RS-25 Engines: Meeting the Need for Speed

Guest blogger Martin Burkey, the SLS strategic communications team’s resident expert on all things engines, returns this week as we prepare for this afternoon’s RS-25 engine testing event at Stennis Space Center – if you’re not already, follow @NASA_SLS on Twitter and our Facebook link below for more info. — David

Rocket engines are among the most amazing machines ever invented. That’s mainly because they have to do one of the most extreme jobs ever conceived – spaceflight – starting with escaping Earth’s deep gravity well. Orbital velocity, just for starters, is over 17,000 mph, and that only gets you a couple hundred miles off the surface. Going farther requires going faster. Much faster.

The RS-25 makes a modern race car or jet engine look like a wind-up toy.

It has to handle temperatures as low as minus 400 degrees where the propellants enter the engine and as high as 6,000 degrees as the exhaust exits the combustion chamber where the propellants are burned.

It has to move a lot of propellants to generate a lot of energy. At the rate the four SLS core stage engines consume propellants, they could drain a family swimming pool in 1 minute.

Graphic showing top speeds for an Indy car and SLS of 230 mph and 22,653, respectively
To be fair, the Indy car probably handles better in the turns.

The most complex part of the engine is its four turbopumps which are responsible for accelerating fuel and oxidizer to those insanely high flow rates. The high pressure fuel turbopump main shaft rotates at 37,000 rpms compared to about 3,000 rpm for a car engine at 60 mph.

The bottom line is that the RS-25 produces 512,000 pounds of thrust. That’s more than 12 million horsepower. That’s enough to push 10 giant aircraft carriers around the ocean at nearly 25 mph.

If the performance requirement to turn massive amounts of fuel into massive amounts of fire wasn’t enough, an engine can’t take up a lot of mass or area in a rocket. A car engine generates about half a single horsepower to each pound of engine weight. The RS-25 high pressure fuel turbopump generates 100 horsepower for each pound of its weight.

But forget mere car engines. The RS-25 is about the same weight and size as two F-15 jet fighter engines, yet it produces 8 times more thrust. A single turbine blade the size of a quarter – and the exact number and configuration inside the pump is now considered sensitive – produces more equivalent horsepower than a Corvette ZR1 engine.

Expanded view of an RS-25 engine
And this is still only the major components of an RS-25 engine.

On the other hand, when you chug fluids that fast, a hiccup is a bad thing. In the case of a rocket engine, that hiccup is called cavitation. At the least, it robs the engine of power. At worst, it can cause catastrophic overheating and overspeeding. So rocket engineers spend a lot of time making sure fluids flow straight and smooth.

That’s also why they test rocket engines on the ground under highly instrumented and controlled conditions. It’s a lot less costly to fail on the ground than in flight with a full rocket carrying people on board and/or a one-of-a-kind multi-million- or multi-billion-dollar payload.

As rocket engines go, the RS-25 may be the most advanced, operating at higher temperatures, pressures, and speeds than most any other engine. The advantage comes down to being able to launch more useful payload into space with less devoted to the rocket structure and its propellants.

In addition to its power, another key consideration for SLS was the availability of 16 flight engines and two ground test engines from the shuttle program. It’s much harder and more expensive to develop a new engine from scratch. Using a high-performance engine that already existed gave NASA a considerable boost in developing its next rocket for space exploration.

Top of an RS-25 engine during a test firing
The RS-25 handles a wide range of temperatures – super-cold on top, super-hot at the bottom.

The remaining shuttle engine inventory will be enough for the first four SLS flights. As for the maturity part, the RS-25 design dates to the 1970s and the start of the Space Shuttle Program. But it’s undergone five major upgrades since then to improve performance, reliability, and safety. If only we could all upgrade 5 times as we age. Further, much of the knowledge and infrastructure needed to use the available engines and restart production already existed. Another hidden savings in time and money.

In its next evolution, the RS-25 design will be changed to make it a more affordable engine designed for just one flight and certify it to even higher thrust – which it is very capable of – to make SLS an even more impressive launch vehicle.


Next Time: The Engine Experience

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How Is A Test Stand Like A Space Ship?

I’d like to introduce a special guest blogger this week, Martin Burkey. Martin is the SLS strategic communication team’s resident expert on all things engines. As we prepare for next week’s RS-25 engine testing event at Stennis Space Center – if you’re not already, follow @NASA_SLS on Twitter and our Facebook link below for more info – Martin will be filling in this week and next to talk about our engines and how we test them. — David

None of the test stands at NASA’s Stennis Space Center look anything like a spaceship. But they operate a lot like a spaceship, even though none of them will ever leave the ground.

A test stand is designed to make a fire-breathing rocket engine think it’s a spaceship, while at the same time keeping it from taking off for space the way it was made to. Those two requirements account for why test stands look and operate as they do… and why they aren’t as compact, light and sleek as a rocket.

Fire streaming from a test stand during an RS-25 test
Burning of the liquid hydrogen and liquid oxygen during an RS-25 test can actually cause the formation of rain clouds.

Most of the big Stennis test stands were built in the 1960s to test Saturn rocket engines. Over the years, they’ve continued to serve the nation by testing Space Shuttle Main Engines and other government and commercial rocket engines. Today, the center is buzzing with test activities in support of NASA’s Space Launch System.

The A-1 Test Stand is the focus of testing to adapt the RS-25 –formerly known as the shuttle main engine – to new SLS performance requirements and environments. It will also be used to test engines with new components for the second SLS mission. The B-2 Test Stand is being readied to support testing of the massive 200-foot SLS core stage with its four RS-25 engines.

Thousands of tons of hulking, ungainly concrete and steel hundreds of feet above and dozens of feet below the Mississippi soil keep the engine locked in place for testing under way this year. An intricate network of tanks, pipes, cables and other equipment provides the engine with all the propellants, power, pressure, vacuum, fluids, cooling, data management and other services to – safely and accurately – simulate a full rocket mission from pre-launch preparations to nearly 200 miles in space.

A worker inspects the machinery of an RS-25 engine
A test stands interface with the engine mimics the interface between the engine and an actual launch vehicle.

Why test the RS-25, an engine that flew successfully for more than 3 decades and has over a million seconds of operating time? Because the RS-25 is a complex and finely tuned piece of equipment that requires thorough understanding various component interactions and responses under different conditions. Like other rocket engines, it operates at extremes of temperature, pressure, vibration, etc. that have to be monitored. More specifically, the RS-25 needs to be adapted to SLS requirements and environments such as higher propellant inlet pressure and lower temperature and integrates technology like a new engine controller unit. The test stand environment allows controlled testing of abnormal conditions and more thorough monitoring and observation than the flight environment. Ultimately, it’s better to find problems on the ground than in flight. There’s a saying in rocket testing – “Test like you fly, and fly like you test.” In other words, make the engine do everything on the ground it’s going to have to do in flight, and don’t do anything in flight you haven’t made the engine do on the ground. Why are we testing a proven engine? Because Space Launch System is going to make its RS-25s do new things in flight, but not until every one of them has been done successfully on the ground.

The A-1 stand supporting the current test series provides the RS-25 with liquid hydrogen (LH2) and oxygen (LOX) propellants. The stand has its own run tanks for propellants but they can also be filled from a 12-inch line running from LOX and LH barges docked near the test stand during test firing. The B test complex is also served by propellant barges, though stage testing relies on the propellant in the stage alone.

Aerial view of the test area at NASA’s Stennis Space Center
Aerial view of the test area at NASA’s Stennis Space Center.

From a safe distance, the Stennis test stands look like crude, utilitarian structures. Up close, they are intricately woven with a network of pipes, cables, and other hardware designed to exercise rocket engines and extract the important data.

The stand provides various gases such as nitrogen and helium for drying, pressurizing and preventing premature combustion in the engine, as well as breathing air for crews working in the engine nozzle before and after testing.

The stand also provides hydraulic and pneumatic pressure to operate the engine propellant valves. There’s electrical power to run the engine controller. There are data cables that carry engine performance information to and from the engine and the vehicle computers and crew. The high speed data system has 186 channels that can record various conditions at more than 100,000 times per second. Four high-speed cameras can record 250 frames per second. Low-speed video includes infrared, black and white and color cameras.

And, of course, there’s water for fire suppression on the stand and water to cool the stand’s ubiquitous “flame bucket” that directs engine exhaust exiting the nozzle at thousands of degrees away from the stand where it rapidly condenses into steam and then into rain that falls downwind. The A-1 bucket consists of 21 slanted deflector segments that make up the deflector seen in pictures. Each segment gets pressurized water, and each segment is drilled with a specific number and pattern of holes that spray water and keep the deflector cool during test firing. During tests, the stand uses 170,000 gallons of water per minute from a nearby reservoir.

The “crew” of the A-1 stand consists of about 50 people, some on the stand until 30 minutes before a test and others in the hardened Test Control Center 200 yards away from the test stand.

So while rocket engine testing may not resemble any real or fictional spaceship, it’s still very much “rocket science” and a critical part of getting NASA’s next great exploration rocket to the launch pad.


Next Time: RS-25 Engines: Meeting the Need for Speed

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Magic From Metal

And, just like that, metal becomes magic.

The space shuttle was an amazing technical creation, a structure of metal and composites and tile and glass and thermal protection blankets that could rise majestically through the atmosphere in defiance of gravity and could cradle humans safely through the inhospitable void of space and could glide back home in a way far too graceful for her stocky form. Amazing.

But the magic? Walk up to an orbiter today, and it’s amazing, but inert. Metal and composites and glass and tile and thermal protection blankets. A machine. The magic came when she was on the pad, fueled for launch and ready to leap upwards. A living thing. Astronauts who knew the machine intimately were surprised at the shuttle they boarded on launch day – hissing and venting and groaning. Breathing. Coiled with purpose. Visceral. Alive. No longer creation but creature.

Magic from metal.

STS-135 crew members Rex Walheim and Sandy Magnus at the hatch to Atlantis
Astronauts say that boarding a fueled space shuttle prior to flight was an amazing experience. As if that needed to be said.

Last week, the Space Launch System (SLS) program completed its Critical Design Review, a first in almost 40 years for a NASA exploration class rocket. Through the CDR process, review teams go through designs and plans for the vehicle with a fine-tooth comb, and then go through the results of that with a finer-tooth comb.

On paper, in PowerPoints, in CAD designs and mission animations, SLS is amazing. In its completed configuration, there’s nothing like it. Unequaled lift capability. Unequaled launch thrust. Unequaled payload volume. Unequaled power to propel robotic vanguards into the depths of the solar system. A machine that in its very form hints at its purpose – “to strive, to seek, to find, and not to yield.”* Amazing. But magic isn’t in paper.

The thing that surprises many people when we talk to them about SLS is how much metal there already is, how much of the machine already exists. It’s not magic yet, but it’s something else amazing – it’s real.

Right now, the first rocket, the first SLS that will leap from Earth, is still only a glimpse of what she’ll become. The rocket is still in pieces, and those pieces are spread out around the country. Some pieces have yet to be built. But when you go to the places where those pieces are, you begin to understand how real this machine already is, how amazing it will be when finished, and, just maybe, a little bit of the magic this metal will eventually hold.

Core Stage: Bending Metal, Big-Time

Workers at Michoud Assembly Facility with a core stage barrel
“What’d you do at work today?” “Oh, the usual. Moved around a barrel for a 200-foot-tall rocket stage; helped open the solar system for human exploration; that sort of thing. You?”

Work today on the core stage gives a sense of the scale of the vehicle. Michoud Assembly Facility outside of New Orleans is filled with very large pieces that will be used to build something much larger — 27.6-foot-diameter barrel sections, rings, and domes that will form the oxygen and hydrogen tanks for both structural test articles for ground testing and the first flight core stage. This fall, the structural test article tanks will be welded, and by the end of the year, all of the barrel sections for the first flight vehicle will be completed. The test article tanks will be shipped next year to Marshall Space Flight Center in Huntsville, AL, where new test stands are currently being built to subject the tanks to the stresses of launch.

Also at Marshall, testing is taking place on the avionics systems hardware and software that will control the rocket during ascent, integrating the “steering” of the core stage and the boosters.

Stage Adapters: First to Fly

Marshall is also the site of work on two stage adapters. The center has already produced two units of the smaller one, the Orion-Stage Adapter (OSA), which connects the Orion spacecraft to the rocket’s upper Interim Cryogenic Propulsion Stage (ICPS). The first OSA was used as a test article, the second one was the first original SLS hardware to fly into space. It was launched in December 2014 on Orion’s Exploration Flight Test-1 on a Delta IV rocket, which sent Orion 15 times higher than the International Space Station’s orbit. Materials are currently arriving for a third unit, which will fly on the first launch of SLS.

Orion roll-out for stacking for EFT-1
The tapered part at the bottom of the Orion stack in this picture was the first original SLS hardware to launch.

The larger adapter, the Launch Vehicle-Stage Adapter (LVSA), will connect the core stage to the ICPS; welding began this month on the adapter test article.

The ICPS upper stage is being built about 25 miles from Marshall at a United Launch Alliance facility in Decatur, AL. The test article for the stage will be completed and transported to Marshall this fall for testing; work on the first flight stage began this month. The ICPS is a modified version of the upper stage of a Delta IV Heavy rocket, like the one that launched Orion for EFT-1.

Engines and Boosters: Lighting the Way Forward

Sixteen RS-25 engines for the first four flights of SLS are already in inventory at Stennis Space Center, in Mississippi, where engine testing is currently underway. The testing subjects the RS-25 engines, which previously flew on various space shuttle missions, to the different conditions they will experience on the SLS. The engines have been successfully completing tests at the higher SLS thrust level, using a new engine controller unit, with such new conditions as higher flow pressure and lower temperature of the propellant coming into the engines.[/embedyt]

Flight hardware for the solid rocket boosters for the first SLS flights is at the Booster Fabrication Facility. Before the segments there are cast with propellant and assembled into completed boosters, one more qualification test firing of the boosters will be performed at an Orbital ATK facility in Utah. The first of these test firings was conducted in March 2015 at the facility, qualifying changes to the boosters, including the addition of an extra propellant segment to make the boosters more powerful, as well as new insulation, processing and inspection techniques. After a second test next year, the flight boosters will be assembled, incorporating changes demonstrated in the tests.[/embedyt]

In all of these places, you can see pieces of the machine – very real, very concrete. Things we can touch. It’s not the full machine yet. But you begin to get a picture of what the machine will be. You stand at the base of a 22-foot-tall core stage barrel, and you can imagine the immensity of a stack of them almost ten times that tall. You see one RS-25 engine firing, and mentally picture four of them roaring together. You see a barrel in one place and an engine somewhere else and a booster in another place and an adapter in another, and you begin to see a rocket taking shape.

And, sometimes, particularly in the engine and booster tests, you see a little something more – you see the machine come alive, hissing, venting, breathing fire and steam. And you see the incredible – you see engines with the power to summon rain from the sky, you see boosters that can create glass from the desert sand.

For just a moment, you get a glimpse of the magic in the metal.


Next Time: How Is A Test Stand Like A Space Ship?

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* From ”Ulysses,” by Alfred Lord Tennyson, who, in full disclosure, was not actually writing about a rocket.