RLV physicaly impossible ?

I am really amazed by all these optimistic people trying to do rlvs
and i belive in these peoples cause! however it strikes me that these
people all seem a bit to optimistic for ex almoast all the groups i
have investigated are into liquid propellant engines, and they seem to
think they can make these engines totaly reuseble, let me remind you
about the physical laws of thermodynamics (heat) that makes it
impossible to turn the engines on and off forever or atleast alot of
times! for me it seems almoast impossible to make a liquid rocket
engine reuseble. Or am i wrong ? will these people beat the heat laws
of rocket engines somehow ?