Inside most liquid-propellant rocket engines is another rocket engine, a sub-combustor that blasts its incandescent exhaust into a pipe and then a turbine instead of shooting it directly overboard. The arrangement sounds like trouble and, all too often, has proven to be just that. But the great power of the gas from the sub-combustor is needed to drive the turbopump that feeds the main combustion chamber. For more than half a century there has been a fundamentally easier and safer way to create the turbine-drive gas-by expanding cryogenic fuel with heat from the main combustion process. However, this so-called expander cycle has never been powerful enough for the first stage of a major space launcher.
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