"To understand hydrogen is to understand all of physics" - Victor Frederick Weisskopf

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For most people who don't follow the daily news of the space industry, they typically hear about what NASA has been up to only at climactic moments, like when a new telescope is deployed or a spacecraft reaches its destination. So when the Space Launch System (SLS) suddenly appeared on the launchpad at Cape Canaveral, ready to blast off to the moon, I bet most Americans hadn't even heard of the new megarocket until then. No doubt they were disappointed when a hydrogen leak scrubbed the launch, but that disappointment was all the worse for space fans like me who've been following the development of SLS since its inception and have been patiently waiting more than a decade for it to launch! 

Artemis 1 after scrubbing its Aug. 29 launch attempt

Still, I can't say I was surprised Artemis was delayed by leaky hydrogen, given that the rocket was cobbled together from legacy Space Shuttle architecture designed nearly 50 years ago. Per this ArsTechnica article, they tabulated that Shuttle scrubbed on average nearly once every launch attempt, with hydrogen leaks frequently being the culprit. Not only was it a tremendous headache for the launch operations team, but it was also incredibly expensive! Here are two notable instances I found across Shuttle's 30-year history where hydrogen really derailed the mission:

STS-35

In 1990, the Shuttle program appeared to be back on its feet following the Challenger disaster; Discovery had just lofted the brand new Hubble Space Telescope, and NASA was hoping to break their record of most launches in year. Columbia was slated for May to fly the ASTRO-1 suite of ultraviolet astrophysics experiments on STS-35. However, launch was scrubbed after propellant fill caused excessive buildups of hydrogen around both the quick disconnect assembly of the external tank, and the umbilical assembly of the orbiter aft section. They attempted a tanking test a few days later to isolate the leak, but it became clear the leak could not be fixed right there on the pad, so Columbia was rolled back to the Vehicle Assembly Building for repairs (sounds just like SLS). They swapped out the suspect umbilical assembly (taking the replacement from Endeavour, which was still under construction at the time), and rolled back to the pad in early August 

Locations of the excessive hydrogen buildup: External tank quick disconnect (A) and orbiter aft section umbilical (B)

Alas, the problem persisted around the orbiter aft end! The umbilical assembly had already been replaced, so NASA determined there must be another independent leak in the aft end. They tried replacing the hydrogen recirculation pumps, as well as a Teflon seal on main engine 3, but nothing could stop the leak. At a loss, they rolled Columbia back to the VAB again (passing Atlantis on the way, in the image below) while Shuttle Program Manager and former astronaut Bob Crippen assembled a "tiger team" to solve the problem once and for all. Over the next several months, a huge team of engineers pored over an intricate fault tree of all of Columbia's hydrogen fluid systems, which ultimately led them to the main engines themselves. Apparently, after Columbia's previous mission, STS-32, the main engines had needed to be completely disassembled and reassembled for maintenance, and when they did that, tiny glass beads had contaminated the hydrogen disconnect hardware. Having now found the true problem, the team painstakingly leak checked every component on the engines, and finally after yet another full tanking test, they declared Columbia leak-free. After all that effort, STS-35, originally scheduled to launch in May, had a flawless liftoff in December 1990. It was one of the most delayed launches of the Space Shuttle Program

Iconic image of two shuttles passing in the night: Columbia (left) returns back to the VAB for repairs on STS-35, while Atlantis (right) crawls towards the pad for STS-38. August 9, 1990

STS-93

In 1999, NASA deployed the Chandra X-ray Observatory on STS-93. Coincidentally on Columbia again, this mission suffered a particularly scary hydrogen leak because it actually happened on ascent. During ignition, a gold pin on the right engine came loose and struck the engine's inner nozzle, severing three cooling tubes containing hydrogen. Additionally, an electrical short from bad wiring on an exposed screw disabled both the center engine and right engine's digital controller. Fortunately, the backup controllers kicked in and saved the crew from potential disaster, since a two-engine shutdown would've required a very risky abort sequence. However, because the controllers recognized the decrease in thrust due to the leaky hydrogen, they tried to compensate by opening the oxidizer valves more than normal to get more propellant into the engines. Not only did this force the engines to burn hotter than normal, but all three engines cut out early because the external tank ran out of oxygen sooner than it should have. Columbia and her crew still managed to get to orbit and execute the mission, albeit at 15ft/s less than expected

You can actually see the damage to the right engine. It appears as a bright streak in the nozzle's inner wall, causing a distortion in the blue shock diamond of the exhaust plume

So given the history of problems, why use hydrogen at all for SLS, why not use an easier fuel like RP-1? Well there's two reasons, one grounded in engineering, one grounded in... politics. The first reason, and the more legitimate one, is that all things equal, hydrogen has the highest specific impulse of any rocket fuel. Said differently, it's the most efficient fuel, meaning that molecule for molecule, it's more energetic than burning methane or kerosene if you're able to wrangle with it. Of course, some of that efficiency is offset by the fact that hydrogen is super light, so it's hard to pack a lot of it into a tank. But the high specific impulse is one of the reasons why Shuttle opted for hydrogen

The other reason is simply that Congress mandated SLS adapt Shuttle architecture to make use of the leftover hardware. But the idea that we could take all the finicky and expensive systems of Shuttle, alter them somewhat and stack them into SLS, and expect to see cost savings, was certainly silly and overly optimistic in retrospect. No doubt the contractors that supplied NASA in the Shuttle days were all too eager to keep the same infrastructure going for Artemis. So we'll see how many more attempts it takes to get SLS off the ground. Hopefully the hydrogen cooperates and stays in the tank!