Second SLS Green Run Test in Late February

Second SLS Green Run Test in Late February

NASA will conduct a second Green Run test of the Space Launch System (SLS) core stage and its four RS-25 engines after an earlier attempt aborted about one minute into the eight minute test.  The second test will take place at the end of February, which almost certainly will delay the first launch of SLS, Artemis-1, into 2022.  It most recently was scheduled for November 2021 after years of delays.

The Saturn V-class SLS is designed to take humans back to the Moon and on to Mars as well as robotic spacecraft far into the solar system.  Congress directed NASA to build SLS in the 2010 NASA Authorization Act after President Obama cancelled the earlier Ares V program.  Managed at NASA’s Marshall Space Flight Center in Huntsville, AL, it has strong support from the Alabama congressional delegation including Senator Richard Shelby (R-AL) who chaired the Senate Appropriations Committee for many years and is expected to be its Vice Chair in the 117th Congress.

Boeing is the prime contractor for SLS and builds the core stage and its upper stage.  Aerojet Rocketdyne builds the RS-25 engines, originally developed for the space shuttle program. Northrop Grumman builds the side-mounted Solid Rocket Boosters (SRBs), derived from the space shuttle’s SRBs.

Artist’s illustration of the launch of a Space Launch System (SLS) rocket.  The core stage is in orange. Credit: NASA

The “Green Run” is actually a series of eight tests of the core stage outfitted with its four RS-25 engines.  The eighth and final test is called a “hot fire” where all four engines are ignited while the core stage is held in place on an enormous test stand at NASA’s Stennis Space Center near Bay St. Louis, Mississippi.  The hot fire test is intended to last for 485 seconds, about 8 minutes, the duration the engines must fire to reach space during an actual launch.

The original test took place on January 16, but automated systems shut it down after 67 seconds because they detected an anomaly in engine 2’s  hydraulic Core Stage Auxiliary Power Unit (CAPU) during a gimble test.

The problem was not with any of the engines themselves.

NASA has been considering the pros and cons of whether or not to do a retest.  In a blog post today, NASA said the answer is yes and it will take place “as early as the fourth week in February.”

Conducting a second hot fire test will allow the team to repeat operations from the first hot fire test and obtain data on how the core stage and the engines perform over a longer period that simulates more activities during the rocket’s launch and ascent. To prepare for the second hot fire test, the team is continuing to analyze data from the first test, drying and refurbishing the engines, and making minor thermal protection system repairs. They are also updating conservative control logic parameters that resulted in the flight computer ending the first hot fire test earlier than planned.  — NASA

If the test is successful, NASA added that it will take about a month before the core stage is ready to ship to Kennedy Space Center (KSC), which would mean late March.

Components of the Space Launch System (SLS) with an Orion spacecraft. Credit: NASA

Earlier the plan had been to complete the hot fire test by the end of 2020 and ship the core stage this month. That would allow for it to be integrated with the rest of the SLS system to meet a November 2021 date for the first SLS launch, Artemis-I, an uncrewed flight of an Orion capsule around the Moon.

The Artemis-1 launch date has slipped many times 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 it seems clear that now will slip again to 2022.

In the blog post today, NASA skirted the issue by saying the core stage would be mated with other components of the system that are being prepared for launch this year. That is factually correct. The other components are being prepared for launch later this year.  But they cannot launch until the core stage is ready.

A lot is riding on this first launch. SLS has many critics because it has taken so long and cost so much to develop. The Office of Management and Budget estimates it will cost $2 billion per launch. It may be an overstatement to say that the future of the SLS program depends on the success of Artemis-I since it has strong congressional support and many rockets encounter problems on their first trip to space, but a catastrophic failure nevertheless could doom the program.

NASA has been striving to meet the Trump Administration’s deadline of putting astronauts back on the Moon by 2024.  The Biden Administration has not indicated in its first few days in office whether it supports the Artemis program and, if so, on what time scale.  Many have been skeptical that the 2024 date could be met in any case for budgetary and technical reasons especially after Congress provided only 25 percent of the funding NASA said it needed in FY2021 for Human Landing Systems to get down to and back from the lunar surface.

Assuming the 2024 deadline is relaxed, a slip to 2022 for Artemis-I may not make much of a difference except that the development cost will keep rising.

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