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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).

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It's not the boiling point, it's the specific heat capacity. Temperature ≠ heat!

Kerosene's cp is only 2 kJ/(kg K)) while hydrogen's is 14.31 kJ/(kg K)), literally an order of magnitude better.

That means kerosene is a terrible coolant, and

For expander cycle engines, the maximum size and thrust of the engine are limited by the amount of energy (heat) that can be absorbed by coolant from the combustion chamber during operation.

(quoted from here)

So kerosene isn't used because of its poor heat transfer properties. Expander cycles are limited anyway by the amount of energy that can practically be supplied by the vaporized fuel1; choosing a coolant that's worse by an order of magnitude makes the cycle impractical.

1 Sutton, 4th edition, p. 216

Oh, and if that weren't bad enough, there's coking. If you heat RP-1 enough, it tends to "coke", or deposit solid materials on the flow passage walls. This makes the heat transfer process even worse, so you'd have to keep the temperature down to prevent this.

This coking mechanism is predominant at temperatures above 825 K and occurs when the fuel is heated enough to decompose into reactive fuel radicals, leading to the eventual formation of coke.

(source)

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    $\begingroup$ The heat of vaporization of H2 is also almost twice that of kerosene. $\endgroup$ – Bob Jacobsen Jul 14 at 2:08
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    $\begingroup$ I'm not sure this answer really gets to the heart of the matter. The limitation of expander-cycle isn't so much how much heat energy can be removed (you have that issue also in gas-generator engines) as how much energy can be removed and then used for doing mechanical work. The cp seems largely irrelevant for this; mostly the problem is that RP-1 does no useful work at all while below its boiling point, and can't be heated much more above this point without coking. $\endgroup$ – leftaroundabout Jul 14 at 9:27
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    $\begingroup$ @BobJacobsen “the engine needs a specified amount of heat removed by cooling” – but that's what you want in an expander-cycle engine. The essential problem, the reason why EC doesn't scale to large first-stage engines, is that the engine does not need enough cooling. If the fuel is easy to heat up, that's actually good because you quickly start extracting mechanical energy. For an extreme bad example, if you were to cool the engine with molten salt, you would be able to cool it well, but wouldn't be able to use that energy for driving the pump. $\endgroup$ – leftaroundabout Jul 14 at 12:19
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    $\begingroup$ The last link in my answer is a paper about coking in RP-1. $\endgroup$ – Organic Marble Jul 15 at 4:37
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    $\begingroup$ @Sean all long-chain hydrocarbons will eventually coke; RP-1 is just optimised to do it relatively little. $\endgroup$ – leftaroundabout Jul 15 at 5:33

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