SLS Avionics: The Brain Without a Body

By Martin Burkey

If you compared NASA’s powerful Space Launch System (SLS) rocket to a human body, the avionics and software would be the nervous system and brain that monitor the body’s condition and makes and sends decisions. Just a few of the hundreds of operations that they make include: liquid propellant flow, engine throttling, engine and booster exhaust nozzle steering, trajectory updates, receiving and sending data to the crew and ground control, and responding to off-nominal issues such as wind gusts or an engine failure.

The avionics are required to work in environments of temperature, pressure, sound, etc. that no human body – and actually few machines – could tolerate. So everything from the boxes, to the boards, to the individual processors are “ruggedized” and tested at every step in development to survive launch.

Ultimately the avionics boxes and software have to work perfectly. But how can you be sure without putting it on the world’s largest rocket and seeing how it works? That’s the focus of the Integrated Avionics Test Facility – or IATF – at NASA’s Marshall Space Flight Center, where the computer, routers, processors, power, and other black boxes and software collectively known as “avionics” are being tested in preparation for the planned 2018 first flight of SLS.

Possibly the coolest thing about the test facility is that it can create an artificial vehicle operating in an artificial world and virtually “fly” SLS hundreds of times – from pre-launch activation and checkout to liftoff to core stage separation at about 17,500 miles per hour and 100 miles in space – to test the entire avionics package.

Expanded view of SLS showing various avionics locations.
Location of avionics aboard SLS Block 1.

Avionics can be found all over SLS: in the booster aft skirt and forward skirt, the core stage engine controllers mounted on the engines themselves, in the core stage engine section, intertank, and forward skirt, in the launch vehicle stage adapter, and in the Interim Cryogenic Propulsion Stage. Of course, avionics for the Orion crew vehicle are also linked in to the performance of the whole vehicle. So basically top to bottom.

An overhead view of the SLS IATF at Marshall Space Flight Center.
The heart of the Integrated Avionics Test Facility at NASA’s Marshall Space Flight Center. The Systems Integration Test Facility-Qualification is shown left background. The System Integration Lab is in the foreground. The SLS booster Hardware In the Loop facility is in the middle background.

Inside the test facility, the vehicle avionics boxes are mounted on a semi-circular, 18-foot-tall frame in the same relative position they will be inside SLS – right down to the length of the connecting cables. Outside the frame, several large towers house the equipment for simulating the SLS “world” and running test after test.

The virtual world of SLS is created by a pair of software tools, ARTEMIS and MAESTRO. They stand for A Real-Time Environment for Modeling, Integration and Simulation (ARTEMIS) and Managed Automation Environment for Simulation, Test, and Real-Time Operations (MAESTRO). (How do engineers come up with this stuff?) ARTEMIS is a suite of computer models, simulations and hardware interfaces that simulates the SLS and its virtual “world.” For instance, it simulates the Earth’s rotation, gravity, propellant tank sloshing, vehicle bending in flight, engine and booster pressure, temperature and thrust, and weather, from hot sunny days to cold stormy nights, and inputs from the Orion crew vehicle and launch facilities. In fact, ARTEMIS has far more lines of software code than SLS itself. MAESTRO serves as the test conductor for the virtual missions. This software configures and controls test operations, sets up the external conditions, monitors the tests, and archives all test data for analysis. That’s where engineers and software writers find out if their code needs fixing or supplementing.

The actual flight avionics for SLS will never be tested in this facility – only their flight-like equivalents. The actual flight avionics will be installed directly into the core stage at the Michoud Assembly Facility in Louisiana and tested there prior to flight. The test team at Marshall can already say that they’ve flown SLS “virtually” thousands of time to help ensure that SLS flies safely on its first real mission in a couple of years.


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Behind the Scenes at QM-2: Getting Ready to Test the World’s Largest Solid Rocket Motor

By Beverly Perry

For two monumental minutes on June 28, the Space Launch System (SLS) solid rocket boosters — the largest ever built for flight — will fire up in an amazing display of power as engineers verify their designs in the last full-scale test before SLS’s first flight in late 2018. Each piece of hardware that’s qualified and each major test — like this one, dubbed QM-2 — puts NASA one step closer on its Journey to Mars.

The smoke and fire may last only two minutes, but engineers at NASA’s Marshall Space Flight Center in Huntsville, Alabama and Orbital ATK in Promontory, Utah have been preparing weeks — even months — in advance for the static test of Qualification Motor 2 (QM-2). Here’s a behind-the-scenes look at what goes into getting ready to fire up the largest and most powerful solid rocket motor ever built for flight.

T (for test) minus weeks and months. In the months prior to the test, propellant-filled segments began arriving at Orbital ATK’s Test Bay T-97 after being cast in nearby facilities. Many of these segments are veterans of space shuttle flights. In fact, the various metal case segments that comprise the five-segment QM-2 motor flew on 48 shuttle flights!

T minus 14 days. In the two weeks leading up to the test, Orbital ATK engineers begin dry runs that simulate the final test as closely as possible (without the smoke and fire). They put the motor and associated systems through their paces no fewer than 11 times before the big day to ensure not only that all systems are functioning as expected, but also that the test will be executed properly. “We only get one shot at firing the rocket motor,” says Dr. Janica Cheney, Orbital ATK’s director of Test Operations. “All the dry runs and other preparations that take place ahead of time are critical to ensuring we get the data we need from this test firing.”

NASA and Orbital ATK test SLS Qualification Motor-2 (QM-2) before first flight.
Are you ready? It’s time for the final full-scale test before the first flight test of the SLS solid rocket motor June 28 at 10:05 a.m. EDT (8:05 a.m. MDT). Many cameras record data during the test, such as this one which captures nozzle plug performance during the test.

T minus 24 hours. For this final full-scale static test, engineers have 82 goals, or test objectives, they need to measure and evaluate. One day before the test, it’s crunch time; caffeine’s flowing as engineers work around the clock the day before the test to ensure all systems function properly and all necessary data can be collected.

T minus 8 hours. Game day. There’s focus — and excitement. There are two more dry runs leading up to the test. Engineers, technicians and operators are “on station,” — present and accounted for at key locations such as the test bay, the instrument rooms and the control bunker. When you hear “control bunker,” think mission control — a command and control center that directs every aspect of the test, similar to what you see at mission control during a launch. Time flies during the final eight hours before the test.

Orbital ATK’s Test Bay housing rolls back to reveal Qualification Motor-2 (QM-2).
At T minus 6 hours with a “go” decision for testing QM-2, engineers at Orbital ATK will roll back the booster test bay housing so the massive motor can be fired.

T minus 6 hours. At 4:05 a.m. EDT (2:05 a.m. MDT), engineers and managers at Orbital ATK and NASA will make a “go” or “no go” decision on testing that day. Assuming the test’s a go, technicians “roll back” Orbital ATK’s specially designed moveable test bay housing and begin running final checks to make sure everything is ready. “We check the status of all the data and control systems, the test bay, the motor preparation and weather conditions,” Cheney says.

Weather is one variable that can halt the QM-2 test. “We make sure there’s no lightning in the area; no high winds; no storms,” explains Orbital ATK Fire Chief Blair Westergard. “We also establish fire breaks. Along with the Box Elder County Fire District, we’re prepared to extinguish any secondary wildfires too.”

Engineers also make sure cameras are ready to film and all data recording systems are online and functioning properly. Orbital ATK Security ensures the area around the test is clear.

T minus 3 hours. Crowds begin to gather as the public viewing area near Promontory off State Route 83 opens at 7:30 a.m. EDT (5:30 a.m. MDT). Orbital ATK Security directs traffic with the help of the Utah Highway Patrol and provides crowd control support to ensure everything remains orderly — vital when 10,000 people are in attendance.

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T minus 1 hour. The formal countdown commences; the public address system broadcast begins. The crew in the test bay begins final procedures to prepare the booster for testing.

T minus 9 minutes. Final system and timing checks are underway.

T minus 4 minutes. A “go for test” announcement sounds from the public address system.

T minus 1 minute. A siren begins; it will blare through T minus 20 seconds.

T minus 45 seconds. The “Safe and Arm” system sequence begins, which arms the motor. The Safe and Arm device is remotely activated from the “safe” position into the “armed” position, allowing the motor to ignite when the “fire” command is given.

T minus zero. At 10:05 a.m. EDT (8:05 MDT), two minutes of pure awesome commence as the gigantic motor burns through about five and a half tons of propellant each second during the approximately two-minute test. Inside the control bunker, there will be jubilation — and relief. “This is serious business — this is rocket science,” Cheney emphasizes. “But there’s nothing better than the smoke and fire and the data that comes with it when you’ve had a successful day. Our success is NASA’s success — we don’t do it alone.”


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Three Cool Facts About QM-2

By Beverly Perry

The countdown to the last full-scale test firing of the massive Space Launch System (SLS) solid rocket boosters has begun. Mark your calendars: June 28, 8:05 a.m. MDT.

Expect two minutes of shock and awesome as the flight-like motor burns through about six tons of propellant each second during the test. With expanding gases and flames exiting the nozzle at speeds in excess of Mach 3 and temperatures reaching 3,700 degrees Fahrenheit, say goodbye to some of the sand at Orbital ATK’s test facility in the Utah desert because after the test, the sand at the aft, or rear, end of the booster motor will be glass.

NASA and Orbital ATK are rolling back the house and rocking the Utah desert for QM-2 June 28.
NASA and Orbital ATK are rolling back the house and rocking the Utah desert for QM-2 June 28.

The 154-foot long Qualification Motor 2 (QM-2) consists of the five propellant-filled segments in the middle of the booster; the aft skirt is also part of the test, but the forward assembly (nose cap, forward skirt) won’t be. (See our Boosters 101* infographic if you need a refresher on booster parts and assemblies). The test will broadcast live on NASA TV and our Facebook page. We will also live tweet from @NASA_SLS on Twitter.

For those watching at home (or work), here are three cool things that might not be so obvious on the screen, in countdown order.

3. This motor’s chill. QM-2’s been chilling — literally, down to 40 degrees — since the first week in May in Orbital ATK’s “test bay housing,” a special building on rails that moves to enclose the booster and rolls back so the motor can be test-fired. Even though SLS will launch from the normally balmy Kennedy Space Center in Florida, temperatures can vary there and engineers need to be sure the booster will perform as expected whether the propellant inside the motor is 40 degrees or 90 degrees (the temperature of the propellant during the first full-scale test, Qualification Motor 1 or QM-1).

2. This booster’s on lockdown. If you happen to be near Promontory, Utah on June 28, you can view the test for yourself in the public viewing area off State Route 83. And don’t worry, this booster’s not going anywhere — engineers have it locked down. The motor is held securely in place by Orbital ATK’s T-97 test stand.

During the test, the motor will push against a forward thrust block with more than three million pounds of force. Holding down the rocket motor is more than 13 million pounds of concrete — most of which is underground. The test stand contains a system of load cells that enable engineers to measure the thrust the motor produces and verify their predictions.

Solid rocket booster test burns so hot it turns sand to glass.
The solid rocket motor test firing will burn so hot the sand at the aft end of the motor will turn to glass.

Putting out the fire at the end of the test is the job of the quench system, which fills the motor with carbon dioxide from both ends of the test stand. A deluge system sprays water on the motor to keep the metal case from getting too hot so the hardware can be re-used. Both the quench and deluge systems had to be upgraded to handle the heat and size of the big five-segment boosters.

1. Next time, it’s for real. These solid rocket boosters are the largest and most powerful ever built for flight. They’ve been tested and retested in both full-scale and smaller subsystem-level tests. Engineers have upgraded and revamped vital parts like the nozzle, insulation and avionics control systems. They’ve analyzed loads and thrust, run models and simulations, and are nearing the end of verifying their designs will work as expected.

Most of this work was necessary because, plainly put, SLS needs bigger boosters. Bigger boosters mean bolder missions – like around the moon during the first integrated mission of SLS and Orion. So the next time we see these solid rocket motors fire, they will be propelling SLS off the launch pad at Kennedy Space Center and on its first flight with Orion. For real.

Next time: Behind the Scenes at QM-2: Getting Ready to Test the World’s Largest Solid Rocket Motor.


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We’ve Got (Rocket) Chemistry, Part 2

Editor’s note: This is the second in a two-part series on the chemical reactions at the heart of rocket propulsion. Last week, we talked about the liquid engines of the SLS core stage , this week we’ll talk about the boosters.

By Beverly Perry

To give SLS extra power to get it off the ground, twin five-segment rocket boosters, built by Orbital ATK, tower more than 17 stories tall, burn six tons of solid propellant each second and help SLS break free from the clutches of Earth’s gravity.

Solid rocket fuel is the original rocket fuel, dating back to the early fireworks developed by the Chinese centuries ago. For the SLS boosters, aluminum powder serves as the fuel and a mineral salt, ammonium perchlorate, is the oxidizer.

Artist rendition of boosters and engines of during launch
The powerful aluminum-ammonium perchlorate reaction fuels the twin SLS solid rocket boosters.

Aluminum doodleAluminum is the most abundant metal on Earth. It’s also highly reactive. Aluminum is so reactive, in fact, that it’s not found naturally in its pure form but only in combination with other minerals. It’s this ability to readily combine with other metals that makes aluminum so useful. Every day, we use products made of aluminum alloys, or mixes with other metals, for things like beverage containers, covering leftovers, or iPhones. Amazingly, it’s this same stuff that fuels solid rocket boosters.

Ammonium perchlorate, the salt of perchloric acid and ammonia, is a powerful oxidizer (read: majorly explosive). In the boosters, the aluminum powder and ammonium perchlorate are held together by a binder, polybutadiene acrylonitrile, or PBAN. The mixture, with the consistency of a rubber eraser, is then packed into a steel case.

: Interior of booster case after firing
This is what the inside of the empty booster case looked like after the first qualification motor test in March 2015. Preparations are already well underway for the second qualification test this summer.

When it burns, oxygen from the ammonium perchlorate combines with aluminum to produce aluminum oxide, aluminum chloride, water vapor and nitrogen gas – and lots of energy.

Nitrogen doodleThis reaction heats the inside of the solid rocket boosters to more than 5,000 degrees Fahrenheit, causing the water vapor and nitrogen to rapidly expand. Just like in the liquid engines, the nozzle funnels the expanding gases outward, creating thrust and lifting the rocket from the launch pad.

Compared to liquid engines, solid motors have a lower specific impulse – the measure rocket fuel efficiency that describes thrust per amount of fuel burned. However, the propellant is dense and burns quite quickly, generating a whole lot of thrust in a short time. And once they’ve burned their propellant and helped propel SLS into space, the boosters are discarded, lightening the load for the rest of the spaceflight.

So that’s it really. Make water and shoot off an enormous firecracker and you’ve got: Rocket Chemistry. On this scale though, you can’t try this at home. Watch the real show when SLS launches in 2018.


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We’ve Got (Rocket) Chemistry, Part 1

Written by Beverly Perry

What do water and aluminum have in common?

If you guessed that water and aluminum make SLS fly, give yourself a gold star!

Chemistry is at the heart of making rockets fly. Rocket propulsion follows Newton’s Third Law, which states that for every action there is an equal and opposite reaction. To get a rocket off the launch pad, create a chemical reaction that shoots gas and particles out one end of the rocket and the rocket will go the other way.

What kind of chemical reaction gets hot gases shooting out of the business end of a rocket with enough velocity to unshackle it from Earth’s gravity? Combustion.

Whether it’s your personal vehicle or a behemoth launch vehicle like SLS, the basics are the same. Combustion (burning something) releases energy, which makes things go. Start with fuel (something to burn) and an oxidizer (something to make it burn) and now you’ve got propellant. Give it a spark and energy is released, along with some byproducts.

For SLS to fly, combustion takes place in two primary areas: the main engines (four Aerojet Rocketdyne RS-25s) and the twin solid rocket boosters (built by Orbital ATK) that provide more than 75 percent of thrust at liftoff. Combustion powers both propulsion systems, but the fuels and oxidizers are different.

RS-25 engine during testing
Steam clouds, the product of the SLS main engines’ hydrogen-oxygen reaction, pour from an RS-25 engine during testing at NASA’s Stennis Space Center.

The RS-25 main engines are called “liquid engines” because the fuel is liquid hydrogen (LH2). Liquid oxygen (LOX) serves as the oxidizer. The boosters, on the other hand, use aluminum as fuel with ammonium perchlorate as the oxidizer, mixed with a binder that creates one homogenous solid propellant.

Making water makes SLS fly

Hydrogen doodleHydrogen, the fuel for the main engines, is the lightest element and normally exists as a gas. Gases – especially lightweight hydrogen – are low-density, which means a little of it takes up a lot of space. To have enough to power a large combustion reaction would require an incredibly large tank to hold it – the opposite of what’s needed for an aerodynamically designed launch vehicle.

To get around this problem, turn the hydrogen gas into a liquid, which is denser than a gas. This means cooling the hydrogen to a temperature of ‑423 degrees Fahrenheit (‑253 degrees Celsius). Seriously cold.

Oxgen doodleAlthough it’s denser than hydrogen, oxygen also needs to be compressed into a liquid to fit in a smaller, lighter tank. To transform oxygen into its liquid state, it is cooled to a temperature of ‑297 degrees Fahrenheit (‑183 degrees Celsius). While that’s balmy compared to LH2, both propellant ingredients need special handling at these temperatures. What’s more, the cryogenic LH2 and LOX evaporate quickly at ambient pressure and temperature, meaning the rocket can’t be loaded with propellant until a few hours before launch.

Once in the tanks and with the launch countdown nearing zero, the LH2 and LOX are pumped into the combustion chamber of each engine. When the propellant is ignited, the hydrogen reacts explosively with oxygen to form: water! Elementary!

2H2 + O2 = 2H2O + Energy

This “green” reaction releases massive amounts of energy along with superheated water (steam). The hydrogen-oxygen reaction generates tremendous heat, causing the water vapor to expand and exit the engine nozzles at speeds of 10,000 miles per hour! All that fast-moving steam creates the thrust that propels the rocket from Earth.

It’s all about impulse

But it’s not just the environmentally friendly water reaction that makes cryogenic LH2 a fantastic rocket fuel. It’s all about impulse – specific impulse. This measure of the efficiency of rocket fuel describes the amount of thrust per amount of fuel burned. The higher the specific impulse, the more “push off the pad” you get per each pound of fuel.

The LH2-LOX propellant has the highest specific impulse of any commonly used rocket fuel, and the incredibly efficient RS-25 engine gets great gas mileage out of an already efficient fuel.

But even though LH2 has the highest specific impulse, because of its low density, carrying enough LH2 to fuel the reaction needed to leave Earth’s surface would require a tank too big, too heavy and with too much insulation protecting the cryogenic propellant to be practical.

To get around that, designers gave SLS a boost.


Next time: How the solid rocket boosters use aluminum – the same stuff you use to cover your leftovers – to provide enough thrust to get SLS off the ground.

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