[ta1234567890]

Exactly

From the reference article:

> Travelling from the surface of Earth to Earth orbit is one of the most energy intensive steps of going anywhere else. This first step, about 400 kilometers away from Earth, requires half of the total energy needed to go to the surface of Mars.

Which means that if we use something like a balloon/blimp in the first stage, it would be a lot more energy efficient.

Anyone knows why it's not done that way already?

Also, whatever happened with the plane+rocket Virgin Galactic project?

[akiselev]

The Pegasus rocket used to launch satellites from a modified 747 for between 1990 and 2016 [1]. It just doesn't work out well in practice because the cost of the modified plane far exceeds the cost savings from a first stage unless you're doing half a dozen or more launches per year.

[1] https://en.wikipedia.org/wiki/Pegasus_(rocket)

[isostatic]

Virgin Orbital are doing something similar - satellite launching from a 747.

[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).

[yourbandsucks]

Orbit isn't a height, it's a speed. 7 km/sec for low earth orbit. A balloon floating you higher while not making mach 20 isn't much of a shortcut.

[kadoban]

How poor of a shortcut is it? I'm not really clear on what the math would look like. Would it just be better by approximately the ratio of gravity's strength at the surface vs the height of the balloon?

[usrusr]

Better than the miniscule gain in potential energy (height x mass x gravity) because you also save a lot of aerodynamic drag not punching through the lower atmosphere at high speed and some "hovering losses" (rise time x mass x gravity, I'm sure there is a better term for that). But still not worthwhile due to the difficulty of floating a full size rocket. If you had a very very high mountain or tower, moving that full size rocket to the peak could be worthwhile. Theoretically, a civilisation trapped in a gravity well too deep for a solution to the rocket equation could dig themselves out by reshaping their planet from a roughly spherical geoid into a disc or rod (or into a torus for the really adventurous)

[saagarjha]

I doubt such a planet would be hydrostatically stable, though…

[outworlder]

> "hovering losses"

Gravity drag

[yourbandsucks]

Basically just saves what you would've lost to wind resistance and gravity while getting to that height, while adding the complication of accelerating from 0 to orbital before falling back into the atmo.

Space elevator ideas usually have the hop-off point all the way out at geosynchronous orbit to solve the velocity problem. Which is.. a really tall elevator.

[crubier]

Because the problem with space is not at all the altitude necessary to reach it, but it is the speed necessary to remain in it.

So a balloon/plane/blimp/very high building will only help very marginally.

https://what-if.xkcd.com/58/