add vertical space between equations

Question

I am using the following code to combine two equations with one number in parentheses but the two equations are very close to each other and using '\vspace{} not working

How can i add a vertical space between these two equations ?

\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}
\end{aligned}
\end{equation}

Answer 1

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}

Answer 2

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

Answer 3

I think the easiest way to provide a vertical space between two equations could be adding a line which has no numbering using \nonumber command, just as below:

\begin{flalign}
    T_{P} = K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\ 
    \nonumber \\
    Q_{P} = K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \nonumber
\end{flalign}

I realized you want to set just one number for both equations, in this method you should manually control each line to have numbering or not.

Answer 4

Infinitely simpler method:

    \begin{equation}
        \begin{aligned}
            T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\ \noalign{\vskip1pt}
            Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\ \noalign{\vskip10pt}
            N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\ \noalign{\vskip100pt}
            K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
        \end{aligned}
    \end{equation}

This uses low-level TeX commands (\noalign and \vskip) and so requires no loading of extraneous packages. You can find this method in the TeXBook.