Helium gas has been the primary driver in the X3 and X2 operations for some time. Using hydrogen may prove to produce more substantial shock waves in the applications of Scramjet testing. Research and analysis of the operating conditions of the X3 have been conducted for the possible inclusion of a hydrogen driver gas or mixture.
Research suggests that hydrogen produces stronger shocks over a longer period than helium. Hydrogen shows more attenuating effects which seem to be cancelled out by its stronger shock production. However was not substantiated through analytical evaluation, due in part to flaws in derivations.
Using an equation that describes a moving shockwave, it was found that hydrogen produced shocks with a higher velocity than that of helium, at lower pressure ratios. Showing Hydrogen at Mach 4 to have a pressure ratio of 18.5, and U2/C1 of 3.125. Whereas helium for the same Mach number requires a pressure ratio of 19.75 and only produces U2/C1 of 2.812. This and a one-dimensional code for shock tunnels establishing that hydrogen may have more strength as a driver gas.
A brief combustion approximation of driver gas yielded 520 kPa pressure, which is not very substantial but may cause some degradation to internal walls. For precaution against combustion the inclusion of an inert gas, preferably nitrogen, in dump tank operations would prove useful in avoiding reaction. Both in initial test and during venting up to 1 atm. Mixtures of hydrogen and helium are to be addressed with caution.
Hydrogen has been established as a strong producer of shock waves when used as a driver gas. I recommend further analysis be carried out, tests be developed and trials made, for the inclusion to X3 operations.