What happened
On Monday, August 29th, Artemis I first launch attempt got scrubbed despite the efforts of the launch team.
The major milestone of the countdown began at L-7 hours to liftoff with a slight delay due to a thunderstorm weather rule violation. As Green Run Testing and Wet Dress Rehearsal showed, the SLS fueling operation is a critical phase that requires extreme caution. The cryogenic temperatures at which Liquid Hydrogen (LH2) is stored inside the rocket put the core structure at high stress. Moreover, LH2 is pumped inside the vehicle after almost 50% of the Liquid Oxygen (LO2) is already inside the main tank to avoid issues.
The first problem came up at L-5hours when launch controllers detected a higher than normal concentration of H2 inside a purge can that covers umbilical connections into the SLS rocket and is designed to detect leaks. The team was able to solve the issue by using a revert mode, tracing the leak, manually initiating a slow fill, and then gradually increasing the pace of fueling.
At little more than L-2 hours, SLS was fully loaded with topping procedures undergoing. Shortly after, two significant issues came up. The first one involved a crack in the foam insulation or a flange on the ‘intertank’, which connects the liquid hydrogen and liquid oxygen tanks; after in-depth evaluations, the ground team determined that it involved a piece of insulation and that the countdown could proceed.
The second issue was significantly more severe and ultimately cost the launch; the Bleed System of RS-25 engine number 3 failed to reach the nominal pressure to achieve adequate engine cooling. Despite the hard work, the launch team couldn’t find any solution in time, and with precipitations approaching Launch Complex 39B, the only solution was to call it off for the day.
RS-25 Bleed System
When dealing with cryogenic propellants, a crucial part of achieving successful ignition and operations lies in the correctness of the engine element’s temperature. An elevated difference in temperature between the fuel and the mechanical components could result in high-stress loads on the components themselves, which ultimately leads to catastrophic failures.
The role of the Bleed system is to lower the engine temperature before ignition reducing stresses; the system achieves this by letting a certain amount of propellant flow inside the engine.
What are the next steps for Artemis I?
As per the NASA calendar, a second launch opportunity is set for Friday, September 2nd. Of course, this is highly influenced by the engine issue. Having failed to complete a full WDR, the launch team never got to the point of Bleed System activation; thus, it may require additional time to review all the data and develop a solution.
A second press conference is set for Tuesday 30th, at 12 pm CEST (6 pm EST); during the event, we’ll learn whether the team was able to solve the issue or if more time / “Revert To Vertical Assembly Building” is needed.
However, should NASA aim for a September 2nd launch, a major role will be played by the weather. The current forecast shows a 60% possibility of violating weather rules with concerns due to cumulus clouds, electric fields at the surface level, and precipitations along the flight path.
