Launching space missions from LEO vs the ground

Some people have suggested building a spacecraft, for example for a Mars mission, in LEO rather than building it on the ground. Obviously, this would be greatly more expensive and more difficult. However, it does have a big advantage for the mission itself, because the mission starts out having already overcome a large chunk of earth’s gravity.

This may be a silly question, but it’s something I’ve wondered about off and on for some time.

If a Saturn V were assembled in LEO, would it have enough fuel for a trip to Mars? Or (more unlikely yet) if it were assembled on the Moon, how would that affect a trip to Mars?

One issue with your example is that you still haven’t gotten around the need and expense of lifting the entire mass of a Saturn V up into LEO – just because you’re assembling it up there doesn’t mean that the materials are magically already up there. And, that includes lifting all of the fuel (or developing a way to generate the fuel in LEO).

But, it’s not just the mass of the vehicle – you would also have to build an entire orbital factory in LEO in order to assemble the vehicle.

Edit: Sorry, I misread your OP question – you weren’t ignoring those issues, you acknowledged them, and your question was purely about the fuel.

You have to remember the 9/1 ratio rule, for every pound of stuff that you lift into orbit you use nine pounds of fuel. For an orbital construction site regardless of what ever your constructing, you need a habitat with a storm shelter. Orbital construction sites are still a work place, so we don’t even know how OSHA will rule. Just thinking about how Ralph is gonna station lawyer up and say i gots the right to refuse working conditions, gets me amused.

So get your Kerbils and design the spaceship and the only reason you build it in space, is because your going to armor it and not just have a coke can with a thruster on the ass end. Otherwise its simpler to build 10 or 12 sections and send them up individually to be mated and that can be done remotely. After that, you move the ship to a higher orbit and add the reactor and nuclear warheads.

Its expensive, but less work and more stable working conditions to build it on earth. For the asteroid belt, obviously that would be a different story.

This graphical chart gives a general idea of the energy cost of classical manoeuvres in the Solar System. If you start out in LEO, you would “save” 9.4 or so km/s , which is a lot, but you will have had to somehow haul all that stuff up there in the first place.

I also figure that for a space/vacuum launch you would engineer a rocket optimized for those conditions, not recycle a 100-year-old design.

Maximum speed of a Saturn-V will depend on the mass of the payload- obviously it is not going to be empty. There are people here who have run the numbers and can tell you how much stuff you need to make it to Mars (robotic mission? manned?)

Keep in mind that for Apollo the Saturn V’s third stage burned for 2 1/2 minutes to complete the Earth to Orbit sequence and then for 6 minutes for the Trans-Lunar Injection. This was enough to send the Command Module, Lunar Module and Service Module to the Moon at a velocity of 25,053 mph. This is barely above Escape Velocity, but they were aimed at the Moon and the Moon’s gravity provided significant acceleration for the 3 day trip. For Mars you don’t have this assist.
However, to answer your question a Saturn V is a monster if launching from LEO.