They aren't. The helium is cooled by the hydrogen going to the engine. You can't run the pre-cooler on the hydrogen directly, because the hydrogen infiltrates

They aren't. The helium is cooled by the hydrogen going to the engine. You can't run the pre-cooler on the hydrogen directly, because the hydrogen infiltrates the welds (and everything else) causing hydrogen embrittlement, which will quickly tear the heat exchanger apart. http://en.wikipedia.org/wiki/Hydrogen_embrittlement http://en.wikipedia.org/wiki/Hydrogen_embrittlement