Years after shuttle, NASA rediscovers the perils of liquid hydrogen

"Every time we saw a leak, it pretty quickly exceeded our flammability limits."

KENNEDY SPACE CENTER, Fla.—America's space agency on Saturday sought to launch a rocket largely cobbled together from the space shuttle, which itself was designed and built more than four decades ago.

As the space shuttle often was delayed due to technical problems, it therefore comes as scant surprise that the debut launch of NASA's Space Launch System rocket scrubbed a few hours before its launch window opened. The showstopper was an 8-inch diameter line carrying liquid hydrogen into the rocket. It sprang a persistent leak at the inlet, known as a quick-disconnect, leading on board the vehicle.

Valiantly, the launch team at Kennedy Space Center tried three different times to stanch the leak, all to no avail. Finally at 11:17 am ET, hours behind on their timeline to fuel the rocket, launch director Charlie Blackwell-Thompson called a halt.

What comes next depends on what engineers and technicians find on Monday when they inspect the vehicle at the launch pad. If the launch team decides it can replace the quick-disconnect hardware at the pad, it may be an option to perform a partial fueling test to determine the integrity of the fix. This may allow NASA to keep the vehicle on the pad ahead of the next launch. Alternatively, the engineers may decide the repairs are best performed inside the Vehicle Assembly Building, and roll the rocket back inside.

Due to the orbital dynamics of the Artemis I mission to fly an uncrewed Orion spacecraft to the Moon, NASA will next have an opportunity to launch from September 19 to October 4. However, making that window would necessitate fixing the rocket at the pad, and then getting a waiver from the US Space Force, which operates the launch range along the Florida coast.

At issue is the flight termination system, which is powered independently of the rocket, with batteries rated for 25 days. NASA would need to extend that battery rating to about 40 days. The space agency is expected to have those discussions with range officials soon.

If the rocket is rolled back to the Vehicle Assembly Building, which would be necessary to service the flight termination system or perform more than cursory work at the launch pad, NASA has another Artemis I launch opportunity from October 17 to October 31.

A tiny, tiny element

The space shuttle was an extremely complex vehicle, mingling the use of solid-rocket boosters—which are something akin to very, very powerful firecrackers—along with exquisitely built main engines powered by the combustion of liquid hydrogen propellant and liquid oxygen to serve as an oxidizer.

Over its lifetime, due to this complexity, the shuttle on average scrubbed nearly once every launch attempt. Some shuttle flights scrubbed as many as five times before finally lifting off. For launch controllers, it never really got a whole lot easier to manage the space shuttle's complex fueling process, and hydrogen was frequently a culprit.

Hydrogen is the most abundant element in the universe, but it is also the lightest. It takes 600 sextillion hydrogen atoms to reach the mass of a single gram. Because it is so tiny, hydrogen can squeeze through the smallest of gaps. This is not so great a problem at ambient temperatures and pressures, but at super-chilled temperatures and high pressures, hydrogen easily oozes out of any available opening.

To keep a rocket's fuel tanks topped off, propellant lines leading from ground-based systems must remain attached to the booster until the very moment of launch. In the final second, the "quick-disconnects" at the end of these lines break away from the rocket. The difficulty is that, in order to be fail-safes in disconnecting from the rocket, this equipment cannot be bolted together tightly enough to entirely preclude the passage of hydrogen atoms—it is extremely difficult to seal these connections under high pressure, and low temperatures.

NASA, therefore, has a tolerance for a small amount of hydrogen leakage. Anything above a 4 percent concentration of hydrogen in the purge area near the quick disconnect, however, is considered a flammability hazard. "We were seeing in excess of that by two or three times that," said Mike Sarafin, NASA's Artemis I Mission Manager, said of Saturday's hydrogen leak. "It was pretty clear we weren’t going to be able to work our way through it. Every time we saw a leak, it pretty quickly exceeded our flammability limits."

Twice, launch controllers stopped the flow of hydrogen into the vehicle, in hopes that the quick-disconnect would warm a little bit. They hoped that, when they restarted slowly flowing cryogenic hydrogen on board the rocket, the quick-disconnect would find a tighter fit with the booster. It did not. Another time they tried applying a significant amount of pressure to re-seat the quick disconnect.

NASA officials are still assessing the cause of the leak, but they believe it may have been due to an errant valve being opened. This occurred during the process of chilling down the rocket prior to loading liquid hydrogen. Amid a sequence of about a dozen commands being sent to the rocket, a command was sent to a wrong valve to open. This was rectified within 3 or 4 seconds, Sarafin said. However, during this time, the hydrogen line that would develop a problematic quick-disconnect was briefly over-pressurized.

Deferring to the experts

So why does NASA use liquid hydrogen as a fuel for its rockets, if it is so difficult to work with, and there are easier to handle alternatives such as methane or kerosene? One reason is that hydrogen is a very efficient fuel, meaning that it provides better "gas mileage" when used in rocket engines. However, the real answer is that Congress mandated that NASA continue to use space shuttle main engines as part of the SLS rocket program.

In 2010, when Congress wrote the authorization bill for NASA that led to creation of the Space Launch System, it directed the agency to "utilize existing contracts, investments, workforce, industrial base, and capabilities from the Space Shuttle and Orion and Ares 1 projects, including ... existing United States propulsion systems, including liquid fuel engines, external tank or tank related capability, and solid rocket motor engines."

During a news conference on Saturday, Ars asked NASA Administrator Bill Nelson whether it was the right decision for NASA to continue working with hydrogen after the agency's experience with the space shuttle. In 2010, Nelson was a US Senator from Florida, and ringleader of the space authorization bill alongside US Sen. Kay Bailey Hutchison, of Texas. "We deferred to the experts," Nelson said.

By this Nelson meant that the Senate worked alongside some officials at NASA, and within industry, to design the SLS rocket. These industry officials, who would continue to win lucrative contracts from NASA for their work on shuttle-related hardware, were only too happy to support the new rocket design.

Among the idea's opponents was Lori Garver, who served as NASA's deputy administrator at the time. She said the decision to use space shuttle components for the agency's next generation rocket seemed like a terrible idea, given the challenges of working with hydrogen demonstrated over the previous three decades.

"They took finicky, expensive programs that couldn't fly very often, stacked them together differently, and said now, all of a sudden, it's going to be cheap and easy," she told Ars in August. "Yeah, we've flown them before, but they've proven to be problematic and challenging. This is one of the things that boggled my mind. What about it was going to change? I attribute it to this sort of group think, the contractors and the self-licking ice cream cone."

Now, NASA faces the challenge of managing this finicky hardware through more inspections and tests after so many already. The rocket's core stage, manufactured by Boeing, was shipped from its factory in Louisiana more than two and a half years ago. It underwent nearly a year of testing in Mississippi before arriving at Kennedy Space Center in April 2021. Since then, NASA and its contractors have been assembling the complete rocket and testing it on the launch pad.

Effectively, Saturday's "launch" attempt was the sixth time NASA has tried to completely fuel the first and second stages of the rocket, and then get deep into the countdown. To date, it has not succeeded with any of these fueling tests, known as wet dress rehearsals. On Saturday, the core stage's massive liquid hydrogen tank, with a capacity of more than 500,000 gallons, was only 11 percent full when the scrub was called.

Perhaps the seventh time will be a charm.

Source: Ars Technica

https://arstechnica.com/science/2022/09/years-after-shuttle-nasa-rediscovers-the-perils-of-liquid-hydrogen/