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| Artemis II mission SLS rocket "wet dress rehearsal" testing (photo credit: NASA/Sam Lott) |
Liquid hydrogen (LH2) related news this week saw heavy coverage on the Artemis II mission fueling tests that NASA performed on the Space Launch System (SLS) rocket. Called a wet dress rehearsal, these tests involve loading the SLS with LH2 and liquid oxygen (LOx) on the launch pad to check out various systems and operations before clearing the rocket for launch.
NASA reported some hydrogen leaks that they are troubleshooting, which of course is one of the key reasons for a wet dress rehearsal. This has resulted in a rescheduled launch target in March. It's worth noting that SLS has only flown one other time and these types of fixable issues are extremely common in new launch vehicles.
Nonetheless, armchair rocketeers in traditional and social media have predictably jumped into the fray. The authors often have some combination of: lack of any relevant competence; vested interests threatened by hydrogen; and/or are drawn to the siren call of easy clickbait. Below are a few facts to help filter out this noise from unqualified critics.
Workhorse Fuels for Heavy Launch Vehicles
There are many parameters to consider in the propulsion trade studies performed for any given rocket stage. Program and mission architectures, economic and technical assumptions and constraints, system requirements, concept of operations, flight heritage, supply chains, and a host of other considerations all come into play.
Two workhorse fuels have been widely and successfully used for heavy launch vehicles: kerosene and LH2. These fuels are generally coupled with LOx as the oxidizer resulting in "kerolox" or "hydrolox" rocket propulsion systems. Other solid, liquid, and hybrid fuels are used for boosters and various stages, but none are more ubiquitous than kerosene and hydrogen.
Some examples of rocket stages using hydrogen that have successfully launched large payloads and astronauts on over 500 missions over the past six decades include:
- NASA Apollo Saturn rocket (second and third stages)
- Centaur upper stage (atop Atlas, Titan, and Vulcan* first stages)
- NASA Space Shuttle (external tank)
- ESA Ariane 5 and now 6 rockets (both core stages)
- ULA Delta IV M+/Heavy (both core stages)
- NASA SLS rocket (both core stages)
- China Long March rockets (various configurations and stages)
- JAXA H3 rocket (both core stages)
- Blue Origin New Glenn* rocket (upper stage)
- ISRO LVM 3 and next generation rockets (upper stages)
The reason LH2 has been used (and continues to be used) for so long on so many rockets from so many countries is quite simple. It provides the highest specific impulse among practical rocket fuel options and has provided decades of reliable and safe operations**.
Liquid Methane (LCH4) is Not Inherently Better
Methalox (LCH4 and LOx) heavy launch vehicle stages are still relatively new despite decades of testing at NASA going back to the 1960s. So far, there have been only three successful orbital launches with rockets fueled by liquid methane: Landspace ZQ-2 in 2023 (Chinese), SpaceX Starship in 2024 (US), and Blue Origin New Glenn first stage in 2025 (US)*. While the continuing development of these vehicles is promising, it's a very limited number of launches so far compared to other rocket fuels.
In regards to safety, methane and hydrogen have many similar risks and associated mitigations. Hydrogen has never been the root cause of a NASA launch failure, nor would the use of methane have prevented any of the failures that have occurred. However, methane has been the root cause of several launch failures of vehicles under development, many of them due to leakage of the fuel.
It's also worth noting that methane is a 25 times more powerful greenhouse gas over one hundred years than carbon dioxide (CO2), and much worse in shorter timeframes. As a result, while methane may represent lower capex and opex for a clean sheet vehicle design with a high launch cadence, the extrinsic cost to the global community of any releases to the air are high.
Finally, if a mission architecture requires in-situ resource utilization (ISRU) to produce more fuel on the moon or Mars, hydrogen can be generated by melting ice deposits followed by electrolysis. In contrast, methane requires all of that since hydrogen is a necessary feedstock, plus processing the hydrogen with a source of carbon to produce methane (e.g., Sabatier process with CO2 and H2). In the case of the moon, that requires bringing the carbon along or collecting a great deal of astronaut respiration over a long time.
Despite all of the above - and the significantly lower performance of methalox vs hydrolox propulsion - there are a gaggle of armchair rocketeers who start writing about the demise of hydrogen and how it should be replaced by methane every time there is a hiccup with a hydrolox rocket stage. This is nonsense.
The Problem with Umbilicals
The only association most people have with umbilicals is the biological kind that allows vital fluids to pass between a mother and her fetus. At birth, the mammalian umbilical is severed and the remaining stump dries up and falls off leaving a sealed useless belly button to wash (and perhaps get a piercing in).
Launch vehicle LH2 umbilicals are another matter. They must be easily mated, well insulated, flexible, tightly seal, allow very large flowrates, de-mate on command, and rapidly retract away from the vehicle at launch. And do all of that reliably at very low within-spec allowable leakage rates. No other cryogenic fluid interface outside of the aerospace and defense sector has to deal with these kinds requirements.
As a result, rocket umbilicals can (and often do) leak regardless of the fluid being used. The question is how much is too much, and how to stay below that threshold via design and operational procedures. And when you're dealing with a human rated vehicle, the stakes are too high to take any chances when sensors indicate you might be at or over the threshold during testing. Failing fast and iterating is not an acceptable option when lives are at stake.
Hydrogen Will Take Us to the Moon Again
Minimizing loss of life risks must take precedence over launch schedules, external pressure, or the "optics" of poor media coverage. NASA has learned and relearned this painful lesson three times in its history: Apollo I (1967), Challenger (1986), and Columbia (2003). We must never forget the ultimate sacrifice of those astronauts.
Artemis II will launch when it's ready. And all the noise from this week's news and social media coverage will be immediately forgotten as we send humans around the moon again for the first time since Apollo.