NASA is Bringing the Space Shuttle Engine Back to Launch the World’s Largest Rocket

RS-25 Engine test fire on the A-1 test stand at NASA's Stennis Space Center. Photo: Aerojet Rocketdyne

The first reusable rocket engine in history, the RS-25, proved its worth during NASA’s 30-year space shuttle era, helping power the orbiters uphill from 0 – Mach 25 in just 8 minutes, with a 100% success rate over the course of the program (the losses of Challenger and Columbia were not related to the main engines).

Often referred to as the Ferrari of rocket engines, the liquid hydrogen/liquid oxygen fueled RS-25 is one of the most tested large rocket engines ever made, with more than 3,000 starts and over one million seconds (nearly 280 hours) of total ground test and flight firing time over the course of 135 shuttle missions.

Shuttle Discovery on pad 39A awaiting launch. Photo: Mike Killian

Now, with the shuttle fleet retired several years ago, and a new heavy-lift rocket to launch deep-space crews in development, the engines that proved their worth time and time again are being called upon to serve the United States one more time for NASA’s colossal Space Launch System (or SLS).

Just like shuttle, two tall solid rocket boosters will provide most of the thrust during launch and ascent to reach space (we will have a story on those later). But the main engines are just as critical, and Aerojet Rocketdyne (the manufacturer) currently has 16 flight engines in inventory; 14 are veterans of numerous shuttle missions and 2 are brand new, plus there are 2 development test engines as well.

But differences between the SLS and space shuttle require that the RS-25s now undergo several modifications to adapt to the new environment they will encounter with SLS, to meet the giant 320-foot-tall rocket’s enormous thrust requirements.

Locked down on the A1 test stand at the agency’s Stennis Space Center near Bay St. Louis, Miss., the RS-25s have been undergoing hot fire tests now since early 2015, providing engineers with critical data on the engine’s new state-of-the-art controller unit, or the “brain” of the engine, which allows communication between the vehicle and the engine itself, relaying commands to the engine and transmitting data back to the vehicle.

The new controller also provides closed-loop management of the engine by regulating the thrust and fuel mixture ratio while monitoring the engine’s health and status, thanks to updated hardware and software configured to operate with the new SLS avionics architecture.

Higher inlet pressure conditions, thanks to the engines upgrades, are also evaluated.

We’ve made modifications to the RS-25 to meet SLS specifications and will analyze and test a variety of conditions during the hot fire series,” said Steve Wofford, engines manager at NASA’s Marshall Space Flight Center in Huntsville, Alabama, where the SLS Program is managed for the agency.

The engines for SLS will encounter colder liquid oxygen temperatures than shuttle; greater inlet pressure due to the taller core stage liquid oxygen tank and higher vehicle acceleration; and more nozzle heating due to the four-engine configuration and their position in-plane with the SLS booster exhaust nozzles.”

For shuttle flights the engines pushed 491,000 pounds of thrust during launch—each—and shuttle required three to fly, but for SLS the power level must increase to 512,000 pounds of thrust per engine (more than 12 million horsepower). The SLS will require four to help launch the massive rocket and its payloads with a 70-metric-ton (77-ton) lift capacity that the initial SLS configuration promises (later variants will be even bigger and more powerful).

The RS-25 can 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.

Some cool facts from Aerojet Rocketdyne on their RS-25s:

There is nothing in the world that compares to this engine,” said Jim Paulsen, vice president, Program Execution, Advanced Space & Launch Programs at Aerojet Rocketdyne. “It is great that we are able to adapt this advanced engine for what will be the world’s most powerful rocket to usher in a new space age.”

The engines currently in stock are already assigned their spots to fly the first four SLS missions, but unlike their former lives as reusable engines, these will be their final launches. The SLS is being made as an expendable launcher designed from heritage hardware and ideas; the RS-25 is now one-time use.

NASA awarded Aerojet Rocketdyne a $1.16 billion, nine-year contract to restart production of an expendable version of the RS-25 for SLS in late 2015.

RS-25 test fire for SLS. Photo Credit: Mike Killian

Meanwhile, development of the rocket itself is well underway across the country. NASA is hoping to launch the first mission with an un-crewed Orion capsule to the moon and back on a shakedown flight in 2019, before launching the first crewed Orion mission sometime between 2021 and 2023.

Once the engines are finished testing individually, they will be integrated with an SLS first stage and mounted atop another test stand, to test fire the engines for a full-duration launch. Engineers need to make the engines THINK the rocket is really flying a launch ascent profile, in order to verify everything will operate as expected on launch day.

That test is expected to occur in 2018.

For now, here’s a little preview; some incredible video from NASA test firing the Saturn V first stage, whose five F-1 engines launched men to the moon on the Apollo missions:

And just think, when the SLS stage test fires, it will do so for 500 seconds…

As the rocket evolves over the 2020s and 2030s, it will become the largest and most powerful rocket ever made, but the initial SLS missions will only have half the lifting power of the Apollo Saturn V moon rockets.

The elephant in the room is whether the SLS program will keep getting the funding and political support it needs to put people on Mars in the next 20 years.

Meanwhile, SpaceX is developing the Falcon Heavy rocket, and plans to launch two paying customers to circle the moon and back before 2020, with hopes of Mars missions by 2030.
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