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

Liquid hydrogen and liquid oxygen are used in NASA's space launch system. The same attributes that make this fuel a liability are what make it compact and powerful.

The second launch attempt of SLS had to be called off on Saturday, September 3, after engineers failed to resolve a hydrogen leak in a quickDisconnect, an 8 inch inlet that connects the liquid hydrogen fuel line to the rocket's core stage. SLS probably won't launch until October at the earliest because of the setbacks. The Artemis 1 mission, which will send an uncrewed spaceship to the moon, 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 eventually called off after a faulty sensor mistakenly indicated that an engine hadn't reached the required ultra- cold temperature. Engineers tried to fix the leak on Saturday but none of them worked. The Artemis mission manager told reporters that the leak was not manageable.

The rocket needs to return to the Vehicle Assembly Building to have a safety check done on it. The rocket may need some hardware fixes because of an accidental command. Engineers are trying to figure out if the over-pressurization contributed to the leak.

Inheriting the hydrogen problem

NASA has been leaking hydrogen for a long time. Hydrogen leaks were the cause of Scrubs of Space Shuttle Launches. Ground teams spent more than six months searching for a hydrogen leak that grounded the Shuttle fleet in 1990. It's possible that hydrogen-related scrubs could have been predicted because SLS is a lot like the Space Shuttle. NASA doesn't have a choice but to manage this limitation of its mega Moon rocket.

Jordan Bimm is a space historian at the University of Chicago.

Bimm told me that NASA has used contractors around the U.S. to maintain political support and funding for space exploration. The first liquid hydrogen rocket was developed in the 1960's. NASA was mandated by the U.S. Congress to use technologies from the Shuttle in their next- generation launch system. He said that it was a political decision meant to keep contractor jobs in key political districts.

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

The Space Shuttle's engine, along with its reliance on liquid hydrogen and liquid oxygen, would have to be carried over to SLS. Four of the 16 engines collected by NASA are currently affixed to the SLS rocket at the Kennedy Space Center in Florida.

Bimm said that this situation reminded him of the 1983 film The Right Stuff, in which the character Buck Rogers said no bucks, no Buck Rogers. He said that NASA must often prioritize political support from Congress. Bimm said that the ongoing use ofRS-25 engines is an example of how mundane fuel choice can be political and how often the most straightforward and desirable solutions are not politically viable for a large national agency.

NASA used a mixture of liquid hydrogen and liquid oxygen to power its rocket. Liquid methane and liquid oxygen are used as oxidizers in the upcoming Starship by the company. Bimm said that the goal of the project was to be able to extract liquid methane on Mars as a form of cost-saving resource utilization. When designing SLS, the U.S. space agency worked under a different set of principles.

The proposed SLS design is the lowest near-term costs, soonest available, and the least overall risk path to the development of the next heavy lift launch vehicle, according to a preliminary project report. The SLS architecture would mean that a new liquid engine in the near term would not need to be developed, thus shortening the time to first flight as well as likely minimizing the overall cost of the SLS.

It's ironic that SLS, which was supposed to fly in 2017, has yet to launch, and its total development costs, including the crew capsule, have now exceeded $50 billion. The cost for each launch of SLS is estimated. The hydrogen problem has been inherited by NASA.

A beneficial but pesky molecule

As a rocket fuel, hydrogen is very useful. When combined with liquid oxygen, it burns with a high intensity. NASA says that liquid hydrogen yields the highest specific impulse or efficiency in relation to the amount of propellant consumed. It is possible to cram hydrogen into a rocket when it is chilled to 423 degrees. Bimm said that the advantages of liquid hydrogen as a fuel is its efficiency at storing the energy you want to release to propel the rocket, as well as its low weight.

SLS on the pad at Kennedy Space Center.

Liquid hydrogen was used in the shuttle's three main engines. The Ariane 5 rocket is a good example of how hydrogen can be used for the second stage. Atlas, Boeing, and Blue Origin all use liquid hydrogen in their rockets.

Bimm said that the disadvantages of hydrogen are that it is difficult to move around and control due to the small molecule size of hydrogen which leads to leaks and the need to keep it in a liquid state. Hydrogen can burn in large quantities when it is in a liquid state. It's the lightest known element. NASA explains how hydrogen can be used as fuel.

To keep it from evaporating or boiling off, rockets fuelled with liquid hydrogen must be carefully insulated from all sources of heat, such as rocket engine exhaust and air friction during flight through 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, venting is necessary to prevent the tank from exploding. Metals exposed to the extreme cold of liquid hydrogen become brittle. Moreover, liquid hydrogen can leak through minute pores in welded seams.

When designing SLS, NASA chose liquid hydrogen despite some challenges.

New rocket, same old problems

Changes to the rocket's structure are caused by the sudden influx of hydrogen. When filled with ultra-cold liquid, the hydrogen tank shrinks by about 6 inches in length and by about 2 inches in diameter. The components attached to the tank need to compensate for the sudden contraction. NASA uses accordion-like bellows and other components to achieve this.

Even the smallest openings in the universe are home to hydrogen. The fuel lines can't be hard-bolted to the rocket. The quickDisconnects are meant to break free from the rocket during launch. As the rocket takes flight, the seal needs to let go so that it doesn't leak under high pressures. The leak in the vicinity of the quickDisconnect exceeded NASA's flammability limit on Saturday. NASA was unable to resolve the leak.

It's a cause for concern that NASA hasn't fully fueled the first and second stages. Hopefully the space agency's engineers will come up with a solution to deal with hydrogen leaks again.

It is a frustrating beginning to the Artemis era. NASA needs SLS as it seeks a sustainable return to the lunar environment as well as a future human mission to Mars. NASA is going to have to make SLS work, and it might have to do a lot of scrubbing at once.