The thundering roar of a rocket leaving the launch pad is a familiar sight. Much less familiar is the job of the smaller upper stage engines that do their job mostly beyond eye and camera range, but give spacecraft the big, in-space push they need to venture into deep space.

NASA's new rocket, the Space Launch System (SLS), will rely on a proven upper stage engine - the RL10 - for its first mission with the agency's Orion spacecraft in late 2018. The SLS Block 1 rocket will use one RL10B-2 engine, the same engine currently used by the Delta IV rocket, as a part of the interim cryogenic propulsion stage (ICPS).

As the rocket evolves to a more powerful Block 1B configuration, an exploration upper stage (EUS) will be added. The EUS will use four RL10C-3 engines, and the upgraded rocket will send astronauts tens of thousands of miles beyond the moon to explore deep-space, paving the way for NASA's Journey to Mars.

To achieve these deep space missions, NASA recently contracted with Aerojet Rocketdyne of West Palm Beach, Florida for the production of 10 RL10C-3 engines for the rockets second and third flights with Orion, as well as two spare engines.

"The RL10 is a very technically mature engine design," said Steve Wofford, SLS Liquid Engines manager at NASA's Marshall Space Flight Center in Huntsville, Alabama, where the SLS Program is managed.

"It has been the nation's upper stage workhorse engine for more than 50 years and is second to none in performance and demonstrated reliability. It also leverages existing propulsion technology to provide SLS with a robust engine in a timely manner and avoids costs associated with a new engine development program."

The RL10 rocket engine was first developed by Pratt and Whitney in late 1950s and was first flown in 1963. The RL10 has sent spacecraft to every planet in our solar system, including Voyager 1, the first craft that reached interstellar space. Now, the RL10 will power the vehicle that will send people farther away from Earth than humans have ever traveled before.

The engine has evolved and improved over time and has a stellar record: flown more than 400 times; logged approximately 15,000 hot fires; and accumulated more than 2.3 million seconds of hot fire operation time with a demonstrated reliability greater than 0.999 throughout its history.

The thrust and specific impulse - think gas mileage - of the RL10C-3 version makes it ideal as an upper stage engine for NASA's human exploration missions to deep space.

"Engines are one of the most complex rocket elements and we're pleased to be working with Aerojet Rocketdyne to build these flight engines," said James Burnum, NASA SLS Liquid Engines RL10 manager.

"Starting with the second mission, humans will be traveling into deep space further than ever before. We need a reliable engine with a proven track record that has the performance to power humans to deep space."

The $174 million contract with Aerojet Rocketdyne covers the management, testing, certification and delivery of the engines for human spaceflight and will continue through Feb. 29, 2024.

Engine testing will be performed at NASA's test facility in West Palm Beach, Florida, as well as a green run of the EUS with four RL10 engines at NASA's Stennis Space Center near Bay St. Louis, Mississippi. T

he green run will be the first time the engines are assembled into a single configuration with the EUS and fired under simulated flight conditions. This will test the compatibility and functionality of the system to ensure a safe and viable design. The SLS will launch from NASA's Kennedy Space Center in Florida.