On March 18th, 2021, on a cloudless day above the B-2 Test Stand, the largest single stage of a rocket ever built waited patiently. At 3:37 pm CDT, 4 RS-25 engines roared to life, bringing this massive stage under load once more. They burned the fuel on board at for the full duration of nominal flight of 485 seconds before shutting down. However this isn't the first time this stage attempts this test.
On January 16th, this very same SLS Core Stage attempted to conduct its 8 test for the Green Run, the hot fire test, but once the engines were lit, a few issues arose. Non were severe issues, but in fact were all simply due to larger margins than normal operations made to keep the vehicle safe. The reason for these larger margins, is that unlike most hot fire tests, this is the actual Core Stage that will lift off no later than January 2022 for the Artemis-1 mission. Therefore, Stennis Space Center made the margins higher on a multitude of components, so the vehicle will be safe under most conditions and if an anomaly where to occur, it could be detected earlier and increase it's chances of being tested again. However, these safety concerns would lead to the stage only firing for just over a minute, leading to a two month delay and risking the originally set November launch date.
After a grueling investigation on the Solid Rocket Boosters (currently stacked at Kennedy Space Center), NASA was able to extend the NLT date to January 2022, as that is when the SRB's hit their "expiration" date and must be unstacked. Additionally, NASA was able to accelerate many other aspects of SLS, giving the core stage additional time to complete its test.
After a failed attempt at doing a hot fire in February, on the day after St. Patrick's day the great men and women out at Stennis Space Center got lucky and were at last able to accomplish every milestone for this test and more. A few of the test conducted involved throttling the engines from 109% to 99% at the 1 minute mark only to be throttled up to 105% at 90 secs and finally down to 85% just before shutdown. Another important test involved the gimballing of the engines, to test for steering of the stage, something that made for an interesting sight on the live stream. In the end they got much more data than they bargained for, but from the looks of it, it seems as if everything went perfect.
Now that the Green Run is complete, the core stage will be inspected, fixed and then lowered by mid April and it will be shipped to KSC aboard the Pegasus Barge. Once it arrives, it shall be stacked on the Mobile Launch Platform and readied for flight later this year to early next year.
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