STENNIS SPACE CENTER, Miss. -- The space shuttle's hydrogen-fueled rocket engines will soon roar again after receiving upgrades to fly on the Space Launch System, a heavy-lifting mega-rocket NASA hopes will take astronauts on journeys farther than humans have ever traveled.

An RS-25 engine is installed on a test stand in Mississippi to begin testing for NASA's Space Launch System heavy-lift rocket. Credit: NASA
 
The first unmanned SLS flight is set for the end of 2017, at the earliest, but testing is already beginning to wring out the rocket's engines.

Each Space Launch System flight will use four of the space shuttle main engines on its core stage. Two strap-on rocket boosters, derived from the solid rocket motors that flew on the space shuttle, will help the massive launch vehicle get off the ground.

Astronauts will strap into an Orion capsule on top of the second SLS flight around 2021, launching a crew beyond low Earth orbit for the first time since 1972. The mission could visit a captured asteroid near the moon, assuming NASA's proposed asteroid retrieval mission is technically and politically successful by then.

With an initial version capable of lifting 70 tons into low Earth orbit -- and rockets producing 8.4 million pounds of thrust at liftoff -- the Space Launch System will be the most powerful booster ever built, according to NASA.

NASA kept parts for 16 shuttle rocket engines after the program's last mission in 2011. Officials shipped the powerplants from Kennedy Space Center in Florida to the Stennis Space Center, NASA's primary rocket engine test facility in southern Mississippi, for storage and upgrades for the Space Launch System.

In parallel with the engine modifications, construction crews are beefing up a mammoth test stand at Stennis for the most powerful liquid-fueled rocket engine test firing conducted in the United States in nearly 50 years.

Some time in 2016, the massive core stage of the first SLS vehicle will be hoisted into the test stand for at least two "mission duty cycle" test firings, each lasting more than 8 minutes with four rocket engines, sending a thunderous sound wave across southwest Mississippi.

But engineers first plan a series of single-engine ground test firings to check out the upgraded space shuttle main engines. When fitted with upgrades for SLS flights, the engines are called RS-25s.

The first such engine was mounted on the A-1 test stand here July 17, kicking off a multi-year campaign aimed at ensuring the Space Launch System's main stage propulsion system is ready for flight.

The used powerplant, designated Engine No. 0525, is a pathfinder for SLS flight engines that will be tested here.

Engine No. 0525 will never fly. It is a holdover from the space shuttle program, which used it for checkouts of new components before they were introduced for real launches.

The engine has a similar purpose for the Space Launch System.

"It's going to get a new engine control unit installed on it, and the main objective of the test series that we're going to run is to test out this new controller," said Gary Benton, NASA's RS-25 rocket engine test project manager. "It will be on stand for four, five or six months, or however long it takes to get through the test series. There will be two of these engineering model controllers that get tested, and that will lead to the design of the new controllers that are actually going to fly."

Benton said the engine will start "hotfire" testing about six-to-eight weeks after it was moved to the test stand.

The primary change to the shuttle-era engines is a new controller, a computer avionics box that is similar to units already used on the RS-68 engine, a disposable hydrogen-fueled powerplant built by Aerojet Rocketdyne, the same company that holds the RS-25 contract with NASA.

"We can save money there and have a common supplier building these parts for multiple engines," Benton said.

Supplied by Honeywell, the controller is the brain of the engine, serving as the intermediary between the rocket's flight computer and the engine's myriad parts.

"It's actually telling the engine valves what position to be in, how fast to open or close," Benton said. "It's looking at sensors on the engine and determining what it needs to do. It's controlling everything on the engine -- purges, valve positions, all that kind of stuff."

The RS-25 engine generates 418,000 pounds of thrust at maximum power at sea level, according to Aerojet Rocketdyne's website. The engines consume cryogenic liquid hydrogen and liquid oxygen propellants.

If all goes according to plan, the sound of a shuttle main engine should again reverberate across the pine forests here by the fall.

"We're excited to get back into testing," Benton said. "It's a very powerful engine, and we're ready to get back into testing them again."

Artist's concept of the Space Launch System. Credit: NASA
 
Engine No. 0525 will complete several full-duration firings -- each nearly 9 minutes long -- to check out the two engine controllers, along with new flow meters and other hardware upgrades installed on the A-1 test stand since the end of the space shuttle program.

"All of the tests planned are full-duration tests. Those will tests of 500-some odd seconds with over 4,000 seconds planned total," Benton said.

Then engineers will install each of the four engines slated to fly on the first Space Launch System mission into the A-1 test stand for a series of acceptance tests to verify they are ready for flight.

The engines assigned to the first SLS mission, called Exploration Mission-1, powered space shuttle crews into orbit on more than a dozen flights.

The engines tapped for the first SLS mission, including their flight heritage, are:

• Engine No. 2060 -- (STS-127; STS-131; STS-135)
  
  
• Engine No. 2058 -- (STS-116; STS-120; STS-124; STS-119; STS-129; STS-133)
  
  
• Engine No. 2045 -- (STS-89; STS-92; STS-102; STS-105; STS-110; STS-113; STS-121; STS-118; STS-127; STS-131; STS-135)
  
  
• Engine No. 2056 -- (STS-104, STS-109, STS-114; STS-121)

The 16 space shuttle engines are in storage in a climate-controlled Aerojet Rocketdyne facility at Stennis. The engines rest inside shipping containers and on mobile dollies.

Before each engine goes on the test stand, technicians will add the new Honeywell controller and put the engine through a thorough inspection. Benton said each engine will be ignited on the test stand about six times before being cleared for flight.

Once they pass testing, the engines will be transported to NASA's Michoud Assembly Facility in New Orleans for integration with the Space Launch System's Boeing-built core stage.

Fifteen of the engines are mostly intact, and parts exist to assemble a 16th engine -- designated Engine No. 2063 -- to have enough powerplants for four SLS missions.

At NASA's current budget, officials expect the fourth SLS mission to fly in the mid-2020s.

Because the Space Launch System is not reusable like the space shuttle, NASA and Aerojet Rocketdyne will have to restart the engine production line for later missions.

While technicians work on the RS-25 engines, construction crews have descended on an Apollo-era test stand here to make the facility ready for an all-up eight-minute firing of the Space Launch System's core stage in 2016.

The B-2 test stand is surrounded by pine thickets and has canal access to receive the 27.6-foot-diameter SLS core stage.

The budget for the restoration project is approximately $100 million, according to Bryon Maynard, lead systems engineer for the B-2 test stand. So far, he said, the rehabilitation effort is running about 25 percent under budget.

The huge concrete facility is divided into two parts. The B-1 side is leased to Aerojet Rocketdyne for test firings of every RS-68 engine produced here before delivery to United Launch Alliance for Delta 4 flights. Engineers used the B-2 half of the test stand for firings of the Saturn 5 moon rocket's first stage in the 1960s, full-up tests of the space shuttle's three-engine main propulsion system in the 1970s, and qualification of the Delta 4's cryogenic core stage in the early 2000s.

The B-1/B-2 test stand at Stennis. The left half of the facility is leased to Aerojet Rocketdyne for testing of the Delta 4 rocket's RS-68 engine, and the right side is being restored for Space Launch System testing. Credit: Stephen Clark/Spaceflight Now
 
Maynard said The B-2 stand has rusted and deteriorated since it was last used more than a decade ago in support of the development of the Delta 4 launcher.

NASA built the facility in the 1960s, and it is strong enough for rocket engine tests of up to 11 million pounds of thrust. The Saturn 5's first stage produced 7.5 million pounds of thrust, and four RS-25 engines with the Space Launch System core will generate nearly 1.7 million pounds of thrust at sea level.

Construction is due to wrap up by January 2016, according to Maynard, when the test stand will be ready to receive the SLS first stage.

The "green run" testing of the Space Launch System core vehicle will include three phases. First, teams will fill the stage with cryogenic super-cold liquid hydrogen and liquid oxygen in a wet dress rehearsal. Then the rocket's four engines will be fired twice for approximately 500 seconds, the expected duration they will burn during a real launch.

The base of the rocket will be suspended 105 feet above the ground. With a height of 200 feet, the stage will stand 30 stories above the forested Mississippi coastal plains.

During the test stand restoration project, workers will move a behemoth 1.2 million-pound metal structure and upgrade and extend a heavy-lifting crane. Crews already replaced a device called an aspirator, which cools down the super-heated air around the engine plume during a test firing.

If the first SLS liftoff remains set for late 2017, the core stage will need to arrive at Stennis in 2016.

In a money-saving move, NASA opted not to construct a ground test article of the SLS first stage. The vehicle to be tested in Mississippi will next go to Kennedy Space Center for launch preparations.

That is one reason why engineers plan a limited test series on the SLS core stage: to avoid excess wear-and-tear on the rocket that will fly.

KSC launch teams will travel to Stennis to participate in the ground tests in a familiarization exercise to hone countdown procedures ahead of the first flight.

The SLS green run test will be the most powerful ground firing of liquid-fueled rocket engines since 1970, when testing ended on the Saturn 5.

"When you stand out there and we fire it, you're going to feel it in your chest," Maynard said.

Follow Stephen Clark on Twitter: @StephenClark1.