True, but if it is going to become a launcher, you don't want any factors that are going to uncessarily inflate airframe volume, which is what LH2 does, or consume limited airframe volume that could go to cargo or passenger space, which LH2 also does. The X-43A's fuel tank contained only gaseous hydrogen, which is why it only had 10 seconds of combustion. The B and C models were supposed to be hydrocarbon burning models, which is why I believe they were cancelled by the pro-hydrogen mafia in NASA: they would have exposed the claims of hydrogen proponents as lies.<br /><br />LH2 has several times the BTU/kg that RP-1 has, but RP-1 has three times the BTU/Liter that LH2 has. This means its easier for an RP-1 scramjet SSTO to meet the mass fraction requirements of its average Isp than it is for an LH2 scramjet SSTO to meet its mass fraction requirements.<br /><br />The only claimed practical advantage that LH2 has is that it can be used as a coolant for the hot structures of the airframe before being used as fuel, however denser hydrocarbons can do the same thing. Methane, propane, methylacetylene, cyclopropane, etc are all dense hydrocarbons that have superior coolin capacity, and JP-10's anti-coking additives allow it to be used as a coolant to significant temperatures, due to its high flashpoint. <br /><br />BTW: I'm currently compiling all my thoughts, ideas, and arguments in this area for a major report on my X-106 project that will be made available on my website. When that happens, I'll post something to space.com.