What is parameter 'p' in the formula? If somebody knows any better source to study this part... welcome to suggest. Thanks a lot.
Probable Number of Fragments Hitting the Target
It can be proven that the fragments from a typical warhead are generally lethal at long range, far in excess of the lethal effects from blast weapons of equivalent size. Drag reduces the energy slowly. For example, fragments from a hand-grenade can be dangerous to a range of about 100 m. However, the likelihood of being struck by a fragment at 100 m is small. There are only so many fragments that are distributed in all directions. The average number striking a target will reduce proportionally to 1/R^2, where R is the range. We can express this in the following formula:
N_hits = A(N_o/4pR^2)
where:
N_hits is the expected number of fragments hitting the target;
N_o is the initial number of fragments from the warhead;
A is the frontal area of the target presented to the warhead; and
R is the range of the target to the warhead.
You can follow the link and search for 'fragments hitting the target' here https://fas.org/man/dod-101/navy/docs/es310/dam_crit/dam_crit.htm