Which is basically impossible. 1. Just starting from the point of grabbing the payload, the rocket is going close to the size of existing rockets to push the spacecraft and payload back "towards" the moon. The altitude saves only a few percent propellant. 2. Conversely, the propellant to slow down the spacecraft propulsively* to grab the payload is also going to be near the same amount it took to get to the moon. 3. So now but since the spacecraft which is on a (large) return rocket slowed down to pickup the payload, the rocket that is doing #2 has to be much more massive.* ignoring any aerobraking
Doesn't matter about water on the moon, the link up wont work
Okay the skimmer aerorbrakes down to near nothing speed and pops up its own hydrogen balloon. It drifts lowly and peacefully over to the payload. A big robot arm takes the payload and puts it inside the pod bay. HAL then accelerates from zilch to 7 km per second into orbit. (I don't really think it is necessary to do it this way but the point is I do think something can be engineered.)
I don't care so much how much propellent it saves. I just want to get the payload to where I can accelerate it with propellant from the Moon, not the Earth. In many cases the payload will be going to leo or geosynchronous orbit. At least at first.
there is not as much "abundant" water on the moon as you think and it doesn't matter, it doesn't work. There is no way of taking a rocket from the moon and picking up a payload within the earth's atmosphere. It doesn't matter if the propellant is free.
Let's concede for a moment that I'm an ignorant loon. You think it is physically impossible to send a rocket from the Moon and retrieve a payload in the earth's atmosphere?
I have no intention to 'stop in space' whatever that might mean. I appreciate everyone explaining that I can't do what I never intended to do.
It does not matter if the total work done is more than the work done by blasting the payload off the Earth's surface into orbit, because the fuel is far cheaper,
the rocket is reusable,
and the effort of getting the rocket and reaction mass into orbit from the Moon is a tiny fraction of what it would be from the Earth's surface.
...1. I don't care so much how much propellent it saves. I just want to get the payload to where I can accelerate it with propellant from the Moon, not the Earth. In many cases the payload will be going to leo or geosynchronous orbit. At least at first....
So let's say the bungee cord version is on hold. Instead we have a rocket on the Moon. It is nuclear powered. No I don't think nuclear power is magic and has unlimited energy. It uses water for reaction mass.Now we move from the Moon's surface to low earth orbit. For efficiency sake we have a smaller stage, a skimmer, that decelerates in the upper atmosphere, hooks the payload and accelerates it back to orbital speed. The skimmer then connects back to the Moon rocket, perhaps to repeat its process several times. It does not matter if the total work done is more than the work done by blasting the payload off the Earth's surface into orbit, because the fuel is far cheaper, the rocket is reusable, and the effort of getting the rocket and reaction mass into orbit from the Moon is a tiny fraction of what it would be from the Earth's surface.With abundant water on the Moon, the economics of space travel will change.
Which is a classic reverse bomber approach. If you have stupid amounts of fuel you can pick up small popup payloads occasionally. But the scenarios where you can even have a reverse bomber begin to approach contrived; you need an extensive propellant production infrastructure in orbit and/or on the moon which implies a significant space presence, which if have such a thing, means you aren't picking up small payloads rarely in a functioning business economy or a massive national program.
So the story starts sliding towards things like picking up payloads from uncommon/unscheduled/unfixed places, or it's a fiction about terrestrial apocalypse lunar survivors picking up something/somebody from an earth with zero space infrastructure. Otherwise the rational economic choice for a major space transportation system is a high throughput infrastructure system such as momentum exchange tethers which can reduce the unit cost.
Blowing through several hundred tons of precious lunar water...
(rather than MX tethers using lunar surface slingshot chucked sandbags) to pick up one guy is an egregious and vulgar display of kinetic energy, which is why a reverse bomber tickles my redneck rocket engineer funny bone but not much else. There's a pull quote from the book "The Cassini Division" that applies well here, "That is the most shocking waste of delta-vee I have ever seen".
bring water from the moon in Space Zeppelins
Quote from: bradjensen3 on 08/10/2017 12:04 ambring water from the moon in Space ZeppelinsYour ideas are intriguing to me and I wish to subscribe to your newsletter.
Quote from: bradjensen3 on 08/10/2017 12:19 amLet's concede for a moment that I'm an ignorant loon. You think it is physically impossible to send a rocket from the Moon and retrieve a payload in the earth's atmosphere? It is.https://forum.nasaspaceflight.com/index.php?topic=42119.msg1635027#msg1635027https://forum.nasaspaceflight.com/index.php?topic=41207.0Need means of propulsion other than chemical or NTR.And it is silly to bring "abundant" lunar water (even if it was free) deep into the earth's gravity well and atmosphere. There is water already available. There is no advantage.
No I am not proposing anything that violates physical laws or includes magical engineering.
Now everyone who assume that water is scarce on the Moon is probably wrong.
Of course, my whole argument and proposal is based on the idea that water is not precious and scarce on the Moon. I think I have mentioned that several times. Your -as far as I can tell- unjustified assertion that water is scare on the Moon negates everything I am saying. While you might be with the majority in thinking that there is hardly in water on the Moon, that does not make it so. Until the Clementine returns, people thought there was virtually no water on the Moon. Based on the two bodies we have actually set foot on, the Earth and the Moon, and the one that we have observed most closely and deeply, Ceres, water seems to be everywhere. (The Mars probes so far are pretty much robot dune buggies.) If it were me, I would be looking for a planetary process that generates water. The notion that most or all of the water on the ground came from the sky is hard to defend I would think.
...It is nuclear powered...
Quote from: bradjensen3 on 08/10/2017 08:03 pmOf course, my whole argument and proposal is based on the idea that water is not precious and scarce on the Moon...... The water is too useful for the general space economy as a hydrogen source
Of course, my whole argument and proposal is based on the idea that water is not precious and scarce on the Moon...
My proposal is water heated by a small nuclear reactor.