NASA's delayed Artemis rocket launch to Moon shows perils of liquid hydrogen fuel

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Plans to launch NASA's mega Moon rocket have slipped to late September — at the earliest — to allow more time for testing.

The second attempt to lift off was scrubbed at the beginning of this month due to a large fuel leak from the Space Launch System (SLS) rocket.

Artemis 1 crews have since repaired the leak, but the rocket still needs to undergo final tests next week to see how the new parts perform when the rocket is fuelled up.

If everything goes to plan, the rocket could take off on Wednesday, September 28 at 1:37am AEST (although NASA is also looking at Monday October 3, Australian time).

If it misses that window, then the next opportunity to fly won't be until mid-October.

"We'll go when it's ready," NASA's chief administrator Bill Nelson said after the last attempt.

"This is part of a space program. Be ready for the scrubs."

Senator Nelson endured four scrubs before flying onboard Space Shuttle Columbia in 1986.

Hydrogen was the shuttle era's fuel of choice, and back then, leaks were common. In 1990, NASA spent six months chasing down leaks in what was known as the "summer of hydrogen".

NASA's giant SLS rocket uses the same type of fuel and engines flown by the shuttle program.

So why are liquid hydrogen rockets so finicky, and how do they stack up with other technologies?

On the plus side, hydrogen is very efficient as a fuel, said Haydn Scott-Kilsby, a propulsion engineer at Gilmore Space Technology, which plans to launch a rocket from Australia next year.

"Hydrogen and liquid oxygen produces the best performance of any conventional fuel we know today," Mr Scott-Kilsby said.

"You get the most thrust for the mass you have."

The SLS is the most powerful rocket ever built, capable of sending more than 27 tonnes into orbit around the Moon.

But there are also some big drawbacks to using hydrogen as fuel.

It's very light, even when it is cooled to -258 degrees Celsius and in liquid form.

For instance, one litre of liquid hydrogen weighs only a bit over 70 grams.

That means you need bigger tanks to hold it.

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The 65-metre-high orange rocket core holds two tanks: one for hydrogen, the other for oxygen.

"Look how big SLS is; that core is absolutely massive," Mr Scott-Kilsby said.

"The hydrogen tank is twice the size of the oxygen tank. But there's twice as much oxygen in the tank by weight."

And the bigger the tank, the more weight at lift off.

But by far the most challenging problem is working in cryogenic conditions.

Hydrogen can slip through the smallest of cracks, especially when it is chilled under pressure, and ultra-low temperatures can make materials — even metal — brittle.

"If you hit steel at that temperature, it will just shatter," Mr Scott-Kilsby said.

"It's one of the reasons that SpaceX and a lot of new companies have moved away from hydrogen, because while you get that peak performance, those additional challenges offset the benefits."

Commercial and other national space organisations use a range of fuel combinations depending upon the type of rocket and engines.

"A lot of people use liquid kerosene and liquid oxygen," Mr Scott-Kilsby said.

These include reusable rockets for low-Earth missions such as SpaceX's Falcon 9 and Falcon Heavy, as well as the deep-space work horse Atlas V, used to lift NASA's Mars missions.

The liquid kerosene and oxygen combination has been around for more than 50 years.

It is denser than hydrogen so you don't need a big tank, it's cheaper to use, stable at room temperature, and allows companies to pack more in.

But it comes with a significant disadvantage. The rocket must be fuelled after it is loaded with crew and cargo.

It also creates a film of gunge inside the engines, so they have to be cleaned before they can be reused.

Space X is also developing Starship, a mega-rocket that uses liquid methane and oxygen.

Methane gives even better performance than kerosene and is a lot cleaner (for the engine at least).

China is also considering using methane for its Long March 9 mega rocket. 

So do kero or methane have enough oomph to put a big payload into deep space orbit?

Anything is possible with the right engine and rocket design, Mr Scott-Kilsby said.

"If you make your rocket engine bigger, you can make as much thrust as you want from any particular fuel," he said.

In addition to liquid rockets, the SLS uses twin boosters, which provide 75 per cent of the lift at launch.

The boosters use solid propellants, which combine fuel and oxidiser into one tank.

"It's like a giant firework," Mr Scott-Kilsby said.

Solid fuels are simple to make and easy to store.

But they can be dangerous: "Once you light that candle, it's going. There's nothing you can do."

He said most newer space companies were developing rockets with multiple stages, smaller tanks and no boosters.

Companies are also developing hybrid engines that combine the benefits of solid fuels with the ability to flick a switch and turn off liquid-fuelled engines.

Hydrogen has been used in NASA's crew-rated missions since Saturn V ferried the Apollo astronauts to the Moon.

NASA is not alone. Russia's Yenisei heavy rocket also uses hydrogen, as does Blue Origin's New Shepard (demonstrating another one of the downsides).

In 2010, Congress mandated that NASA had to work with the same contractors it used for the shuttle program.

The revamped SLS system uses four R25 engines, three of which have been refurbished from the shuttle program. Its boosters are also a shuttle legacy.

Despite the thrifty reuse of parts, the Artemis mission is anticipated to cost $US93 billion all up.

Earlier this year, NASA's inspector general Paul Martin called out the estimated $4.1 billion cost per launch.

"Relying on such an expensive single-use, heavy-lift rocket system will, in our judgement, inhibit if not derail NASA's ability to sustain its long-term human exploration goals of the Moon and Mars," he said in his report on the program.

Unlike many large-lift rockets now in development, none of the components are reusable.

It is wonderful that the SLS has heavy capacity options, said David Flannery, who is developing technology for Moon and Mars missions at the Queensland University of Technology.

"But I have a feeling it is unlikely to be used for many missions, simply because of its cost."

"You could assemble something in orbit with smaller rockets, for example. Also if a competitor comes along with a reusable rocket, why would we use the SLS?"

Right now, there are no other rockets ready to go that are powerful enough to lift the Orion capsule — which is at least 10 times heavier at launch than the latest Mars rover — all the way to the Moon. 

"When you send a human, there's a lot more mass that needs to be sent, like life support systems," Dr Flannery said.

One future contender, SpaceX's Starship reusable super-heavy rocket, is gearing up to do its first orbital launch.

But it can't take a heavy load to the Moon in one go.

"We know that their model requires refuelling in low-Earth orbit in order to deliver crew and cargo to the Moon," NASA's Mike Sarafin said.

Starship will also develop the upper stage rocket and lander for the Artemis 3 mission.

NASA is continuing to work on the launch pad, testing the seals around fuelling pipes that are removed right at the last second before launch.

Leaks can only be detected in ambient temperatures, so NASA will do a full check of the tank when it is fully loaded next week.

The system can't be clamped together to prevent leaks 100 per cent, but the issues that occurred during the last attempt were much larger than NASA anticipated.

The other factor to consider is the flight termination system — a battery-powered bomb on board the SLS set to detonate if the rocket goes off course after launch.

The battery is certified for 20 days, but it will have run at least double that time by September 27.

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If NASA isn't given permission to fly without recharging the battery, the rocket will roll back to the vehicle assembly building, which would delay the launch even further.

As disappointing as that may sound, it's just par for the course, Mr Scott-Kilsby said.

"There are just so many individual parts that can go wrong, and when you are launching for the first time, you are going to find something until you get to that countdown and hit the engine."

Even then, as demonstrated by the early launches of Starship, things don't always go to plan.

"There are so many things that need to work in perfect harmony to get a successful launch," Mr Scott-Kilsby said.

And there's a lot riding on this mission getting off the ground.

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