3 ms·

Yes, escape velocity is required. Mere position won't do it. Remember that velocity is what carries a spacecraft away from Earth's gravitational field. Anythin

by lutusp 13y ago

Yes, escape velocity is required. Mere position won't do it. Remember that velocity is what carries a spacecraft away from Earth's gravitational field. Anything less than escape velocity and the spacecraft is pulled back into the atmosphere. > What if instead of a rocket to get out of the atmosphere, you went to a specific location in the stratosphere and some sort of tow cable would be launched from a platform(maybe anchored to the moon if needed?) ... You need to think about how far away the moon is. The mass of the cable would very far exceed the mass of the spacecraft, and it would certainly cost more than a chemical rocket, however inefficient. > ... but wouldn't a space elevator need to at some point exceed the escape velocity for earth? The space elevator idea relies on tapping earth's own rotational velocity, by having a counterweight located sufficiently far away that it is held perpetually aloft by centrifugal force, countered by the tension on the elevator cable. That means the counterweight must be substantially more than about 22,300 miles above the earth's surface (the geostationary orbital height). This is another idea that requires very ingenious engineering to prevent the mass of the cable from ruining the overall plan. I think eventually people will think of something that none of us have thought of yet, using advanced and possibly future technologies, to solve this kind of problem. But it won't depend on a long cable stretching to the moon.

chvvel843 13y ago
This is completely wrong. Escape velocity is the speed, which is required to break free of the gravitational pull, BUT in the case that no additional propulsion is available. In other words, you shoot an object with that speed from the surface of the earth and provide no additional force to accelerate it or keep the speed constant. You can leave the earth's gravitational pull with ANY speed. Assume that there was a magically (geostationary) floating platform 300 km above earth. There are absolutely no problems for someone on that platform to pull you up with a rope. Imagine a 1km high skyscraper... Do you need to reach certain speed if you were to be pulled to the top of the skyscraper?! No, speed is irrelevant. What is important is to have a constant force, which is in the opposite direction of the gravity which is equal or greater than G.
- lutusp 13y ago
  > This is completely wrong. On the contrary, it's completely right. > Escape velocity is the speed, which is required to break free of the gravitational pull, BUT in the case that no additional propulsion is available. You just changed the subject, and we could always branch out into a discussion of perpetual acceleration sources, but that would depart the realm of reality. The OP's question asked about reality. > You can leave the earth's gravitational pull with ANY speed. As a matter of fact, no, that's not possible -- unless you have a continuous energy source and don't care how much energy you expend. Which is your new topic, not the topic of current propulsion methods, the topic of the OP's post. Without benefit of something exotic like a space elevator you can stay in place, stationary, 100 kilometers above the earth's surface, motionless with respect to the surface, in perpetuity, but only by a constant expenditure of energy. But that's a different topic. > No, speed is irrelevant. Without invoking technologies that don't exist, speed is completely relevant. > What is important is to have a constant force ... Be careful that you don't confuse static forces and those created by a dynamic source of acceleration that require an expenditure of energy. Static forces require no energy, but most accelerations involving motion do (apart from the class of friction-free orbits). My reply answered the OP's inquiry, not the science-fiction idea that you could have an infinite, inexhaustible energy source.
  - chvvel843 13y ago
    I am not talking about sci-fi either ... only giving a simple example about how it actually works. Anyone with even basic knowledge in physics will know that you are wrong. Please research a little bit to see why you are wrong. An object can leave the earth's gravitational pull at ANY speed. I just googled the topic, one of the first results: https://uk.answers.yahoo.com/question/index?qid=20070904045946AAzXoFb https://uk.answers.yahoo.com/question/index?qid=200709040459...
    - lutusp 13y ago
      > An object can leave the earth's gravitational pull at ANY speed. Yes, but as I have already explained without effect, science fiction is not the topic. You need to understand the topic under discussion before replying, not after. In a discussion like this, it seems someone must always suggest building a beanstalk that reaches to orbital heights. Fine, but not constructive, and not the topic. > Anyone with even basic knowledge in physics will know that you are wrong. Please troll somewhere else. The OP asked for practical suggestions given the realities of present-day aerospace engineering, a field in which I am more than qualified. I can also wave my hands in the air, but that doesn't get hardware into space. -- Former NASA Space Shuttle engineer.

by lutusp 13y ago

chvvel843 13y ago

This is completely wrong. Escape velocity is the speed, which is required to break free of the gravitational pull, BUT in the case that no additional propulsion is available. In other words, you shoot an object with that speed from the surface of the earth and provide no additional force to accelerate it or keep the speed constant. You can leave the earth's gravitational pull with ANY speed. Assume that there was a magically (geostationary) floating platform 300 km above earth. There are absolutely no problems for someone on that platform to pull you up with a rope. Imagine a 1km high skyscraper... Do you need to reach certain speed if you were to be pulled to the top of the skyscraper?! No, speed is irrelevant. What is important is to have a constant force, which is in the opposite direction of the gravity which is equal or greater than G.

lutusp 13y ago
> This is completely wrong. On the contrary, it's completely right. > Escape velocity is the speed, which is required to break free of the gravitational pull, BUT in the case that no additional propulsion is available. You just changed the subject, and we could always branch out into a discussion of perpetual acceleration sources, but that would depart the realm of reality. The OP's question asked about reality. > You can leave the earth's gravitational pull with ANY speed. As a matter of fact, no, that's not possible -- unless you have a continuous energy source and don't care how much energy you expend. Which is your new topic, not the topic of current propulsion methods, the topic of the OP's post. Without benefit of something exotic like a space elevator you can stay in place, stationary, 100 kilometers above the earth's surface, motionless with respect to the surface, in perpetuity, but only by a constant expenditure of energy. But that's a different topic. > No, speed is irrelevant. Without invoking technologies that don't exist, speed is completely relevant. > What is important is to have a constant force ... Be careful that you don't confuse static forces and those created by a dynamic source of acceleration that require an expenditure of energy. Static forces require no energy, but most accelerations involving motion do (apart from the class of friction-free orbits). My reply answered the OP's inquiry, not the science-fiction idea that you could have an infinite, inexhaustible energy source.
- chvvel843 13y ago
  I am not talking about sci-fi either ... only giving a simple example about how it actually works. Anyone with even basic knowledge in physics will know that you are wrong. Please research a little bit to see why you are wrong. An object can leave the earth's gravitational pull at ANY speed. I just googled the topic, one of the first results: https://uk.answers.yahoo.com/question/index?qid=20070904045946AAzXoFb https://uk.answers.yahoo.com/question/index?qid=200709040459...
  - lutusp 13y ago
    > An object can leave the earth's gravitational pull at ANY speed. Yes, but as I have already explained without effect, science fiction is not the topic. You need to understand the topic under discussion before replying, not after. In a discussion like this, it seems someone must always suggest building a beanstalk that reaches to orbital heights. Fine, but not constructive, and not the topic. > Anyone with even basic knowledge in physics will know that you are wrong. Please troll somewhere else. The OP asked for practical suggestions given the realities of present-day aerospace engineering, a field in which I am more than qualified. I can also wave my hands in the air, but that doesn't get hardware into space. -- Former NASA Space Shuttle engineer.