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Oil-fired superheat lets you run superheated steam, which PWRs can't do, so gets you theoretically better performance from your neutrons but you're dependent on having oil fuel for good performance. It does seem to be what the Soviets did, and is probably the best way to do CONAS.
Separate boilers feeding the same turbines requires that the turbines run on saturated steam, which is an inefficient way to burn oil fuel and not great for turbines. Nuclear plants deal with this because they have to, and because fuel efficiency is less of a concern for them. AFAIK this is how the West thought the Soviet CONAS plants worked, but it seems to just be bad.
Entirely separate oil-fired and nuclear plants geared onto the same shaft adds a whole bunch of weight and complexity, but makes the plant design and operation effectively independent. At this point, you've just got an oil-fired boost plant. CONAG is a much lighter way to do that.
If the RN was talking about a 20,000 shp nuclear plant and a 20,000 shp steam plant, I suspect they were thinking in terms of the third option.
I can't say exactly how the 50/50 CONAS plant would have worked but I am certain that it was an option considered only briefly under a much wider nuclear propulsion study. The study that was ongoing at the time ultimately concluded that CONAG would be the optimum solution for a 75,000 SHP plant for guided missile destroyers. Specifically, a single reactor delivering 60,000 SHP and two 7,500 SHP G6 gas turbines to provide emergency power, rapid start-up and boost from 80% power to 100% power. The logic was:
- The nuclear part of the system was heavy, anything to make it lighter was good, e.g. having alternative emergency power meant only one reactor was needed and using the Gas Turbines for boost meant that the single reactor could be smaller and lighter
- The study assumed the ship had a double bottom, the double bottom would have to be filled with liquid anyway so that liquid may as well be fuel
- Like conventional boilers, nuclear reactors were slow to raise steam, adding gas turbines solved for this, allowing the ship to get underway faster
Note that this was all 1956-58.
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