Which rocket launch would generate more velocity?
The argument is this:
Two identical rockets prepared to launch, one from the lunar surface, and one from an orbitting space station above. Which rocket achieves greater velocity, and thus arrives at its’ destination quicker?
The rocket at the orbiting space station is already at orbital velocity, so it has a significant head start.
This is a meaningless question, because in practice, rocket performance is limited by how much propellant you have onboard.
Well, if the rocket on the station has X kilograms of propellant, that propellant must have come from somewhere, probably a planetary body like the earth or the moon.
So in effect there is no difference between launching from the moon with a big rocket with multiple stages and waiting in orbit for a bit, or docking with the station and launching the main mission later.
I assume that the rocket is in lunar orbit? As long as the initial burn is in the same direction as the moon’s orbit, the forces will add, and it will go faster.
Yes, and if the burn is in the opposite direction as the orbit, the orbiting rocket will be slower. But if you don’t want this to happen just wait half an orbit and you can add your orbital velocity rather than subtract.
“Zero gravity” just means that you’re falling at the same rate as the objects around you and therefore experience no acceleration. If you’re orbiting the Earth you feel zero gravity, but that’s not because the Earth’s gravity doesn’t affect you. The Earth’s gravity is pulling you towards Earth, it’s just that you have a sideways vector that exactly matches the amount you’re falling.
But, to achieve lunar (or Earth, or Mars, or whatever) escape velocity you have to expend fuel, and all that fuel does is get you to zero. So if you start at zero you’re better off because you didn’t need to use rocket fuel to get to zero.
So is something on the Moon.
No, an object on the surface of the moon is not at lunar orbital velocity. It may be traveling around the earth at the same speed as the moon, but when the OP referred to an orbiting space station above, I assume he meant orbiting the moon. Though the OP wasn’t exactly clear on his hypothetical.
I read the OP as a rocket at an earth orbiting space station. One, because an earth orbiting space station seems more logical and two, it would be a rather obvious answer if we were comparing a rocket orbiting the moon, and a rocket sitting on the moon.
Sorry gang, had to sleep…(rigors of night shift work)
What I was attempting to determine was poorly worded.
Identical rockets, identical fuel, same destination, same distance to travel.
Will the Lunar launch, achieving thrust from the solid foundation of the moon’s surface, reach greater velocity during it’s journey, or will the rocket launch from the stillness of zero gravity space, docked at a space station do so? Or would there be no difference at all ?
First question: Where is the space station? If it’s in low Earth orbit, it’s got a lot of gravity well to climb out of. If it’s in lunar orbit, the answer is more obvious.
The launch from the lunar surface needs to expend fuel to get out of the moon’s gravity well. It’s a lot shallower than Earth’s, but will still use up fuel that won’t later be available for acceleration in deep space.
A rocket engine doesn’t push against anything so the surface of the moon is inconsequential as far as what you’re picturing. The rocket from the moon needs to fight gravity as it comes out of the gravity well so will expend significantly more energy just to get to orbit. The rocket in orbit is still in the moons gravity well but is already moving at orbital speeds and has a full tank of gas. The rocket in orbit has significant advantage.
It seems like you’re under the misapprehension that rockets need something to push against for maximum efficiency. They do not. The only push is off of its own propellant as it leaves the engine.
You still haven’t spelled out the conditions exactly, but it’s all about delta-V, or the velocity increase required to get to where you want. The moon’s surface is inside the moon’s gravity well, and requires about 1.9 km/second of delta-V to get out of (to lunar orbit).
As others have said, whether that’s more or less delta-V than some other place depends on where that place is. Getting from Low Earth Orbit to, say, Mars requires more delta-V than from the surface of the moon. However, it’s less than if you started in Geostationary Earth Orbit (there’s a nice table here).
You’ve also still got the misconception that “zero gravity space” (such as being docked to a space station) is “still”.
A space station, despite having zero gravity, is not still. It is orbiting the Earth (or the moon, or Mars, or the Sun, or whatever). To orbit the Earth you need to be traveling at a speed that exactly matches your falling speed.
If you’re talking about launching from the Moon vs launching from Earth orbit, think about this.
At the surface of the Earth, you feel an acceleration of 9.8 m/s^2. On the surface of the moon, you feel about one sixth that, or about 1.6 m/s^2.
So to have an advantage launching from Earth orbit, you’d have to be in an orbit where your escape velocity is smaller than the escape velocity at the lunar surface. Earth’s gravitational force decreases gradually. At low earth orbit, like where the International Space Station is, it’s greater than lunar. At high orbits, like geosynchronous, it’s lower.
So it depends on how high out of Earth’s gravity well you’ve already climbed. At some point you’ve climbed high enough that it’s exactly equal to the lunar surface, and higher than that and it is smaller.
Orbits are one of those things that are hard to picture intuitively. Many years ago Microsoft put out a program called “Space Simulator” that I messed around with for a while. It was a great teaching tool and really helped me to understand how orbits work. I’ve not seen Kerbal Space Program yet but I understand that it’s very good at teaching as well. If you have any interest in space then check one of those programs out - the only real downside is that you’ll never watch space movies the same way again!
I can verify that Kerbal Space Program is fun as hell and a good use of your entertainment dollars, if you have a computer that can run it. (It’s still in development, and poorly optimized.)
Take a look at some screenshots.
Depends largely on which direction you want to go. “Velocity” includes not only speed, but direction.
First, there is no “zero gravity”. There is “free fall”, where all the gravity pulling on you is unopposed by the surface of some planet or moon, which is the closest thing to “zero gravity”, except possibly being between galaxies, where the total gravitational forces on you would be the smallest (but still not zero).
Second, unless the destination is on the moon, the rocket in orbit around the moon has the advantage, because the rocket on the moon has to overcome the moon’s gravity and the one in orbit around the moon is past that hurdle (or has sidestepped it by coming from somewhere else). Of course, Musicat’s point is also very important! Furthermore, a destination has a velocity that you have to match. Regardless, in most cases, the one in orbit has a big advantage.
It is all about “Delta-V”, which is the energy required to get from one orbit to another. Google for more info.
That said, there are some really nifty tricks that allow zipping from one place to another with similar delta V, which I learned about here on the SDMB, but forget the technical term for. Unfortunately, though, “zipping” means getting there with minimal energy expenditure, but not necessarily quickly.
There’s also “Simple Rockets” on ios and android which is a more simplified 2D version but still teaches you the fundamentals of orbital mechanics.