A shuttle-era main engine roared to life in a ground-shaking Mississippi test firing Thursday, the latest in a series of full-duration burns to make sure the upgraded powerplants can handle the higher temperatures, pressures and thrust levels needed for the agency's new Space Launch System -- SLS -- super rocket.
Bolted into the Apollo-era A-1 test stand at NASA's Stennis Space Center near Bay St. Louis, Miss., RS-25 engine No. 0525 ignited at 5:01 p.m. EDT, throttling up with a 120-decibel roar as it gulped more than 300 gallons of liquid oxygen and hydrogen rocket fuel per second.
With the propellants burning at some 6,000 degrees Fahrenheit -- hot enough to melt lead -- the Aerojet Rocketdyne engine's exhaust plums shot out its liquid hydrogen-cooled nozzle at 13 times the speed of sound, blasting a billowing cloud of white steam high into the sky above the test stand.
Six seconds after ignition, the engine's new flight computer was programmed to command valve settings that increased the thrust level to 109 percent of the power originally intended for shuttle launchings. The test plan called for the thrust level to drop to around 80 percent before eventually throttling back up above 90 percent.
Unlike an actual launch, where the sound quickly fades away as the rocket climbs away toward space, the RS-25 test firing went on for a full eight minutes and 55 seconds -- the time needed for the new SLS rocket to reach orbit -- before the engine finally shut down, putting on a memorable show for reporters and a throng of social media representatives.
"No exaggeration to say you feel it more than hear it," said Stephen Clark, a reporter for Spaceflightnow.com. "And hearing protection is actually needed."
Steve Swofford, NASA's SLS engines manager, said telemetry indicated "the initial results are great. We ran full duration for 535 seconds, we met our test objectives, didn't note any anomalies at this time. So now we get the fun part of going through the data."
Once known as the space shuttle main engine, or SSME, the compact powerplants flew three at a time aboard NASA's winged orbiters, along with a pair of four-segment solid-fuel boosters that provided the lion's share of the initial push out of the deep lower atmosphere.
For NASA's gargantuan 322-foot-tall Space Launch System booster, needed for eventual piloted flights into deep space, two more powerful five-segment boosters will be used, along with four renamed RS-25 engines.
The SLS first stage initially will develop about 8.4 million pounds of thrust -- 10 percent more than NASA's legendary Saturn 5 moon rockets -- with about 2 million pounds of push coming from a quartet of RS-25s. The initial version of the rocket will be capable of lifting 70 metric tons to space while a later version with a more powerful second stage engine will be able to lift 130 metric tons.
NASA has 16 left-over shuttle engines that are being upgraded for use in the new heavy-lift booster. The agency is in negotiations with Aerojet Rocketdyne to purchase another four-engine flight set and two spares.
First flight of the SLS is planned for 2018 when the rocket will boost an uncrewed Orion capsule around the moon and back to an ocean splashdown. The second test flight, Exploration Mission No. 2, or EM-2, is planned for launch around 2021. As currently envisioned, it will carry a crew of two to rendezvous with a boulder plucked from the surface of an asteroid.
NASA's projected budget covers one SLS flight every other year or so, but agency managers hope to improve efficiency and processing enough to improve that rate to one mission per year with an ability to launch up to three flights in 12 months if warranted.
Using Thursday's test to provide an update on SLS development, program managers and engineers told reporters the RS-25 test firings have been going well and that the SLS design is now complete pending final acceptance of a critical design review conducted earlier this summer.
Todd May, NASA's SLS program manager, said a major objective for 2015 was getting the RS-25 back onto the test stand at Stennis. Thursday's test firing was the sixth in a series of seven using engine 0525.
"These things have a brand new state-of-the-art controller that is a fraction of the cost of the controllers we used during the space shuttle program," he said. "We also have to put them through the paces with the environments that will see on the SLS."
The SLS engines will be arranged in a square pattern at the base of the booster, putting them closer to the nozzles of the solid-fuel boosters than they were in the shuttle program when three engines were arranged in an offset triangular pattern
The SLS configuration will subject the engines to higher radiated temperatures and new insulation was tested during Thursday test firing to help protect the nozzles during launch.
Another major difference between the SLS and shuttle versions of the engine is a higher propellant inlet pressure where where liquid oxygen rushes in from a tank at the top of the first stage.
"We've got a very, very tall liquid oxygen tank on SLS, and that tall a tank gives you a greater pressure down at the bottom of the tank," Swofford said earlier. "It's like being deep in a swimming pool, the pressure's pretty high down at the bottom.
"So it's higher than the engines are used to seeing. They can handle it, but we test to prove they can handle it."
Additional objectives include test runs at higher throttle settings. Thanks to upgrades during the shuttle program, the engines typically operated at 104.5 percent the thrust level they were initially designed for and were certified to run at up to 109 percent in certain abort scenarios.
The SLS RS-25s will run at 109 percent as a baseline throttle setting, generating up to 512,000 pounds of thrust each. Engineers are studying ways to increase performance to 111 percent and possibly even to 113 percent. But only if the budget supports the ground testing that will be needed to prove it's safe.
"We have discussed, for future blocks of the rocket once we get up and flying, seeing if we want to chase performance," May said. "We have a very robust design, and we actually have a lot of performance margin (but) we don't want to get into a situation where we're chasing performance. Because that last little bit of performance is what really drives your costs."
But unlike the space shuttle, the SLS is not reusable and the rocket's stages, including the engines, will crash into the ocean after use. Because the engines will never be reused, the higher throttle settings, and the stress they put on the engines, is not as big a concern as it was in the shuttle era.
Higher throttle settings are "a little harder on the engine, we're running it a little closer to redline, but it's safe, especially for short durations on a mission like SLS," Timothy Duquette, an SLS engineer at NASA's Marshall Space Flight Center, said in a video presentation.
In the same video, Kathryn Crowe, a propulsion engineer, described the RS-25 as "the Ferrari of rocket engines."
"When you're looking at designing a rocket engine, there are several different ways you can optimize it," she said. "You can optimize it through increasing its thrust, through increasing the weight-to-thrust ratio or increasing it's overall efficiency and how it consumes your propellants.
"With this engine, they maximized all three of those. Anytime you maximize a variety of categories, it increases the complexity greatly. So even though it's the most complex engine, it's also the most efficient engine that's currently in use."