>>12417143>what's a dual expander cycle?It's like the full-flow-staged-combustion cycle of expander engines.
An expander engine in principal simply uses waste heat generated by the combustion chamber to boil a propellant, which becomes the supply of high pressure gas that spins the turbines, which pump the liquid propellants through the engine. The turbine exhaust then dumps into the main combustion chamber and is burned, which generates a shitload of heat as you'd expect, which means the combustion chamber gets hot. This heat is what boils the coolant, you get it.
In a normal expander cycle, only one propellant is used to drive the turbopump (in the RL-10 it's hydrogen). This is more simple but has the disadvantage of needing one propellant stream to be able to run both pumps; it's the same problem that oxygen-rich staged combustion engines have, and fuel rich staged combustion engines too. In fact in the RL-10 the problem is especially bad because hydrogen has a very low density and oxygen has a very high density; the engine is using low density gas to spin a high density liquid pump, which basically means the turbine on the RL-10 is really big.
In a dual-expander cycle, you have two separate turbopumps and two separate cooling loops. One system is using boiled hydrogen to run the hydrogen pump, the other is using boiled oxygen to run the oxygen pump. This system allows the engine to reach higher combustion chamber pressures and greater mass flow rates, which means the thrust goes up significantly and the Isp goes up a bit, too. It's a better design, you just need two pumps.
