And the lower right illustration handily shows why if we get serious about building Solar Power Satellites in GEO, the moon is a better source for the raw materials.
For me this really drove home just how “far away” GEO and lunar trips are from LEO.
The local football team joke threw me off a little, but cool representation!
LEO should be halfway up Earth’s gravity well, not just above the surface. A suborbital lob would be the only thing that low- all orbits MUST be at least halfway up their respective wells.
IANAE. The peaks, like the one between Mars and Ganymede, represents…solar escape velocity?
“And the lower right illustration handily shows why if we get serious about building Solar Power Satellites in GEO, the moon is a better source for the raw materials.”
What’s the cost of energy to launch manufactured components from Earth’s surface, vs. the cost of first creating the necessary manufacturing infrastructure in space or on the Lunar surface, before you have the first of those components?
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere (fueled by H2 and O2 derived from the water-ice we now know to exist there [though, I suspect, in quantities and concentrations that would make it more valuable for life support than expended as propellant]) and/or mine uranium detected there (also in low concentrations) for local and spacecraft nuclear power, rather than ‘risk’ launching it from Earth.
They never seem to consider what it takes to get from raw ores to finished spacecraft (or reactor grade uranium), disregarding the non-metal components, or that your people *still* must come from Earth at a reasonable economic cost.
Meanwhile, we live on a world with plentiful fuel resources, many of which can be handled under shirtsleeve conditions, which more than offsets the fact that it takes somewhat more of them to get out of Earth’s gravity well, as opposed to the Moon.
All the places humans are likely to personally go (as opposed to probes…I doubt any of us want to ride down int the atmosphere of Jupiter) in this solar system are less massive than Earth (a few of them even permit aerobraking), therefore if we finally get good at getting in and out of *this* gravity well, everywhere else is a relatively straightforward operation.
If we don’t learn to at least get to LEO affordably, the rest doesn’t matter.
Gravity wells (and the energy costs involved) are important, but not the only figure of concern to space travel. And fuel, on Earth, is cheap.
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere
Where have you seen that? I’ve heard a lot of people put it forth as a strawman, but I’ve never actually seen it. I’m not saying that no one has ever proposed it — certainly there people who might be that foolish, but I’ve never actually seen a real-life example.
Manufacturing propellants on the moon, on the other hand, doesn’t seem obviously unreasonable.
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere (fueled by H2 and O2 derived from the water-ice we now know to exist there [though, I suspect, in quantities and concentrations that would make it more valuable for life support than expended as propellant]) and/or mine uranium detected there (also in low concentrations) for local and spacecraft nuclear power, rather than ‘risk’ launching it from Earth.
It all depends on scale. If you’re putting ten tons of stuff in orbit, it’s a lot easier to do it from Earth. If you’re putting ten million tons of stuff in orbit, the Moon is a better place for that (especially given it’s absence of atmosphere and smaller gravitational well makes a wider variety of launch technologies feasible).
LEO should be halfway up Earth’s gravity well, not just above the surface. A suborbital lob would be the only thing that low- all orbits MUST be at least halfway up their respective wells.
Looks to me like they are a little over halfway — the surface is at the radius of the planet (6378 km) already.
Wait a minute…that lower right diagram seems screwy. First of all, you don’t need to reach Earth’s escape velocity to go to the Moon. It’s sufficient to put yourself in a closed (i.e. elliptical) orbit with apogee at the Moon’s orbit. Secondly, the energy needed to get to LEO is a substantial fraction of what you need to get to the Moon.
Hmm…if I’ve got my formulas right, I find:
velocity to LEO (ISS height, 278 km) = 7740 m/s, equivalent to escaping a 3054 km “gravity well.”
velocity to lunar orbit = 10851 m/s, equivalent to escaping a 6004 km “gravity well.”
So I think the ISS should be about halfway up the gravity well shown.
Titus, that was a glorius link.
Nah, Nick — he says he measures the gravity well from the surface of the planet — which he should, since very few objects on the surface of the Earth are actually traveling at the correct orbital velocity for their distance from the Earth’s surface. (Jerome Bixby wrote a funny sf short story, “The Holes Around Mars,” in which an expedition discovers a moon of Mars that is traveling at orbital velocity about 3 feet above the surface of Mars. It’s made of neutronium or something, so it can punch through obstacles.)
Damn, I meant “for their correct distance from the Earth’s center” of course. Grr.
Absolutely right — I hadn’t taken the few seconds to run the math in my head. And I also might argue that the wording on the chart was a little imprecise.
Frank, the people who actually ran the numbers on this concluded that as soon as you had built 30 SPS, you would have reached the crossover point where the savings in launch costs would have paid off the costs of establishing the lunar and orbital infrastructure. I think if one had a goal of making a significant contribution to global new electrical needs, 30 SPS would be just getting off to a good start.
I agree with you that a lunar Cape Canveral makes less sense, given the much smaller tonnages involved. I agree with Rand that providing propellant makes more sense.
Gravity wells are naive representations in that the depth of the well tells little about the difficulty of getting from one point to another. Energy efficient deep space transportation depends on a subtle understanding of weak stability boundaries and the related interplanetary superhighway.
See Ed Belbruno and Martin Lo in “New Trends in Astrodynamics and Applications III” from the American Institute of Physics.
“Where have you seen that? I’ve heard a lot of people put it forth as a strawman, but I’ve never actually seen it.”
Typically, this comes up in the Space.com article comment threads now and then. (one can take that as seriously or not, as one likes) Guys who want to take the long way around (and don’t seem to realize it), just to launch a finished ship out of a shallower gravity well.
And I recall, way back in my FidoNet BBS days on the Space Development Echo, an exchange with someone who proposed getting uranium (with zero idea at the time, of how much might be there) for nuclear thermal rockets…rather than, as I said, improve the safety reliability of launching fissionable materials from Earth.
This, too, resurfaced in Space.com comments, when word recently came down that some meaningful concentrations of uranium do exist on the Moon. I made note of it here not long ago, as well:
“Manufacturing propellants on the moon, on the other hand, doesn’t seem obviously unreasonable.”
Agreed. Lunar water may be too rare to turn into rocket fuel/oxidizer, but oxygen alone, is virtually unlimited in the regolith, and also has life-support and other uses. Go for it
Typically, this comes up in the Space.com article comment threads now and then.
I meant serious proposals, not clueless commenters.
This is so smart it actually moved me. I love when XKCD falls into a “perceived” rut with ceiling jokes and big stretches for punchlines… and then this. It’s like Randall’s been throwing fakes and now he got me in the chin with the right hook.
And the lower right illustration handily shows why if we get serious about building Solar Power Satellites in GEO, the moon is a better source for the raw materials.
For me this really drove home just how “far away” GEO and lunar trips are from LEO.
The local football team joke threw me off a little, but cool representation!
“An even more glorious dawn awaits!”?
I see wut they did there…
Once you reach low Jupiter orbit, there is no going back.
You guys should be reading xkcd all the time. (Well, Monday, Wednesday, and Friday, anyway.)
Two (out of many) for engineers:
http://xkcd.com/670/
http://xkcd.com/651/
LEO should be halfway up Earth’s gravity well, not just above the surface. A suborbital lob would be the only thing that low- all orbits MUST be at least halfway up their respective wells.
IANAE. The peaks, like the one between Mars and Ganymede, represents…solar escape velocity?
“And the lower right illustration handily shows why if we get serious about building Solar Power Satellites in GEO, the moon is a better source for the raw materials.”
What’s the cost of energy to launch manufactured components from Earth’s surface, vs. the cost of first creating the necessary manufacturing infrastructure in space or on the Lunar surface, before you have the first of those components?
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere (fueled by H2 and O2 derived from the water-ice we now know to exist there [though, I suspect, in quantities and concentrations that would make it more valuable for life support than expended as propellant]) and/or mine uranium detected there (also in low concentrations) for local and spacecraft nuclear power, rather than ‘risk’ launching it from Earth.
They never seem to consider what it takes to get from raw ores to finished spacecraft (or reactor grade uranium), disregarding the non-metal components, or that your people *still* must come from Earth at a reasonable economic cost.
Meanwhile, we live on a world with plentiful fuel resources, many of which can be handled under shirtsleeve conditions, which more than offsets the fact that it takes somewhat more of them to get out of Earth’s gravity well, as opposed to the Moon.
All the places humans are likely to personally go (as opposed to probes…I doubt any of us want to ride down int the atmosphere of Jupiter) in this solar system are less massive than Earth (a few of them even permit aerobraking), therefore if we finally get good at getting in and out of *this* gravity well, everywhere else is a relatively straightforward operation.
If we don’t learn to at least get to LEO affordably, the rest doesn’t matter.
Gravity wells (and the energy costs involved) are important, but not the only figure of concern to space travel. And fuel, on Earth, is cheap.
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere
Where have you seen that? I’ve heard a lot of people put it forth as a strawman, but I’ve never actually seen it. I’m not saying that no one has ever proposed it — certainly there people who might be that foolish, but I’ve never actually seen a real-life example.
Manufacturing propellants on the moon, on the other hand, doesn’t seem obviously unreasonable.
I see similar arguments elsewhere from those who want to manufacture spaceships on the Moon to go elsewhere (fueled by H2 and O2 derived from the water-ice we now know to exist there [though, I suspect, in quantities and concentrations that would make it more valuable for life support than expended as propellant]) and/or mine uranium detected there (also in low concentrations) for local and spacecraft nuclear power, rather than ‘risk’ launching it from Earth.
It all depends on scale. If you’re putting ten tons of stuff in orbit, it’s a lot easier to do it from Earth. If you’re putting ten million tons of stuff in orbit, the Moon is a better place for that (especially given it’s absence of atmosphere and smaller gravitational well makes a wider variety of launch technologies feasible).
LEO should be halfway up Earth’s gravity well, not just above the surface. A suborbital lob would be the only thing that low- all orbits MUST be at least halfway up their respective wells.
Looks to me like they are a little over halfway — the surface is at the radius of the planet (6378 km) already.
Wait a minute…that lower right diagram seems screwy. First of all, you don’t need to reach Earth’s escape velocity to go to the Moon. It’s sufficient to put yourself in a closed (i.e. elliptical) orbit with apogee at the Moon’s orbit. Secondly, the energy needed to get to LEO is a substantial fraction of what you need to get to the Moon.
Hmm…if I’ve got my formulas right, I find:
velocity to LEO (ISS height, 278 km) = 7740 m/s, equivalent to escaping a 3054 km “gravity well.”
velocity to lunar orbit = 10851 m/s, equivalent to escaping a 6004 km “gravity well.”
So I think the ISS should be about halfway up the gravity well shown.
Titus, that was a glorius link.
Nah, Nick — he says he measures the gravity well from the surface of the planet — which he should, since very few objects on the surface of the Earth are actually traveling at the correct orbital velocity for their distance from the Earth’s surface. (Jerome Bixby wrote a funny sf short story, “The Holes Around Mars,” in which an expedition discovers a moon of Mars that is traveling at orbital velocity about 3 feet above the surface of Mars. It’s made of neutronium or something, so it can punch through obstacles.)
Damn, I meant “for their correct distance from the Earth’s center” of course. Grr.
Absolutely right — I hadn’t taken the few seconds to run the math in my head. And I also might argue that the wording on the chart was a little imprecise.
Frank, the people who actually ran the numbers on this concluded that as soon as you had built 30 SPS, you would have reached the crossover point where the savings in launch costs would have paid off the costs of establishing the lunar and orbital infrastructure. I think if one had a goal of making a significant contribution to global new electrical needs, 30 SPS would be just getting off to a good start.
I agree with you that a lunar Cape Canveral makes less sense, given the much smaller tonnages involved. I agree with Rand that providing propellant makes more sense.
Gravity wells are naive representations in that the depth of the well tells little about the difficulty of getting from one point to another. Energy efficient deep space transportation depends on a subtle understanding of weak stability boundaries and the related interplanetary superhighway.
See Ed Belbruno and Martin Lo in “New Trends in Astrodynamics and Applications III” from the American Institute of Physics.
“Where have you seen that? I’ve heard a lot of people put it forth as a strawman, but I’ve never actually seen it.”
Typically, this comes up in the Space.com article comment threads now and then. (one can take that as seriously or not, as one likes) Guys who want to take the long way around (and don’t seem to realize it), just to launch a finished ship out of a shallower gravity well.
And I recall, way back in my FidoNet BBS days on the Space Development Echo, an exchange with someone who proposed getting uranium (with zero idea at the time, of how much might be there) for nuclear thermal rockets…rather than, as I said, improve the safety reliability of launching fissionable materials from Earth.
This, too, resurfaced in Space.com comments, when word recently came down that some meaningful concentrations of uranium do exist on the Moon. I made note of it here not long ago, as well:
http://www.transterrestrial.com/?p=21139
“Manufacturing propellants on the moon, on the other hand, doesn’t seem obviously unreasonable.”
Agreed. Lunar water may be too rare to turn into rocket fuel/oxidizer, but oxygen alone, is virtually unlimited in the regolith, and also has life-support and other uses. Go for it
Typically, this comes up in the Space.com article comment threads now and then.
I meant serious proposals, not clueless commenters.
This is so smart it actually moved me. I love when XKCD falls into a “perceived” rut with ceiling jokes and big stretches for punchlines… and then this. It’s like Randall’s been throwing fakes and now he got me in the chin with the right hook.