> So...you couldn't just like launch it then fly it...because of the weight? Y Precisely. Rockets use liquid oxygen and cryogenic, extremely weight efficient f

> So...you couldn't just like launch it then fly it...because of the weight? Y Precisely. Rockets use liquid oxygen and cryogenic, extremely weight efficient fuels (usually liquid hydrogen or liquid methane). They are pretty much at the edge of practical chemical energy density. > Does it make any difference if you just have to fly in atmosphere, and not escape Earth's gravity? But they have to fly for a much longer time, with atmospheric drag. If you look up examples like the Me 163 and the X15, you will see that their range is extremely short: https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/North_American_X-15 https://en.wikipedia.org/wiki/North_American_X-15 The reasons rockets use rockets (and not jets) is because their flight time in the atmosphere is even shorter, and they need a ton of thrust in that short period, not because they are particularly efficient.