NASA completed a full fueling and simulated countdown of the Space Launch System rocket at Kennedy Space Center on February 19, 2026, pumping more than 700,000 gallons of cryogenic propellant into the vehicle during the second Artemis II wet dress rehearsal. The test ended as planned at T-29 seconds and 10:16 p.m. ET, clearing a significant technical hurdle that had stalled the first attempt weeks earlier. For the four astronauts assigned to fly around the moon on Artemis II, this rehearsal was the last major ground demonstration standing between hardware checkout and actual launch preparations.
Mission managers emphasized that this was a full-up systems test, not a simple fueling exercise. The rocket, mobile launcher, ground support equipment, and launch control software all operated in the same configuration they will use on launch day, with controllers in Firing Room 1 working from real procedures and timelines. In effect, the team treated the rehearsal as a dress rehearsal for the entire launch campaign, validating that the vehicle can be loaded, held, recycled, and safed without encountering the leak that cut the first attempt short.
What 700,000 Gallons of Supercooled Fuel Looks Like
The sheer volume of propellant involved in fueling the SLS is difficult to overstate. More than 700,000 gallons of cryogenic propellant, roughly 2.8 million liters of supercooled liquid hydrogen and liquid oxygen, flowed into the rocket’s core stage and interim cryogenic propulsion stage during the rehearsal. Loading began after a chilldown sequence that pre-conditions the plumbing and tank walls to handle fluids stored hundreds of degrees below zero, reducing thermal shock and minimizing the risk of cracks or sudden pressure spikes as the ultra-cold liquids enter warmer lines and tanks.
Once the tanks were full, ground teams shifted all SLS stages into replenish mode, a process that continuously tops off the tanks to replace propellant that boils away during lengthy countdown holds. Without replenish mode, the rocket would slowly lose fuel as cryogenic liquids warm and convert to gas, changing tank pressures and propellant levels. That steady topping-off is what allows NASA to keep the vehicle fully loaded through multiple built-in pauses in the countdown sequence, and it explains why the total volume pumped can exceed what the tanks actually hold at any single moment, especially over several hours of terminal count and planned recycles.
A Countdown Built to Break on Purpose
The second wet dress rehearsal was not simply about filling tanks. NASA designed the test to walk through a full countdown with deliberate holds and recycle procedures, stress-testing the launch team’s ability to stop, restart, and ultimately drain the rocket safely. The timeline included planned holds at T-1 minute 30 seconds and T-33 seconds, followed by a recycle back to T-10 minutes before the clock ran down a final time and stopped near T-30 seconds. Per NASA, the rehearsal concluded precisely at T-29 seconds, just before the point where the ground launch sequencer would hand control to the rocket’s onboard computers.
That choreography matters because real launch days rarely go smoothly. Weather, sensor anomalies, and range conflicts can force a launch director to pause or reset the clock, sometimes more than once. Practicing those interruptions under realistic conditions, with a fully fueled rocket and live ground support systems, gives the team muscle memory that a tabletop exercise cannot replicate. According to a NASA coverage advisory, the simulated launch window opened at 8:30 p.m. ET, and a post-rehearsal news conference was scheduled for February 20 with Artemis II launch director Charlie Blackwell-Thompson, SLS program manager John Honeycutt, and NASA associate administrator Lori Glaze to discuss how the countdown logic and ground systems performed under the planned stresses.
How a Hydrogen Leak Derailed the First Try
The success of the second rehearsal is best understood against the failure of the first. During an earlier wet dress rehearsal attempt, the countdown was terminated at T-5 minutes 15 seconds after sensors detected a liquid hydrogen leak at the tail service mast umbilical interface, the connection point where ground equipment feeds propellant into the rocket’s base. Elevated hydrogen concentrations at that interface forced an immediate halt, and the team drained the vehicle without completing the countdown sequence, sacrificing the chance to practice the final minutes before engine start in order to protect both the hardware and the launch pad.
NASA traced the problem to seals within the TSMU quick-disconnect hardware and replaced two seals at the interface before clearing the rocket for a second attempt. That repair work, along with inspections of surrounding components and additional analysis of leak signatures, added roughly two weeks to the rehearsal timeline. The fix appears to have worked: the second rehearsal loaded the same propellant volume and ran through a longer, more complex countdown without triggering the hydrogen alarms that cut the first test short, suggesting that the leak path identified in the original configuration has been effectively mitigated under full cryogenic conditions.
Why the Fix May Matter Beyond Artemis II
Most coverage of the wet dress rehearsal has focused on whether the test “passed,” treating it as a binary gate for Artemis II. That framing misses a broader consequence. The TSMU interface is not unique to this mission; it is part of the mobile launcher and ground support architecture that NASA will reuse for Artemis III and beyond. If the original seals were prone to failure under cryogenic thermal cycling, the replacement hardware and the inspection protocols developed during the troubleshooting process could reduce the odds of similar scrubs on future flights. NASA has not published specific reliability projections tied to the seal swap, so any percentage improvement remains speculative, but the engineering logic is straightforward: a known failure mode that has been diagnosed, repaired, and retested under flight-like conditions is less likely to recur.
For the Artemis II crew, the practical takeaway is that the rocket can now be loaded, held, recycled, and drained without leaking hydrogen, a flammable gas that poses both safety and schedule risks. According to reporting from the Associated Press, NASA loaded the SLS with its full complement of liquid hydrogen and liquid oxygen during the second attempt and held the vehicle at various points in the countdown without seeing the kind of rising hydrogen concentrations that would trigger an automatic cutoff. That performance gives mission planners more confidence that Artemis II’s launch window can be used effectively, rather than being consumed by repeated scrubs and drain operations driven by ground-side leaks.
What Comes Next for the Artemis II Campaign
With the wet dress rehearsal complete, NASA’s focus shifts from fueling tests to data review and configuration for flight. Engineers will pore over telemetry from the rehearsal, examining temperatures, pressures, and valve positions across the core stage, upper stage, and ground systems to confirm that everything behaved within expected margins. Any small anomalies, such as slower-than-predicted chilldown in a line or minor sensor noise during a hold, can now be cataloged and, if necessary, corrected while the rocket is still in a test posture. A separate NASA briefing on early results is planned to outline the initial findings and describe how the rehearsal data will feed into launch readiness reviews.
Operationally, the completion of this test clears one of the last major programmatic hurdles before setting a firm launch period for the mission. The SLS and Orion stack will transition from a test configuration into a launch-ready state, with final closeouts, ordnance connections, and software loads proceeding in parallel with crew training for ascent, translunar injection, and emergency procedures. As summarized in NASA’s pre-test planning updates, the agency has framed the wet dress as a prerequisite for entering the formal launch campaign; having met that condition on the second attempt, Artemis II can now move from practicing countdowns to preparing for an actual journey around the moon.
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*This article was researched with the help of AI, with human editors creating the final content.
