This calculation is totally wrong. We know that Bora and it’s derivatives are quasi ballistic missiles.
Yes, in order to travel around 660 km in total and achieve an average speed of 4.3 Mach, this missile flies well in to hypersonic speeds in most of it’s trajectory. But towards the end of it’s mid course flight it loses a lot of momentum and slows down and dives towards earth gaining speed. But as all free falling masses it is subject to Terminal Velocity laws. It’s terminal speed, hence, is governed mainly by it’s weight and it’s shape ( aerodynamic structure related drag coefficient - please see below constants)
We can do a little calculation as well by assuming few parameters:
-let us say empty weight of missile is 1750kg.
-drag coefficient of missile is 0.1 (assuming missile is diving at 90 degrees
-Air density value is 1.2 kg/m3
-diameter of missile squared is 0.36
then the terminal velocity of the missile is 890 m/s = 2.6 Mach
If we can increase the empty weight to say 2250kg and reduce the missile drag coefficient to 0.05 ,
Terminal velocity becomes 1430m/sec = 4.2Mach
It is due to Terminal Velocity that you can have a person with special suit jumping from near stratosphere, gaining speed and travelling at near supersonic speeds until he hits denser air layers whence his speed decreases below 200 km/hour (belly down) and can thus open a parachute.
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