Hydrogen is responsible for the postponement of Artemis I: why does NASA still cling to it?

Hydrogen rocket engines are victims of repeated leaks. But most launchers continue to use them, including for the Artemis I mission, which has already been postponed twice. Why so much attachment to this technology which seems so random?

If the first mission of the Artemis program is still grounded, it is because of a fuel leak: liquid hydrogen escaped from a seal and led to the postponement of the launch. Even if this news disappointed the many people who were impatiently awaiting this lunar mission, the postponement of Artemis I was not surprising for specialists in the sector, accustomed to this type of breakdown.

Let’s go back in time: on December 2, 1990, the space shuttle Columbia took off on its tenth mission with seven astronauts on board. A huge relief, since the launch was scheduled for May 16 of the same year! Before being pushed back to the 30th, then in September after a return to the workshop, and finally at the beginning of December after another unsuccessful passage on the launch pad. In short, cascading postponements due to a hydrogen leak that seemed impossible to repair.

Hydrogen, a difficult fuel to master

Since then, almost nothing has changed. This type of fuel always seems to pose so many problems for NASA. ” Hydrogen has many qualitiessummarizes for Numerama Nathalie Girard, project manager for advanced launcher concepts at Cnes, but it is also a very difficult fuel to handle. »

Launch of the Challenger space shuttle. // Source: Flickr/CC/Nasa Johnson (cropped photo)

If this technology is so used by most active launchers, it is because the performance is much better: up to 20% higher than that of kerosene, for example. From the beginning of space conquest, industrialists understood that to fly heavier rockets, higher, they needed a powerful fuel capable of projecting all this mass. The first Apollo missions already used hydrogen, but the Russian Soyuz launchers run on kerosene or derivatives.

On the other hand, if hydrogen is so efficient, it comes with a few constraints that are sometimes difficult to solve.

  • The first corresponds to temperature that must be reached to make it liquid. Usually in a gaseous state, it must be brought down to −252 degrees Celsius to make it liquid, which is not so simple. At such temperatures, the surrounding materials tend to shrink and become brittle. It is therefore necessary to have a carcass adapted to this extreme cold, and if the joints start to move, even if only a little bit, it is a sure leak! Which brings us to the second constraint: size.
  • Hydrogen is the very first element in the periodic table, so it is the lightest. A hydrogen atom is particularly tiny, but also very inviscid, with the consequence of flowing very easily. Under these conditions, even the smallest of openings will be fatal and all the hydrogen will rush in.
  • Finally, a last constraint and not the least: it is extremely flammable. Beyond a hydrogen concentration of 4% in the atmosphere, the slightest spark can cause a disaster. ” The small advantage says Nathalie Girard, is that hydrogen is also very volatile and disperses rapidly in the atmosphere. But it is no less dangerous. »

There is no revolutionary solution

On the other hand, kerosene is easier to handle because it is liquid at room temperature. However, it is less efficient, which prompts choices to be made when building launchers. However, another possibility exists: methane. ” It’s been in the works for decadessays Nathalie Girard, but for the moment no methane engine is used. »

Some are trying it, in Europe with the Prometheus engine project, but also the SpaceX Raptors which are to equip the Starship when it is finished. Blue Origin also tried its luck with its BE-4 engines. All these devices are in more or less advanced stages of development, but are not completely finished.

SN11 Starship engine cut
A controlled flameout of a Raptor engine. // Source: SpaceX

Methane would have the advantage of being a little more efficient than kerosene (but less than hydrogen), while being more manageable with a more pronounced viscosity, and a liquefaction temperature of “only” −180 degrees. ” Methane would not be revolutionaryshade Nathalie Girard, as with every new fuel, there is a pro/cons balance. »

So, given the recurring leak concerns for hydrogen, could NASA have made the effort to switch to methane for such an important mission? Not that easy. Developing a new launcher is very expensive, especially for a project as pharaonic as the SLS (Space Launch System). When the American Congress had accepted the financing of the rocket, NASA was asked to rely as much as possible on the already existing technology, namely that of the shuttles which had proven itself, despite the few breakdowns. Nasa Deputy Administrator Lori Garver opposed it at the time, calling for the development of all-new engines to move to the next generation of launchers, but without success.

To have a methane engine, we would have to imagine the technology capable of cooling it properly, the materials of the tank that will support it, the technique to ignite everything during takeoff, etc.

All of this requires costs in terms of research which are not an immediate priority, knowing that hydrogen remains an almost certain value. NASA would therefore have the choice between keeping a technology that works globally, despite some flaws, or risking embarking on the creation of new, completely different launchers, without guarantee that these will be ideal.

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Astronauts in front of the SLS.  // Source: Flickr/CC/NASA/Kim Shiflett (cropped photo)

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