Large rocket engines cannot be reusable

Large rocket engines cannot be reusable because they are
damaged by large Reynolds number. The Reynolds number =
Re = V*D*S/N whe
V = gas velocity
D = diameter of the chamber
S = gas density
N = gas viscosity

Gas viscosity is primarily a function of temperature. The
impact of pressure is minor and the viscosity correction for
pressure is less than 10% for up to 3.5 MPa. This means that
the Reynolds number is proportional to the chamber's diameter
and to the gas density, which is proportional to its pressure.
The large Reynolds number is the primary cause of turbulence,
combustion instability, and cavitation. Turbulence disturbs
the protective layer of cool gas adjacent to the chamber's wall.
Cavitation and vibration damages turbopumps. All of these
fatal diseases are associated with large combustion chambers
and large turbopumps. They do not exist in small combustion
chambers and well designed, small turbopumps because their
Reynolds number is small. This means that large rocket engines
cannot be reusable and they are suitable for nuclear missiles
only.

Small rocket engines have another important advantage: they
have superior thrust-to-weight ratio due to the Cube-Square
Law which states that as scale is reduced, properties which
are a function of volume (mass) will decrease faster than
those which are a function of area (thrust and strength).

Small, simple engines can be made by robots, so they can
be very cheap. Large engines must be made by rocket plumbers.

A rocket launcher made of a few hundred small engines is
very reliable because the failure of a few engines out of
a few hundred is not a catastrophe.

The small engines have a small flaw: their small combustion
chambers do not mix fuel and oxidizer well. This means that
only volatile propellants (e.g., oxygen and methane) can be
used in those engines, and the injector holes must be small.
Small holes are more difficult to make than large holes and
they may plug up with dirt.

The best example of small, robust engines that can be made
by industrial robots are engine clusters. They are robust
enough to survive reentry, splashdown, and handling on a
bobbing ship:
http://www.islandone.org/LEOBiblio/S...engine_cluster