Oh my god. I live aerospikes, and i love expander cycles, even though neither of them are actually practical, so the idea that each one might be able to solve the main problem with the other one has given me an enormous space boner.
What about expander cycle engines is not practical? The RL-10 which uses an expander cycle has been used for a very long time(1962) and it's future looks bright as well[1].
Ah, i was inaccurate, sorry. They are very practical - but there's been a limit to how big you can make them, based on the need to get enough surface area in contact with the exhaust to vapourise the fuel. AIUI, that's why they're great for upper stages, but haven't been used for first stages. Combining them with an aerospike might let them scale up to first-stage size.
Expander cycle engines are limited because you need heat to run the turbo pump. And the heat is provided by running propellant around engine to cool it off. The advantages Tim Dodd mentions for larger engines (propellant flow increases faster than surface area of the engine), are exactly the things that make expander cycle engines hit their limit: not enough area to heat up the propellant to run the pumps enough for the amount of propellant you need for the thrust you want.
Aerospikes have more surface area, which means more area to cool... which is more area for the propellant to flow through to get heated up to run the engine, which means more power available to give more fuel.
