Is there any reason that weather balloons can't be used to assist the launch of space vehicles?

You are actually mentioning a technology that WAS put to use in the early days of space exploration. See the following video to learn more about it: http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-35/ The bottom line to your question is: Been there...seen it...done that. We've already reached the limits of what we think balloon based space exploration can practically deliver. If you want to restrict your exploration to just the upper Earth atmosphere, which will STILL have some atmosphere above you...by the way...then by all means, go with balloon based exploration. BUT, if you want the craft to remain in orbit OUT of Earth's atmosphere, then use of a propellant is a MUCH better solution. If you tried to balloon-assist that spacecraft, you would need ridiculously large balloons. Weather balloons as our technology is today simply COULDN'T be of large enough diameter to lift the massive spacecraft+fuel units...EVEN THOUGH they have a semi-reduced requirement for fuel.

You are actually mentioning a technology that WAS put to use in the early days of space exploration. See the following video to learn more about it: http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-35/ The bottom line to your question is: Been there...seen it...done that. We've already reached the limits of what we think balloon based space exploration can practically deliver. If you want to restrict your exploration to just the upper Earth atmosphere, which will STILL have some atmosphere above you...by the way...then by all means, go with balloon based exploration. BUT, if you want the craft to remain in orbit OUT of Earth's atmosphere, then use of a propellant is a MUCH better solution. If you tried to balloon-assist that spacecraft, you would need ridiculously large balloons. Weather balloons as our technology is today simply COULDN'T be of large enough diameter to lift the massive spacecraft+fuel units...EVEN THOUGH they have a semi-reduced requirement for fuel.

Balloons have been used in the past to launch small rockets but it would take an impossibly large balloon, or group of balloons, to lift a large rocket like a Delta or Atlas, much less something as large as the space shuttle.

Balloons have been used in the past to launch small rockets but it would take an impossibly large balloon, or group of balloons, to lift a large rocket like a Delta or Atlas, much less something as large as the space shuttle.

The primary job of the rocket boosters is not to escape the atmosphere but to accelerate the payload to the speed required to remain in orbit.An object in low Earth orbit has to reach a speed of 17,500 mph if it is not to come crashing back down to Earth. You may be able to use weather balloons to lift things to great heights, but the object would still have to be accelerated to 17,500 mph or else it would simply crash back to Earth. Since the speed on a weather balloon is negligible compared to that, you gain no benefit from it at all, and the fuel requirements remaining to accelerate your satellite to the required speed make the thing too heavy to lift with helium balloons.

There've been a couple of proposed projects that employ that very principle. The only thing preventing it is the weight of the vehicle; even though you may be launching from 10 ro 20 miles up, you still need to carry an enourmous amount of fuel to boost the craft to orbital velocity. To my knowledge the "Balloon-Roc" project got as far as model testing, but no actual craft were launched.

Balloons rise due to buoyancy, once you get into the upper atmosphere there isn't enough medium to give any more buoyancy or thrust. I suppose you could use the balloon as a stepping stool, but I doubt a balloon would be able to lift the necessary fuel to get you the rest of the way out of the Earth's pull.

Balloons rise due to buoyancy, once you get into the upper atmosphere there isn't enough medium to give any more buoyancy or thrust. I suppose you could use the balloon as a stepping stool, but I doubt a balloon would be able to lift the necessary fuel to get you the rest of the way out of the Earth's pull.

There've been a couple of proposed projects that employ that very principle. The only thing preventing it is the weight of the vehicle; even though you may be launching from 10 ro 20 miles up, you still need to carry an enourmous amount of fuel to boost the craft to orbital velocity. To my knowledge the "Balloon-Roc" project got as far as model testing, but no actual craft were launched.