# How can a rocket land on the moon?

Discussion in 'Physics & Math' started by Spencer666, Dec 26, 2019.

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1. ### Janus58Valued Senior Member

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Your statement about the car is just silly. Of course you can't use car brake in a free fall, I never said you could, in free fall, you use rockets for braking. The point is that even though rockets and cars use different physical means to "apply the brakes", they both would use a method of shedding speed gradually rather than all at once. You completely missed the point in order to concentrate on something totally irrelevant.
The Lunar module had plenty fuel for a landing/takeoff. The descent module carried some 8000 kg of propellant mass, and an engine with an specific impulse of 311 sec. The fuel was in 4 67.4 cubic ft tanks. It is easy to work out that this amount of fuel would fit in those tanks and that those tanks would fit in the descent module. Also knowing the Total mass of the Lunar module and the rocket equation, this is enough to work out just how much velocity change this would produce for the module. Also knowing the mass of the Moon (1/81 that of the Earth), its radius ( between 1/3 and 1/4 of the Earth's) and initial orbital height, you can worked out how much velocity change is needed to land on the Moon. The calculated performance of the Lunar module falls in this range. How can you claim that it couldn't carry enough fuel when it is obvious that you have no clue as to how to figure out how much fuel it would require? (Your "gut feeling" is not sufficient argument).

As already noted the the Moon is much less massive than the Earth and has a much weaker gravity. Its escape velocity ( which determines how hard it is to land on or leave) is about 1/5 that of the Earth's. So it is that much easier to land on the Moon vs. the Earth.
It's not that we couldn't use rockets to land on the Earth, it would just be very wasteful to do so. It would increase the total fuel requirements immensely. Not only do you need to add the extra fuel needed for the landing, you need to add the fuel to lift that fuel in the first place. Something like the Saturn V could, in theory, lift something to orbit by rocket and then bring it back by rocket, but only at the cost of making that something be really small. We make use of the Earth's atmosphere because it is there and we can use it to reduce fuel requirements. It allows us to make more of the initial rocket mass payload vs fuel load. Why use rockets to land on the Earth when its easier and cheaper to use parachutes? It's a matter of using the best method for the given circumstance and taking advantage of what nature has already given us.

Not having an atmosphere for parachute braking would make spaceflight much harder in practice, but not totally impossible.

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3. ### Spencer666Registered Member

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You mentioned the car firstly ! I was pointing out your car example was irrelevant .

It isn't a gut feeling , more of an observation . The camera would not be able to observe the moon service on landing if the module was ejecting a huge amount of mass on descent t slow down .

~1 min 50.s in the provided video .

Free fall is still 9.82m/s2 is it not ?

Cross referencing my other thread , to me if they did land , the view I observe in the video , is they used some form of electromagnetic distortion to land !

Perhaps provided by the ''outside'' world , a more advanced version of ourselves . They then reverse engineered this tech and broke it .

I am presently trying to find the you tube video where a NASA person explains they dissembled the module .

''We had the technology and destroyed it '' is the words he uses !

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7. ### exchemistValued Senior Member

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The acceleration due to gravity on the moon's surface is 0.167 (16.7%) that of the Earth. So it is 9.82 x 0.167 = 1.64m/secĀ²

This is because the mass of the moon is a lot less than that of the Earth.

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8. ### Spencer666Registered Member

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Thank you for providing that information !

Do you know the terminal velocity of the free fall ?

9. ### Spencer666Registered Member

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True , the thrust is also in free fall .

There is no medium to push against , the physics does not work .

There isn't any recoil in a constant thrust .

10. ### Gawdzilla SamaValued Senior Member

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One thing OP seems to have missed is that the astronauts matched speeds with the Moon, THEN landed. Relative velocity zero just before touchdown.

11. ### Spencer666Registered Member

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By what physics did they slow down ?

A thrust falls with the module .

The moon landing story is similar to a donkey following a carrot on a stick .

12. ### Spencer666Registered Member

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Further investigations , a moon lift off !

Where is the thrust ?

13. ### Gawdzilla SamaValued Senior Member

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The parts you don't understand, of course, the parts with the numbers in them.

14. ### exchemistValued Senior Member

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I'll play along, since it is Boxing Day.

Terminal velocity is the velocity of a falling body at which the force of gravity is exactly matched by the force due to air resistance. This prevents any further acceleration as there is no longer any net force acting on the body to accelerate it. The value of terminal velocity thus depends on the mass and shape of the body and, most obviously of all, depends on the existence of an atmosphere.

It thus has no meaning on the moon, where there is no atmosphere. And where there is an atmosphere, it has no meaning in the absence of information about the specific body in question. For example, a free-falling parachutist can alter his terminal velocity by the attitude of his arms and legs.

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15. ### Gawdzilla SamaValued Senior Member

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Is there a better word than "conspiracy atheist" for this kind of rebuttal?

16. ### Janus58Valued Senior Member

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No, you completely missed the point of the analogy being made to focus on a part that was not relevant.
Its like someone saying that relative to their respective sizes, walking six inches for an ant would be the equivalent of walking 600 ft for a human, and you came back to say " but an ant has six legs and a human only two" . The mode of locomotion is irrelevant, as the the comparison was only meant in relation to their respective sizes.

By the same physics that causes the recoil of a gun. Pushing the bullet out of the barrel also produces a force back on the gun. This does not require any air outside of the gun to work and you would get the same recoil in space.
The rocket ejects gasses at high speed out the nozzle and the craft "recoils" upward against gravity. A constant flow of gas at a high enough speed is enough to slow or even stop the descent. So for the Lunar module a constant "recoil" that would produce an acceleration of 1.64m/m/sec if the craft were far away from any gravity, would exactly cancel out gravity near the surface of the Moon, allowing your craft to hover, or descend at a constant slow speed to touchdown.

17. ### originHeading towards oblivionValued Senior Member

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I will bet you a gazillion dollars spencer666 won't understand anything you said.

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18. ### Gawdzilla SamaValued Senior Member

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Willful ignorance is not that uncommon, yes.

19. ### JeevesValued Senior Member

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I think he's pretty much exhausted his entertainment value.

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20. ### exchemistValued Senior Member

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My guess is he will disappear, now that the inconvenient facts are emerging.

We always get a Christmas crank of some description, don't we? But I'm mildly disappointed we have not had a free energy crank for a long time now. I used to treat it as a Christmas puzzle, finding out what was wrong in this years' contrived scenario.....

21. ### Janus58Valued Senior Member

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Put's me in in mind of a song:

"Oh Christmas crank, Oh Christmas crank, how faulty is your logic,
Oh Christmas crank , Oh Christmas crank, how faulty is your logic,

Your plan has one big fatal flaw,
It breaks the first and second law,
Oh Christmas crank, oh Christmas crank, why did you not learn physics?"

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22. ### billvonValued Senior Member

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How interesting that you say that! Watch the film of the first Moon landing from the pilot's perspective. The camera could not see the Moon's surface for the last few seconds because the lunar lander's engine was ejecting a huge amount of mass to slow down its descent, and it was blowing dust like crazy. He even comments on it in the audio recording - at 40 feet he says "kicking up some dust" in that understated way that astronauts often use. Fortunately he could see the spacecraft's shadow on the blowing dust and could therefore tell his altitude visually.
Nope. Not on the Moon.
What a fascinating (and silly) conspiracy theory!

23. ### RainbowSingularityValued Senior Member

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a new Russian hyper-sonic missile ?
very quickly i should imagine lol

Russia probably now has the capability to put a lunar orbital nuclear powered missile platform on the moon.
ideal for shortening the response time for defending against asteroids.
maintaining planetary defense systems in such harsh conditions must be very very complicated, technical and expensive to produce.

a lot cheaper than having a large city wiped out by some errant late detected asteroid though.