At a time when cell phones and automobile features are outdated after a few short years, it may seem impossible that any technology would remain virtually unchanged over decades. But the world’s first cryogenic fuel-powered rocket engine, a NASA spinoff, remains the most-used upper-stage rocket engine in the United States more than 50 years after its creation.
The RL10 rocket engine, first successfully flown in 1963, has been crucial to NASA’s space exploration and has also put hundreds of commercial and military payloads into orbit, enabling satellite communications and satellite-based defense operations. What’s more, after more than half a century, only a handful of countries have the technology, pioneered under the program, to power rockects with liquid hydrogen and oxygen.
The RL10 was matured under a contract between NASA’s Lewis Research Center, now Glenn Research Center, and a division of Pratt & Whitney Aircraft, now part of Aerojet Rocketdyne. Both entities had previously worked on the technology independently.
In the 1940s, Lewis, then part of NASA’s predecessor, the National Advisory Committee for Aeronautics (NACA), had carried out extensive testing on high-energy liquid rocket propellants, including liquid hydrogen. Cutting-edge propulsion and cryogenic-handling technology remain two of the center’s specialties today.
Pratt Whitney’s work with liquid hydrogen began in the 1950s, when Lockheed Corporation subcontracted with the company to create a liquid hydrogen-powered airplane engine, part of a top-secret project for the Department of Defense. To have space for noisy engine testing, Pratt & Whitney opened a facility on a large tract of land near West Palm Beach, Florida, where Aerojet Rocketdyne still builds RL10s.
Known as the SUNTAN project, the work was eventually abandoned as the team determined liquid hydrogen was too unwieldy for use in an airplane engine, at least with existing technology for handling cryogenics. However, a modestly funded liquid hydrogen engine test program within SUNTAN, known as Project Bee, was established at Lewis. Project Bee was a success and solidified the center’s reputation as an expert institution in handling liquid hydrogen for propulsion.
The project to build the first rocket powered by cryogenic engines, the Centaur upper stage, began in 1958, the year NASA was created. It started as a Department of Defense project, with the aim of putting heavy payloads into orbit, but the Space Agency took over the work a year later, moving it to Marshall Space Flight Center, with Pratt & Whitney designing and building the RL10 engines that would launch it.
After the Centaur exploded during its first test launch in 1962, Marshall officials were prepared to kill the program, but Abe Silverstein, center director at Lewis, convinced NASA Headquarters to move it to Lewis, whose test data and designs from years before in Project Bee had heavily influenced the Centaur and SUNTAN teams.
One of the engineers who carried out some of Lewis’ early cryogenic work was Bill Goette, who worked on injectors for different propellant combinations at Lewis under NACA and later would spend a decade heading the RL10 program.
Among other takeaways from the early Lewis research, Pratt & Whitney ended up adapting a concentric tube injector design created at the center.
“It was all on a research basis—there was never a specific end use in mind,” Goette says of the early work under NACA. “That was NACA’s role, working on basic research, taking risk out, and giving companies a heads-up on how to do things.”
Even after the Centaur program moved to Lewis, the engine work initially stayed at Marshall, because the RL10 was also planned to be used on the upper stage for the Saturn 1 launch vehicle, predecessor to the Saturn V that was to launch the Apollo missions. But Goette and others at Lewis, as well as contractor General Dynamics, worked to ensure the engine would also meet Centaur’s needs.
In 1963, the work paid off. Centaur launched on top of an Atlas booster rocket, marking the first successful flight of a cryogenic rocket engine. A few years later, NASA decided to shift direction for Apollo’s upper-stage rocket, but Centaur and the RL10 project lived on at Lewis, with Goette in charge of the engine program.
“Before it came to Lewis, they had built a number of the engines and tested them,” he says. “A lot of the bugs had been fixed.”
He says the engine was and remains remarkable for its efficiency. The high-performance combination of liquid hydrogen fuel and a liquid oxygen oxidizer generates more thrust per unit of fuel burned than any other propellant combination. That means a rocket can carry less fuel, which keeps the weight down, allowing more payload to be carried to orbit.
But the RL10’s expander cycle goes one step further in fuel efficiency and simplicity by eliminating the need to burn fuel to turn turbines that drive the fuel pumps. Instead, the cryogenic liquid hydrogen is used to cool the combustion chamber and nozzle, where it picks up heat and turns into hydrogen gas. This expansion of the hydrogen gas drives the turbine, which powers the pump.
“That’s essentially free energy,” Goette says, comparing the engine’s expander cycle to using the heat generated by a car engine to warm the interior. “I don’t think anyone else has built an engine with that kind of cycle.”
Following Centaur’s success, the liquid hydrogen propulsion technology developed under the RL10 program was also used to create the J-2 upper-stage engines for Saturn V that enabled the United States to put astronauts on the moon. Critical technologies and knowledge such as injector design, mitigating combustion instability, inhibiting propellant slosh, and hydrogen gas venting were essential for the success of the Saturn V. Eventually, liquid hydrogen and oxygen became the Nation’s go-to fuel for boosters and upper stages alike, including the Space Shuttle main engines and the Air Force’s most powerful launch vehicle, Titan IV. Liquid hydrogen is slated to be the fuel for both the core and upper stages of NASA’s planned Space Launch System. Centaur continues to fly today as the upper stage for Atlas V, and the RL10 also flies on Delta IV.
One challenge that remained when the program came to Lewis was that the engines needed to be capable of multiple starts in space to meet mission needs. But without gravity in orbit, the remaining fuel tended to float aimlessly in the tank. The Centaur already used small hydrogen peroxide thrusters to control its orientation, and Goette says a few more thrusters at the rear of the tank fixed the problem by creating enough forward momentum to keep the liquid propellants settled at the bottom of the tanks where the engine inlets were located.
Other changes included extending the engine’s nozzle and narrowing the thrust chamber throat to increase power and efficiency. Over the nearly 30 years that Lewis oversaw the Centaur program, both the rocket design and the RL10 engine continued to change and evolve.