There has been a consistent drive to improve the volumetric energy content of liquid fuels to extend the flight distance, flight speed and loading capacity of aerospace vehicles. Compared with conventional jet fuels like aviation kerosene and rocket kerosene, high-density liquid hydrocarbon fuels can provide more propulsion energy and thus are specifically important to promote the performance of volume-limited aerospace vehicles. In this review, the molecular characteristics of high-density fuels and the strategy to synthesize them are first discussed. Then the important progress in synthesis of several typical fuels is summarized, including multi-cyclic and alkyl-diamondoid fuels synthesized by cycloaddition, hydrogenation and isomerization reactions, highly-strained fuels such as cyclopropanated hydrocarbons, quadricyclane and pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane synthesized via cyclopropanation and photoisomerization, high-density biofuels synthesized from cyclic biomass-derived compounds via polymerization, condensation and alkylation reactions, and nano-suspension fuels prepared by surface-modification and stabilization of nanoparticles(aluminum, boron and carbon) in liquid fuels. Specifically, the catalysts and reaction mechanism involved in these processes are highlighted for better controlling the reactions towards higher synthesis efficiency. Also the parameters of typical fuels synthesized using the above-mentioned processes are listed to show the potential for practical application. Finally, an outlook for the synthesis of high-density fuel is given.

Contents
1 Introduction
2 Molecular characteristic of high-density fuels
3 Synthesis of polycyclic hydrocarbon fuels
3.1 Oligomerization reaction
3.2 Hydrogenation reaction
3.3 Isomerization reaction
4 Synthesis of highly strained fuels
4.1 Cyclopropanated hydrocarbons
4.2 Quadricyclane
4.3 Pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane and dimer
5 Synthesis of high-density biofuels
5.1 Pinene-derived fuels
5.2 Lignocellulose-derived fuels
6 Synthesis of nano-fluid fuels
7 Conclusion