Airplanes take off without difficulty, so why are rockets so sensitive to weather?

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Rockets are powerful and imposing, but also sensitive. When a launcher like the one on the Artemis I mission has to lift off, no criteria is left to chance. In particular, that of time. NASA’s second attempt to launch Artemis I to the Moon is approaching. Due to a technical problem, the space agency was forced to cancel the launch of its Space Launch System (SLS) rocket, originally scheduled for Monday, August 29, 2022. The liftoff of the Artemis I has been postponed to Saturday 3 September. Shortly after the announcement of the lift-off postponement, the date of Friday 2 September was first considered as a “possible option”. However, the risk of storm obviously decided that NASA opted for September 3, in order to launch the first mission of its Artemis program. It may seem surprising that the launch of such an imposing device as a rocket is so sensitive to the criterion of weather, while airplanes can sometimes fly in bad conditions. “An airplane is heavy, a rocket is much more sensitive,” explains Christophe Bonnal, expert in the direction of CNES launchers, to Numerama. We cannot afford to make a rocket with the same technologies as an airplane. It wouldn’t take off, because it would be too heavy. Consequently, launchers are sensitive to various weather criteria. Space agencies prefer not to engage with any of them. “We never take any chances,” confirms the expert. Not too hot, not too cold Monitoring weather conditions is essential in the first place to respect the launch parameters of a rocket. “In the case of Artemis, the temperature must be above 5°C (for now, it’s easy), but also below 35°C. Otherwise, the thrusters [le carburant, ndlr] evaporate”, explains Christophe Bonnal. An example is, unfortunately, famous in the history of the conquest of space: the disintegration of the shuttle Challenger, on January 28, 1986. The climatic conditions were too cold, which caused the accident, in which the seven members of the crew lost their lives. “The seals had frozen”, recalls the expert. This had generated the leakage of hot gas from one of the boosters, which had caused the ‘main tank explosion. » Space Shuttle Challenger Launch The crash occurred 73 seconds after launch. // Source: Flickr/CC/Nasa Johnson (cropped photo) Wind: A key criterion for launching a rocket No it’s enough to have the right temperatures: you also have to be wary of the wind, especially during the launch.” When taking off, the thrust is minimal. Also, it is the time when one is the heaviest. This is therefore the moment when the take-off acceleration is the weakest,” explains Christophe Bonnal. In other words, the take-off is relatively slow. However, “when you take off slowly, you are very sensitive to side winds. It can generate side forces, which cannot be compensated by rocket engines. In the case of Artemis, they are limited to 15 meters per second, or 50 km/h. » The wind force is not only checked near the launch pad. It is also analyzed at altitude, up to 60 km, by sounding balloons sent up before liftoff. Radiosondes are also made (sending radio waves into the atmosphere and then analyzing their return ), to infer crosswind speeds. Wind can not only influence the way the launcher moves, but it can also have a big impact if the takeoff unfortunately goes wrong. “If the launcher blows up, you have to protect the people on the ground and the facilities This protection must take into account ab only the wind If a rocket explodes, experts fear 3 phenomena, which are therefore foreseen at every take-off. Christophe Bonnal lists them for us: “The distant and high-speed projection of small metal pieces. “To do this, we establish a perimeter around the launcher, which the public cannot enter (at the Guiana Space Center, in Kourou, it is 3 km); “The fireball during an explosion can be very hot, so that you avoid creating a hot blast wave. “The safe zone around the launcher also serves to prevent this danger. “There may be a toxic cloud. It doesn’t necessarily come from the launcher itself, but from the satellites, which often have very toxic fuels, such as hydrazine. “Here, the criterion of the direction of the wind is decisive: it is necessary to avoid that the wind is in the direction of the public during the launch, so as not to send towards them this possible toxic cloud. To go further Casters are immune to lightning (but you never know) What about rain? By that standard, rockets aren’t that different from airplanes: a downpour doesn’t necessarily stop them from going (that said, we still take precautions, if space agencies see a deluge or a big storm coming). What is feared, however, are the risks of hail or lightning. For this, the teams use field mills, which make it possible to determine the variation of the magnetic field around the launch pad, up to several kilometers. “Normally, launchers are of a size insensitive to lightning, Christophe Bonnal points out. But we are not playing with fire. As for the SLS, NASA checks that there is no risk of lightning at 18 kilometers, during lift-off .The clouds the rocket might pass through while flying are also closely guarded. It’s best to avoid flying a launcher right into the middle of a large cumulonimbus cloud, where hailstones the size of a human hand can move at full speed. Therefore, agencies do not hesitate to stop the countdown to a launch, waiting for a large cloud to advance a little further. “Let’s not play with fire” Christophe Bonnal Finally, there is a type of weather that should not be overlooked: space weather, that is, the impact of the Sun’s activity on our planet’s environment. During rocket launches, “we check that the Sun is not very active, because avoid disturbances to the launcher’s electrical system.In every rocket launch, it’s the same prot ocol: all these weather criteria are controlled. If a single parameter is unsatisfactory, it is better to refrain from launching the rocket. “We may very well have a day when all the criteria are good, but we have a bit of wind, so we don’t start,” emphasizes Christophe Bonnal. Let’s hope the weather isn’t capricious as we spend our evening this Saturday, September 3, awaiting the launch of the long-awaited Artemis I mission.
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