Modify letterspacing in \chapter, \section etc

Question

I try to create a design similar to this one:

As you can see, the space between each letter is a bit longer than by default, which is great when typesetting something in capitals. I saw the following topics: Enlarging tracking (= letter spacing) but I was not able to adapt it with my document.

\documentclass{article}

\usepackage{sectsty}
\usepackage{xcolor}

% Create a new command, \addspaces, ta receive a string and format it as                                                 
% all uppecase with more spaces between letters.                                                                    
\def\theLetterSpace{0.5pt}
\def\theWordSpace{2\LocalLetterSpace}                      
\newlength\LocalLetterSpace
\newcommand\spaceout[2][\theLetterSpace]{%
\setlength\LocalLetterSpace{#1}\expandafter\spaceouthelpA#2 \relax\relax}                                             
\def\spaceouthelpA#1 #2\relax{%
    \spaceouthelpB#1\relax\relax                
}                                                                                                                       
\def\spaceouthelpB#1#2\relax{%
    #1%
    \ifx\relax#2\else                         
    \kern\LocalLetterSpace\spaceouthelpB#2\relax%               
    \fi
}     

\newcommand{\addspaces}[1]{
    \textsc{\large\spaceout[2pt]{#1}}
}

\sectionfont{\normalfont\LARGE\scshape\color{red}\raggedright}

\begin{document}
\section{†est}
\addspaces{Hello World}
\end{document}

Thank in advance for your help.

Answer 1

You have to use \addfontfeatures, which however needs to declare fonts explicitly.

\documentclass{article}

\usepackage{fontspec}
\usepackage{sectsty}
\usepackage{xcolor}

\setmainfont{Latin Modern Roman}[
  SmallCapsFont={* Caps},
]

\newcommand{\spacedsc}{\scshape\addfontfeatures{LetterSpace=20}}

\sectionfont{\normalfont\LARGE\spacedsc\color{red!80!blue}\raggedright}

\begin{document}

\section{Special Relativity in Four Sentences}

\end{document}

Answer 2

Under pdfLaTeX and LuaLaTeX, you could load the letterspace package and add \lsstyle to the list of parameters set by \sectionfont.

\documentclass{article}
\usepackage{sectsty,xcolor}
\usepackage{letterspace}  % for "\lsstyle" macro

\sectionfont{\normalfont\LARGE\scshape\lsstyle\color{red}\raggedright}

\begin{document}
\section{Special Relativity in Four Sentences}
\end{document}