Flawless Second Artemis II Launch Rehearsal Shows Readiness, Despite Setbacks
After validating repairs to the Liquid Hydrogen umbilicals on Mobile Launcher-1, On Tuesday, February 17th, NASA picked up the countdown for the second Artemis II Wet Dress Rehearsal. Pre-tanking operations proceeded smoothly, as did the pre-tanking briefing and poll. Launch Director Charlie Blackwell-Thompson gave the go to begin tanking at approximately 9:27 am EST on Thursday, February 19th. Chilldown of the propellant lines to the rocket began on-time at 9:40am, with slow fill of both Liquid Hydrogen and Liquid Oxygen underway by 10:30.
Liquid Oxygen transitioned to fast fill as planned, but as operators prepared to transition to Liquid Hydrogen fast fill, a key inflection point that would set the tone for the rest of the day, the primary communications system suffered a fault at approximately 11:10am. The launch team switched to a backup system while engineers troubleshooted the primary (eventually traced to a network configuration issue), but it was decided to wait to begin LH2 fast fill until after the communications system had been fixed. By 11:45 the system had been fixed and LH2 fast fill was underway.
Chilldown and propellant loading of the Interim Cryogenic Propulsion Stage (ICPS) was without issue, and by 1:07pm the Core Stage Liquid Hydrogen tank was in stable replenish. At a press conference the day after the test, Charlie Blackwell-Thompson stated that the maximum hydrogen leak recorded during the entire tanking phase was 0.4%, while even on Artemis I’s final launch attempt the leaks reached 6%. This placed levels well below the original Artemis I limit of 4%, along with the current limit of 16%. During fast fill, the vehicle also successfully performed the “kick-start”, a procedure that partially pressurizes the liquid hydrogen tank in order to force propellant through the RS-25 engines and out of the 4-inch bleed line to the Mobile Launcher to start chilling them down. During Artemis I, this moment was the source of many leaks, but did not cause any issues in this test. At 2:41, NASA announced that all propellant tanks on both stages were in stable replenish, allowing final checks to occur before dispatching the closeout crew and pad rescue teams.
The closeout crew successfully simulated seating the crew, closed both the Orion Crew Module and Launch Abort System hatches, and prepared the Crew Access Arm for retraction before departing Launch Complex-39B at 7 pm. The final planned hold at T-10:00 had to be extended to allow the engine section temperatures to settle into the right range, but at 8:32 pm EST, the Artemis II vehicle began its first terminal count.
Going into the test, the focus was on the two planned terminal countdown runs. The plan was to first proceed down to T-1:30, just before the core stage goes to internal power. Between T-6:00 and T-1:30, the count can hold for up to three minutes before needing to recycle to T-10:00 and restart terminal count. After T-1:30, no in-place hold is available and a recycle must happen if any cutoff occurs. After three minutes, the count would continue until T-33 seconds, when it would be aborted again to practice recycling. After resetting to T-10:00, a full terminal count would be performed, allowing handover to the onboard Automated Launch Sequencer (ALS) at T-30 seconds and for ALS to abort approximately one second later. An abort after handoff mandates a scrub of the launch attempt, so controllers would proceed into safing and detanking to end the test.
The first terminal count went nominally up to the T-1:30 hold point, with the core stage hydrogen leak peaking at 1.6%, whereas tank pressurization at this point in the first rehearsal caused a leak to reach a concentration of 16%, forcing a cutoff and scrub at T-5:15. At approximately T-1:30, a genuine problem did appear and stopped the countdown; the booster power 001 metric combines two ground and one vehicle sensor to measure the health of the booster’s onboard batteries. The vehicle reported a healthy value, but there was a discrepancy with the ground sensors. This was not a constraint on the wet dress rehearsal as no flight was planned, so the count was picked up after just over three minutes and proceeded to the cutoff as planned. After a 57 minute recycle, the countdown again proceeded through terminal count, this time not stopping until ALS cutoff at T-29 seconds, or 10:16pm, with more time remaining in the simulated launch window. The maximum hydrogen leak concentration during the second count was 0.7%.
This test was the first time that SLS‘s in-place hold capability was demonstrated and the first time a recycle from an aborted terminal count was performed. A change from the first wet dress was the addition of extra hold time both before and after tanking in order to be able to thermally cycle the umbilical seals if required. In order to let the seal warm up and then restart flow requires 20-30 minutes, so enough time was added to allow this to be done without major delays.
Ultimately, the WDR was a massive success and a showcase of the propellant loading process and systems, for arguably the first time, working as designed. Despite the brief communications outage, the launch team demonstrated that they are experienced and capable of executing a launch attempt with or without contingencies.
Large quantities of data were collected throughout, informing both future countdown revisions, future training opportunities, and the ongoing investigation into the behavior of the Tail Service Mast Umbilical seals. At the press conference following the test, SLS Program Manager John Honeycutt hinted that rollout loads affecting the seals were a potential contributor to the leaks, though he declined to speculate further and said that “We’re gonna let the teams go off and look at the data that we got over the day yesterday and see how that addresses things that we might need to do different moving down the road, or maybe we just look at operationally if we do something different.” Blackwell-Thompson also stated that the filter problem experienced on the hydrogen integrity test was the observation of an unexpected pressure difference across one of the filters in Mobile Launcher’s “cryo skid”. Unsure if the filter was clogged with debris, frozen, or affected by some other fault, the test was terminated and the filter was checked once it had warmed up. After a close inspection no debris was located, leaving the most likely cause as air being ingested into the system and then frozen. The system was purged and the filter was replaced before Thursday’s WDR.
While not strictly part of the WDR, there was an additional first time process to go through. SLS‘s Flight Termination System (FTS) must be certified and approved by the US Space Force’s Eastern Range in order to launch. This certification depends on the system being tested (with ordnance disconnected) to confirm it is operating as designed, with a success giving 25 days of time before the FTS must be re-tested to confirm its safety. This test process takes a few days, and the initial pre-rollout test had lapsed by February 11th. To launch in March, the FTS would need to be re-tested, but a cycle of rolling back to the VAB, re-testing it there, rolling back out, and returning to a launch configuration would burn up too much time to make March viable. To cover this scenario, following Artemis I the Exploration Ground Systems program designed two platforms, known as Contingency Pad Access, that could be hoisted by a mobile crane and attached to Mobile Launcher-1 at the pad.
Despite looking like arms, these platforms are static and must also be removed by crane before a launch attempt. Both provide access to the forward assembly of one of the Solid Rocket Boosters and their FTS electronics, with one also allowing access to the Core Stage’s intertank and its FTS systems. The ability to re-test the FTS at the pad enables much greater launch availability, as a rollback/rollout cycle can add weeks and mandate skipping at least one entire launch period. Following the second WDR, one of the Contingency Pad Access platforms was lifted into place to allow access to the intertank and one booster, but unfortunately was short-lived.
On the morning of Saturday, February 21st, NASA announced that overnight the flow of helium to the ICPS had been interrupted for a few hours. Detailed investigation and repairs would require a rollback to the Vehicle Assembly Building, definitively ruling out the March period. See Space Scout’s coverage of this issue for more details. The platform was removed and NASA now plans to rollback the vehicle to the VAB on Tuesday, February 24th. If the issue is resolved quickly, the April launch period will be feasible due to FTS re-test and ICPS battery replacements occurring during the VAB stay as well.
Space Scout will continue Artemis II coverage as events develop.
