Why hydrogen leaks remain a major problem for NASA launches

SLS on the launch pad at Kennedy Space Center in Florida.

SLS on the launch pad at Kennedy Space Center 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 properties also make this fuel a liability.

The second launch attempt of SLS must be cancelled On Saturday, Sept. 3, after engineers failed to fix the hydrogen leak with a quick disconnect — an 8-inch inlet connecting the liquid hydrogen fuel line to the rocket’s core stage. Due to setbacks, SLS may not launch until October at the earliest.This Artemis 1 questone of the unmanned Orion spacecraft that will go to the moon and back, will have to wait.

Ground team able to fix hydrogen leak in the meantime first failed launch attempt On Monday, Aug. 29, the launch was eventually canceled because sensors erroneously indicated that the engine had not reached the required supercooled temperature.Saturday’s leak proved harder to contain, engineers tried three Fixes, none of them worked. “This was not a controlled spill,” 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 for mandatory safety checks related to its flight termination system. The rocket may need some hardware fixes, as an unintentional command briefly raises the pressure within the system. Accidental overpressure could lead to leaking seals, and engineers are currently evaluating this possibility.

Inheriting the hydrogen problem

Hydrogen leaks are nothing new for NASA. Shuttle launch scrubs occur with disturbing regularity and are often the result of hydrogen leaks. One of the most notorious episodes was “hydrogen summer,” when ground teams spent more than six months trying to find an elusive hydrogen leak that grounded the space shuttle in 1990. SLS was largely modeled on the space shuttle, including the use of liquid hydrogen propellant , so hydrogen-related scrubbing can of course be predicted. But SLS is just that, and NASA has no choice but to manage this limitation on its giant lunar rocket.

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

“Since NASA’s inception in 1958, the agency has used contractors located across the United States to maintain broad political support and funding for space exploration in Congress,” Beam told me. “The first system to use liquid hydrogen was the Centaur rocket, developed in the 1950s and 1960s. In 2010, Congress, in its authorization bill to fund NASA, required the agency to use the space shuttle’s existing technology.” He added: “This is a political decision to maintain contractor jobs in key political districts, as well as congressional funding and support for NASA.”

Space Shuttle Endeavour's maiden flight, May 7, 1992,

Space Shuttle Endeavour’s maiden flight, May 7, 1992,
photo: NASA

This development means that the retired space shuttle’s RS-25 engine, and its reliance on a liquid hydrogen/liquid oxygen mixture, will have to be moved to the SLS. In total, NASA managed to collect 16 engines from decommissioned space shuttles, of which There are currently four stickers on the SLS rocket Standing on the launch pad at Kennedy Space Center in Florida.

Beam said the situation was reminiscent of the slogan of the 1983 film the right thing“No money, no Buck Rogers.” He said that NASA “must often prioritize political support in Congress to sustain its exploration programs.” The continued use of the RS-25 engine “is another example of something like How something as mundane as fuel choice is political, and the most immediate and ideal solutions are often politically infeasible for large state institutions created in the Cold War era. Science’,” Beam said .

Instead of propellants like methane or kerosene, NASA opted to use a mixture of liquid hydrogen and liquid oxygen to power its heavy-lift rockets. In contrast, SpaceX’s upcoming Starship uses liquid methane, with liquid oxygen as the oxidant. “Setting its sights on Mars, SpaceX chose liquid methane, hoping to extract this element [when] As a cost-saving form of resource utilization on Mars,” Beam explained. NASA, which has been underfunded and had to please politicians, adopted a different set of principles when designing the SLS.

“Based on current information and analysis, [proposed SLS design] represents the lowest near-term cost, fastest available, and overall least risky path to developing the next generation of domestic heavy-lift rockets,” NASA wrote in a 2011 article Preliminary Project Report“Choosing this SLS architecture means there is no need to develop a new liquid engine in the short term, thereby reducing first flight time and potentially minimizing the overall … cost of the SLS.”

Ironically, the SLS, originally scheduled for test flight in 2017, has yet to launch, and its total development cost, including the Orion crewcapsule, with Now over $50 billion. This does not include the estimated $4.1 billion cost per SLS rollout. And by inheriting components from the space shuttle, NASA also inherited the hydrogen problem.

A beneficial but annoying molecule

Hydrogen is very useful as rocket fuel. It is readily available, clean, lightweight, and when combined with liquid oxygen, burns with extreme intensity. “Combined with oxidants such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency relative to the amount of propellant consumed, compared to any known rocket propellant,” according to to NASA. When cooled to -423 degrees Fahrenheit (-253 degrees Celsius), hydrogen can be stuffed into rockets, providing a lot of fuel for depressurization. “The advantages of liquid hydrogen as a fuel are its efficient storage of the energy you want to release to propel a rocket, and its light weight, which has always been a consideration in spaceflight,” Bimm said.

SLS on the Kennedy Space Center pad.

SLS on the Kennedy Space Center pad.
photo: NASA

NASA’s Apollo-era Saturn rockets used liquid hydrogen for the second stage, as do the space shuttle’s three main engines. Hydrogen is typically used in the second stage (Europe’s heavy-duty Ariane 5 rocket is a good example) and as the liquid fuel needed to maneuver spacecraft in orbit. Rockets currently using 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.

“Hydrogen has the disadvantage of being difficult to move and control, because the small molecular size of hydrogen can lead to leaks, and it needs to be kept in a liquid state, which requires cooling to extremely low temperatures,” Bimm said. What’s more, hydrogen is extremely volatile in liquid state, It can burn a lot.Ones the lightest known element, also very leaky.NASA explain The many challenges of using liquid hydrogen as a fuel:

To prevent it from evaporating or boiling, rockets fueled by liquid hydrogen must be carefully isolated from all heat sources, such as rocket engine exhaust and air friction during flight. Once the vehicle reaches space, it must be protected from the sun’s radiant heat. Liquid hydrogen absorbs heat and expands rapidly; therefore, ventilation is necessary to prevent the tank from exploding. Metals exposed to extremely cold liquid hydrogen become brittle. Additionally, liquid hydrogen can leak through tiny pores in the weld.

Despite these challenges, NASA chose liquid hydrogen when designing the SLS, and it is now paying the price.

New rockets, old problems

When refueling the SLS, a sudden influx of cryogenic hydrogen can cause major changes in the rocket’s physical structure. A 130-foot-tall (40-meter-tall) hydrogen tank would shrink about 6 inches (152 mm) in length and about 1 inch (25.4 mm) in diameter when filled with supercooled liquid, according to to NASA. Components attached to the tank, such as pipes, vents, and brackets, must compensate for this sudden contraction. To achieve this, NASA uses connectors with accordion-like bellows, slotted joints, telescoping sections, and ball joint hinges.

But hydrogen — the smallest molecule in the universe — often passes through the tiniest opening. Fuel lines are especially problematic because they cannot be hard bolted to the rocket.as their As the name suggests, the quick disconnect is designed to disengage the rocket during launch while providing a tight seal. This seal must prevent leakage under high pressure and ultra-low temperature, but it also needs to be loosened while the rocket is in flight. On Saturday, the concentration of the leak near the quick disconnect device was well above the 4 percent limit, exceeding NASA’s flammability limit. Unable to resolve the leak, NASA called in to scrub.

NASA has yet to fully fuel and count down the first and second stages, which is a real cause for concern. The space agency has dealt with hydrogen leaks before, so hopefully its engineers will once again devise a solution to move the project forward.

Still, it was a frustrating start to the age of Artemis. NASA needs SLS as it seeks a permanent and sustainable return to the lunar environment with an eye toward future humans Mars mission. NASA will have to make SLS work, and it will likely have to do a more severe scrubbing time and time again.

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