Why can’t the space shuttle just go slow enough to not be heated up by friction with earths atmosphere on re-entry?
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Real life orbit is just falling fast enough in a sideways direction that your altitude is maintained above the friction of earth’s atmosphere, so you dont slow down, (but not *too* fast or you’ll escape orbit). This is very different from most examples of future spaceships we see in sci-fi, which have seemingly limitless energy reserves to magically overcome the incredibly strong and constant pull of a planet’s Gravity.
Irl, Without a very high sideways speed relative to earth you need to spend constant fuel to accellerate 9.8 m/s² moving away from Earth to counteract the 9.8m/s² of gravity pulling on you, as you float nearby in space.
But if you start out with your speed moving sideways fast enough, with the direction pointed in a tangent direction to the earth (i.e. above the atmosphere tracing the equator), then you can stop spending fuel once you’re in outer space. The sideways momentum doesn’t need fuel to maintain it once there’s no more air molecules to slow you down. The constant pull of gravity keeps your ship curving around in orbit at a stable altitude so you wont fly straight off into the sun or out of the solar system. The pull is constant so by speeding up or slowing down sideways, you orbit higher or lower.
It would be mathematically impractical or impossible to carry enough fuel to descend straight down into atmosphere from above. The gravitational pull simply too constant and atmosphere is too thin to go slow enough at a direct angle down (like a helicopter / elevator). The gradual descent at a slight angle is the only way to reliably not become a meteorite at the bottom. It gives you time to release all the momentum stored up when you first launched into orbit, as friction as you decelerate.
So basically we turn the energy from launch and getting into orbit as a reserve of momentum that keeps the ship above the planet for free, but we have to eventually lose the momentum to descend. It wouldn’t make sense to spend fuel slowing down from 8000 m/s sideways to 0 sideways while also spending fuel thrusting up to keep from dropping like a rock. Instead we use the atmosphere as a aerobrake so that the only fuel needed to decend is the fuel needed to move into some atmospheric air molecules from the zone above them.
If you’re interested in learning this stuff about orbital mechanics intuitively I have to recommend the sim game Kerbal Space Program, which is like a combination spaceship builder, launch & orbit simulator, and space explorer management game.
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