[QUOTE] I never ceased to amaze me what the massive increase in cost was with anything to do with the cryogenic pumps, feed lines or couplings for an Ariane 5. Not forgetting the techniques used to shift 3 tons of cryogenic fuel into an engine per second.[/QUOTE] Cryogenic materials. Plenty money. I remember when we looked at using liquid hydrogen to cool the intake air of a gas turbine (More dense air = increased mass flow) and then feed it to the burners. Apart from the cost, what put us off the idea IIRC was the fact that hydrogen had a long residence time before combusting which meant that in an aeroderivative engine it wouldn't ignite until it was half way down the turbine. Plenty molten splatter. Hydrogen being a smaller atom can diffuse in to the surface driven by a concentration gradient. Then, because it has low solubility in non close packed structures, it will readily come out of solution where defects/voids occur and become dihydrogen gas, hence exerting a huge pressure internally leading to rupture. Thus everything would have to be made out of more expensive and often weaker (and therefore thicker) FCC materials such as Austenitic stainless steels, nickel based alloys etc. [URL]http://en.wikipedia.org/wiki/Hydrogen_embrittlement[/URL]
