Pratt Whitney eventually took over management of the engine, but by then the engine that had arrived at Lewis with problems restarting was fully developed in several versions.
“The early 1960s version of the RL10 that was developed and matured under Lewis’ stewardship was a revolutionary prototype. But a lot of additional effort went into maturing that early model into the RL10A-3-3 that became a reliable workhorse engine during the 1970s,” says Jeff Breen, head of RL10 evolution at Aerojet Rocketdyne and formerly at Pratt & Whitney. “We have since evolved the RL10A-3-3 model six more times to achieve better performance and durability. However, I’d suggest that the current model, the RL10C-1, retains a large percentage of the heritage that can be attributed to the Lewis and Pratt & Whitney development partnership.”
Both the Centaur and the RL10 proved immensely popular from the late 1960s to the early ’90s, when the United States dominated commercial space launches. Breen estimates RL10 engines helped put into space about 90 percent of the large commercial satellites launched during that period. Today, it remains the upper-stage engine of choice to launch payloads for the U.S. military and other performance-demanding civilian missions. Many of the GPS satellites, an Air Force program that has come to play huge roles in industry and daily life, were inserted into orbit by RL10 engines.
NASA has used the Centaur to launch countless lunar and interplanetary exploration missions, from Surveyor 1, the first successful moon lander, to the Viking landers that were the first to explore the surface of Mars, to the twin Voyager spacecraft that explored Jupiter, Saturn, Uranus, and Neptune and now are the first probes to enter interstellar space. Current RL10-launched missions include the Juno probe at Jupiter, the Curiosity rover on Mars, Cassini in orbit at Saturn, and the New Horizons spacecraft that flew to Pluto and is now on its way to the Kuiper Belt.
A major reason for the RL10’s popularity has been its exceptional performance capability, which has only increased over time. Measured as “specific impulse”— the ratio of thrust per unit of propellant mass flow rate—the engine’s original thrust of 424 seconds has increased to about 465 seconds today. While the first model could produce 15,000 pounds of thrust, present models produce up to 25,000 pounds.
Its unique pumping method and other design elements also bestow reliability. “It has proven itself to be the most reliable engine ever built,” Breen says. “There’s been only one failure attributed to the engine in all its flights.”
Goette says this is because, in an era before computer design tools, Pratt & Whitney’s approach was to come up with a conservative design “and then test, test, test, and when it breaks, fix it.” During testing, for example, the engine had to run 10 times as long as it would need to run in space, and the valves were cycled many times more than they would in flight. “It was designed with beaucoup margin, and they tested the hell out of it and got rid of any failure points,” Goette says.
Given the cost and importance of most rocket payloads, whether they’re satellites or astronauts, reliability is the biggest consideration, Breen says. “There’s a lot riding on each launch. That’s why customers come to you—they know the engine can both deliver the performance and reliably place the payload into the correct orbit. The RL10 engine provides customers with confidence in mission success.”
The ability to make multiple restarts in space, which increases the performance capability and allows longer launch windows, is also a major benefit that was not easily achieved. If it’s difficult to start a car on an Alaskan winter night, it’s harder to restart an engine in the frozen void of space, Breen says. While there are other rockets with that capability, the RL10 is the only one that has restarted up to seven times during a single mission.
All this has made the engine and the RL10-powered Centaur the premier upper-stage engine and the most-used upper stage, respectively, in U.S. rocketry. The year 2009 marked the 400th flight powered by RL10 engines.
The ability to harness liquid hydrogen gave the U.S. civilian Space Program, the commercial aerospace industry, and the military a distinct edge over other nations. Recent military uses include launches of the Navy’s Mobile User Objective System satellite constellation and the Air Force’s X-37 space planes.
Commercially, throughout the early decades of the space age, virtually all U.S. satellite television, radio, and phone applications, as well as some weather, Earth-observation, and navigational satellites, got an assist from the RL10 engine.
The engine, however, has never been used in human spaceflight, despite its distinguished track record. That’s about to change. Boeing’s CST-100 Starliner and Sierra Nevada’s Dreamchaser both plan to use dual-engine Centaur upper stages to carry astronauts into space, and NASA’s Space Launch System will carry out crewed missions with the help of four RL10 engines on its Exploration Upper Stage.
“We’re eager to finally fly astronauts,” says Breen. “Adding the human spaceflight milestone will complete the RL10’s legacy.”
While it’s still based on the same design that was matured at Lewis in the 1960s, the engine has been updated for the 21st century. The RL10 A and B series are being phased out and replaced with the RL10C-1, which was completed in 2014 and incorporates the best aspects of the older lines, says Breen.
And in spring of 2016, Aerojet Rocketdyne successfully test-fired an RL10 engine with a 3D-printed core main injector, which reduced the part’s cost and production time by about half. By 2019, Breen expects about 95 percent of the engine’s complex geometrical parts to be 3D printed. Helping with that effort are the Air Force and, once again, Glenn.
“3D printing is a new manufacturing approach, but the upgrade will maintain the soul of the RL10 engine—its expander cycle—intact. So we keep the performance and simplicity that has made the engine so reliable, but now it’s significantly more affordable to manufacture,” Breen says.
Such updates ensure that the world’s first cryogenic engine isn’t going away anytime soon.
“It’s just hard to improve on it,” says Goette.