Is it me being stupid or someone drops LPG out of the equation
OK, if hydrogen does not have the energy density and liquid hydrocarbons are too difficult what is the problem in using good old propane? Liquid propane is easy to store, has very reasonable energy density and can be used as a coolant in "hot" parts of the plane so by the time it reaches the scramjet injector it is guaranteed to be gas. From there on it ignites as easily as hydrogen. In fact if energy density of "plain" propane is not enough there is plenty of "more complicated" options like cyclobutane, mixtures of cyclopropane with less reactive liquid low-MW hydrocarbons, etc which should all outright self-ignite on contact with the hypersonic airflow.
Is it me being thick or I am missing something here?