The propellants were hypergolic and consisted of hydrazine and either fuming red nitric acid or nitrogen tetroxide depending on the evolution of the system. Nasty stuff.
http://www.titan-ii.com/RFHCO.html
I think the Titan program was ended specifically because of the expensive and toxic nature of hypergolic fuel such as hydrazine. Scramjet tech seems like a perfect one to follow but I doubt Zero will support it in favor of every cent going to fuel government jobs and abortion clinics.
Recall that the Germans had several test pilots who literally melted from exposure to the nearly pure hydrogen peroxide used as fuel for the Me 163 Komet rocket fighter ...
Also, when they were being retired and cleaned up for museum displays, the shuttles were referred to as “Mach 25-capable Superfund sites.”