Hi shaheen!
I am not quite sure what you're referring to here,But let me explain you little bit of Re-entry dynamics based on what I studied.Kindly note,I am not an RV expert--instead I am more into computers and electronics.
First off,There are two aspects of RV that we should be concerned with at the moment and those are-
1) Ballistic Coefficient or Beta given by W/Cd*A--because it is the single most important parameter governing the trajectory and terminal kinetics
2) Stability of RV during flight
#1)
An RV design with higher beta will have higher terminal velocities,hence making it difficult for normal radars to accurately track them.Below are some of the plots from research papers.-
View attachment 376924
Picture Courtesy:Author John C Adams.
In the above picture one can clearly see that a RV with higher beta will have higher terminal velocity upon impact. In the graphs above a beta=5000lb/ft^2 yields something like 8-9M upon impact. So the design challenge is to reduce Drag coefficient to least possible value along with reference surface area. Modern American RVs have a beta in vicinity of 5000lb/ft^2 or roughly 24000kg/m^2.
Kindly note the initial conditions to solve 4 highly coupled non linear differential eqns were-
(a) Altitude = 250,000 ft or 76.2km
(b) Velocity = 22,500 ft/sec or 6858m/s or close to 20Mach
(c) Flight Path Angle = 12 deg
Kindly note that #b depends on apogee which in turn depends on the velocity at burn-out point! So,obviously in pakistan's or even in india's IRBMs(like Shaheen,Ababeel,Agni-1,2) the velocity with which RV will splash into atmosphere will be much lower than this figure and hence the terminal velocity will be also lower accordingly!
#2
Now coming to the point of stability of RV.
View attachment 376925
View attachment 376926
As you'd have studied in your Flight Dynamics classes that an aerial vehicle that has CG ahead of CP is inherently unstable because the pitching moment tends to move the body away from equilibrium point. or in more mathematical way,the stability derivative C-em-alpha is positive(C-em-alpha should be negative for stability). As we can see,all the RV designs presented above are unstable(however their degree is different).
View attachment 376945
In simple conical design we can clearly see that the Cd is least,and hence it offers highest possible beta. besides the neutral point of a simple conical RV is also decent which means that RV can be filled with mass almost till the rear without making it unstable. From the graph above,if we fill it past that point then the RV will become unstable!