A theory is presented of a new heat propagation mechanism, thermal hyd
raulic quenchback (THQB), in large scale superconducting magnets. The
underlying physics of THQB is discussed and an analytic solution for t
he quench propagation velocity is presented. This solution represents
the first such result for THQB, and is shown to be in excellent agreem
ent with the full numerical simulations of the governing mass, momentu
m, and energy conservation equations for the compressible flow of the
coolant in the conduit. The THQB propagation velocity is observed to b
e as much as an order of magnitude greater than the helium coolant vel
ocity. This is in direct contrast to a standard quench where the two v
elocities are always essentially equal.