  When hypothetical neutrino secret interactions ($\nu$SI) are large, they form
a fluid in a supernova (SN) core, flow out with sonic speed, and stream away as
a fireball. For the first time, we tackle the complete dynamical problem and
solve all steps, systematically using relativistic hydrodynamics. The impact on
SN physics and the neutrino signal is remarkably small. For complete
thermalization within the fireball, the observable spectrum changes in a way
that is independent of the coupling strength. One potentially large effect
beyond our study is quick deleptonization if $\nu$SI violate lepton number. By
present evidence, however, SN physics leaves open a large region in parameter
space, where laboratory searches and future high-energy neutrino telescopes
will probe $\nu$SI.
