3-D printing may be a huge advance for space travel. On long-distance missions (maybe like travel to Mars) it will be easier to carry a printer than loads of spare parts. http://bit.ly/RVAn0z If the material can be made to flow through the printer head, things as various as food and buildings could be printed. http://bit.ly/1eTSKic
The need to haul huge amounts of stuff to Mars always seemed to me to preclude colonization. People will go crazy trying to live in little boxes, so the ability to keep building more and more living space seems crucial. Printing buildings from the dust of Mars would be a perfect solution; along with printing the solar panels and interior lighting needed to make underground living enjoyable. I envision habitats like the shopping malls inside fancy Las Vegas casinos (except with living space and practical little shops instead of haute couture) designed to mimic old European villages with winding streets and blue-painted ceilings to avoid claustrophobia. That would be a cool setting for a science fiction story.
I’ll look forward to 3-D printing experiments on the Space Station. It’s not clear to me they can print parts of mixed plastics and metals yet, so there may be a way to go before we can really print “replacement parts” for machines.