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I have read this article here and it talks about the delta-v cost of travelling to and from the Moon. Importantly it says that getting to an orbit round the Moon, and docking with a space station, and then going down to the Moon, back to space station, back to Earth, has a very high delta-v compared to a straight Earth-Moon-Earth mission.

Can someone explain to me why that is the case?

The reason I'm wondering is that my (basic) knowledge of space flight says that during the Apollo missions, NASA famously had "the loneliest man in the solar system" left orbiting the Moon, while Neil and Buzz went for a stroll on the Moon. Was the orbit of the orbiting space craft that much different to this Luna space station, that the delta-v was lower for earth-moon orbit-moon-moon orbit-earth trip?

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  • $\begingroup$ Delta-v is only proportional to the fuel requirements if you have a single craft that needs to take all of its fuel with it. The Lunar Orbit Rendezvous method probably had a larger delta-v total than direct ascent, but that doesn't mean that it required more fuel - it didn't. $\endgroup$ – Emilio Pisanty Sep 17 '18 at 8:33
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Was the orbit of the orbiting space craft that much different to this Luna space station?

Very different.

The Apollo program used a lunar orbit rendezvous to land a vehicle on the Moon and return the astronauts to Earth, with the lunar orbit at an altitude of about 116 km above the Moon's surface. In contrast, the orbit discussed in the referenced article is not about the Moon. The subject of the referenced article is the proposed Lunar Orbiting Platform - Gateway (LOP-G). This platform is currently planned to be placed in what is called a near-rectilinear halo orbit.

This near-rectilinear halo orbit (NRHO) is neither about the Moon nor about the Earth. It is instead a member of one of several families of pseudo orbits about the Earth-Moon L1 or L2 point in the restricted three body problem. Each of these families of pseudo orbits require the presence of both the Earth and the Moon. The delta V cost from a NRHO to the lunar surface is significantly higher than was the delta V cost from the low lunar orbit to the lunar surface used by the Apollo problem.

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Well, an orbit from earth to the moon is really a very elliptical orbit around the earth, that encompasses the moon. This would mean that if another craft was detached and sent down to the surface from the main craft, this main craft would eventually leave the moons vicinity to take its lap around the earth (and the detached craft would need more fuel to correct for their increased approach velocity).

To avoid this, they need to match their speed to the required one for an orbit around the moon by, at their earth-centered apogee, reduce their speed and thereby reduce their perigee as well, until their orbit instead rotates around the moon. This means that the detached craft will only need to lower its perigee to that of a moon surface intercept, instead of having to reduce it from all the way on the other side of the earth.

On top of all this it would be nice to know that someone is not to far away as well eh? ;)

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