Why has the release of Artemis been cancelled? An engine that did not cool and two more opportunities | Science

In any space launch, delays and postponements are frequent. The flight director, with a $2 billion-per-attempt rocket on the pad, can’t risk giving the go-ahead if all the parameters don’t add up.

In the case of the Space Launch System (SLS), the problem that forced the countdown to stop 40 minutes before “zero” referred to one of the four main rockets: the sensors detected problems in the flow of hydrogen that was to cool the bells of the engines before ignition. This is an essential operation that takes advantage of the cold of the cryogenic liquid to protect the metal from the high temperatures of the exhaust. The engineers asked for 10 minutes to check what was happening, but the countdown did not start again.

The rocket carries 775 sensors that try to measure every last detail of the flight. Most, in the tail section, connected to each other by almost thirty kilometers of cable. Its measurements are coordinated through the flight computer, installed inside the rocket itself. It’s not a very modern processor: a derivative of the G3 that powered Macs more than twenty years ago. Or, if another comparison is preferred, it is between a Pentium II and a Pentium III.

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In addition, cracks have been detected in the insulation lining the section between the hydrogen and oxygen reservoirs, although this was not a sufficient reason for the postponement. The reduction of hydrogen flow in an engine is.

Half a million dollars

When handling cryogenic fuels, the problem of metal expansion and contraction is formidable. When filled with liquid at 250 degrees below zero, the hydrogen reservoir shrinks 15 centimeters in length. All the fluid lines and cables that run alongside it terminate in accordion joints so they can adapt to the change in size. Also the big pipe that carries oxygen from the upper tank to the engines. Except that it runs along the outside of the rocket so it won’t be affected by the low temperatures of hydrogen. If it passed through the interior, the oxygen (which is only 180ºC below zero) would freeze into a solid block.

The postponement will not come cheap to NASA. Emptying and refilling the tanks represents a cost of around half a million dollars (the same amount in euros), plus the same amount in personnel expenses. Much of the fluid is recovered, but it is impossible to avoid losses due to heating caused by the drain pumps. Plus the one that is simply lost by evaporation so that the pressure inside the tank does not increase to alarming levels. They are the white clouds seen rising from his sides while he is on the platform.

The orbit chosen for Artemis I poses many restrictions on the timing of launch. The next opportunity will be on Friday, September 2.

A heavy tradition

Launch delays have been a tradition since the early days of astronautics. Sometimes for technical reasons, usually – like this time – associated with hydraulic systems. Some ships like the now-retired shuttle used three computers to monitor the status of onboard systems. If a failure was detected, the three machines “voted” to decide whether or not to cancel the launch, unless it was a truly catastrophic failure.

The second most common cause of cancellation is weather conditions. It doesn’t just have to be good weather on land. The proximity of electrical storms (Apollo 12 was struck by lightning during takeoff) and strong winds aloft are also unacceptable, not because they threaten to deflect the trajectory, but because they can induce dangerous stresses in the rocket body. This Monday, during a moment of the countdown, the weather conditions would not have allowed the launch either.

That is what happened many years ago, in 1962, at the launch of John Glenn, destined to become the first American to orbit the Earth. Glenn entered and exited the capsule (and this was a process that required hours) on 10 occasions, twice due to fuel leaks and the rest due to worsening weather.

There are few times that a launch is aborted when the rocket is already in the air. In 2018, the Soyuz M-10 carrier rocket failed at low altitude and escape rockets had to blast off the capsule with the two astronauts on board. That probably saved their lives, even at the cost of a good thrashing during the fall.

More shocking was the case of Gemini 6, in December 1965. The rocket’s engines turned on and off in a matter of seconds, when it had not yet risen. The two astronauts saw that the timer was running, but they didn’t feel the push, so they resisted the urge to eject from the capsule.

It was lucky. The Gemini did not use an escape tower, but expelled seats like those on airplanes, based on the strange theory that even if the rocket exploded, the fireball would not be very large and the astronauts could fall far from it. What no one thought is that their diving suits were saturated with oxygen and if they jumped, the flames would probably have set them on fire, with their occupants inside.

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