Also you'd be fighting conservation of momentum - if the material you gather isn't going your speed and direction, you lose some of your speed and direction to

Also you'd be fighting conservation of momentum - if the material you gather isn't going your speed and direction, you lose some of your speed and direction to pick it up. Given how fast anything has to go to reach orbit (about 17000mph for low Earth orbit) and escape velocity is about 41% higher, that speed difference would be a major problem. For atmospheric flight the closest thing is a jet engine which is "air breathing", i.e. requires air to run and works in part by sucking air in, using some of the Oxygen in it for combustion, and shoving the extra air out the back. This gives them much higher efficiency than chemical rocket engines, as they don't have to carry their own oxidizer or the mass to eject for propulsion - but only works because the air is dense enough where they operate. Which is not true for space flight. Commercial jets typically fly a bit under the speed of sound in air which is 767mph. Of course supersonic air craft exist - I think mach 5 is achievable by some military jets - but that's still a fraction of orbital velocity. Anyhow running in the atmosphere means jets don't "scoop up air" in any sense but rather just use it immediately - so any lost momentum can be immediately countered by the engine. Another direction to think about it - a simple model for air resistance says that air resistance increases with the square of your velocity (and with the density of air, which exponentially decays with height though also depends on temperature. I think the height wins out though. Not sure if the equation works at extremely high altitude. But certainly this favors using air at lower altitudes and probably also lower speeds https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air