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21-02-2018, 10:11 AM   #5
eddieb
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Join Date: Aug 2009
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Quote:
 Originally Posted by skulb I would not argue with any of what you said. But on the subject of propulsion, it would take surprisingly short thrust to actually leave the Moon. It is not comparable at all to leaving the earth. A few seconds burst to get you up and then a tiny directional burst to leave the Moon's gravity well before falling almost automatically back to earth. The escape velocity required to leave earth's orbit is about 40 000 km/hr. The escape velocity on the Moon is one seventh of this. And because there is no resistance from an atmosphere like there is when leaving earth, the fuel requirements for the a return trip are much smaller than these escape velocity figures would indicate. You can thrust the engines for a few seconds to get clear of the surface and then spend whatever fuel you have left on the approach burn for earth immediately. And if you leave the Moon retrograde, as one does, the distance you would have to travel to leave the Moon's gravity field is shortened further. The opposite would mean essentially following the Moon along its orbit as one tried to escape its gravity, and is therefore very silly. By following the Moon's orbit retrograde when leaving you basically fall down to earth naturally just as soon as you reach escape velocity from the Moon. People tend to think of motions in space as lines, largely thanks to the imbeciles in Hollywood. But they are always curves, because you're always orbiting something. Thrusting and reverse thrusting essentially expands or shrinks the orbit on the opposite side from you, like inflating or deflating a bubble. Thrust long enough and the orbit is broken there and you can leave when you get round to the other side. Or in the case of the moon you can thrust in earth orbit to expand the projected orbit until it will intersect with the Moon's gravity. Once there you can then reverse thrust to shrink the new orbit around the Moon. Interestingly, the further you are away from the object you are aiming at the less fuel you actually need to expend to correct your approach vector. The tiniest adjustments would be needed from Moon orbit to aim a space craft to exactly where you want it on earth. A lighter's worth of kerosene spent in orbit around the moon can mean a 300 000 km difference in the approach to earth that it would take a full shuttle to achieve if you were close to earth. It's very fascinating. All of this works perfectly fine in theory. Just not with the gear they had at the time and with the radiation belt on the way there. Or for that matter the temperature variations on the Moon's surface. According to NASA's own documentation the "space suits" worn were not adequate protection from either the high temperatures of the sunlit surface or the low temperatures in the shadows on the Moon. And those would be instant as well, because there is no atmosphere. In other words your front would be scorching hot and your shadowed half would be freezing cold at the same time. And NASA's suits were not designed for either, which they are themselves admitting. But they do have engines that can ignite in vacuum. And they can test them. It is important to keep things real here.
Part of the video that I didn't comment on was Von Braun's statement on how much fuel would be required to get to the Moon and back. He said ( according to the video ) the rocket would need to be three ( Yes 3 ) times the height of the Empire State building and it would have to carry hundreds if thousands of llbs of fuel. I think I would be more prone to agree with a man who apparently knew a thing or two about rockets etc than you, no offence offered.
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