add vertical space between equations
You can manually add a vertical distance to each line-break. I hope, this is what you want.
% arara: pdflatex
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\setcounter{equation}{2}
\begin{equation}
\begin{aligned}
T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\[1pt]
Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\[10pt]
N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\[100pt]
K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
\end{aligned}
\end{equation}
\end{document}
You can also modify the length \jot
if you don't want to manually specify the skips for each line. For example
\setlength{\jot}{10pt}
If you want the change to be localised to that equation, insert the line inside the equation
environment, as in the following example
\documentclass{article}
\usepackage{mathtools}
\begin{document}
an equation with more spacing
\begin{equation}
\setlength{\jot}{10pt}
\begin{aligned}
T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\
Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\
N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\
K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
\end{aligned}
\end{equation}
and a normal one
\begin{equation}
\begin{aligned}
T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\
Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\
N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\
K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
\end{aligned}
\end{equation}
\end{document}
Output