Why hydrogen leaks continue to be a major headache for NASA launches

SLS on the Kennedy Space Center launch pad in Florida.

SLS on the Kennedy Space Center launch pad in Florida.
Photo: Nasa

NASA’s space launch system is powered by a mixture of liquid hydrogen and liquid oxygen. Together, these elements provide a compact and extremely powerful rocket propellant, but these same attributes are also what make this fuel a liability.

The second attempt to launch SLS must have been canceled on Saturday, Sept. 3, after engineers failed to fix a hydrogen leak in a quick disconnect — an 8-inch inlet that connects the liquid hydrogen line to the rocket’s core stage. Following the setback, SLS likely won’t launch until October as soon as possible. The Mission Artemis 1in which an uncrewed Orion spacecraft will travel to and from the Moon will have to wait.

Ground crews were able to repair a hydrogen leak during the first failed launch attempt on Monday August 29, but the launch was ultimately canceled after a faulty sensor incorrectly indicated that an engine had not reached the required ultra-cold temperature. Saturday’s leak proved much more difficult to contain, with engineers attempting three fixes, none of which worked. “It was not a manageable leak,” Artemis mission manager Mike Sarafin told reporters after the scrub.

NASA is still evaluating its next steps, but the rocket must return to the vehicle assembly building to undergo a mandatory safety check related to its flight termination system. The rocket may need some hardware fixes due to an inadvertent command that briefly increased the pressure in the system. The unintended overpressure may have contributed to the seal leaking, and this is something engineers are currently evaluating as a possibility.

Legacy of the hydrogen problem

Hydrogen leaks are not new to NASA. Scrubs from space shuttle launches occurred with shattering regularity and were often the result of hydrogen leaks. One of the most infamous episodes was “hydrogen summer“, when ground crews spent more than six months trying to locate an elusive hydrogen leak that grounded the shuttle fleet in 1990. SLS is modeled heavily on the Space Shuttle, including the use of propellant to liquid hydrogen, so hydrogen-related scrubs could certainly have been predicted. But SLS is what it is, and NASA has no choice but to deal with this limitation of its mega moon rocket.

Jordan Bimm, a space historian at the University of Chicago, says NASA continues to use liquid hydrogen for political rather than technical reasons.

“Since NASA’s inception in 1958, the agency has used contractors located in the United States to maintain broad political support and funding for space exploration in Congress,” Bimm told me. “The first system to use liquid hydrogen was the Centaur rocket developed in the 1950s and 1960s. In 2010, the United States Congress, in its NASA funding authorization act, asked the Agency to use existing shuttle technologies in its next-generation launch system. To which he added, “This was a political decision to maintain contractor jobs in key political districts and from this funding and congressional support for NASA.”

The first flight of Space Shuttle Endeavour, May 7, 1992,

The first flight of Space Shuttle Endeavour, May 7, 1992,
Photo: Nasa

This development meant that the retired Space Shuttle RS-25 engine, along with its reliance on a liquid hydrogen/liquid oxygen mixture, would have to be transferred to SLS. In total, NASA managed to recover 16 engines from the retired shuttles, including four are currently affixed to the SLS rocket standing on the launch pad at Kennedy Space Center in Florida.

This situation, says Bimm, recalls the slogan of the 1983 film The good thing: “No money, no Buck Rogers.” NASA, he said, “often must prioritize political support from Congress to maintain its exploration program.” The continued use of RS-25″ engines is another example of how something as mundane as fuel choice can be political and how often the simplest and most desirable solutions are not politically viable for a large national agency created in the Cold War era of ‘Big Science’,” Bimm said.

Instead of opting for propellants like methane or kerosene, NASA chose to use a mixture of liquid hydrogen and liquid oxygen to propel its heavy rocket. By comparison, SpaceX’s upcoming Starship uses liquid methane, with liquid oxygen as the oxidizer. “With their sights on Mars, SpaceX selected liquid methane in hopes of extracting this element. [when] on Mars as a form of economic resource use,” Bimm explained. The US space agency, perpetually strapped for money and having to please politicians, worked by a different set of principles when designing SLS.

“Based on current information and analysis, the [proposed SLS design] represents the lowest, most rapidly available, short-term costs, and the least risky path to development of the next heavy domestic launch vehicle,” NASA wrote in a 2011 draft report. “Selecting this SLS architecture would mean that a new near-term liquid engine would not need to be developed, thereby shortening the time to first flight and likely minimizing the overall cost of SLS.”

The irony is that SLS, which was supposed to fly in 2017, has yet to launch, and its full development costs, including the Orion crew cassume, have now exceeds $50 billion. This excludes the estimated cost of $4.1 billion for each SLS launch. And by inheriting the components of the space shuttle, NASA also inherited the hydrogen problem.

A beneficial but annoying molecule

Hydrogen is extremely useful as rocket fuel. It is readily available, clean, light, and when combined with liquid oxygen burns with extreme intensity. “In combination with an oxidizer such as liquid oxygen, liquid hydrogen produces the highest specific impulse, or efficiency relative to the amount of propellant consumed, of any known rocket propellant”, according at NASA. When cooled to -423 degrees Fahrenheit (-253 degrees Celsius), hydrogen can be packed into a rocket, providing a huge amount of fuel for the money. “The advantages of liquid hydrogen as a fuel are its efficiency in storing the energy you want to release to propel the rocket, as well as its low weight, which is always a consideration in spaceflight,” Bimm said.

SLS on the pad at Kennedy Space Center.

SLS on the pad at Kennedy Space Center.
Photo: Nasa

The second stage of NASA’s Apollo-era Saturn rocket used liquid hydrogen, as did the shuttle’s three main engines. Hydrogen is commonly used for second stages (the European Ariane 5 heavy rocket is a good example) and as the liquid fuel needed to maneuver spacecraft in orbit. Rockets that currently use liquid hydrogen include Atlas’ Centaur and Boeing’s Delta III and IV, while Blue Origin’s BE-3 and BE-7 engines also rely on hydrogen.

“The disadvantages of hydrogen are that it is very difficult to move and control due to the small molecular size of hydrogen, which leads to leaks and the need to keep it in a liquid state, which which requires cooling at extremely low temperatures,” Bimm said. Moreover, hydrogen is very volatile when it is in the liquid state, and it can burn in large quantities. As the lightest element known, it is also very fleeing. Nasa Explain the many challenges of using liquid hydrogen as a fuel:

To prevent it from evaporating or boiling, rockets fueled with liquid hydrogen must be carefully isolated from all sources of heat, such as rocket engine exhaust and air friction during the flight in the atmosphere. Once the vehicle reaches space, it must be protected from the radiant heat of the Sun. When liquid hydrogen absorbs heat, it expands rapidly; thus, ventilation is required to prevent the tank from exploding. Metals exposed to the extreme cold of liquid hydrogen become brittle. Additionally, liquid hydrogen can leak through the tiny pores of the welded joints.

Despite these challenges, NASA opted for liquid hydrogen when designing the SLS, and now it’s paying the price.

New rocket, same old problems

When refueling SLS, the sudden influx of cryogenic hydrogen causes significant changes in the physical structure of the rocket. The 130-foot-tall (40-meter-tall) hydrogen tank shrinks about 6 inches (152 mm) in length and about 1 inch (25.4 mm) in diameter when filled with ultra liquid -cold, according at NASA. Components attached to the tank, such as ducts, vent lines, and brackets, must compensate for this sudden contraction. To do this, NASA uses connectors with concertina bellows, split joints, telescoping sections and ball-and-socket hinges.

But hydrogen, the smallest molecule in the universe, often finds its way through even the smallest openings. The fuel lines are particularly problematic, as they cannot be securely bolted to the rocket. Like their its name suggests, the quick disconnects, while ensuring a perfect seal, are designed to release from the rocket during launch. This seal should prevent leaks under high pressures and ultra-cold temperatures, but it should also release when the rocket takes flight. On Saturday, a leak in the vicinity of the quick disconnect reached concentrations well above the 4% stress, exceeding NASA flammability limits. Unable to fix the leak, NASA called the scrub.

The fact that NASA has yet to fully fuel Stages One and Two and deepen the countdown is a real cause for concern. The space agency has dealt with hydrogen leaks before, so hopefully its engineers will find a solution again to move the project forward.

Still, it’s a frustrating start to the Artemis era. NASA needs SLS as it seeks a permanent and sustainable return to the lunar environment and envisions a human future mission to Mars. NASA is going to have to make the SLS work, and it may have to do it one aggravating scrub at a time.

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