[scrumbledober]

most of the energy spent getting to that 400km low earth orbit is spent accelerating to very very fast lateral speeds. If you use a balloon to get your rocket up as high as you can before launching it you limit the weight of the rocket you can lift up there, and most of the weight of rockets is used to gain that lateral speed. The plane + rocket idea has been shelved for now, as reusable rockets have decreased the price much more than launching a small rocket from 40,000 feet.

[mrkstu]

Isn’t that orthogonal to interplanetary travel though?

If you go straight up far enough you’ll be out of Earth’s gravity well.

You’ll still need acceleration to escape the solar gravity well, but you’ll never need horizontal acceleration necessary for Earth orbit.

[isostatic]

You want to accelerate lower down in the gravity well, however not too low due to atmosphere losses. It’s more efficient that way.

Launching from stationary altitude doesn’t save much at all

[0xffff2]

>If you go straight up far enough you’ll be out of Earth’s gravity well.

While you could theoretically do this by accelerating directly up, You still have to accelerate somehow.

[mrkstu]

Understood, but somehow the discussion ended up around achieving orbital velocity instead of escape velocity (the one relevant to the original sub-thread topic), two very different numbers (though escape velocity is ultimately a higher number.)

https://physicsabout.com/orbital-escape-velocity/

[0xffff2]

Sure, but they're not orthogonal at all. One is just a lot more delta-v than the other. It's the same basic thing. Point the rocket nozzle in the opposite direction that you want to go and turn it on (modulo the fact that the "direction you want to go" might not be intuitive because orbital mechanics).