to orbital velocity for circular orbit can use that and 59km to determine orbital velocity for any altitude.
Note: the function for relating altitude to orbital velocity will be nonlinear. I plotted points which showed a curve occurring. If you want any pictures of that I will need to know how to get pictures posted on this site.
Only that much???