I am curious to know about how rockets or an object lands on the moon when there isn't any air (atmosphere) and the gravity is low.
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27$\begingroup$ Rockets don't need air to work. $\endgroup$– Organic MarbleDec 13, 2019 at 13:16
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5$\begingroup$ related How did skeptics of rocketry think rockets work? $\endgroup$– user20636Dec 13, 2019 at 13:26
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2$\begingroup$ If rocket engines would need air to get close to the Moon and to land softly, there would have been no manned and unmanned landings at all. Even reaching an orbit around the Earth would be impossible. $\endgroup$– UweDec 13, 2019 at 14:35
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4$\begingroup$ Have you seen SpaceX land rockets on Earth? It's pretty neat, and very much the same concept. $\endgroup$– corsiKaDec 14, 2019 at 5:49
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2$\begingroup$ @Ed999 No rocket carries air, some carry oxygen, some use other oxidators. Carring 79 % useless nitrogen would be a big waste. $\endgroup$– UweDec 15, 2019 at 10:31
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3 Answers
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Rocket engines (unlike jet engines and internal combustion engines) are designed to work without drawing air from outside. A moon lander carries 2 tanks: one filled with propellant, the other one filled with an oxidiser. The rocket engine combines these two and ignites the mixture. The exhaust product provides thrust both in an atmosphere and in a vacuum. Thrust may be slightly higher in a vacuum. The thrust is used to reduce the speed of the spacecraft to near zero, enabling a soft landing.
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$\begingroup$ I would prefer the sentence: the exhaust product provides thrust in air as well as in a vacuum. Thrust may be slightly higher in a vacuum. $\endgroup$– UweDec 13, 2019 at 15:28
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1$\begingroup$ I prefer this answer as well, might want to include something about firing in the reverse direction of travel to slow down $\endgroup$– StickyzDec 13, 2019 at 16:14
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3$\begingroup$ I wish you hadn't. I object to "kitchen sink" answers which include far more detail than is necessary to answer the question. $\endgroup$– HobbesDec 13, 2019 at 19:58
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2$\begingroup$ @Hobbes If you don't like an edit to your answer, you are free to roll it back. Personally I wouldn't in this case; in fact I think the detail about "thrust is higher in vacuum" is much more unnecessary detail than changing "zero" to "near zero". $\endgroup$ Dec 15, 2019 at 22:03
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Spacecrafts, including Apollo Lunar Module (LM) lander, use propulsion for retroburns, thus decreasing their velocity as they descend. This method works in the vacuum of space as well as in the atmosphere.
answered Dec 13, 2019 at 13:14
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$\begingroup$ what do you mean by "rocket" $\endgroup$– user20636Dec 13, 2019 at 13:22
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$\begingroup$ By rocket do you mean 'Saturn V' or all the lander entries here en.wikipedia.org/wiki/List_of_missions_to_the_Moon which used rockets to slow to some extent (several have crashed, one slowed and used airbags for the final 50kmh or so) $\endgroup$ Dec 13, 2019 at 13:22
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3$\begingroup$ "A rocket is a missile, spacecraft, aircraft or other vehicle that obtains thrust from a rocket engine" "A rocket also can be a type of engine" $\endgroup$– user20636Dec 13, 2019 at 13:39
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3$\begingroup$ my best guess is the OP is using rocket in the sense of "rocket engine" $\endgroup$– user20636Dec 13, 2019 at 13:44
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7$\begingroup$ For the purposes of this question, a moon lander does count as a rocket. $\endgroup$– HobbesDec 13, 2019 at 15:56
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Rockets thrust works because of conservation of momentum, which is a fundamental law of our universe. When hot gasses are pushed out of the rocket, they exchange momentum with the rocket in the opposite direction. As a result, rockets don't need to "push" against anything on the business end. It's often misunderstood as a wing or helicopter rotors "pushing" against the air, but that isn't how rockets function. For some chemical rockets, if they require an oxidizer that is needed for creating hot gases, then it is carried inside the rocket.
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1$\begingroup$ "For some chemical rickets that require an oxidizer needed for creating hot gases is carried inside the rocket" ???? I think you mean "rockets", and the sentence is still poorly-constructed. $\endgroup$– DanDec 14, 2019 at 23:22