chain or unsaturated alkyl radicals with or without a plurality of double bonds in any order m may be aluminum boron magnesium zinc manganese and others that appear in groups iib iiia and iva in the periodic table of the elements in addition these functional groups may be connected by oxygen to produce metallic ethoxides the preferred substances are triethyl aluminum trimethyl aluminum triethyl boron trimethyl boron dimethyl zinc diisobutyl aluminum hydride triisobutyl aluminum diethyl aluminum hydride di n propyl aluminum hydride tri n propyl aluminum tri n butyl aluminum tri n hexyl aluminum and the like these hypergolic substances are injected with the inflowing liquid oxidizer and ignite it to initiate the necessary combustion in the sequence in addition continuous or repeated injections of these hypergolic compositions enhances vaporization of the liquid oxidant and subsequently prevents the flow of liquid oxidizer onto the solid fuel preventing disruption of the boundary layer combustion zone the hypergolic fluid is injected in an amount sufficient to vaporize all of the liquid oxygen the flow rate can be readily calculated from the temperature of the liquid oxidizer and the heat of vaporization of the oxidizer and the flow rate of the oxidizer for example a hybrid engine using liquid oxygen and a trialkyl aluminum pyrogollic fluid a flow rate of from about 0 1 by weight of the liquid oxidizer is sufficient to vaporize all the oxidizer flow rates higher than 5 by weight of the oxidizer are unnecessary and can lead to unstable burning usually the flow rate is from about 0 5 to 3 by weight of the oxidizer specifically the liquid oxidant and the hypergol are forced through injectors either.