NASA says it is a step closer to completely 3-D-printing a rocket engine - a technology that could dramatically lower the cost of one of the most expensive parts of space flight.

A team working at NASA's Marshall Space Flight Center in Huntsville, Ala., said it fired 3-D-printed parts together in late October to produce 22,000 pounds of thrust. The "engine" used liquid hydrogen and oxygen fuel, just like many NASA rocket engines today.

"We manufactured and then tested about 75 percent of the parts needed to build a 3-D-printed rocket engine," project manager Elizabeth Robertson said Thursday. "By testing the turbo pumps, injectors and valves together, we've shown that it would be possible to build a 3-D-printed engine for multiple purposes such as landers, in-space propulsion or rocket engine upper stages."

NASA has worked with vendors for three years to make 3-D-printed parts such as turbo pumps and injectors. In this test, they connected them but not in a configuration that looks like a typical engine.

"In engineering lingo, this is called a breadboard engine," said testing lead Nick Case. "What matters is that the parts work the same way as they do in a conventional engine and perform under the extreme temperatures and pressures found inside a rocket engine. The turbo pump got its 'heartbeat' racing at more than 90,000 revolutions per minute (rpm) and the end result is the flame you see coming out of the thrust chamber to produce over 20,000 pounds of thrust, and an engine like this could produce enough power for an upper stage of a rocket or a Mars lander."

To make each part, engineers enter a design into a 3-D printer's computer. It builds each part by layering metal powder and fusing it with a laser. It's called "selective laser melting," and the result is a turbopump with 45 percent fewer parts than a welded pump.

Future tests will use liquid oxygen and methane - propellants that might be produced on Mars.