>The most famous one is the under-development SABRE [1], which includes a closed-loop helium cycle. From the linked article: >>The 'hot' helium from the air p

>The most famous one is the under-development SABRE [1], which includes a closed-loop helium cycle. From the linked article: >>The 'hot' helium from the air precooler is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel. It's open-loop regenerative cooling, with helium as an intermediate coolant. The heat still ends up in the propellant, which gets dumped overboard. Not closed-loop. That patent also specifies a heat exchanger to the propellant tank. >I was figuring on dumping the air once done versus worrying about a reservoir. A poorly telegraphed joke. The atmosphere, here, is the reservoir. I have edited my comment.