Today, around 4:30 p.m. local time (CST), NASA reached an important milestone with the development of the Space Launch System (SLS) – the heavy-duty launch system that sends astronauts back to the moon and carries out crewed missions on Mars. As part of a Green Run Hot Fire test, all four RS-25 motors on the SLS Core Stage were fired simultaneously from top to bottom as part of the first integrated test of the stage's systems.
This test is the final hurdle in an eight-step validation process before the core tier can be paired with its Solid Rocket Boosters (SRBs) and sent on its maiden voyage around the moon (Artemis I) – which is currently planned for sometime in November 2021.
The event was held at the Stennis Launch Center near St. Louis, Mississippi and was broadcast live on NASA Live. NASA has also (and continues to do) regular updates to its Artemis page for NASA blogs. The fire test began at exactly 4:27:51 p.m. CST (02:27 PST; 05:27 EST) with engine 1 firing, followed by engines 3, 4, and 2 separated by a few hundredths of a second.
The B-2 test bench at NASA's Stennis Space Center near Bay St. Louis, Mississippi. Photo credit: NASA
The entire test fire lasted a little over a minute, with the engines failing at 4:29:07 CST (a total burn time of approximately 76 seconds). This did not match the 8 minutes (480 seconds) that the ground controllers were aiming for, as it is estimated that it would take 485 to 493 seconds of burn to simulate a start. They also planned to run two 30-second gimbal tests that would simulate flight control commands.
At this point in time, the exact cause of the termination has not yet been disclosed. However, according to an update just posted on the Artemis page on NASA blogs, the shutdown was due to the automated security systems being triggered:
“All four RS-25 engines fired successfully, but the test was terminated prematurely after about a minute. At this point the test was fully automated. During the fire, the on-board software acted appropriately and initiated a safe shutdown of the engines. "
Even after a little over a minute, however, the test was largely successful. Not only was it the first time the four RS-25 engines fired together, but the process also validated many key systems and allowed the mission controllers to collect data from the core phase as the many integrated components worked together.
"During the test, the fuel tanks were pressurized and this data will be valuable as the team plans the way forward," added the NASA statement. "In the days ahead, engineers will continue to analyze data and inspect the core stage and its four RS-25 engines to determine next steps."
The eight-step process for validating the SLS core level. Photo credit: NASA / Kevin O’Brien
As already mentioned, this was the last step in the Green Run, with which the new (also known as "green") hardware is to be tested and the core stage is to be made operational. The first three steps are to simulate the launch conditions on a non-powered core stage and turn on the avionics. In steps four and five, the various engine elements such as the propulsion system components, the thrust vector controls and their hydraulic system are tested.
Steps six and seven involve creating a schedule for the start and simulating a countdown, followed by a stress test that loads and drains approximately 2.65 million liters of propellant from the core stage. The eighth and final test is the Hot Fire Test (also known as "Wet Dress Rehearsal"), in which the core stage is fully loaded with propellant and performs a static fire in place – in this case on the B-2- Test stand in Stennis.
The maximum amount of thrust the four RS-25 engines can produce during takeoff and ascent is over 900,000 kg (2 million pounds) or around 8,900 kilonewtons (kN) of thrust. In hot fire tests on board the B-2 test stand, however, the RS-25 engines achieve a peak value of 725,750 kg (7.67 lbs) or 7,117 kN of thrust. This is the maximum amount of thrust they will generate at sea level (the launch pad).
Upon completion of the tests, the core tier is combined with two expendable Solid Rocket Boosters (SRBs) like the Space Shuttle. Together, these are used for all configurations of the SLS missile grouped by two blocks. For Unit 1, the SLS can generate 4 million kg of thrust and deliver 24.5 to 38 tons (27 to 42 tons) of crew or payloads into orbit or to the moon.
Artist's impression of the various Block 1 and Block 2 SLS configurations. Photo credit: NASA / MSFC
For Block 2, the SLS can produce 5.4 million kg (11.9 million lbs) and is configured to deliver 39 to 41 tons (43 to 46 US tons) or more into 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 retirement of the venerable Saturn V rocket. Similarly, NASA hopes this rocket will be the workhorse of Project Artemis, NASA's program to send "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 gate in orbit and the Artemis base camp on the lunar surface. These enable a “program for sustainable lunar exploration” and preparations for the possible “Moon to Mars” missions.
You can follow the entire event through NASA Live (see below). And await the post-test panel discussion, which begins at 4:30 p.m. PST (7:30 p.m. EST).
Further reading: NASA