This week, a significant milestone in space exploration faced unexpected challenges. If plans had unfolded perfectly, four astronauts would have been returning from a historic 10-day journey around the Moon aboard the Artemis II rocket. Instead, NASA engineers are diving deep into technical issues associated with cryogenic fuel, particularly focusing on a familiar nemesis: leaks of liquid hydrogen.
In early February, just hours before a key wet dress rehearsal, control specialists detected a troubling leak during the fueling process. This discovery raised immediate safety concerns, leading to multiple suspensions of fuel flow throughout the test. Ultimately, the rehearsal went unfinished, prompting over a week of investigations and repairs to get to the root of the problem.
For those familiar with NASA’s history, the recurrence of hydrogen leaks and mission delays may feel frustratingly familiar. In 2022, during the uncrewed Artemis I mission’s test flight, the countdown faced numerous interruptions, with a hydrogen leak almost jeopardizing the launch before engineers managed to fix a faulty valve at the last moment. Such dilemmas echo throughout the decades of NASA’s Space Shuttle program, active from 1981 to 2011, which also dealt with hydrogen-related issues.
Hydrogen leaks are not simply a technical hassle; they present serious safety hazards due to the gas’s highly flammable nature. An excess concentration can lead to catastrophic outcomes, necessitating rigorous safety protocols during operations like the wet dress rehearsal. This raises a crucial question: why does NASA persist in using this notoriously tricky fuel?
Why Hydrogen Remains NASA’s Choice
Hydrogen has a storied history as a rocket fuel since the mid-20th century, and its propensity for leaks is not unique to NASA—it affects most rockets that rely on hydrogen. For instance, the Vulcan Centaur rocket from United Launch Alliance encountered a significant fuel leak resulting in fire during tests in 2023, which led to substantial damage and postponed its launch.
One of the key factors contributing to hydrogen’s leakiness is its lightness. As the lightest element, hydrogen “leaks” from connections more easily than heavier fuels. Adam Svenger, a senior scientist and NASA cryogenic engineer, notes, “Low density is beneficial for performance, but with it comes a trade-off.” This trade-off is crucial to understand as it informs both the engineering challenges and the performance capabilities of rockets.
“Low density is beneficial for performance,” said Svenger. “So there is a certain trade-off.”
A critical measure in assessing fuel choices, known as specific impulse (Isp), indicates how much thrust an engine can produce per unit of fuel consumed. Hydrogen excels in this regard, boasting one of the highest Isp values among propellants available today. Thus, its high efficiency is a significant reason for its ongoing use in missions like Artemis.
However, the decision to use hydrogen for the Artemis program goes beyond technical performance. Legislative decisions have shaped NASA’s path, as Congress mandated the use of Space Shuttle-era hardware and personnel for the Space Launch System (SLS). Casey Dreier, the Planetary Society’s director of space policy, emphasizes that this directive resulted in specific supply chains, all linked to previous missions, influencing costs and logistics.
“Ultimately, it was a Congressional decision, which by law required NASA to use Space Shuttle hardware and workers for the SLS.”
Consequently, hydrogen fuels both the upper and initial stages of the SLS, not just for performance but also due to policy decisions surrounding the preservation of existing infrastructures. This produced a unique set of challenges, as the endeavor to adapt old technology creates complications—hydrogen leaks thus become a symptom of this complex overhaul.
These issues are notably concentrated in the Tail Service Mast Umbilical (TSMU) area, which connects the SLS rocket to ground equipment, leading to the need for regular seal replacements. For safe fueling and subsequent launches, NASA aims to maintain the leak rate below 16%, a crucial parameter monitored closely by NASA Launch Director Charlie Blackwell-Thompson.
In addition to monitoring the leak rates, NASA employs several strategies to mitigate problems, such as identifying the sources of leaks and pre-fueling heating of the fuel lines to facilitate stabilization of the seals prior to re-cooling. In early February, NASA Administrator Jared Isaacman indicated that tests with partially fueled hydrogen tanks showed encouraging progress, with early data suggesting a reduction in leak instances.
Nonetheless, the question of whether SLS will ever achieve complete freedom from hydrogen leaks remains unresolved. Amit Kshatriya notes that as of now, the SLS is not designed for reuse, and the launch pad operations still face “snarls” that complicate processes.
“This is an experimental apparatus.”
The initial Artemis I test and the February Artemis II rehearsal highlighted the sensitivity of operating with cryogenic fuels. Yet, as NASA continues to refine its fueling processes, they are deepening their knowledge and experience, paving pathways for safer flights in the future. As Kshatriya aptly put it, “No one in these seats should call these rockets operational.”
In essence, the battle against hydrogen leaks encapsulates a blend of engineering challenges and political influences. NASA’s ongoing efforts reflect a steadfast commitment to addressing these issues, recognizing that hydrogen remains one of the most effective choices for propellant, especially in the vacuum of space where its unique energy properties are most beneficial for lift-off and maneuverability.