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Aerospace Why no Moon missions

  1. May 22, 2010 #1
    I've been always wondering, if it was possible to visit the moon so many years ago, why is no one bothering today?

    My question is related directly with the fact we have a space station in orbit, so no need to use rockets. Everything needed to reach the moon is a small craft with just enough fuel to navigate in space, it would be so easy reaching the moon from Earth's orbit ISS astronauts can go there every other weekend.
     
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  3. May 22, 2010 #2

    mgb_phys

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    Because we have proved that our Germans are better than their Germans, and there isn't much more reason to go.

    You still need the energy to go from being 300km above earth to being 350,000km above earth.
    Getting above the atmosphere into LEO is a big step in terms of aerodynamics but is nothing in the total energy requirment.
     
  4. May 22, 2010 #3
    Reaching orbit takes almost all the fuel of the craft. Not having to escape the gravity well of the planet is a tremendous saving that you downplay :)

    Also, by leaving the ISS you already are accelerated at its speed - almost 28 000 km/h, or more than 2/3 of the top speed of the fastest Apollo.

    It's 1:44 in the morning here and I am not in math mood, but it is obvious it will need only a fraction of the energy used in the Apollo missions.

    Anyway, nice to see someone actually recognizes the fact it was the Germans all along ;) Sometimes I wonder what would happen if Oppenheimer's father did not move to the US and his son was still in Germany when he made the atom bomb...
     
  5. May 22, 2010 #4

    turbo

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    Given the state of our propulsion technology, we still need to use rockets, and we still need to loft the propellants into LEO. There is no compelling need to return to an arid dusty rock right now, and once our manned craft get outside the Earth's protective magnetic field, the occupants are at risk for getting fried if the Sun throws a tantrum. We still don't have a means of adequately shielding the occupants, and they will almost certainly die if exposed to a large solar event. We got lucky during the Apollo program.
     
  6. May 22, 2010 #5

    mathman

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    Oppenheimer didn't do it by himself. He was the director, but the technical work was done by others, many of whom were refugees from the Nazis.
     
  7. May 22, 2010 #6

    mgb_phys

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    Low earth orbit hasn't escaped the gravity well.
    You are only about 7% of the radius of the earth further away than at the surface.
    Graviational energy (J per kg) = 400,000 / R (km)
    So the difference to LEO is = 400,000/6360 - 400,000/(6360+500)
    Almost the same as the gravitional potential energy at the surface.

    At the moon the object is almost out the earth's gravity at 400,000/406,000 = 1 J/kg

    But not in the direction of the moon

    Everything you need to take to get to the moon will have had to have been lifted through the atmopshere into LEO.

    The atom bomb was largely the invention of Leo Slizard - a Hungarian, what would have happened if the inventors of the Rubik cube had got the bomb
     
  8. May 22, 2010 #7
    Thats the other curious thing, in the last 40 years computers have evolved exponentially, and yet we still use practically the same rocket engines Von Brown made for NASA.

    Heck, a desktop pc today has more computing power than all the space program computers put together back in those days.

    If you are moving the speed of the ISS you already have escape velocity, you simply have to change trajectory and maintain velocity. I know on the ISS gravity is almost the same as here, they only appear in zero g because they are constantly falling :)
     
  9. May 22, 2010 #8

    Hurkyl

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    But nowhere near the amount of thrust. :wink:
     
  10. May 22, 2010 #9
    Who needs thrust ;) It seems that theoretical science, numbers and computers are set to displace practical science... sadly... I'd rather visit the moon, as uninteresting as it may be, instead of reading all those theories which have no practical implications

    Sure, but you can do it only once, and then reuse it a lot of times. Plus you won't need atmosphere reentry shielding and other equipment that's also saving energy.
     
    Last edited: May 22, 2010
  11. May 22, 2010 #10

    turbo

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    :rofl:

    The "back to the Moon'ers" seem to have little appreciation for physical law.
     
  12. May 22, 2010 #11
    That is one of the greatest lines I have ever heard!

    https://www.youtube.com/watch?v=<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/kTKn1aSOyOs&hl=en_US&fs=1&"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/kTKn1aSOyOs&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>
     
    Last edited by a moderator: May 4, 2017
  13. May 22, 2010 #12
    Orbital and escape velocities are very different. In order to 'change trajectory and maintain velocity' it takes a consideral amount of fuel. You have to overcome the Earth's gravity well until you enter the Moon's gravity well in order to get to the moon from orbit.
    The real reason we are not going to the moon anytime soon is that Obama has told NASA that they need to concentrate on missions to the asteroids instead of the Moon and Mars if they want funding. This was presented in a NASA press release a couple of weeks ago.
     
    Last edited: May 22, 2010
  14. May 22, 2010 #13

    Integral

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    Remind me, why do we need to go to the moon? Last time we were there, did they forget something we need?
     
  15. May 22, 2010 #14
    I think someone left their driver's license. Better to go to the moon than your DMV/RMV. :biggrin:
     
  16. May 22, 2010 #15

    mgb_phys

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    Not original I'm afraid - it's from Tom wolfe's The Right Stuff and is an apocryphal explanation of the US-USSR space race to Pres. Johnson.
     
  17. May 22, 2010 #16

    Redbelly98

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    What is the practical implication of going back to the moon?
     
  18. May 22, 2010 #17

    DaveC426913

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    Computers have only been around for 40 years. Rockets have been around for centuries.

    The development curve of a technology tends to be logarythmic over a long time; i.e. it starts off steep and levels off as we perfect the technology. Chemical rockets are currently about as good as they're going to get.
     
  19. May 23, 2010 #18
    The energy needed to go to the moon is a few times the energy needed to go to low earth orbit.

    Chemical rockets are not easily scalable. A rocket that takes 25 tons to low earth orbit (which is where established technology is, more or less) is not necessarily going to scale right away to put 200 tons there, or to put 25 tons on the moon. In fact, NASA estimated that it would take 10 years and billions of dollars to develop a rocket that puts 25 tons on the moon.

    We don't want to go much lower than 25 tons, because, with all the safety equipment and such, a 25-ton spacecraft would have barely enough room for three passengers and not much else.

    We don't have the technology to reassemble and refuel large craft in orbit. Partly because no one bothered to design and test those, partly because it's simply not possible to hold high specific impulse propellants (liquid hydrogen and oxygen) in containers for extended periods of time.

    There is a way out. A 200 kW ion thruster engine is scheduled to be tested in real-world conditions on ISS in 2012. When that technology is sufficiently tested and stable, we'll be able to assemble a mini-ISS in orbit and to use solar panels to power ion thrusters that slowly transfer it from Earth to the Moon or anywhere else we want to go.
     
  20. May 23, 2010 #19
    Why bother feeding yourself when others eat? The same thing about going to the moon - few people might have been there, but you surely haven't. What is the practical implication of going on a trip?


    Honestly, no one of you wants to go to the moon?
     
  21. May 23, 2010 #20

    [PLAIN]http://upload.wikimedia.org/wikipedia/en/c/c8/Apollo_8_acceleration.gif [Broken]

    So Saturn V rocket burns all its 3 stages in less than 12 minutes. The trip to the moon takes more than 3 days, so what you basically say is:

    5 F-1 Engines total of 33 500 000 newtons
    8 Retro motors total of 3 100 000 newtons
    5 J-2 engines total of 5 000 000 newtons
    8 Ullage motors total of 800 000 newtons
    1 J-2 engine of 900 000 newtons
    4 Retro motors total of 600 000 newtons
    plus a bunch of smaller engines I did not include

    Total of about 50 000 000 newtons of thrust, used up in less than 12 minutes is a few times LESS than what the actual Apollo craft was carrying? From the http://upload.wikimedia.org/wikipedia/commons/b/b2/Saturn_v_schematic.jpg" [Broken] it seems all other engines besides the 3 stages of the rocket were not for the actual travel from Earth's orbit to the moon, but for the moon landing itself...

    In other words, the craft was hardly using any energy once it exited Earth's orbit, drifting all the way to the moon.

    And you all say eliminating those 12 minutes which burn well over 90% of the fuel will not be beneficial? Am I missing something here?
     
    Last edited by a moderator: May 4, 2017
  22. May 23, 2010 #21

    Redbelly98

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    Not the same thing. Eating is necessary to live, traveling to the moon isn't.
    If they sent someone there, it wouldn't be me (even if I wanted to), so that argument doesn't hold up. I'm all for robotic exploration of the solar system, sending people isn't worth the extra cost in my view.
     
  23. May 23, 2010 #22

    D H

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    You are missing a whole lot here. You are looking at launch only. So what are you missing?
    • Trans-lunar injection, about 3.1 km/s.
      The maneuver occurred a couple of orbits after launch and was performed by the Saturn V third stage. It placed the combined third stage+command module+lunar lander+ascent vehicle on a trajectory toward the Moon (more specifically, toward where the Moon would be 3.5 days later).
    • Lunar orbit insertion and orbit circularization, about 1 km/s.
      These maneuvers placed the combined command module+lunar lander+ascent vehicle in an elliptical orbit and then a circular orbit about the Moon.
    • Lunar orbit descent and powered landing, about 2 km/s.
      These maneuvers placed the combined lunar lander+ascent vehicle on an elliptic orbit about the Moon with a very low perilune altitude and then a powered descent to the lunar surface.
    • Lunar ascent and docking, about 1.9 km/s.
      These maneuvers launched the lunar ascent vehicle from the lunar surface and then, over a series of maneuvers, docked the ascent vehicle to the command module.
    • Trans-Earth injection, about 1 km/s.
      This maneuver placed the command module on a trajectory that would later intersect the Earth's atmosphere.

    I'm leaving out small correction burns and such.


    You want to go from the ISS to the Moon. First off, the fuel and vehicles needed to accomplish that would need to be launched to the ISS. There ain't no such thing as a free lunch. That fuel and equipment taken into low Earth orbit is dead weight as far as the launch vehicle is concerned. What you are proposing about is one of the most expensive ways to get to the Moon: The low Earth orbit rendezvous option.

    The low Earth orbit rendezvous option is very expensive even if the rendezvous is in the optimal orbital plane, which is the Moon's orbital plane at the time of lunar orbit insertion. The ISS is not even close to this. It has an orbital inclination of 51.6° (relative to the Earth's equator); the Moon's orbit about the Earth has an inclination of 23.4° ±5.1°. Plane changes are extremely expensive, particularly so in low Earth orbit. Using the ISS as a base for lunar operations is lunacy.
     
  24. May 23, 2010 #23
    Even so, that doesn't change the fact space craft burns the majority of its fuel reaching Earth's orbit. The ISS might not be the best solution in that case, but launching an in orbit space station once and using it as a remote launch pad will save not only immense quantities of fuel but also a lot of failed launches. The percent of successful launches is still pretty low, and a failed craft is a much bigger waste, in both time and resources. Even if we talk about robotic probes, they are not that heavy and can be transported in quantities to the orbital station and then separately launched at their destinations, eliminating the need for each of them to be launched separately with big, expensive and risky rockers, that have to carry not only their payload, but their fuel.
     
  25. May 23, 2010 #24

    russ_watters

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    Others have pointed out several misunderstandings here, but have missed an obvious one: there aren't any spacecraft attached to the space station capable of going to the moon. So we'd still need to launch one from earth, go to the ISS, then go from the ISS to the moon. The total trip would be the same as it was for Apollo. [edit: oh, DH caught that - apologies]
    I'm sorry, but you're basing that on speed only. You haven't even begun to grasp the issue of gravity.
    Of course I want to go to the moon. But currently lacking the $500 billion required, I'd be much obliged if you could loan it to me!
     
  26. May 23, 2010 #25
    Yes, you sill have to launch it to the orbit, but that's a one time task, after which you can use it multiple times, saving a lot. The whole idea is about eliminating escaping Earth's gravity and atmosphere reentry each and every time.
     
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