Space Launch System concept visual. Image: NASA
There's a big misconception these days that NASA isn't doing anything but sending astronauts up to the International Space Station on Russian Soyuz spacecraft. That's far from the truth. There are a handful of amazing planetary missions underway, and on the manned side of things the agency is pressing forward with its Space Launch System, the next heavy lift rocket that will be able to send men and machines beyond Earth orbit to the giant planets of our Solar System.
Powering the core stage of this big new rocket is the RS-25 engine. It's an engine NASA debuted in the early 1980s with the space shuttle, but the SLS version is updated and more powerful, and it's currently in testing leading up the first SLS launch currently scheduled for 2017. So how exactly does the rocket engine powering the future of space exploration work?
The RS-25 engine isn't new to NASA; it's the same engine that powered the space shuttle, helping that spacecraft reach orbit from 1981 to 2011. It is a staged-combustion-type hydrogen engine designed to generates very high performance at low altitudes where the atmosphere is thickest. It's a sustainer engine, meaning it burns throughout the vehicle's ascent to orbit. This means it has to power a rocket through very different environments: through sea-level atmosphere at the launch pad and through the thin upper atmosphere.
RS-25 engine test. Image: NASA
To deal with these different flight environments, the RS-25 has a high level of throttle-ability. In the first stages of a flight after launch, a rocket experiences what's known as a "max Q" or maximum dynamic pressure. This is the point when the force of air affecting the rocket is greatest, a combination of high speed flight in dense air. To minimize structural loads at this point, the engines are throttled down deeply for a brief time then brought back up to full power.
At higher altitudes a rocket is put in the opposite difficult dynamic region: the air is thinner so there's less pressure meaning if the engines fire at full power the rocket might accelerate beyond its design limits. A throttle-able engine is a way to ensure the rocket doesn't accelerate beyond its design limits, and by extension the capacity of the astronauts to survive.
Inside this specialized engine is an intricate network of pipes designed to deliver extremely cold rocket propellants—liquid oxygen at -300 degrees Fahrenheit and liquid hydrogen colder than -400 degrees—from storage tanks to the combustion chamber. But the pipes aren't just a delivery system. They have to "move" as the temperature inside the rocket engine changes; the change can be by as much as 500 degrees Fahrenheit when the engine is firing.
One way NASA tests the pipes' ability to withstand this harsh environment is to force liquid nitrogen through it at -320 degrees and see what happens. "A test like this may sound benign since no flammable propellant is used, but it is very significant to make sure we have the proper piping design and setup for engine testing," said Jeff Henderson, A-1 Test Stand director.
But the RS-25 that will be mounted to the SLS rocket's core stage isn't exactly the same engine as the one that was mated to the shuttle. Because the former is designed to carry significantly heavier payloads than the latter, some modifications were needed to work this vintage technology into a modern launch vehicle. "We need more thrust on the SLS than the shuttle, since we have a heavier payload," said Mike Kynard, SLS Liquid Engines program manager at the Marshall Center.
"The core stage is a good bit larger than the external tank on the shuttle. To accommodate the higher thrust level, we increased the number of engines we had from three to four, and increased the power level of each engine." The RS-25 engines on the shuttle ran at 491,000 pounds vacuum thrust. For the SLS the power level was increased to 512,000 pounds vacuum thrust.
Last week, NASA took crucial steps in preparing to test this renewed engine: an RS-25 has been installed on the A-1 test stand at NASA's Stennis Space Center near Bay St. Louis, Mississippi. "This test series is a major milestone because it will be our first opportunity to operate the engine with a new controller and to test propellant inlet conditions for SLS that are different than the space shuttle," said Steve Wofford, SLS Liquid Engines Element manager. "This testing will confirm the RS-25 will be successful at powering SLS."
The first tests will collect performance data on the advanced engine controller, which is essentially the engine's brain. This unit has also been updated from the shuttle days for the SLS. The controller regulates the valves that direct propellant flow into the engine, which in turn corresponds to engine thrust. This controller also regulates the engine startup and shutdown sequences, vital stages in normal and emergency operations. And while a static engine test focusing on controllers might now sound exciting or glamorous, it's the first step in the long road from launchpad to an asteroid, the Moon, or Mars. Or, hopefully, somewhere much further with really awesome robots.
SLS is designed to carry astronauts in NASA's Orion spacecraft deeper into space than ever before, to destinations including an asteroid and Mars. NASA is using existing and in-development hardware and infrastructure, including the RS-25 engine, to the maximum extent possible to enable NASA to begin deep space missions sooner.
Testing of engine No. 0525 begins in the coming weeks on a test stand originally built in the 1960s for Apollo-era engines that helped launch the lunar missions. The stand has since been used for several major testing projects, and NASA spent almost a year modifying the structure to accommodate the RS-25 engine.
The SLS Program is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Aerojet Rocketdyne of Sacramento, California, is on contract with NASA to adapt the RS-25 engines for SLS missions.