SLS Hot Fire Test Should Have Lasted 8 Minutes, Not 1

Today, at close to 04:30 PM local time (CST), NASA achieved a major milestone with the development of the Space Launch System (SLS) – the heavy launch system they will use to send astronauts back to the Moon and crewed missions to Mars. As part of a Green Run Hot Fire Test, all four RS-25 engines on the SLS Core Stage were fired at once as part of the first top-to-bottom integrated test of the stage’s systems.

This test is the last hurdle in an eight-step validation process before the Core Stage can be mated with its Solid Rocket Boosters (SRBs) and sent on its maiden voyage around the Moon (Artemis I) – which is currently scheduled to happen sometime in November of 2021.

The event took place at the Stennis Launch Center near St. Louis, Mississippi, and was live-streamed via NASA Live. NASA also posted regular updates (and continues to do so) on their NASA Blogs Artemis page. The fire test commenced at exactly 04:27:51 PM CST (02:27 PST; 05:27 EST) with the ignition of Engine 1, followed by Engines 3, 4, and 2 – which were separated by a few hundredths of a second.
The B-2 Test Stand at NASA’s Stennis Space Center near Bay St. Louis, Mississippi. Credit: NASA

The total test fire lasted for a little over a minute, with the engines cutting out at 4:29:07 CST (a total burn time of about 76 seconds). This fell short of the 8 minutes (480 seconds) the ground controllers were aiming for since it is estimated that a burn of 485 to 493 seconds is needed to simulate a launch. They also planned to conduct two 30-second engine gimbal tests, which would simulate flight steering commands.

At this time, the exact cause of the abort has not been announced. But according to an update just posted on the NASA Blogs Artemis page, the shut down occurred as a result of the automated safety systems being tripped:

“All four RS-25 engines ignited successfully, but the test was stopped early after about a minute. At this point, the test was fully automated. During the firing, the onboard software acted appropriately and initiated a safe shutdown of the engines.”

However, even at just over a minute, the test was largely successful. Not only was it the first time that the four RS-25 engines fired together, but the process also validated many key systems and allowed the mission controllers to gather data from the Core stage as its many integrated components worked together.

“During the test, the propellant tanks were pressurized, and this data will be valuable as the team plans the path forward,” the NASA statement adds. “In coming days, engineers will continue to analyze data and will inspect the core stage and its four RS-25 engines to determine the next steps.”
The eight-step process for validating the SLS Core Stage. Credit: NASA/Kevin O’Brien

As noted, this was the final step in the Green Run, which is designed to test the new (aka. “Green”) hardware and get the Core Stage ready to enter service. The first three steps consist of simulating launch conditions on an unpowered Core Stage and powering up the avionics. Steps four and five involve testing the various engine elements, like the propulsion system components, the thrust vector controls, and their system of hydraulics.

Steps six and seven involve establishing a timeline for launch and simulating a countdown, followed by a loading test where roughly 2.65 million liters (700,000 gallons) of propellant are loaded and drained from the Core Stage. The eighth and final test is the Hot Fire Test (aka. “wet dress rehearsal”) where the Core Stage is fully-loaded with propellant and conducts a static fire in place – in this case, on the B-2 Test Stand at Stennis.

The maximum amount of thrust that the four RS-25 engines can generate during launch and ascent is over 900,000 kg (2 million lbs) or around 8,900 kilonewtons (kN) of thrust. For hot fire tests aboard the B-2 Test Stand, however, the RS-25 engines will peak at 725,750 kg (1.6 million lbs) or 7,117 kN of thrust, which is the maximum amount of thrust they will generate at sea level (on the launch pad).

Once testing is complete, the Core Stage will be mated with two expendable Solid Rocket Boosters (SRBs) like the Space Shuttle. Together, these will be used for all configurations of the SLS rocket, which are grouped according to two Blocks. For Block 1, the SLS will be able to generate 4 million kg (8.8 million lbs) of thrust and deliver 24.5 to 38 metric tons (27 to 42 US tons) of crew or payloads to orbit or the Moon.
Artist’s rendering of the various Block 1 and Block 2 SLS configurations. Credit: NASA/MSFC

For Block 2, the SLS will be able to generate 5.4 million kg (11.9 million lbs) and configured to deliver 39 to 41 metric tons (43 to 46 US tons) or more to orbit or the Moon. In these latter configurations, the SLS will be the most powerful rocket NASA has built since the Apollo Era and the retiring of the venerable Saturn V rocket. In a similar fashion, NASA hopes that this rocket will be the workhorse of Project Artemis, NASA’s program for sending “the first woman and the next man to the Moon.”

It is also central to their long-term vision of space exploration, which includes deploying the elements of the Lunar Gateway in orbit and the Artemis Base Camp to the lunar surface. These will allow for a “program of sustainable lunar exploration” and preparations for the eventual “Moon to Mars” missions.

You can catch the full event via NASA Live (see below). And stay tuned for the post-test panel discussion, which will be starting at 4:30 PM PST (7:30 PM EST).

Further Reading: NASA