They aren’t going to have to replace the liners in the Orbiter fuel lines; they think that they can be repaired by welding. Atlantis will be the first to be repaired and fly, and it looks like it won’t set back the ISS construction schedule as much as had earlier been feared. But this paragraph is worth noting.

The liners help direct the flow of propellant through the plumbing and past accordion-shaped bellows that give the plumbing needed flexibility for when the supercold fuel causes the pipes to shrink.

That “supercold fuel” is liquid hydrogen. Rocket engineers like hydrogen for a couple of reasons–it has very high performance in terms of fuel economy, and it burns very cleanly with oxygen, with only water as a by product. But it has some engineering design issues as well, and this is one of them. Thermal conditioning of the propulsion system is required to allow it to handle the large temperature variations from ambient to the cryogenic temperatures required to maintain hydrogen in a liquid state. This is one of the things that makes Shuttle so finicky and complex.

Another problem with it is that it is the opposite of dense (I like to use the term “fluffy”), so that very large tanks are required to carry it. For example, even though the Shuttle carries six times as much liquid oxygen as liquid hydrogen, the hydrogen tank is much larger than the oxygen tank. This need for additional tankage mass wipes out a lot of the theoretical performance advantage of lox/hydrogen propulsion.

It may be that ultimately, low-cost space transports will continue to use this propellant combination, but it’s refreshing to see XCOR and others explore more-tractable hydrocarbons, such as kerosene and propane. My suspicion, at this point, given what the real cost drivers are for launch systems, is that at least in the near term, space transports will not be using liquid hydrogen as a fuel.