CNN —
NASA’s plans to send four astronauts on a lunar flyby this week are facing a familiar challenge: fuel leaks. As the original timeline called for a triumphant return with the Artemis II mission, engineers are battling technical issues with persistent leaks in the rocket intended to carry the crew to deep space.
Just hours before a planned “wet dress rehearsal” in early February, launch controllers discovered a significant leak of super-cooled liquid hydrogen on the launchpad, raising safety concerns. The hydrogen leaks continued to appear, forcing NASA to halt fuel flow to the rocket on multiple occasions.
The issue ultimately prevented the agency from completing the full test and led to over a week of investigation and repairs.
The recurring hydrogen leaks and resulting mission delays echo past challenges for NASA.
The launch of an uncrewed test flight around the Moon in 2022, known as Artemis I, faced several delays and was nearly derailed by a similar hydrogen leak before a team of NASA workers in protective suits manually repaired a faulty valve at the last minute. Records also indicate engineers grappling with similar problems throughout the Space Shuttle program, which ran from 1981 to 2011.
Leaks are a major concern on the ground: hydrogen is highly flammable and energetic, meaning too much of it in an area carries the risk of a catastrophic explosion.
As launch controllers prepare for another “wet dress” rehearsal this Thursday, the question remains: why does NASA continue to rely on this notoriously unpredictable fuel?
Engineers first pioneered the use of hydrogen as rocket fuel in the mid-20th century, before it was used in the Apollo lunar rockets—and most launch vehicles have struggled with leaks since.
The Vulcan Centaur rocket, produced by U.S. Military contractor United Launch Alliance and based on decades of legacy technology, also uses hydrogen to power its upper stage. In 2023, a fuel leak caused an explosion during a Vulcan Centaur test in Alabama, damaging nearby infrastructure and delaying the rocket’s inaugural launch.
The tendency for hydrogen to leak can be attributed to the fact that it is the lightest element in the universe. “It tends to identify a way out of anything you endeavor to contain it in,” said Adam Swanger, senior principal researcher and research engineer in cryogenics at NASA’s Kennedy Space Center. “And it has a very low density.”
To position it in perspective, hydrogen is approximately 14 times lighter than air on Earth. But the same properties that make hydrogen demanding to contain also make it an ideal rocket fuel.
“Low density is good for performance,” said Swanger to CNN. “So there’s kind of a trade-off there.”
When selecting fuel for a rocket, the most important consideration is a concept called “specific impulse,” often abbreviated as Isp. It’s a measure of how much thrust—or force—a rocket engine can generate with a given amount of propellant.
To calculate Isp, the expected thrust of a rocket engine is divided by the rate at which it expels propellant weight. And weight is crucial in spaceflight: the more power a rocket must use to lift its own weight, the less capacity the vehicle has to carry valuable cargo or people to orbit.
Hydrogen is known for having a very favorable specific impulse due to its light weight, providing considerable power on liftoff. In fact, it’s the best in the business, boasting the highest efficiency of all rocket fuel options, “which is why we end up using it a lot,” said Swanger.
Though, in the case of Artemis, NASA’s fuel choice goes beyond simple performance.
Some argue that hydrogen causes more problems than it’s worth, given its propensity for causing launch delays.
However, the fuel also offers the best efficiency advantage when used in the vacuum of space. That’s why some rocket manufacturers opt to use hydrogen for the upper stages of launch vehicles, but employ a more manageable fuel for the first stage of a rocket, which houses all the engines providing the initial thrust from the launchpad.
Rockets built by Jeff Bezos’ Blue Origin or Elon Musk’s SpaceX, for example, use alternative fuels—such as methane or RP-1, a type of kerosene—for the first stages of their rockets.
But NASA’s Artemis lunar rocket, called the Space Launch System or SLS, uses hydrogen in both the upper and first stage portions of the vehicle.
And there’s a less obvious reason for this: “Ultimately it was a congressional decision that was established through legislation requiring NASA to use the hardware and workforce and contractor base from the Space Shuttle to build the SLS,” said Casey Dreier, chief policy advisor at the non-profit Planetary Society.
In other words, the SLS uses hydrogen in part because the Space Shuttle also used it, and lawmakers wanted the SLS program to largely preserve the jobs and supply chains from the Shuttle era.
The hydrogen leaks NASA is grappling with today are a symptom of that decision, added Dreier. Attempting to assemble pieces of an old program to build new rockets—rather than starting from scratch—“actually shifted a lot of consequences and costs onto the operation of the rocket.”
And while all rockets that use hydrogen are susceptible to frustrating leaks, NASA’s problems with the SLS may be exacerbated by its political peculiarities.
“It will never operate as well as if they had designed a new rocket,” said Dreier. “It will have large fixed costs. And you’ll have finicky rockets.”
NASA has acknowledged that the SLS may be finicky. But the vehicle is still in the early days of use, after two decades of development.
“It’s an experimental vehicle,” said Amit Kshatriya, NASA’s associate administrator, during a press conference on February 3 alongside several other agency officials.
Notably, NASA doesn’t typically consider a rocket to be fully “operational” until it enters routine service, a milestone the SLS could be some time from reaching considering the low frequency of its flights. Agency officials only deemed the Space Shuttle operational, for example, after it completed its first four missions, all of which were crewed.
Kshatriya added that NASA has found it difficult to anticipate the SLS leaks and determine how to prevent them. The process of bringing the rocket to the launchpad may have contributed to the sealing issue engineers are currently resolving, but the agency hasn’t confirmed the cause.
“It’s fairly complicated from a stress and strain perspective. It’s not an excuse,” Kshatriya noted, but engineers are only beginning to break down how these issues might develop.
During Artemis I and also in the first wet dress rehearsal of Artemis II in early February, the leaks were located in the same area: the Tail Service Mast Umbilical (TSMU), a three-story structure connecting the SLS rocket to ground equipment.
To address the most recent issues detected in the TSMU, NASA said technicians recently replaced seals around two of the propellant lines in that area.
To successfully complete a “wet dress” test and maintain the rocket safe on launch day, NASA must keep the leak rate during filling below 16%, according to NASA launch director Charlie Blackwell-Thompson.
And NASA is employing several methods to try to stay within acceptable limits. In addition to working to identify and fix the source of leaks before filling, the agency may also use a technique during the hydrogen loading process involving briefly warming the fuel lines, hoping the seals can reseat in a desirable position before being subjected to extremely cold temperatures again.
Some of the troubleshooting efforts may be yielding results, as NASA administrator Jared Isaacman announced earlier this month that a test where engineers partially filled the SLS’s hydrogen tanks showed improvements. Based on initial reviews of the data, the agency did not observe some of the leaks that concerned them in the previous “wet dress” rehearsal, Isaacman said.
The ongoing effort to contain NASA’s preferred fuel within the rocket raises the question of whether the SLS will always be prone to hydrogen leaks, or if a permanent fix can be found.
Kshatriya pointed out that while an SLS rocket has flown before, the vehicle is not reusable. That means the SLS on the launchpad today is completely new, and may have its own quirks and peculiarities.
The wet dress rehearsal in early February was “the first time this particular machine has been exposed to cryogenics,” said Kshatriya, referring to the super-cooled fuels. “And how it breathes, how it vents—and how it tends to leak—is something we’re characterizing.”
But eliminating hydrogen leaks altogether may require advances in materials science.
“Rather than asking why hydrogen is difficult to handle, from a materials science
