# How can a rocket land on the moon?

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

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1. ### billvonValued Senior Member

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Nope. The actual footage.
SpaceX and NASA.

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3. ### Spencer666Registered Member

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I'll only make one more comment this subject as clearly naive people are !

Look at the size difference in the exhaust system on a rocket and a module . A massive amount of ejected material is needed for a lift off from V=0 , a stationary frame .

A super-massive amount of ejected material would be needed in free fall when the V=X to land

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5. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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So don't just make a statement

Do the maths, as some teachers used to say "show us your workings"

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7. ### Spencer666Registered Member

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Does Mason logic really require math to demonstrate simplicity ?

It takes an equal and opposing force as the great mind of Newton once said before !

Example - A 1 kg mass would need an equal amount of Newtons of force ejected to hover .

F=ma , etc

8. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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So your not going to the maths?

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9. ### Spencer666Registered Member

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A 1kg mass takes 9.81N of force to lift it off the ground .

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10. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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Earth ground or Moon ground?

?

11. ### exchemistValued Senior Member

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On the moon it takes only ~1.63N.

12. ### Spencer666Registered Member

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Earth ground ! Of course that is also not accounting for a very low effect of air pressure .

13. ### Spencer666Registered Member

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Yes but when we add V=X to the topic (free fall) , the amount of force needed to stop a descent is much greater than the amount of Newtons required for a lift off from V=0 .

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As you are aware m/s2

14. ### exchemistValued Senior Member

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Only 16% of what is needed to do the same with Earth gravity.

15. ### billvonValued Senior Member

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Yep. Now look at the weight difference between a rocket and a module. Now look at the weight difference on the Earth vs on the Moon.

In fact, consider how small these rocket systems are, all designed to land on the Earth, That's because these three vehicles don't have to make it to Earth orbit, and so are much lighter.

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Correct. That's why there was so much dust blown up when they landed that they couldn't see the surface.

16. ### YazataValued Senior Member

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I haven't read this entire thread, so what is Spencer666's basic assertion?

That rocket propulsive landing are impossible in principle? SpaceX seems to have mastered it to an impressive level of reliability:

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That propulsive landings are only possible in atmospheres where there is something for rocket exhaust to "push against"?

Orbital class rocket boosters are large because orbital velocity, the velocity necessary to stay in orbit around the Earth, is something like 18,000 mph (mach 25 or something like that). That's why orbital rockets quickly rise up out of the atmosphere (100 km or so) and pitch downrange, so that most of their rocket burns are horizontal to the Earth's surface in (more or less) vacuum. They aren't trying to go higher, they are trying to go faster.

The Moon has something like 1/6 Earth's gravity and the velocity necessary to orbit the Moon turns out to be something upwards of 3,000 mph. That's closer to jet fighter speed than rocket speed. What's more, there isn't any atmosphere on the Moon and it's possible to orbit it at as low an altitude as necessary to avoid mountains and surface irregularities.

Keep in mind that rocket boosters don't scale precisely to the velocities that they are intended to reach. The size of the rocket isn't just proportional to its intended speed. It also has to lift all the fuel necessary to reach those speeds. They have to lift and accelerate their own mass.

On Earth, they also have to push against the aerodynamic forces of the atmosphere ("max-q" and all that).

Or a somewhat smaller amount if it can achieve a higher specific impulse. That again grants an advantage to the Moon, since a rocket engine's fuel efficiency (so to speak) is in part a function of the difference in pressure inside the engine's combustion chamber and outside. The Earth's atmospheric pressure lowers the efficiency of rocket engines firing at ground level, while the Moon is already in vacuum. So a rocket engine will work better in space or on the Moon.

A rocket would be fighting a planetary body's gravity either way. On ascent, fighting to accelerate out of the body's gravity well. On descent, fighting to prevent the body's gravity from accelerating the vehicle into the surface.

Last edited: Dec 30, 2019
17. ### Janus58Valued Senior Member

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And the reaction mass of the exhaust is accelerated up to velocities of kilometers per sec in a fraction of a second by the energy released by the burning of the fuel. So with F=ma, the acceleration of the exhaust gases if many many time the 9.81 m/s/s due to gravity. So for instance, getting the exhaust up to 3.5 km/sec in 1/10 sec is an acceleration of 35,000 m/s/s and it would only take accelerating 0.28 grams of reaction mass at this rate to produce that 9.81 N of force.
The lunar module descent engine were capable of a thrust of 45,050 N of thrust, the entire fully fueled Lunar module massed only 15,200 kg. With the acceleration due to gravity on the Moon being 1.63 m/sec/sec, this only required 24776 N of force to lift against Moon gravity. And this deceased as the LEM got closer to the surface as the descent module used up its fuel.
Complete and utter BS. You have a rocket sitting on the pad, Without firing the engines, you just let the fuel fall out. it accelerates ( on the Earth) at 9.81 m/s/s relative to the rocket, but produces no thrust.
Now you fire the engines the energy of the burning adds an additional downward acceleration to the burnt fuel exhaust, during the initial acceleration, the fuel accelerates downward due to both gravity and the energy from combustion. The the addtional acceleration due to gravity has no effect on the amount of thrust produced by the engines as that is a result of the acceleration of the exhaust caused by combustion. What additional acceleration is provided by gravity after it leaves the rocket is of no consequence.
Same rocket in a free=fall descent. You just try to let the fuel fall out. Nothing happens, because both the fuel and rocket are already falling at the right speed for free-fall. the fuel isn't going to start falling faster. Again you fire the engines, accelerating the exhaust gases relative to the rocket. The same rocket with the same engine will produce exactly the same upwards thrust slowing the rocket as it did trying to lift off.
In fact, the amount of fuel needed by a rocket to say lift off the surface of the Moon and reach an altitude of 10 km up still moving upwards at 1 km/sec, is exactly the same as the amount of fuel needed to make a soft landing for the same rocket starting at an altitude of 10 km and moving downward at 1 km/sec.

You have accused other here of being "naive", but it has become clear here that it is you that is displaying naivety.* You naivety is that your estimation of your understanding of the subject matter far exceeds your actual grasp of the material. A great deal of the stuff you "Know to be true", just isn't. There is an old saying "Don't try and teach your grandmother how to suck eggs". It basically means that you shouldn't try to lecture people on a subject that they know a great deal more about than you do. And this is clearly relevant in your case.

You've also tried to claim that NASA is lying to us about the Moon landing and a whole bunch of other stuff. But NASA is just one agency for one country. There are plenty of other countries in the world, with their own experts who could easily shoot holes in any in NASA's "lies", especially if they are easy to expose as you seem to think. And a good number of those experts are from countries that would just love to knock the US down a peg or two, and have absolutely no reason to go along with NASA in hiding the "lies". If you go the full "world-wide conspiracy" track, you have to explain how a such a conspiracy can be so efficient to prevent any leaks or whistle blowers from cropping up ( the larger the conspiracy is the harder this is), and yet be so inept in faking the Lunar landings, that, based on your arguments, anyone capable of adding 2 and 2 and getting 4 could see through it.

* Assuming that you are not, as suspected by others, simply a troll, in which case, you are just emotionally stunted rather than ignorant.

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18. ### Spencer666Registered Member

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0.59s in this video !

Please explain how the module can be released and magically maneuver's without any observable propulsion ?

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19. ### billvonValued Senior Member

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?? Boats and submarines have unobservable propulsion; are they magic?

Hydrazine thrusters do not produce any visible flame, since nothing is burning. The thrusters use a catalyst to decompose hydrazine and produce nitrogen, hydrogen and ammonia, all of which are colorless, clear gases in space.

(Oh and BTW the video also shows how the blast of the lander engine completely obscures the landing area by blowing dust around just before they land. Check out the video at 5:01.)

About 30 seconds of Googling would have shown you that. Are you unable to do your own research?

Last edited: Dec 31, 2019
20. ### originHeading towards oblivionValued Senior Member

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He is unwilling.

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21. ### DaveC426913Valued Senior Member

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I think even that is giving him too much credit.

I don't think Spencer is merely an uninformed skeptic - I think he's a troll. I think he knows exactly what to do to get attention.

He's picked a few random unrelated events in history and makes up controversial "conspiracies" about them. From the Moon landing to Titanic. He has hinted that (and thus I predict that) his next conspiracy will be about the WTC on 9/11.

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22. ### KristofferGiant HyraxValued Senior Member

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He already did that.

23. ### exchemistValued Senior Member

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Agreed. There seems to be little doubt. Attention-seeking.