These engines used toxic hypergolic propellants that required special handling precautions. These included Suva refrigerants and Dymel propellants. The Dnepr was a three-stage rocket using storable hypergolic liquid propellants. Many CFCs have been widely used as refrigerants, propellants (in aerosol applications), and solvents. Both are used as propellants in aerosol sprays. Like CFCs, HCFCs are used for refrigeration, aerosol propellants, foam manufacture and air conditioning. TEMED can also be a component of hypergolic propellants. This is their first engine to combust liquid oxygen and liquid methane propellants. The main difficulties with liquid propellants are also with the oxidizers. Surfactants are used to reduce the melting viscosity of sugar propellants. Two varieties of solid rocket fuel propellants exist. The primary use of ammonium perchlorate is in making solid fuel propellants. They are therefore sometimes called storable liquid propellants. Both lithium and fluorine are expensive compared to most rocket propellants. Of the liquid propellants used, density is lowest for liquid hydrogen.