– could the space shuttle just fly into space like a plane, rather than being propelled vertically by rockets?
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As you rise higher above the surface, the air density reduces. This means you need larger, more efficient wings to maintain lift – or you need bigger engines to go faster to get more lift from the same size wing.
Both of these things require more *weight* which sends you back around the same spiral of needing either bigger wings or bigger engines. You also start to reach the limits of materials – for example, the bending forces in the wing structure and wing roots grow, so you either need a stronger material or more weight to keep things strong enough.
To get into orbit, you also need to gain a vast amount of speed – for example the ISS orbits at 7660m/s or about 17,000 mph, at 400km altitude. If you attempt to gain that sort of speed inside earth’s atmosphere the drag losses become unsustainable. For reference, the SR-71 flew at 2400mph at 27km in altitude and the air friction was so severe that the entire plane is titanium.
Therefore, one way to think about it is that the vertical launch profile is the quickest way to punch through the thick air as quickly as possible and minimise the frictional losses. Then you can accelerate to orbital speeds with minimal losses once above ~100km altitude.
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