NOVELTY - Preparing (M1) high density fuel involves: slurring first catalyst selected from one of montmorillonite polyacidic clay or perfluorinated sulfonic acid resin in hexane under a nitrogen atmosphere; adding beta -pinene incrementally to slurry with stirring to form first reaction mixture; heating first reaction mixture to 100 degrees C; stirring heated first reaction mixture until reaction of reactants is complete, followed by removing first catalyst, adding platinum dioxide hydrogenation catalyst and isolating hydrogenated alpha -pinene dimer and beta -pinene dimer from residue.    USE - For preparing high density fuels (claimed) which is used in rocket engines and in jet or turbine engines.    ADVANTAGE - The produced high density fuels reduces dependence on foreign oil, have potential for use in rocket engines and in a wide variety of jet or turbine engines; has higher densities and net heating values than more conventional renewable fuels including biodiesel and bioethanol. The fuels have properties that are competitive with specialized conventional petroleum based fuels. The method uses more environmentally friendly catalysts, or at least catalysts potentially less environmentally damaging than conventional liquid acid systems. The method prepares high density fuel candidates in up to 90% yield from beta -pinene, renewable strained bicyclic compound derived from wood and plant sources. The heterogeneous acidic catalysts promote selective isomerization and dimerization of pinenes under moderate conditions (100 degrees C, atmospheric pressure). The method uses montmorillonite clays as catalysts for number of organic reactions and offer several advantages over classical acids e.g. strong acidity, non-corrosive properties, mild reaction conditions, high yields, selectivity, low cost, and the ease of setting and working-up.    DETAILED DESCRIPTION - Preparation (M1) of high density fuel involves: providing first catalyst selected from one of a montmorillonite polyacidic clay or perfluorinated sulfonic acid resin; slurring the first catalyst in hexane under a nitrogen atmosphere; adding beta -pinene incrementally to the slurry with stirring to form a first reaction mixture; heating the first reaction mixture to 100 degrees C; stirring the heated first reaction mixture until reaction of reactants is complete to form a second reaction mixture; removing the first catalyst from the second reaction mixture; adding a platinum dioxide hydrogenation catalyst to the second reaction mixture under a hydrogen atmosphere; stirring the second reaction mixture to allow contact of reactants until reaction of reactants is substantially complete to form a third reaction mixture containing hydrogenated alpha -pinene dimer and hydrogenated beta -pinene dimer; removing the activated platinum dioxide catalyst from the third reaction mixture; removing the hexane from the third reaction mixture to form a residue; and isolating the hydrogenated alpha -pinene dimer and hydrogenated beta -pinene dimer from the residue to form a high density fuel.