NASA’s Space Launch System rocket and Orion spacecraft are currently on Launch Pad 39B at Kennedy Space Center in Florida, but the fuel set to propel them is notorious for leaking. The question remains: Why does NASA keep using this notoriously fickle fuel?
If all had gone according to plan, four astronauts might have been returning just this week from a history-making, 10-day slingshot trip around the moon.
Instead, NASA’s engineers have been grappling with the rocket and fuel meant to propel the mission, called Artemis II, troubleshooting an all-too-familiar problem.
Just a few hours into a pre-launch test called a wet dress rehearsal in early February, launch controllers found that enough super-chilled liquid hydrogen fuel was leaking at the launchpad to prompt safety concerns. The hydrogen leaks kept cropping up, forcing NASA to halt fuel flow to the rocket multiple times.
The issue ultimately left the space agency unable to complete the full test and led to more than a week of investigations and repairs.
A Tiny Molecule with a Powerful Punch
Engineers pioneered the use of hydrogen as rocket fuel in the mid-20th century before it was used for the Apollo moon rockets — and most of the launch vehicles that have opted for the fuel since have also wrestled with leaks.
Hydrogen’s leaky tendencies can be attributed to the fact that it’s the lightest element in the universe. It “tends to find its way out of things you want to try to contain it in,” said Adam Swanger, a senior principal investigator and cryogenics research engineer at NASA’s Kennedy Space Center. “And it has very low density.”
To put that in perspective, hydrogen is roughly 14 times lighter than the air on Earth. But the same properties that make hydrogen so difficult to contain also make it an ideal rocket fuel.
Political Factors
NASA’s choice of fuel goes beyond just performance. The Space Launch System rocket, called the SLS, uses hydrogen for both the upper and first-stage portions of the vehicle.
And there is one not-so-obvious reason why: “It was ultimately a congressional decision that came through via law that NASA had to use Shuttle hardware and Shuttle workforces and contractors to do the SLS,” said Casey Dreier, the chief of space policy at the nonprofit Planetary Society.
In other words, the SLS uses hydrogen in part because the Space Shuttle also used hydrogen, and lawmakers wanted the SLS program to largely preserve the Shuttle-era workforces and supply chains.
The hydrogen leaks NASA is grappling with today are a symptom of that decision, Dreier added. Opting to attempt to cobble together pieces of an old program for new rockets — rather than starting from scratch — “actually shifted a lot of consequences and cost when it comes to trying to operate the rocket.”
Will Hydrogen Always be a Problem?
NASA’s chronic struggle to contain its fuel of choice within the rocket raises the question of whether the SLS rocket will always grapple with hydrogen leaks — or if a permanent fix can be found.
Kshatriya noted that while an SLS rocket has flown before, the vehicle is not reusable. That means the SLS on the launchpad today is a brand-new, and it may have its own tics and foibles.
But avoiding hydrogen leaks completely may require advancements in material sciences.
“Instead of asking why hydrogen is hard to handle, from a material science point of view, you’re asked, ‘Do any existing materials have good enough fracture toughness?’” explained Jihua Gou, a professor for the University of Central Florida’s department of mechanical and aerospace engineering.
NASA is currently using Teflon polymers called PTFE, he noted, for those seals.
“Teflon is typically used because that’s historically been the one that works best. There’s just not a lot of options,” Swanger said. And “it becomes a really challenging problem with really large interfaces.”
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