Since a ramjet isn't constrained by turbine inlet temperature like a gas turbine is, it seems to me that it should be able to achieve a higher thermal efficiency, given that it's operating at a speed that results in a comparable compression ratio. Is this true?
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$\begingroup$ The problem is, the nozzle is less efficient than a turbine and fan combo, which is why turbofan is more efficient than turbojet and also afterburner isn't very efficient at all. Game is already lost when you are jetting hot stream directly to generate thrust, unless you are at high altitude where air is too thin to be efficiently driven by a fan. $\endgroup$– user3528438Oct 8, 2020 at 18:58
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$\begingroup$ @user3528438 You are talking about propulsive efficiency which is not the same thing as thermal efficiency. $\endgroup$– Peter SOct 9, 2020 at 7:51
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$\begingroup$ To put it another way, exhaust temp is too high for ram jet to be very efficient. $\endgroup$– user3528438Oct 9, 2020 at 10:07
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$\begingroup$ @user3528438 I don't see why the ramjet would have a higher exhaust temperature if it runs at the same or higher pressure ratio as the gas turbine? $\endgroup$– Peter SOct 10, 2020 at 6:35
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$\begingroup$ @PeterS The combustion product in a turbine cools by doing work on the turbine blades (work that either compresses the incoming combustion air or directly propels thrust air via the fan), so is cooler before it enters the tailpipe, even if combustion temperature is the same. Raising combustion temperature in the ramjet makes this worse, and there isn't enough pressure differential to make up the difference with nozzle design. $\endgroup$– Zeiss IkonOct 10, 2020 at 13:35
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The Carnot condition notwithstanding, whether a ramjet actually operates at a higher "hot end" temperature than a turbojet is questionable, as well as whether a comparable percentage of thermal energy in the exhaust stream is converted to kinetic energy.
Further, just because there's no turbine, doesn't mean there's nothing in the hot part of the engine that can melt -- combustion chamber walls are made of real material, as is the flameholder, and due to cost of exotic metals, likely aren't as heat resistant as first stage turbine blades. Even a very simple, low-powered ramjet (see YouTube) can operate at a temperature that makes the hot section glow from heat; one capable of propelling a high speed aircraft will presumably run even hotter, requiring tradeoffs in efficiency to prevent thermal damage.
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$\begingroup$ I know that rocket engines run substantially hotter than gas turbines by using regenerative and film cooling of the combustion chamber and nozzle. The number I'm finding is 3500 K for rockets compared to just about 2000 K inlet temperature for bleeding edge gas turbines. Is there reason why a ramjet couldn't run that hot as well? $\endgroup$– Peter SOct 10, 2020 at 5:52
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$\begingroup$ Rockets use far higher mass flows of fuel and oxidizer than ramjets, which have only fuel flow to apply. Further, if you correctly measure efficiency by "work done" vs. "fuel energy consumed" film cooling with fuel directly reduces efficiency by consuming fuel without doing additional work. Regenerative cooling with kerosene has limits, which largely define the difference between Jet-A/JP-4 and RP-1. Short version, if you're regen cooling with kerosene, you'd better be using RP-1, especially since a ramjet is expected to operate far longer than a rocket engine. $\endgroup$ Oct 10, 2020 at 13:31