Rocket engines for power generation?

Here is an idea I have been thinking about for a while:

Rocket engines are quite efficient at converting chemical energy to linear
motion. They also have an astronomically high power to weight ratio.

Rocket engines have the disadvantage compared to diesel engines and
gas/steam turbines that they need liquid oxygen as an oxidizer instead of
air, but given the fact that liquid oxygen is one of the cheapest liquids
in existence and can be obtained anywhere by liquefying air that should not
be a big deal.

To get a rough estimate of the power conversion efficiency of rocket
engines, lets examine a large stage combustion engine like the russian
RD-170. I am using the data from the encyclopedia astronautica.

It has a vacuum specific impulse of 337s. That means an exhaust velocity of
3305.97m/s and thus 5.46471882045 MJ/kg of mechanical energy. Kerosene has
a specific chemical energy of 42.8 MJ/kg. With the mixture ratio of 2.6:1
that gives a specific chemical energy of 11.888 MJ/kg for the propellant
mixture. The efficiency of conversion from chemical energy to mechanical
energy of the exhaust is 0.46 or 46%. The mechanical energy produced is
slightly over 21GW!

An efficiency of 46% does not sound that good, but the rocket engine would
just be the first stage of energy conversion. A mixture ratio of 2.6:1 as
with the RD-170 results in a very fuel-rich exhaust which could be burnt
with additional oxygen to power a more traditional gas turbine as a second
stage and maybe even a steam turbine as a third stage.

By combining a rocket engine with a traditional gas turbine, you could
easily get an overall conversion efficiency of more than 60%. Not bad for a
device with extremely high power to weight ratio. Since the oxydator is
pure oxygen the combustion would be extremely hot and clean. There would be
no NOx in the exhaust. The exhaust after the second combustion stage would
be pure CO2 and H2O.


The best way to build a rocket engine for power generation would probably be
a rotary rocket engine that uses centrifugal force for pumping the
propellants to very high pressure. It would be disk-shaped and rotating
with a rim velocity of 3000m/s. The combustion chambers would be at the rim
like in the original rotaryrocket designs. It would have to be made from
high tensile strength materials like carbon fiber composite and be tapered
from the hub to the rim to withstand the enormous centrifugal forces. It
would contain some neodymium permanent magnets near the hub for direct
conversion of its rotation energy to electrical energy. These could also be
used for controlling the rotation speed.

It would obviously be quite a challenge to get such a design reliable and
durable enough for power generation, but at least you do not need
oxygen-rich preburners like in the RD-170 because of the centrifugal
pumping. One thing that might be a problem is that the propellant pressure
at the rim would be much too high for normal combustion chamber designs,
but there should be a way to cope with that, for example by putting the
combustion chambers more towards the hub of the disk.


So what do you think. Is this
a) theoretically impossible
b) theoretically possible but practically impossible because of the lack of
suitable materials
c) just not worth doing
or
d) a good idea?

best regards,

Rüdiger