  In cosmological first-order phase transitions, gravitational waves are
generated by the collisions of bubble walls and by the bulk motions caused in
the fluid. A sizeable signal may result from fast-moving walls. In this work we
study the hydrodynamics associated to the fastest propagation modes, namely,
ultra-relativistic detonations and runaway solutions. We compute the energy
injected by the phase transition into the fluid and the energy which
accumulates in the bubble walls. We provide analytic approximations and fits as
functions of the net force acting on the wall, which can be readily evaluated
for specific models. We also study the back-reaction of hydrodynamics on the
wall motion, and we discuss the extrapolation of the friction force away from
the ultra-relativistic limit. We use these results to estimate the
gravitational wave signal from detonations and runaway walls.
