Artemis 1 will be the first comprehensive test of NASA’s deep space exploration systems: the Orion spacecraft, the Space Launch System (SLS) rocket, and ground systems at the Kennedy Space Center. NASA Space is important to us, and that’s why we work to bring you top-notch coverage of the industry and launches in Florida. Such journalism takes time and resources. Support it with a subscription here. — As NASA pushes toward a third launch attempt of the Artemis I mission later this month, the agency’s use of a popular rocket propellant — supercooled liquid hydrogen — has become the focus of attention because of its delicate nature. Now set to take off no earlier than 6:47am. EDT Friday, Sept. 23, the ground support equipment of the 322-foot-long Space Launch System rocket is under repair at the Kennedy Space Center. Two previous attempts to launch an unfilled Orion capsule to the moon had been scrubbed due to hydrogen-related cooling and power problems. Teams at pad 39B are currently investigating what caused a severe hydrogen leak during the fueling process on September 3. The massive rocket’s liquid oxygen tank reached 100% full, but hydrogen only hit 11% during the countdown, forcing teams to clean up and investigate the problem contained in a quick disconnect, or QD. Frequent problems with hydrogen, many of which trace back to the space shuttle program, are common. That it requires more refrigeration – liquid hydrogen must be stored at minus 423 degrees Fahrenheit – than other propellants ultimately means brittleness or weakening of components such as metal storage tanks tends to be the main factor in hardware problems. It must also be pumped at high pressures, easily exposing even the smallest leaks. The upside, though, is that hydrogen provides more performance than other rocket fuels. More: NASA retargets late September for Artemis I launch as teams work with hardware NASA’s massive Artemis launch: It will be loud, but how loud? It depends “There’s certainly no question that hydrogen is a challenging molecule, but it’s worth it,” John Blevins, NASA’s SLS chief engineer, said during a post-scrub briefing this week. “Hydrogen is the most efficient molecule, and if you look at the mission we’re doing, it calls for using that fuel.” “It wants that sustained, high performance to get out of what is really the highest performance rocket engine on the planet: that’s the (four RS-25 main engines),” Blevins said. Artemis I is part of NASA’s overall program to return astronauts to the Moon. If all goes well with this uncrewed test flight, astronauts are expected to fly a similar out-and-back mission known as Artemis II sometime after 2024. NASA hopes to put two humans on the surface before 2030 and then establish a permanent presence before moving to Mars. FLORIDA TODAY spoke with Jim Brenner, an associate professor of chemical engineering at Florida Tech with extensive experience in hydrogen, about rocket propellants and how they compare. Note: This Q&A has been edited for length and clarity. FLORIDA TODAY: Kerosene, methane and hydrogen are some of the most popular rocket propellants today. Can you give a brief overview on how to rank? Brenner: Kerosene has a lower density – that’s probably the best way to put it – than methane, and hydrogen has the highest of the three. But as you go from kerosene to methane to hydrogen, the boiling point will drop significantly. This means that to keep it as a liquid, and you have to do it to minimize the amount of space taken up in the rocket, you have to go much, much colder. And the colder you go, the more likely the container you’re putting it in is brittle.

Differences in temperature

FT: At that point, you’re probably experiencing temperature fluctuations as well, right? Like the difference in temperatures between these cryogenic propellants, the stuff that’s been heated by the hot Florida air, and so on? Brenner: That’s a factor. Certainly temperature variations both within the day and between when a tank is full versus when it is not have their effects. Going back to the first two space shuttle disasters, both were caused by problems related to thermal expansion. For the second space shuttle disaster (Columbia in 2003), the conclusion from one of the dissertations I was part of here was that the polymer foam on the outside of the space shuttle should have been replaced … and ultimately caused the second disaster. This is probably the best example that the general public could remember of what I call cryogenic fragility. When you get to a low enough temperature, even metal will become brittle. Hydrogen is also different from some materials in that it will embrittle the metal independent of temperature (a well-documented phenomenon caused by metals absorbing hydrogen). So you can have embrittlement problems at any temperature with hydrogen, but they are obviously much worse at lower temperatures.

The yield of hydrogen

FT: When it comes to Artemis I and the Space Launch System, NASA officials say hydrogen is required for performance. So, of the three popular propellants mentioned, does hydrogen have the highest efficiency once put to use? Brenner: That’s right. And that’s why hydrogen has always been the preferred fuel for it. If you don’t care about reuse (like SLS since it will be spent after launch) then it’s probably the best choice. But if you are going to reuse things, then you need to make sure that after each filling and emptying that the cryogenic embrittlement and cyclic fatigue associated with repeated use is not going to cause a cumulative effect that makes problems similar to those experienced with Artemis. There’s only so many times you can cool things down that low before you end up running into problems. While Artemis has had such problems, it is far from the first time the space enterprise has had this problem. They’ve had problems with liquid hydrogen and liquid oxygen for a long time.

The popularity of kerosene in reuse

FT: So looking at something like SpaceX’s Falcon 9 rocket, is the difference between kerosene and hydrogen that big that SpaceX can fly over and over again without so much trouble? Brenner: That’s definitely the working case. I couldn’t tell you how many cycles it might last, but that’s why they’re going this route. Because it’s not as cold, you won’t have as many problems as you would with liquid hydrogen. Their case is logical.

Hydrogen as the most abundant element

More: Artemis I onlookers flock to Space Coast but launch attempt foiled FT: When we discuss hydrogen as a fuel, you will certainly hear about how it must be a logical choice, as it is the most abundant element in the universe – and how it could one day be produced on the moon before missions to Mars. Is there any practical truth to this? Brenner: You can’t mine hydrogen on the moon. If we’re going to have a spacecraft return from either the Moon or Mars, we’re either going to have to send a vehicle with the fuel to go back – economically, that’s a loser – or we’re going to have to mine the resources necessary to get us there back a propellant. There are many people looking at mining aluminum from the moon or Mars, turning it into nanoparticles and using it as a propellant. This is honestly quite dangerous because you are literally trying to store an explosive. The hydrogen is also not in a high enough concentration to be able to do practically much with it. Yes, it is the most abundant thing in the universe, but it is not in a form that you can easily use. Water here on Earth is easy enough to use because it’s in liquid form, but trying to concentrate something that’s in the gas phase so that it’s easy to store makes a big difference. That’s why kerosene, or gasoline or diesel for that matter, is better for ground vehicles than natural gas or hydrogen. Yes, you can operate natural gas or hydrogen vehicles here on Earth too. But when you do that, you have to either cool them to a very low temperature or press them to a very high pressure – and often both. This is not as easy to do as it sounds on paper.

SpaceX uses methane for Starship

Artemis | Five facts about the Space Launch System rocket Five facts you need to know about the Space Launch System rocket. Rob Landers, Florida Today FT: How does SpaceX’s Starship system – also massive in size and deployed for deep space – and the use of methane fit into that? Brenner: Methane is between kerosene and hydrogen. It’s not as dense an energy source, but you don’t need to cool it as much as hydrogen, so you won’t have as many issues with reuse as you do with hydrogen.

Missing material data

FT: You mentioned that one of the reasons we see these issues is a lack of data and experience. Can you expand on that? Brenner: There really isn’t that much data in the public domain about the reliability of the materials under the conditions that we abuse our materials in East Central Florida. No place in the world abuses our materials like we do here. The rocket business is a very unforgiving environment. We’re taking materials up to very high temperatures and very low temperatures, and there’s just not a lot of material reliability data under the conditions we’re talking about. Until someone develops a database that is public about the reliability of materials under such conditions, you will have problems like what Artemis is facing. You can’t look for this kind of thing on the Internet – it just doesn’t exist. We cannot afford to have another space disaster. If anyone has a problem with material reliability and was…