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Wouldn't it be possible if we launched from the moon instead? Most of the trust of rocket is loss to get out of the earth gravity well. (Not that we're anywher

Wouldn't it be possible if we launched from the moon instead? Most of the trust of rocket is loss to get out of the earth gravity well. (Not that we're anywhere near having a rocket factory on the moon... but maybe one could be assembled and fueled in orbit?)

ars 7y ago
The only reason to land on the moon would be if you could find fuel there. Which you can't. So that's a no for the moon, but something from orbit could make sense. Have a staging area and just accumulate tons of fuel from multiple launches.
- outworlder 7y ago
  > Which you can't Of course you can. > By atomic composition, the most abundant element found on the Moon is oxygen. It composes 60% of the Moon's crust by weight, followed by 16-17% silicon, 6-10% aluminum, 4-6% calcium, 3-6% magnesium, 2-5% iron, and 1-2% titanium. You have oxygen. You have aluminum. You can now make a solid rocket. There's some magnesium there too if you want to use that instead.
  - rbanffy 7y ago
    And aluminium, magnesium and titanium are great for building spacecraft, if we can get the mining and manufacturing capability there.
  - saalweachter 7y ago
    It's kind of hilarious that the hardest thing to find in the inner solar system, off Earth (and, of course, the Sun), isn't precisely water but hydrogen.
  - seppel 7y ago
    > You have oxygen. You have aluminum. Not exactly. You have aluminium oxide, which you could split into aluminium and oxygen using huge amounts of energy, like we do on earth. Helium-3 could be interesting, though.
    - behringer 7y ago
      Energy isn't a problem when you can build nuclear power plants without worry.
      - AnimalMuppet 7y ago
        You then have the problem of building the nuclear power plant, though...
btilly 7y ago
Most of the thrust of rocket is loss to get out of the earth gravity well. I believe that this is wrong. The Earth weighs 5.96e+24 kg and has radius 3.37e6 meters. The Sun weighs 1.98e30 kg and our orbit has radius 1.496e11 meters. That means that the potential well for getting away from Earth is about 11,800,000 joules/kg while for getting out of the Sun's gravity well is about 882,800,000 joules/kg. Assuming that I did the math right, that's about 7.5 times as hard. It therefore takes a lot more energy to climb out of the Solar System than it does to climb out of Earth's gravity well. Voyager got a LOT of energy from those gravitational slingshots.
- GlenTheMachine 7y ago
  You're trying to compare apples to apples here, when the situation we have is actually apples to oranges ;). A probe has to launch from the surface of the earth. But counterintuitively the probes start out already in solar orbit, even before they're launched -- because the earth is in solar orbit. More than half the energy required to achieve escape velocity is needed just to get into a roughly circular relatively low orbit around the gravitating body you're trying to escape from, and by virtue of being launched from the earth the probes get that velocity for free. Furthermore, the earth's orbit isn't really "low" with respect to the sun. We're fairly far out there, so the fraction of the energy needed to go from earth orbit to a solar escape trajectory is even less. Put another way: a probe launched from the earth gets no help leaving earth's gravity well. But once it does it gets a huge automatic gravity assist from the earth itself as it enters solar orbit.
  - Jeff_Brown 7y ago
    That was very helpful to read, thanks.
- outworlder 7y ago
  There's a saying among KSP players: "Orbit is halfway to anywhere". It takes a lot of energy to escape Earth's gravity well. It takes a comparatively tiny amount to escape the Sun's, as a sibling poster pointed out. It would be actually harder to visit, say, Mercury, as you now have to shed all the energy Earth has given you for free, in order to "fall" into the Sun's well. Voyager wanted to visit multiple planetary bodies – changing orbital parameters is not cheap. But if all it wanted was to get out of the system, burning straight out would probably be cheaper (in Delta-V terms). The closer to Earth the better, for the Oberth effect.
rbanffy 7y ago
> Wouldn't it be possible if we launched from the moon instead? It would be cheaper in fuel terms, and require less thrust, but it'd also require us to manufacture propellant on the lunar surface and fly everything we can't build there from Earth (and landing on the Moon is purely propulsive).

ars 7y ago

The only reason to land on the moon would be if you could find fuel there. Which you can't. So that's a no for the moon, but something from orbit could make sense. Have a staging area and just accumulate tons of fuel from multiple launches.

outworlder 7y ago
> Which you can't Of course you can. > By atomic composition, the most abundant element found on the Moon is oxygen. It composes 60% of the Moon's crust by weight, followed by 16-17% silicon, 6-10% aluminum, 4-6% calcium, 3-6% magnesium, 2-5% iron, and 1-2% titanium. You have oxygen. You have aluminum. You can now make a solid rocket. There's some magnesium there too if you want to use that instead.
- rbanffy 7y ago
  And aluminium, magnesium and titanium are great for building spacecraft, if we can get the mining and manufacturing capability there.
- saalweachter 7y ago
  It's kind of hilarious that the hardest thing to find in the inner solar system, off Earth (and, of course, the Sun), isn't precisely water but hydrogen.
- seppel 7y ago
  > You have oxygen. You have aluminum. Not exactly. You have aluminium oxide, which you could split into aluminium and oxygen using huge amounts of energy, like we do on earth. Helium-3 could be interesting, though.
  - behringer 7y ago
    Energy isn't a problem when you can build nuclear power plants without worry.
    - AnimalMuppet 7y ago
      You then have the problem of building the nuclear power plant, though...

btilly 7y ago

Most of the thrust of rocket is loss to get out of the earth gravity well. I believe that this is wrong. The Earth weighs 5.96e+24 kg and has radius 3.37e6 meters. The Sun weighs 1.98e30 kg and our orbit has radius 1.496e11 meters. That means that the potential well for getting away from Earth is about 11,800,000 joules/kg while for getting out of the Sun's gravity well is about 882,800,000 joules/kg. Assuming that I did the math right, that's about 7.5 times as hard. It therefore takes a lot more energy to climb out of the Solar System than it does to climb out of Earth's gravity well. Voyager got a LOT of energy from those gravitational slingshots.

GlenTheMachine 7y ago
You're trying to compare apples to apples here, when the situation we have is actually apples to oranges ;). A probe has to launch from the surface of the earth. But counterintuitively the probes start out already in solar orbit, even before they're launched -- because the earth is in solar orbit. More than half the energy required to achieve escape velocity is needed just to get into a roughly circular relatively low orbit around the gravitating body you're trying to escape from, and by virtue of being launched from the earth the probes get that velocity for free. Furthermore, the earth's orbit isn't really "low" with respect to the sun. We're fairly far out there, so the fraction of the energy needed to go from earth orbit to a solar escape trajectory is even less. Put another way: a probe launched from the earth gets no help leaving earth's gravity well. But once it does it gets a huge automatic gravity assist from the earth itself as it enters solar orbit.
- Jeff_Brown 7y ago
  That was very helpful to read, thanks.
outworlder 7y ago
There's a saying among KSP players: "Orbit is halfway to anywhere". It takes a lot of energy to escape Earth's gravity well. It takes a comparatively tiny amount to escape the Sun's, as a sibling poster pointed out. It would be actually harder to visit, say, Mercury, as you now have to shed all the energy Earth has given you for free, in order to "fall" into the Sun's well. Voyager wanted to visit multiple planetary bodies – changing orbital parameters is not cheap. But if all it wanted was to get out of the system, burning straight out would probably be cheaper (in Delta-V terms). The closer to Earth the better, for the Oberth effect.

rbanffy 7y ago

> Wouldn't it be possible if we launched from the moon instead? It would be cheaper in fuel terms, and require less thrust, but it'd also require us to manufacture propellant on the lunar surface and fly everything we can't build there from Earth (and landing on the Moon is purely propulsive).