Why the rocket fuel that will power Artemis II is so hard to handle

The Artemis II mission, which was initially supposed to launch on Feb. 8, has now been delayed by at least a month after NASA found a hydrogen leak while fuelling the tank. 

The first Artemis mission also faced several delays because of fuel leaks.

It is difficult to imagine a substance that is hundreds of degrees below freezing yet still a liquid. Such is the strange nature of the liquid hydrogen and liquid oxygen that are the components of rocket fuel, and why they tend to leak so easily.

All liquids have a boiling point, where they change from a liquid into a gas. For water, it’s 100 C. 

Oxygen, which is naturally a gas at room temperature, has an extremely low boiling point of -183 C. If you can keep it below that temperature, it will stay in a liquid form that looks like pale blue water.

Liquid hydrogen is even colder, with a boiling point of -253 C, which is just above absolute zero, the coldest anything can get. Because of these extremely frigid temperatures, these liquids will instantly freeze anything they touch.

The equipment that handles these super cold liquids must be able to function without freezing or cracking from the huge temperature difference between the fuels and the Florida air, where the rockets are launched. 

Keeping these fuels in liquid form is the equivalent of trying to keep a glass of water from boiling inside an oven set to 200 C. You would have to put the water in an insulated container to protect it from the high temperature and keep it from boiling. For these cryogenic liquids, our air is an oven that is constantly trying to boil them back into vapour form.

That is why the core stage of the gigantic Space Launch System (SLS) rocket is orange. It’s covered in insulating foam which was sprayed onto the entire exterior to keep the cold fuel inside protected against the warm air. They also have to be protected against the warm metal of the rocket itself, which is why the fuel lines and tanks must be pre-chilled before the fuel is poured in. 

All these efforts are only partially successful. The boiling cryogenic fuel, like steam, creates pressure that builds up. You’ll notice plumes of white vapour coming off the side of a fully fueled rocket, which is the vapour being allowed to escape.

The combination of ultra-cold temperatures and vapour pressure naturally leads to leaks. Hydrogen in particular is very good at escaping, because it’s the smallest molecule that exists. It can squeeze between even the smallest cracks. So every pipeline, every joint, every valve, is a source of leakage. And there are many potential sources of leaks on the SLS rocket.

The fuels are stored in tanks close to the launch pad, then piped up through the tall launch tower beside the rocket, through hoses to the rocket itself, then through the plumbing leading to the fuel tanks and eventually to the engines.

The leak that appeared during the recent wet dress rehearsal was from the connector that attaches the hose on the launch tower to the side of the rocket. This is a quick disconnect part that you can see letting go right at the moment of launch, as the rocket begins to rise off the pad.

This connector also leaked on the Artemis I flight and the rocket had to be rolled back to the Vehicle Assembly Building (VAB) several times for repairs, causing delays lasting months. 

Engineers think they can fix this leak at the launch pad instead of trucking the rocket back to the VAB, saving time and resources. Another wet dress rehearsal will be required before finally committing to a launch, now planned for no earlier than March 6.

Hydrogen leaks have been an issue through the entire space program, as NASA continues to use it as a fuel. Private companies such as SpaceX, as well as the Russian space program, have abandoned liquid hydrogen in favour of kerosene, which isn’t cold and much easier to handle. 

SpaceX’s new Starship, which is intended to land on the moon and possibly Mars, avoids the hydrogen issue by using methane fuel.

The giant SLS rocket’s use of the more difficult liquid hydrogen and oxygen fuel is based on technology that dates back to the space shuttles that were developed in the 1970s.

The fuel provides the most thrust for its weight, which is useful for heavy lift vehicles, but it also ensures that contractors from many different states that have built NASA technology in the past will have jobs. 

But critics of the rocket say it’s unsustainable because it’s very expensive to fly and completely disposable. In other words, the entire launch system is thrown away on every flight. Only the tiny crew capsule returns to Earth, which is contrary to the far less expensive re-usable modern rockets flown by private companies that operate at a fraction of the cost.

The future of the SLS rocket depends on improving the technology to fix the hydrogen leaks but also balancing the cost of flying the rocket to the benefit of saving the jobs of those who build it.