One way is to set a maximum speed for him in space, such as say V
max, say 50,000 mph.
Then make an assumption that his speed depends upon the density of air.
Then utilizing the air density correlation with altitude ( a search and you will find some information on this ), one can determine his speed at that altitude.
Make a graph of speed vs density, and you can then pick off the altitude points and label them on the graph at the matching density to get his speed.
That's not entirely correct though since we did not take into account how drag depends on speed - the faster he goes the more drag, and since he most likely has a maximum power output, this again limits his speed through a medium. In fact, the drag is going to depend upon his velocity squared. One can see that to achieve the same speed as he did from the above graph, the density has to much less. Double the speed, decrease density by 4. Triple the speed, decrease density by 9.
Of course, one can see a sticky point. In space where friction is quite negligible, by him outputting power continuously, he would accelerate to a much greater velocity than the first proposed V
max. Maybe the aliens told him that if he goes faster he turns into a mushroom :).
since the guy is supersonic,
You can read some interesting prose about supersonic in a series,
https://leehamnews.com/2018/02/09/bjorns-corner-aircraft-drag-reduction-part-16/
https://leehamnews.com/2018/08/17/bjorns-corner-supersonic-transport-revival-part-2/
Especially note, in series 16,
The volume drag of a needle body, like in the picture, is proportional to the inverse square of the body length divided by its diameter. The body length divided by the diameter is called the fineness ratio of the body.
The guy is quite blunt - his fineness ratio is about 3.
Volume drag, wave drag, which I haven't talked about, would predominate for our hero.
You can work that in also after reading up on the subject.
