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Post by asdf on Mar 15, 2006 13:29:44 GMT -4
The landed on the surface so they could have a greater thrust velocity. They could but they would have to land vertically so upon liftoff , they would have thrust against the surface of the moon. Note that the some of the new designs for spacecraft now are to land vertically onto the surface.
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Post by gwiz on Mar 15, 2006 13:36:07 GMT -4
The landed on the surface so they could have a greater thrust velocity. They could but they would have to land vertically so upon liftoff , they would have thrust against the surface of the moon. Note that the some of the new designs for spacecraft now are to land vertically onto the surface. They landed vertically because it is safer to stop and then land than it is to land and then stop, particularly if there isn't a runway available. hplasm is right about your understanding of rocketry. I suggest you try learning how they work before you give us another display of ignorance.
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Post by scooter on Mar 15, 2006 14:11:54 GMT -4
asdf, Exactly how much education do you have? So far, your statements seem to indicate very, very little. More inputs for a "tribute" thread...(maybe that's where he's getting his "data"...) Dave
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Mar 15, 2006 14:30:08 GMT -4
They create most of their thrust against the surface of the earth's and the atmosphere. This is completely false. Rockets work because of the law of conservation of linear momentum. There is no need for an external gas or surface to push against. From this Web page we have, "Isaac Newton stated in his third law of motion that "for every action there is an equal and opposite reaction." It is upon this principle that a rocket operates. Propellants are combined in a combustion chamber where they chemically react to form hot gases which are then accelerated and ejected at high velocity through a nozzle, thereby imparting momentum to the engine. The thrust force of a rocket motor is the reaction experienced by the motor structure due to ejection of the high velocity matter. This is the same phenomenon which pushes a garden hose backward as water flows from the nozzle, or makes a gun recoil when fired."
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Post by BertL on Mar 15, 2006 14:59:34 GMT -4
LOL.
asdf, how old are you? What education did (are) you follow(ing)?
Your lack of understanding physics won't help you in this discussion.
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Post by tofu on Mar 15, 2006 15:53:13 GMT -4
He's about 12, maybe less.
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Post by twinstead on Mar 15, 2006 15:59:06 GMT -4
He's about 12, maybe less. God, I hope so. IMO, In many ways a 12 year old could be excused for arrogant ignorance.
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Post by JayUtah on Mar 15, 2006 17:22:03 GMT -4
Asdf, do you realize that there are people on this board who build and operate commerical rocket-powered spacecraft? This isn't just some knitting club.
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Post by JayUtah on Mar 15, 2006 17:38:10 GMT -4
They could but they would have to land vertically so upon liftoff , they would have thrust against the surface of the moon.
Then at what altitude above the moon does that effect stop working?
Let's say I'm near Mars. But I point my rocket engine back at Earth's moon, millions of miles away. Does the rocket exhaust still "push" against the distant lunar surface and let me travel?
In fact, let's say I'm in deep space, nowhere near a planet or moon or any solid object. If I point my rocket engine in some arbitrary direction, aren't there good odds that I'm pointing at some solid object, no matter how distant?
But you seem to think that in deep space the exhaust has nothing to push against. So obviously those faraway surfaces don't count. That would mean that you get thrust when you're close to a surface, but less thrust the farther you get away from it.
So please supply for me the equation that makes thrust, F, a function of distance, d, from a surface and show me how you derived it.
Do another experiment for me. Stand a couple of feet away from a wall. Put out your arms and push against the wall. Did you nearly fall backwards? Now turn so there's nothing in front of you. Push just as hard against the air. No tendency to fall down backwards?
That suggests that thrust depends on the density of the medium against which you push. So please revise the equation above to account for r, the mass density of the medium.
And continue to think about it. You say the space shuttle and satellites etc. orbit fly within the vestiges of Earth's atmosphere. But you saw an object lesson in the degree of force needed. Against a solid object you could easily push with enough force to knock you off balance. But in normal Earth air you could not. You'd need to push with more force in order to generate sufficient backward thrust. So if you go way up high where the air is very thin -- only a very small fraction of the density it is at sea level -- then clearly you would need thousands of times more force from the rockets in order to have any effect whatsoever.
So why are the smallest rockets on these spacecraft reserved for use at altitude?
Note that the some of the new designs for spacecraft now are to land vertically onto the surface.
That's because that's the only way you can land on the lunar surface with our current technology. We can't use parachutes or wings, which are our only other ways to generate forces that oppose gravity.
If you were able to fly horizontally across the lunar surface, and it were sufficiently free of obstacles, you could actually achieve orbit that way.
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Post by Fnord Fred on Mar 15, 2006 17:59:36 GMT -4
Does he really believe this ****!?! I mean, sure, he's a woowoo, but how depraved do you have to get to think that space rockets behave under the same principles of hopping boat to boat at a dock? You know, remedial science internment camps are looking better and better...
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Post by sts60 on Mar 15, 2006 18:18:43 GMT -4
They create most of their thrust against the surface of the earth's and the atmosphere.
No, and no. This is completely wrong at the most fundamental level. Thrust against an external surface accomplishes nothing, and thrust against the atmosphere is counterproductive. We already explained this to you.
The velocity needed is reached during this time. The rest is just coasting through space and slingshotting.
No, and no again. A great deal of the speed built up by any object going into orbit, or an escape trajectory, is developed by firing rockets in a vacuum (10^-6 torr or lower). Without the ability to effect a change in velocity in a vacuum, no spacecraft would ever get into its intended orbit, or leave Earth, or go into orbit around or land on another planet.
According to your idea of rockets work, there can be no such thing as a communication or navigation satellite, because they must be boosted (and re-boosted) in a vacuum. Do you deny that people get television signals from a satellite? Or that transcontinental phone calls are routed through satellites? Do you deny that GPS devices work by satellite? Do you deny that there is such a thing as the International Space Station? Logically, if you insist that rockets must "push against" air or a surface, you must deny all of these things.
Using gravity of planets, moons, etc. to increase and decrease their velocitites and directions.
It is meaningless to speak of increasing or decreasing a direction. A velocity is a vector consisting of both speed and direction. And, while the gravitational attraction of celestial objects is used to effect velocity changes, this cannot be done usefully without the ability to change your own velocity, in a vacuum, with rockets.
You are contradicting your very first post. By your, ah, reasoning, it is impossible to bring back samples from the Moon. Your hypothetical robot spacecraft couldn't get there, couldn't land, and couldn't take off.
You must understand that if you think rockets need "something to push against", anything else you say regarding getting into or moving around in space is simply invalid.
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Post by sts60 on Mar 15, 2006 18:23:28 GMT -4
The landed on the surface so they could have a greater thrust velocity.
This doesn't even mean anything. What are you trying to say? That they could have landed somewhere other than a surface?
They could but they would have to land vertically so upon liftoff , they would have thrust against the surface of the moon.
No. Rockets don't need anything to push against, again. Furthermore, by your reasoning, they would lose thrust as soon as they got a little ways away from the surface, and fall back. You have contradicted yourself again.
Note that the some of the new designs for spacecraft now are to land vertically onto the surface.
You must not have been paying attention. Every spacecraft ever flown, with the obvious exception of the Space Shuttle, has been designed to land vertically.
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Post by sts60 on Mar 15, 2006 18:30:17 GMT -4
asdf, if rockets need something to push against, how is it possible to move a three-ton satellite in a vacuum (approximately one ten-millionth of atmospheric pressure) with a two-ounce thruster?
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Post by pzkpfw on Mar 15, 2006 20:18:46 GMT -4
I like to try to "connect the dots" in HB thinking; to see where they are comming from.
Could this "you need the rocket to push against a surface" thing be part of why the last HB (was it MoonMan?) though the LM blew its entire propellant load at once on lift-off?
----
My suggested experiment:
Get a brick and a piece of wood - make the wood the same size as the brick.
Stand on a skateboard with the brick. Throw it. See how far you move in the opposite direction.
Then try again with the wood.
Expected result: you move further using the brick, as the mass you chuck is higher.
Makes sense?
Cheers,
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Post by scooter on Mar 15, 2006 20:51:00 GMT -4
I'm confused... Here on this thread, asdf is talking nonsense about how the missions couldn't happen, but on other threads on this site, he talks like the HB's are kooks. So, which is it? Methinks he is very, very young...
Dave
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