  We comment on the note "Low Q nuclear fusion in a volume heated mixed fuel
reactor" by Ruhl and Korn in which hydrodynamic life-time considerations are
included in estimates of ignition energy for uncompressed fusion fuel targets.
For the case of DT fusion, the authors arrive at a required hot-spot energy of
1MJ. We point out that their Q = 1 estimate is not relevant for transition into
a thermonuclear fusion regime, and that it is based on DT implanted in a
boron-proton matrix without accounting for all consequences. Applying the
proper corrections leads to an increase in the needed laser energy to initiate
useful fusion power production, even for DT, into the GJ range. The aim of
Marvel Fusion is the use of p-11B fusion reactions, which would require already
according to Ruhl and Korn's optimistic estimate, a hot-spot energy of 1 GJ. If
this were to be provided for by a mixed pBDT fuel or a staged explosion scheme
(rather than by laser deposition), it would imply a high associated production
rate of fast neutrons, and the need for tritium breeding, and push the energy
produced by a single laser pulse into non-manageable dimensions.
