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I read SpaceX’s biggest rival is developing “space trucks” to ferry cargo in an orbital economy in Quartz which linked to this April 2016 outline of ULA's view of Transportation Enabling a Robust Cislunar Space Economy.

This is quite an exciting outline of what the logistics of a a cis-lunar space economy might look like, and there is a lot of ideas and new ways of thinking to take in. One of them seems here to be to not have to cary your 2nd stage full of fuel into space between your first stage and your payload - am I understanding this correctly?

How does bringing propellants up from the surface of the Earth (the other sources are pretty far out there) allow one to avoid Earth's deep gravitational well? Are they going to collect water from clouds instead? I'm missing something here!

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above: Slide number 8 from ULA's Transportation Enabling a Robust Cislunar Space Economy annotated later to indicate the connection between transport of terrestrial water to orbit while avoiding Earth's deep gravitational well.

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  • $\begingroup$ Where in the sources you linked to does it say they want to bring water from earth surface? Water can be converted to liquid hydrogen and liquid oxygen when you have the necessary equipment and a lot of energy available. So water on asteroids or the moon can be converted to rocket fuel. But when it comes to supply from Earth, it would be far more economical to make the LH and LO on Earth and bring that. $\endgroup$ – Philipp Aug 27 '16 at 16:30
  • $\begingroup$ @Philipp good point - thanks! I'll adjust my wording. I'm just thinking about kilograms of hydrogen and oxygen and gravity, but I should be careful with the chemical forms. These are supposed to be advanced cryogenic devices that can keep things cold much longer than current storage systems, so possibly they can "cache" LH2 for a while at least. $\endgroup$ – uhoh Aug 27 '16 at 16:34
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For missions that require more fuel than the standard ACES stage contains, they propose to launch 2 rockets, one with the ACES stage and payload, one with extra LOX en LH.

They also propose using the ACES stage and XEUS lander as space tugs, keeping them in space for a long time instead of having to launch a new one for every mission. That's what the 'avoid Earth's gravitational well' is about.

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  • $\begingroup$ The video is much easier for me to absorb than the dense pdf I started with. I like the way you can pack everything so concisely into a few sentences. This is a lot of help thanks! $\endgroup$ – uhoh Aug 28 '16 at 0:02
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I believe the point is that the spacecraft itself only has to get out of Earth's gravity well once.

Until ISRU propellant sources on the moon are up and running, propellant still needs to be brought up from Earth's surface, but regular bulk propellant-only launches to a fuel depot in LEO have different engineering requirements that may allow for cost reduction. This is sometimes referred to as the "big dumb booster" concept. The payload is cheap and easily replaceable, so the launcher doesn't need to be as reliable; the payload doesn't care about g-force, so higher-g trajectories (which lose less energy to gravity because of their shorter ascent time) can be considered, making everything more efficient.

Because propellant is generally a large majority of a spacecraft's fueled mass, keeping the spacecraft out of the gravity well while still having to truck propellant isn't a huge win, but it can be a win.

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  • $\begingroup$ You actually answered my question per se, addressing the use of the term "gravity well". Forgive me just this once for accepting the other answer - it has "bonus" information that may be more helpful to future readers. But this raises some interesting points about the benefits of the "big dumb booster" optimizaiton for a "big dumb payload." So I've asked a separate question about it. $\endgroup$ – uhoh Aug 28 '16 at 0:22

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