SLS teams scrub second launch attempt for Artemis I as hydrogen leaks persist

by Thomas Burghardt & Chris Gebhardt

NASA has scrubbed the second launch attempt of the Artemis I mission from the Kennedy Space Center in Florida after a liquid hydrogen (LH2) leak was detected at the vehicle-to-ground interface on the LH2 Tail Service Mast Umbilical (TSMU).

This was the fourth time teams loaded liquid hydrogen on the vehicle, and the fourth time something in the LH2 TSMU leaked hydrogen.

The Mission Management Team (MMT) is meeting to address the situation; however, numerous sources indicate that the only real option is to roll back to the Vehicle Assembly Building to investigate the leak source before targeting the next available lunar launch window that, from a timing perspective, is likely the one that opens on October 17.

Additional information will be available after a post-MMT briefing scheduled for approximately 4 PM EDT / 20:00 UTC.

Update from Monday’s (Aug. 29) attempt

Monday’s launch attempt resulted in a scrub due to an issue with one sensor on one of four liquid hydrogen (LH2) bleed lines that transfer LH2 into the engines as part of their thermal conditioning into their appropriate start temperature range, known as the “start box.”

Ideally, the engines should be at -420°F (+/- 40°F) [-251.1°C +/- 22.2°C] for the LH2 inlet temperature, as related by Jim Blevins, SLS Chief Engineer.

On Monday, the faulty sensor on the Core Stage bleed line for Engine 3 returned a temperature higher than other sensors located in the engine and higher than liquid hydrogen should be for the system.

This mismatch and the need to fully understand what was happening and confirm that the single sensor was indeed the one malfunctioning resulted in the scrub.

After officially calling off Monday’s attempt, the SLS launch team kept the vehicle fueled to gather as much data as possible on the issue.

This process provided the data needed for the team to have confidence that all four RS-25 engines reached their start box temperature range on Monday and that all data points from the engines themselves are good and valid.

For Saturday’s attempt, the team will monitor the sensor they believe to be bad and compare it with data from the engines themselves.

Should the same issue with the Core Stage bleed line sensor for Engine 3 reoccur Saturday, launch teams will look at all data points and are expected to simply ignore that faulty sensor.

The sensor in question is not a flight instrument or development flight instrument but rather used for engineering analysis.

As such, this is not a sensor used by the computer systems on the ground and rocket to verify vehicle health.

Therefore, there is no way this sensor could cause an issue with engine ignition or liftoff, nor would it have any effect on Engine 3’s performance during flight.

Launch count and liftoff

This is only the second time in history that Pad 39B at the Kennedy Space Center has prepared to hosted a launch to the Moon. The only other flight to do so was the Apollo 10 dress rehearsal for the first lunar landing.

For SLS, the final major configuration of the vehicle takes place with fueling operations to load approximately 730,000 gallons of liquid hydrogen and liquid oxygen into the Core Stage.

This major operation began with chilldown of the liquid oxygen and liquid hydrogen transfer lines and the propulsion lines in the core stage.

As the Wet Dress Rehearsal campaign showed, the fueling operation is complex and must follow a specific sequence, specifically so that the liquid oxygen Core Stage tank is no more than 50% full by the time liquid hydrogen loading begins. This has to do with the thermal and mass characteristics of the Core Stage.

This process was seen during the Wet Dress Rehearsal campaigns when liquid hydrogen leaks forced a stop to liquid oxygen loading.

The liquid hydrogen fueling and data connection lines (left). This is one of two Tail Service Mast connections to the rocket. (Credit: Jack Beyer for NSF)

As LH2 loading enters “fast fill” operations, teams closely monitored the purge can in the LH2 Tail Service Mast Umbilical that leaked during Monday’s attempt. No leak re-appeared here on the Sept. 3 attempt.

Heading into the final part of the count in the morning hours of September 3, the Exploration Ground Systems (EGS) launch team were cautiously optimistic that the liquid hydrogen leaks had been addressed. This proved to not be the case.

Core Stage fueling was to be followed by Interim Cryogenic Propulsion Stage (ICPS) fueling with liquid hydrogen and liquid oxygen — with the ICPS being the last portion of the vehicle to be fueled for flight.

However, before tanking operations begin, teams needed to successfully transition from air purge over to gaseous nitrogen purge. Issues establishing this nitrogen purge during the Wet Dress Rehearsal campaigns resulted in the launch team extending the hold before fueling. 

Lightning strikes at LC-39B on Aug 27 have all been cleared by the launch team. (Credit: Brady Kenniston for NSF)

This added time worked as intended on Monday and Saturday, and no issues with the gaseous nitrogen changeover were noted on either launch attempt.

During this period, as propellants were to be flowing into the vehicle, thermal conditioning of the four RS-25 Core Stage engines was to commence 45 minutes earlier than during Monday’s attempt. The team never got to this point on Saturday.

This lengthy process would ensure the engines are within the proper “start box” at the end of the countdown.

The four RS-25 engines on Artemis I were still in service at the end of the Shuttle program.

But for Artemis I, at least one component on each of the Core Stage engines comes from the three engines that powered Columbia to orbit on STS-1 on April 12, 1981.

“It might be a valve, it might be a bolt, for others, it’s pieces of wiring, little things like that,” said Aerojet Rocketdyne’s Bill Muddle, RS-25 lead field integration engineer, in an interview with NASASpaceflight. “But there is something from the STS-1 engines on each of these [for Artemis I].”

The RS-25 engines in test firings at the Stennis Space Center in Mississippi. Single engine test (left). Core Stage Green Run firing (right). (Credit: NASA)

Some changes to the engines for fit and function on SLS have resulted in a slight change to their ignition timing from Shuttle. Instead of igniting at T-6.6 seconds, the four Core Stage RS-25s will begin their ignition sequence at T-6.36 seconds.

Muddle confirmed that the engines will start 120 milliseconds apart from each other for acoustic and vibration considerations just as they did in the Shuttle era.

Ten seconds before engine start, nearly 400,000 gallons of water would have begun dumping into the flame trench of LC-39B and the RS-25 and Solid Rocket Booster flame holes in the Mobile Launcher.

Six seconds before engine start, the Hydrogen Burn Off Igniters (HBOIs) would have fired across the plane of the RS-25 engine nozzles and upward at the Core Stage Auxiliary Power Unit exhaust ports.

This would have ensured that excess hydrogen was safely burned off before the engines started and that any hydrogen was safely burned off should there have been an engine shutdown and abort on the pad.

The water tower at LC-39B. This 400,000-gallon tank is emptied in less than 30 seconds during launch operations for the sound suppression system. (Credit: Stephen Marr for NSF)


From engine start down to T0, the engines will build up to 100% rated performance thrust within three seconds, after which health checks will confirm the engines are good for the next major event that commits the vehicle to flight: Solid Rocket Booster (SRB) ignition.

At T0, a rapid sequence of events will trigger the swing arms to move away from the vehicle, for the Tail Service Mast fueling and data lines to pull back into their protective doghouse covers, and for the final fire sequence command to be sent to light the SRBs.

The moment the boosters ignite, SLS is committed to lifting off the pad under its own 8.8 million pounds of thrust.

SLS will rise vertically off its Mobile Launcher perched atop LC-39B for seven seconds before the pitch and roll program will maneuver the rocket onto the proper azimuth for its flight to the Moon.

The roll program will initiate at a velocity of 35 meters per second (79 miles per hour).

Max-Q, the moment of maximum stress on the rocket, will be reached at T+ one minute and 10 seconds as the vehicle climbs through 12.9 km (42,500 feet) and accelerates through 447 meters per second (1,000 mph).

This first stage of flight will see SLS utilize an open loop guidance system, where the rocket follows a pre-programmed pitch profile based on its velocity, altitude, and initial orbital insertion targets.

After constantly changing its pitch as required to reach the desired orbit, an attitude hold will be commanded on the SLS rocket just before the first major event in the launch sequence, SRB separation.

At approximately T+ two minutes and 12 seconds, the twin five-segment SRBs from Northrop Grumman will separate from the Core Stage, having done their job.

This event will be preceded by a lowering of the pressure inside the SRB chamber which will lower the thrust — an event known as “tail off.” This will ensure the boosters have just enough remaining thrust to safely separate.

According to Muddle from Aerojet Rocketdyne, additional thermal coating was needed on small portions of the outside of the engine nozzles of the RS-25s because the SRB separation motors, when fired, will impinge on those locations on the engines.

A cylindrical Solid Rocket Booster separation motor on the aft skirt. In order to ensure proper separation from the Core Stage, the aft motors point at the engine nozzles of two of the four RS-25 engines. (Credit: Nathan Barker for NSF)

“Here, [the RS-25s] are literally a couple feet away [from the Solid Rocket Booster separation motors]. So we had to be very careful,” said Muddle.

“The booster separation motors actually point at our engines for about a second, and so working with Northrop Grumman and Boeing, we had to figure out what the heat loads were going to be. And so basically, once you know that, where on the nozzle the heat load is going to be, and how much, then you design more insulation, more ablatives and those types of things.”

At SRB separation, under a nominal mission profile, SLS will be traveling around 1,417 meters per second (3,170 mph) while being 48.1 km (158,000 feet) in altitude.

Approximately one minute later, with the vehicle now flying above the majority of Earth’s atmosphere, the aerodynamic elements protecting Orion can safely be jettisoned.

The three fairing panels surrounding the European Service Module (ESM) will separate at T+ three minutes and 13 seconds, followed by the Launch Abort System (LAS) at T+ three minutes and 19 seconds.

The ESM fairing panels and LAS separate from Artemis I. (Credit: Mack Crawford for NSF/L2)

The timing of these events, especially those later in the mission, are approximated based on predicted vehicle performance. The changing trajectory options, based on when in the window that liftoff occurs, can also slightly affect the timing of staging events and burn times.

The core stage will continue to power the ascent for several more minutes, targeting an unstable 30 x 1805 km orbit. The 30 km perigee, while above the Earth’s surface, is well within the atmosphere. This trajectory ensures that the Core Stage safely reenters during its first orbit, breaking apart over a designated area of the Pacific Ocean. The 1805 km apogee gives the combined ICPS and Orion stack enough energy to, with two burns, reach the Moon.

After Main Engine Cutoff (MECO) at T+ eight minutes and four seconds, the ICPS and Orion stack will separate from the core stage at T+ eight minutes and 16 seconds. For ICPS and Orion to avoid the same fate as the core stage, the stack will coast up to apogee before performing the first of two ICPS burns. This perigee raise burn will increase the perigee to over 800 km.

During this coast phase, Orion’s four solar arrays will deploy, beginning about 18 minutes and 20 seconds after liftoff. Solar array deployment takes approximately 12 minutes.

The ICPS, itself a slightly modified Delta Cryogenic Second Stage (DCSS) from United Launch Alliance’s Delta IV rocket family, is powered by a single RL-10-B-2 engine. According to Aerojet Rocketdyne’s Nicole Cummings, RL-10 & Exploartion Upper Stage Deputy Program Manager, in an interview with NASASpaceflight, this will be the final RL-10-B-2 engine to fly.

ICPS conducts the Trans-Lunar Injection burn, sending Orion on a six-day journey to the moon. (Credit: Mack Crawford for NSF/L2)

“The ICPS, it is historic to watch it fly our last B-2,” said Cummings.  But engine upgrades to the RL-10-C line for better performance are planing and integrated for the ICPS components for Artemis II and III before the RL-10-C-3 engines take over on the Exploration Upper Stage for Artemis IV and beyond.

The RL-10-B-2 for Artemis I will begin the perigee raise burn at T+ 51 minutes and 22 seconds, and the engine will shut down 22 seconds later.

Following another coast phase for ICPS and Orion until perigee, the Trans-Lunar Injection (TLI) burn will occur. This maneuver raises the orbital apogee out to the Moon, allowing Orion to later insert itself into the desired Distant Retrograde Orbit. This second ICPS burn begins at T+ one hour and 37 minutes and lasts approximately 18 minutes.

Orion will separate from ICPS at T+ two hours, six minutes, and 10 seconds. Shortly after, and T+ two hours, seven minutes, and 31 seconds, Orion’s thrusters will fire briefly to distance the spacecraft from ICPS.

With Orion delivered to its Trans-Lunar trajectory, ICPS conducts one final burn at T+ three and a half hours to safely dispose of itself into a heliocentric orbit.

Orion’s next maneuver will not occur until almost six hours later when the service module’s main engine ignites for a planned Outbound Trajectory Correction-1 burn. After a nominal liftoff, ascent, and a handful of on-orbit maneuvers, Orion will coast for a few days before Artemis I action picks right back up at the moon on flight day six.

(Lead image: SLS on the pad ready for launch. Credit: Thomas Burghardt for NSF)

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