Expert: Steve Nelson - 12/7/2011

Question
QUESTION: I wonder why they continue to launch deep space missions from the earth's surface when the vehicle has always to contend with maximum earth gravity plus air-resistance.
It is obvious that a huge amount of fuel is needed just to achieve escape velocity and get into orbit. Since now we have expertise at moving large amounts of hardware into orbit, wouldn't it make more sense to  build the vehicle (assemble) several hundred miles above the earth so that all that fuel could be used to achieve a very high velocity? For a Mars expedition for example.

ANSWER: You still have to get the materials up to orbit to build the vehicle out of.  That takes more energy than just getting the finished product up to orbit.  And orbital construction is tricky under the best of circumstances.  You'd have to bring up tools and machinery that you could've left on Earth.  In short, it would cost ridiculous amount MORE energy to build something in orbit unless you could find the materials already up there...and probably even then.

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QUESTION: Thank you for your answer. But it looks like I did not ask my question properly; sorry about that.
In this case, I am not concerned about energy expenditures. (for now,- can we make this academic instead of practical?) Let me put it this way instead. Lifting off from the earth's surface, using X boosters, and X amount of fuel, what is the maximum velocity of our space vehicle by the time it gets say, one million miles from earth? (ballpark would suffice!)
Now, (second part for comparison):- If that same ship with the same boosters/amount of fuel, was launched from a point in space, say- three hundred miles away from earth, - what is the velocity it would achieve when it reached a million miles from earth? The purpose of my question really is to find out if higher velocities could be achieved in order to significantly reduce the time it takes to get to a very distant object within the solar system.
P.S. I know what you are saying about the difficulties about assembly in space, but a modularly-designed ship might be the answer (using a similar assembly-method as in the space station) Many thanks- Alan

Answer
You should be concerned about energy expenditures, they're the primary consideration in spaceflight.  And since the Earth's diameter is 6,380,000 m and orbital radius that's safe for astronauts to work in (radiation blocked by Earth's magnetic field) is under 100,000 m, it's practically on the surface and there's no significant energy difference.  If you're talking about building it on the Moon, then you have an idea, but the energy difference of launching a ship from orbit vs. from the surface is practically nothing.  And that's on top of the difficulties I already mentioned, but really the energy difference is almost nothing and really less than that.  If you have to assemble it in space you still have to get the parts up there, which means you have to get the rockets that lift them up there, which means you actually lose out, energy-wise, just lifting the components to orbit separately more than nullifies the energy savings of pushing it out to its final destination.