NASA’s space launch system is fueled by a mixture of liquid hydrogen and liquid oxygen. Together, these elements provide a compact and extremely powerful rocket propellant, but these same characteristics are also what make this fuel a liability. The SLS’s second launch attempt had to be canceled on Saturday, Sept. 3, after engineers failed to resolve a hydrogen leak at a quick disconnect — an 8-inch inlet that connects the liquid hydrogen fuel line to the rocket’s core stage. As a result of the setback, the SLS likely won’t launch until October at the earliest. The Artemis 1 mission, in which an Orion spacecraft will travel to the Moon and back, will have to wait. Ground teams were able to fix a hydrogen leak during the first failed launch attempt on Monday, August 29, but the launch was ultimately aborted 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. “This was not a manageable spill,” Mike Sarafin, director of the Artemis mission, told reporters after the brush-up. 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 require some hardware repairs due to an inadvertent command that briefly increased the pressure within the system. Unintended overpressurization may have contributed to the seal leak and is something engineers are currently evaluating as a possibility.
Inheriting the hydrogen problem
Hydrogen leaks are nothing new to NASA. Space shuttle launches occurred with disturbing regularity and were often the result of hydrogen leaks. One of the most infamous episodes was “the summer of hydrogen,” when ground teams spent more than six months trying to locate an elusive hydrogen leak that grounded the Shuttle fleet in 1990. The SLS is largely modeled after the Space Bus, including the use of liquid hydrogen propellant, so hydrogen-related scrubs could certainly have been predicted. But SLS is what it is, and NASA has no choice but to manage this limitation of the big moon rocket. G/O Media may receive a commission 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 creation in 1958, the agency has used contractors located around the US as a way 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 US Congress, in the authorization act that funds NASA, directed the Agency to use existing technologies from the Shuttle in the next-generation launch system.” In which he added: “This was a policy decision intended to preserve contractor jobs in key policy areas and from this funding and congressional support for NASA.” The first flight of Space Shuttle Endeavour, May 7, 1992, Photo: NASA This development meant that the RS-25 engine from the retiring space shuttle, along with its reliance on a liquid hydrogen/liquid oxygen mixture, would have to be transferred to the SLS. In total, NASA managed to collect 16 engines from the retired Shuttles, four of which are currently attached to the SLS rocket sitting on the launch pad at the Kennedy Space Center in Florida. This situation, Bimm said, is a reminder of the catchphrase from the 1983 movie The Right Stuff: “No bucks, no Buck Rogers.” NASA, he said, “often must prioritize securing political support from Congress to maintain its exploration program.” The continued use of RS-25 engines “is another example of how something as simple as fuel choice can be political, and how often the simplest and most desirable solutions are not politically viable for a large national service created in the Cold Age War of “Big Science,” said Bimm. Instead of choosing propellants like methane or kerosene, NASA chose to use a mixture of liquid hydrogen and liquid oxygen to power the heavy-lift rocket. By comparison, SpaceX’s upcoming Starship uses liquid methane, with liquid oxygen as an oxidizer. “With their eyes on Mars, SpaceX has chosen liquid methane in hopes of being able to extract this element [when] to Mars as a form of cost savings in resource use,” Bimm explained. The US space agency, perpetually cash-strapped and having to please politicians, was working with a different set of principles when designing the SLS. “Based on current information and analysis, the [proposed SLS design] represents the lowest near-term cost, shortest available, and least overall risk for development of the next, domestic heavy-lift launch vehicle,” NASA wrote in a 2011 preliminary project report. “Selecting this SLS architecture would mean that in the short term a new liquid engine will not need to be developed, thus shortening the time to first flight as well as likely minimizing the total…cost of the SLS.” The irony is that SLS, which is supposed to fly in 2017, has yet to launch and its total development cost, including the Orion crew capsule, has now exceeded $50 billion. That doesn’t include the estimated $4.1 billion cost attached to each SLS launch. And by inheriting Space Shuttle components, NASA has also inherited the hydrogen problem.
A beneficial but troublesome molecule
Hydrogen is extremely useful as a rocket fuel. It is readily available, clean, light and, when combined with liquid oxygen, burns with extraordinary intensity. “Combined with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific thrust, or efficiency, relative to the amount of propellant consumed, of any known rocket propellant,” according to NASA. When cooled to -423 degrees Fahrenheit (-253 degrees Celsius), hydrogen can be squeezed into a rocket, offering a huge amount of fuel for the dollar. “The advantages of liquid hydrogen as a fuel are its efficiency in storing the energy you want to release to propel the rocket, and its light weight, which is always important in spaceflight,” Bimm said. The SLS on the pad at the Kennedy Space Center. Photo: NASA NASA’s Apollo-era Saturn second-stage rocket used liquid hydrogen, as did the Shuttle’s three main engines. Hydrogen is commonly used for second stages (Europe’s Ariane 5 rocket is a good example) and as a liquid fuel needed to maneuver spacecraft into orbit. Rockets currently using liquid hydrogen include Atlas’s 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 because of the small molecular size of hydrogen that leads to leaks and the need to keep it in a liquid state that requires cooling to extremely low temperatures,” Bimm said. Furthermore, hydrogen is very volatile when in liquid form and can burn in large quantities. As the lightest element known, it is also very leaky. NASA explains the many challenges of using liquid hydrogen as a fuel: To prevent it from vaporizing or boiling, liquid hydrogen-fueled rockets must be carefully insulated from all heat sources, such as rocket engine exhaust and air friction during flight through the atmosphere. Once the vehicle reaches space, it must be protected from the Sun’s radiant heat. When liquid hydrogen absorbs heat, it expands rapidly. Therefore, venting is necessary 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 in welded seams. Despite these challenges, NASA opted for liquid hydrogen when designing the SLS, and is now paying the price.
New rocket, same old problems
During the SLS tank, the sudden influx of cryogenic hydrogen causes significant changes in the physical structure of the rocket. The 130-foot-tall (40-meter) hydrogen tank shrinks about 6 inches (152 mm) in length and about 1 inch (25.4 mm) in diameter when filled with the super-cold liquid, according to NASA. Components attached to the tank, such as ducts, vent lines, and brackets, must compensate for this sudden contraction. To achieve this, NASA uses accordion-like bellows joints, slotted joints, telescoping sections, and ball-and-socket hinges. But hydrogen—the smallest molecule in the universe—often finds its way into even the tiniest of openings. The fuel lines are particularly problematic, as they cannot be rigidly attached to the rocket. As their name suggests, quick disconnects, while providing a tight seal, are designed to be released from the rocket during launch. This seal must prevent leakage under high pressures and extremely cold temperatures, but must also be released as the rocket flies. On Saturday, a leak in the fast disconnect area reached concentrations well beyond the 4 percent limit, exceeding NASA’s flammability limits. Unable to fix the leak, NASA called in the scrub. The fact that NASA has not yet fully fueled the first and second stages and is not deep into the countdown is a real cause for concern. The space agency has dealt with hydrogen leaks in the past, so hopefully its engineers will again come up with a solution to move the project forward. Still, it’s a disappointing start to the Artemis season. NASA needs SLS as it pursues a permanent and sustainable return to the lunar environment and as it looks to a future human mission to Mars. NASA will have to make the SLS work, and it may have to give it an aggravating rub at a time.
