Why rocket launches rarely succeed on the first try

The countdown is on, only a few minutes left until the start; but then Elon Musk grabs his cell phone and tweets. “A pressure valve appears to have frozen,” he writes on Monday. “So no launch today if it doesn’t go into operation soon.” His giant rocket “Starship”, with which astronauts are to fly to the moon and Mars in the future, is meanwhile on the launch pad in Texas, shrouded in white haze. Your first test flight is on the brink. A few minutes later it’s official: she won’t be taking off that day. The valve is still frozen. Musk and his SpaceX team have no choice but to postpone the launch by a few days. The next attempt is to follow this Thursday.

More and more often, it seems, rockets have to stay on the ground. Starship is just one example. The European Space Agency ESA postponed the flight of the Ariane 5 rocket with the Juice spacecraft on board by one day last week. The US space agency NASA had to postpone the Artemis 1 mission, which was supposed to initiate the return of humans to the moon, several times, before it could start in November last year. And the newly developed H3 carrier rocket is also causing problems for the Japanese space agency Jaxa: At the beginning of March, the second attempt to launch failed, during which the rocket ultimately destroyed itself.

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“It’s rocket science,” says Volker Schmid in a nutshell. What the graduate engineer from the German Aerospace Center means by this is: Bringing a rocket into space is not child’s play, but rather complicated. “Everything has to be spot on so that there is no room for error.” Such a development is “the spearhead of technology”. But it is precisely this technology that can become a dreaded problem for aerospace engineers.

Rockets are becoming more and more technically complex

The more space history advances, the more technology is in the rockets. The missiles are becoming larger, more digital, have more engines or require more fuel. Elon Musk’s Starship is the symbol of progress: it is the world’s most powerful rocket ever built. It is 120 meters tall, can carry loads of 100 to 150 tons and has a fuel capacity of almost 5000 tons.

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“That’s a house number,” says Schmid. “To test such a large rocket, nothing can be left to chance.” That’s why it was right that SpaceX aborted the launch at short notice. After all, rocket launches are not just about ensuring safety (which is a priority for missions with people), but also about investment. The frozen pressure valve was a clear no-go for the launch and, in the worst case, could have caused the rocket to explode. “That would destroy millions of dollars,” the aerospace engineer knows.

It is not known which pressure valve it is. Martin Tajmar, Director of the Institute for Aerospace Technology at the Technical University of Dresden, points out that so far there has only been Musk’s tweet naming the problem. “It was the first test of the rocket, which should show whether everything really works as planned,” he explains. “So it’s not surprising when a valve freezes.”

The problem of refueling

But why was the valve frozen at all?

This is where one of the main sources of interference in rocket launches comes into play: refuelling. In order for rockets to take off from Earth, they need propellants that release large amounts of energy. The Artemis missions’ SLS rocket uses liquid oxygen and liquid hydrogen, which react with each other. However, liquid hydrogen is very delicate, requires extensive cooling and is also prone to leaks. Musk’s Starship rocket uses liquid methane instead. It is cheaper, less prone to leakage, burns with almost no residue, can also be produced on Mars and does not have to be cooled down quite as much as hydrogen. Nevertheless, it is a cryogenic fuel that can freeze parts of the rocket, such as valves.

Before the fuel can be loaded into the rocket, however, another challenge awaits. Before they are launched, the missiles have usually been on the launch pad for a long time, during which time humidity collects in the line system. The fuel lines to the engines must therefore first be “flushed” with nitrogen to displace the humidity. This is because the water vapor can impair combustion.

In order to then pump the fuels into the combustion chambers, the appropriate pressure is required. The volume and mass flows of the fuels would have to be handled by turbopumps that run flawlessly secured, says Schmid. Otherwise there is a risk of a false start. The same happens when the thrusters are not working in sync. Vibrations then arise which can be so strong that the structure fails. The first stage of the XXL rocket Starship, called Super Heavy, has 33 engines alone that have to work together. “If an engine falters, for example due to unstable combustion, this can have fatal consequences and lead to a crash.”

Why the weather has to play along too

Rockets are highly complex systems – and comparable to clockwork. If just one part doesn’t work properly, it can mess up the whole system. “With new rockets, you have to relearn the operation, the special features, the character of the rocket, so to speak, the handling around it,” says Schmid. Therefore, it usually takes two to four test flights before the engineers and operators are familiar with the system and can optimize it. “So it can happen that the first flight does not succeed in all respects right away, despite the best preparation.”

A failed attempt does not always have to be due to the technology. The weather has to be right too.

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Thunderstorms with lightning, wind at high altitudes – these are bad conditions for a rocket launch. Although the launch site and the Lauch Tower have lightning rods, the problem is more the static charge in the air. “Today’s new rockets have digital cockpits and a lot more electronics and avionics than before,” says Schmid. “If lightning suddenly causes more overvoltage than permitted, that can become a problem.” Short circuits could occur, important on-board systems could fail or be damaged. And shear winds – i.e. air currents that suddenly change in direction and strength in a small area – can also affect the flying metal colossuses. They can significantly affect trajectory and control, which is why allowable limits have been set that should not be exceeded.

There’s no use using the crowbar now.

Martin Tajmar, Director of the Institute for Aerospace Technology at the TU Dresden

Every failed attempt is a learning opportunity

“Safety always comes first,” says Schmid. In the end, no start is without risk. But the decision for or against going into space should not be a gut decision. “It has to be objectively based on valid data.” If something speaks against the start – be it poor technology or bad weather conditions – it just has to be canceled and postponed. “You can learn from that, too,” says the aerospace engineer.

After the unsuccessful maiden flight of his Starship rocket, Elon Musk also stated: “Learned a lot today.” However, it cannot yet be ruled out that the second attempt at launch will also fail. “There’s no use using the crowbar now,” says space systems expert Tajmar. The rocket should not launch until it is ready to launch. Unlike the SLS rocket and its test flight to the moon, there is no time pressure when launching, and there are no multi-billion dollar subsidy programs in the rocket. “That’s a completely different design approach,” says Tajmar, namely: learning by doing. “And if Starship launch succeeds, it will catapult us into a whole new rocket age.”

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