The expander cycle family of rocket-engine power cycles involve using waste heat from the engine's combustion chamber and/or nozzle to vapourise some or all of the engine's fuel, and using the now-gaseous fuel to drive the engine's turbopump(s), before either feeding it into the combustion chamber (closed-cycle or classic expander cycle) or venting it overboard (open-cycle expander cycle or expander bleed cycle).
One often hears the claim that expander-cycle engines can only use cryogenic fuels, such as liquid hydrogen or liquid methane. For example, to quote the above-linked Wikipedia article:
[...] All expander cycle engines need to use a cryogenic fuel such as hydrogen, methane, or propane that easily reach their boiling points.
However, when one considers the temperatures reached in a rocket engine's combustion chamber, it seems like one should be easily able to use even the common non-cryogenic liquid fuels, such as RP-1, in an expander-cycle engine:
- H2 boiling point: 20.3K
- CH4 boiling point: 111.7K
- RP-1 boiling point: ~500K
- H2 combustion temperature: ~2,750K
- CH4 combustion temperature: ~3,250K
- RP-1 combustion temperature: ~3,700K
As one can see, the difference between the boiling points of (say) hydrogen and RP-1 is completely overshadowed by the difference between their boiling points, on the one hand, and the combustion temperatures of either of those fuels, on the other.1 In short, even though non-cryogenic fuels are harder to boil than cryogenics, your typical rocket-engine combustion chamber is hot enough to boil even non-cryogenics with great ease.
So what prevents the development of expander-cycle engines using RP-1 or other room-temperature fuels?
1: Indeed, going by the numbers, RP-1 could, conceivably, be a better choice for an expander-cycle engine than hydrogen or methane, as its higher boiling point is more than offset by its much higher combustion temperature, and the limiting factor for large expander-cycle engines is the amount of heat available for fuel-expansion purposes per unit time (which is limited primarily by the engine's combustion temperature).