SLS Hot Fire Shutdown Due To Conservative Testing Parameters, Not Engine Malfunction


NASA said today that the early termination of the Space Launch System (SLS) hot fire test on Saturday was due to the conservative parameters imposed to prevent damage, not because of an underlying flaw. The test lasted only 67 seconds instead of 485. Officials said prior to the test that it needed to run for at least 250 seconds to get the data they needed. No decision has been made, but NASA is considering skipping a retest in order to maintain schedule, but such a decision would be quite controversial.

NASA is developing the Saturn V-class SLS rocket primarily to send humans back to the Moon and on to Mars. Boeing is the prime contractor and is building the SLS core stage, which is powered by four Aerojet Rocketdyne RS-25 engines.  Previously known as Space Shuttle Main Engines (SSMEs), the RS-25s were reusable and 16 that remain will be used for the first four SLS missions.

On Saturday, January 16, NASA conducted a “hot fire” test of the first core stage outfitted with four RS-25s at Stennis Space Center, Mississippi. The core stage is attached to Stennis’ giant B-2 test stand to hold it in place while the RS-25s generate 1.6 million pounds of thrust.  The hot fire test is the last of eight in the “Green Run” series.

A Space Launch System (SLS) core stage, attached to the B-2 test stand at Stennis Space Center, MS, is fired on January 16, 2021 during the ‘hot fire” Green Run test series. Screengrab.

This is the exact core stage and engines that will be used for the first SLS space mission, not demonstration or test articles. To ensure no damage could be done during the test, parameters were set conservatively.

On the Tuesday before the test, Boeing Vice President and SLS Program Manager John Shannon told reporters that although the full duration of the test was 485 seconds (8 minutes 5 seconds), if it ended early for any reason, they would have all the engineering data they needed after 250 seconds.

When the test took place, however, it shut down after 67.2 seconds. During the test and in a post-test media briefing the day of the test, two problems were identified: a Major Component Failure (MCF) on engine 4 and a “flash” where thermal protection blankets attach to engine 4.

In a blog post update today, however, NASA said it was engine 2 that caused the abort. At that moment, the engines were underdoing a gimble test where they are rotated in different directions just as they must do during ascent to steer the rocket.  Actuators in the Thrust Vector Control system that generate the force to gimble an engine are powered by hydraulic Core Stage Auxiliary Power Units (CAPUs).  The CAPUs for engine 2 exceeded pre-set test limits and the computer system automatically shut down the test as it was designed to do, but NASA said it would not have been a problem during a launch.

“The specific logic that stopped the test is unique to the ground test when the core stage is mounted in the B-2 test stand at Stennis. If this scenario occurred during a flight, the rocket would have continued to fly using the remaining CAPUs to power the thrust vector control systems for the engines.”  — NASA

The engines themselves performed as expected.

Initial data indicate that the MCF and flash on engine 4 were unrelated to the shutdown, but NASA will continue to investigate them.

NASA has been striving to get the first SLS launch, Artemis-I, off the ground this year.  It will send an uncrewed Orion spacecraft around the Moon.  The first SLS flight with a crew, Artemis-II, is scheduled for 2023.  Under the Trump Administration’s plan, Artemis-III would launch in 2024 and deliver the first woman and the next man to the lunar surface.

Many have been skeptical of the 2024 landing date since it was announced by Vice President Pence in March 2019. That is partially because the Human Landing Systems (HLS) to get the crew from lunar orbit down to and back from the surface are only in a preliminary design phase with just three years to go, and cost more than Congress is willing to appropriate.

But another reason is that SLS has experienced so many delays already.  In 2014, NASA committed  to the first launch in November 2018. That slipped to December 2019-June 2020, then to mid-late 2021.  More recently, NASA was saying November 2021, but that was premised on completing the Green Run tests and shipping the core stage to Kennedy Space Center (KSC) in mid-January 2021. At last Tuesday’s pre-test briefing, NASA SLS Program Manager John Honeycutt referred only to launching by the end of this year, not in November, and that was if all went well with the test.

Everything did not go well, however, and NASA is now deciding whether to move forward anyway to maintain schedule or redo the test.

NASA Administrator Jim Bridenstine told Spaceflightnow.com yesterday that it might be possible to forego a second test and do whatever else is needed once the core stage is at KSC, but the decision has not been made.

During a teleconference with reporters late this afternoon, Bridenstine and other officials reiterated they are still analyzing data and no date has been set for making that decision.

Bridenstine, a former Navy pilot and Congressman, is a passionate advocate for the Artemis program and the need for SLS to return astronauts to the Moon. He also is in his final day as NASA Administrator.  A Trump appointee, his tenure will end tomorrow at noon with the change in administrations.  To the end, he remains committed to the 2024 date, insisting that although Congress appropriated only 25 percent of the requested funding for HLS for FY2021, the support is bipartisan and encouraging.

At this afternoon’s briefing it was clear that he is not convinced another hot fire test is needed. Kathy Lueders, head of NASA’s human spaceflight program, stressed that the reason for doing the test in the first place is to reduce risk and the decision on whether to retest must be based on what will be gained versus the additional risk to the vehicle itself, which is needed for Artemis-I.

Lueders previously headed NASA’s commercial crew program that led to the success of SpaceX’s launches of astronauts to the International Space Station this year, albeit years later than planned.  In many interviews, including today, her message is to take everything a step at a time.  The current step is assessing the data, the next step is deciding the path forward.  Step by step.

One factor in deciding whether to do another test is the number of times the core stage can be “tanked” — loaded with liquid oxygen and liquid hydrogen. Boeing’s Shannon said today the number is nine.

This core stage has been tanked three times already at Stennis, for two wet dress rehearsals and the hot fire test.  At least one test will be needed after the stage is at Kennedy Space Center and then the final tanking for the launch itself, exhausting five of them.

A decision not to conduct a retest would be controversial, however.  Shannon laid out the test program in some detail at last Tuesday’s briefing, explaining why getting to 250 seconds is important.

“When the engines start and throttle up we will do what’s called a gimbal profile at 60 seconds, in other words the engine nozzles themselves will move in a pre-programmed set of movements. The engines will throttle down, they’ll throttle back up. This is the most heavily instrumented vehicle we will ever fly.

“So we’ll get a tremendous amount of engineering data on vibration, temperature, stress, acoustics. That’ll all be collected as we continue. At about two minutes and 30 seconds there’s what’s called a sine sweep of those nozzle engines, so they’ll actually start moving in a sinusoidal pattern to check the structural response of those rocket engines moving.

“Once that’s done, we’ve just about collected all the engineering data we need. The rest of the profile is much more a flight profile where we continue to burn. We do one late gimballing of the engines where we move those rocket nozzles again to verify when there’s just a little bit of propellant in the tanks that we can still effectively have enough pressure to move those nozzles. That’ll happen in about seven and a half minutes, and it’ll take about about 30 seconds.

“We get all the engineering data we need for this vehicle by about 250 seconds, but we’re going to go ahead and put it through the entire flight profile as long as everything’s looking Ok.” — John Shannon, Boeing

Wayne Hale, a former NASA flight director who chairs the NASA Advisory Council’s Human Exploration and Operations Committee told SpacePolicyOnline.com that “the baseline is to retest and complete the objective but engineering review — always a good practice — is looking at whether they gathered enough data to go forward without another test.  I expect another test in probably 3-ish weeks.”

In a tweet, he went further, stressing that schedule should not be the driver.

NASA also gets advice from its Aerospace Safety Advisory Panel (ASAP), established by Congress following the fatal Apollo 1 fire in 1967.  ASAP member George Nield, the former head of the FAA’s Office of Commercial Space Transportation, told SpacePolicyOnline.com via email that ASAP is already scheduled to brief Bridenstine this afternoon on its annual report “but I suspect the Green Run will also come up. I’m sure the ASAP will want to understand NASA’s rationale for whatever they decide to do on the need for a redo.”

ASAP was influential almost two years ago in convincing NASA to do the Green Run tests when the agency was considering skipping them entirely or shortening them in order to speed up the schedule.  In April 2019, ASAP Chair Patricia Sanders emphatically called on the agency to retain the full test:

“There is no other test approach that will gather the critical full scale integrated propulsion system operational data required to ensure safe operation. … I cannot emphasize more strongly that we advise NASA to retain this test in the program.” ASAP Chair Patricia Sanders, April 2019

At this afternoon’s briefing, however, Lueders joined Bridenstine in remaining upbeat.  “We have a shot at flying this this year.  We need to get through this next step. We always take it one step at a time.”

Note: this article was updated following a 5:30 pm ET media teleconference.