Why hydrogen leaks are still a big headache for NASA launches

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

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

NASA’s Space Launch System operates with a mixture of liquid hydrogen and liquid oxygen. Together, these elements provide a compact and extremely powerful rocket propellant, but it is these same qualities that also make this fuel a burden.

It should be the second launch attempt of SLS canceled On Saturday, September 3, after engineers failed to resolve a hydrogen leak in a quick disconnect–an 8-inch inlet connecting a liquid hydrogen fuel line to the rocket’s core stage. as a result of the setback, The SLS probably won’t be released until October on the nearest. The Artemis mission 1as the unmanned Orion spacecraft will travel to the moon and back, you will have to wait.

Ground teams managed to fix the hydrogen leak during Failed first launch attempt Monday, August 29, but the launch was eventually called off after a faulty sensor incorrectly indicated that the engine had not reached the required super-cold temperature. Saturday’s leak proved more difficult to contain, as engineers tried three Repairs, none of them worked. “This was not a manageable leak,” Mike Sarafin, Artemis’ mission manager, told reporters after the scan.

NASA is still assessing its next steps, but the rocket must return to the Vehicle Assembly Building to undergo a mandatory safety check related to the flight termination system. The missile may require some hardware repairs due to an unintended command that briefly increased stress within the system. Unintended excessive pressure may have contributed to the seal leak, something that engineers are currently assessing 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 famous episodes washydrogen summerWhen ground teams spent more than six months trying to locate the elusive hydrogen leak that grounded the shuttle fleet in 1990. The SLS is modeled largely on the space shuttle, including using liquid hydrogen fuel, so it’s certainly true that tools Hydrogen-related cleanup has been expected, but the SLS is what it is, and NASA has no choice but to manage this limitation on its massive lunar rocket.

Jordan Beam, a space historian at the University of Chicago, says that NASA continues to use liquid hydrogen for political, not technical, reasons.

“Since the creation of NASA in 1958, the agency has used contractors located throughout the United States as a means of maintaining 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, the US Congress, in the NASA Funded Authorization Act, mandated the agency to use existing technologies from the shuttle in its next-generation launch system.” He added to him: “This was a political decision aimed at preserving contractor jobs in key political areas and from this funding and congressional support for NASA.

The first flight of the space shuttle Endeavor, May 7, 1992,

The first flight of the space shuttle Endeavor, May 7, 1992,
picture: NASA

This development means that the RS-25 engine from the retired Space Shuttle, along with its reliance on a liquid hydrogen/liquid oxygen mixture, must be ported to the SLS. In total, NASA was able to collect 16 retired shuttle engines, of which Four of them are currently installed on the SLS missile Standing on the launch pad at the Kennedy Space Center in Florida.

Bem said that this situation is a reminder of the logo of the 1983 movie correct types: “No bucks, no Buck Rogers.” He said that NASA “should often prioritize bolstering 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 most obvious and desirable solutions are politically unworkable for a large national agency created in the Cold War era.” Bem said.

Instead of choosing a propellant like methane or kerosene, NASA chose to use a mixture of liquid hydrogen and liquid oxygen to power its heavy rocket. By comparison, SpaceX’s upcoming spacecraft uses liquid methane, with liquid oxygen as an oxidizing agent. “With their sights set on Mars, SpaceX has chosen liquid methane in the hope that it will be able to extract this element [when] on Mars as a form of cost-effective resource use,” Bem explained. The US space agency, perpetually cash-strapped and forced to please politicians, was operating on a different set of principles when designing the SLS.

Based on current information and analysis, [proposed SLS design] represent the lowest near-term costs, the closest available, and the lowest overall risk path for developing the next domestic heavy lift launch vehicle,” NASA wrote in 2011. Initial project report. “The choice of this SLS architecture means that in the near term a new liquid engine will not need to be developed, thus shortening the first flight time as well as potentially reducing the overall cost of the SLS.”

The irony is that the SLS, which was supposed to fly in 2017, has not yet been launched, and the total development costs, including the crew of Orion capsule, have It has now exceeded 50 billion dollars. This excludes the estimated cost of $4.1 billion per SLS launch. By inheriting the components of the Space Shuttle, NASA also inherited the hydrogen problem.

Useful but annoying molecule

Hydrogen is very useful as a fuel for rockets. It is easily available, clean and lightweight, and when combined with liquid oxygen, it burns very badly. “In combination with an oxidizing agent such as liquid oxygen, liquid hydrogen produces the highest specific boost, or efficiency in relation to the amount of fuel consumed, of any known rocket propellant,” according to to NASA. When hydrogen is cooled to -423 degrees Fahrenheit (-253 degrees Celsius), the hydrogen can be crammed into a rocket, saving a massive amount of fuel for the buck. “The advantages of liquid hydrogen as a fuel lie in its efficiency in storing the energy you want to release to propel the rocket, as well as its light weight, which is always a consideration in spaceflight,” Beam said.

SLS on the podium at the Kennedy Space Center.

SLS on the podium at the Kennedy Space Center.
picture: 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 in second stages (Europe’s heavy-lift Ariane 5 rocket is a good example), and as a liquid fuel needed to maneuver spacecraft into orbit. Rockets that use liquid hydrogen currently include the Centaur Atlas and Boeing’s Delta III and IV, while the Blue Origin BE-3 and BE-7 engines are also hydrogen-based.

“One disadvantage of hydrogen is 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 very low temperatures,” Beam said. Furthermore, hydrogen is highly volatile when it is in a liquid state, It can burn in large quantities. alightest known elementIt’s also very leaky. NASA explain The many challenges of using liquid hydrogen as a fuel:

To prevent it from evaporating or boiling, liquid hydrogen rockets must be carefully isolated from all heat sources, such as rocket engine exhaust and air friction as they fly through the atmosphere. Once the car reaches space, it must be protected from the sun’s radiant heat. When liquid hydrogen absorbs heat, it expands rapidly; Thus, venting is necessary to prevent the tank from exploding. Metals exposed to the extreme cold of liquid hydrogen become brittle. Moreover, liquid hydrogen can seep through the micropores in welded welds.

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

New missile, same old problems

When the SLS is stored, a sudden influx of cooled hydrogen causes significant changes to the rocket’s physical structure. A hydrogen tank that is 130 feet high (40 m long) shrinks about 6 inches (152 mm) in diameter and about 1 inch (25.4 mm) in diameter when filled with the ultra-cold liquid, according to to NASA. Components attached to the tank, such as ducts, ventilation lines, and brackets, must compensate for this sudden shrinkage. To achieve this, NASA uses accordion-like bellows connectors, slotted joints, telescopic sections, and ball joints.

But hydrogen – the smallest molecule in the universe – often finds its way through even the smallest openings. Fuel lines are particularly problematic, as they cannot be firmly attached to the rocket. as their The name indicates that the rapid separations, while providing an airtight seal, are designed to break free from the missile during launch. This seal should prevent leakage under high pressures and extremely cold temperatures, but it also needs to be left in while the missile is in flight. On Saturday, a leak in the vicinity of the rapid detachment reached concentrations exceeding the 4% limit, exceeding NASA’s flammability limits. Unable to solve the leak, NASA called a scrubber.

NASA’s failure to fully refuel Phase 1 and Phase 2 and going deeper into the countdown is real cause for concern. The space agency has dealt with hydrogen leaks before, so we hope its engineers will once again devise a solution to push the project forward.

However, it is a frustrating start to the era of Artemis. NASA needs the SLS as it seeks a permanent and sustainable return to the lunar environment, and it looks to the human of the future mission to Mars. NASA will have to make the SLS work, and you may have to do it one at a time.

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