Aerospace was one of the first industries to take up three-dimensional (3D) printing. This is because 3D printers are good at making things which are complex and lightweight. The machines work additively, building up layers of material only where it is needed. This allows multiple parts to be consolidated into a single item, rather than expensively assembled with the additional weight of nuts, bolts and welds. So far, 3D-printed aerospace parts tend to be used in non-critical areas, such as brackets or ducts. Now NASA has shown that the technology is capable of a far more demanding role: making rocket engines. NASA recently ground-tested a small rocket at its Marshall Space Flight Centre in Huntsville, Alabama. It used an injector made by selective laser sintering, a 3D-printing method which builds up objects by selectively melting successive layers of metallic powder, in this case a tough nickel-chromium alloy. The injector was designed by NASA and printed by Directed MFG, a firm based in Austin, Texas. It consists of just two parts instead of the 115 individual components used to make a similar injector with traditional manufacturing.In the test-firing the printed injector was used to mix liquid oxygen and gaseous hydrogen, and then channel the propellants into the combustion chamber where they are ignited to produce thrust. Although the rocket was small, the 3D-printed injector was based on a type used in large rockets, such as those which would be needed to launch a manned trip to Mars.The results were impressive. The jet engine generated 20,000 pounds of thrust and operated flawlessly at temperatures up to 3,300°C, said NASA. The space agency will continue tests to see how durable the printed injector is. Companies will be watching the results closely. GE, for one, has plans to use additive manufacturing to make parts for its future jet engines.NASA’s engineers think 3D printing could be used to reduce the cost of making rockets. But it also has other plans for the technology. NASA recently completed tests with Made in Space, a Californian company, on a 3D printer in a reduced-gravity plane that simulates near-weightless conditions (see picture). This printer makes things by extruding heated filaments of plastic, similar to the way many small desktop 3D printers operate. Next year the plan is to send a 3D printer to the International Space Station to make spare parts and tools.Further into the future the space agency is looking at the possibility of 3D printing food on manned deep-space missions. This suggests that one day it may well be possible for Babbage to do what Captain Jean-Luc Picard used to do in Star Trek: say “Tea. Earl Grey, hot” to a machine called the replicator. It would make him one, cup and all.