As I understand it, it's a practicality issue: with enough extra stages you can always escape, but the payload size rapidly approaches zero. (See: http://space.stackexchange.com/a/5383)

The size, complexity, and costs of extra stages make it impossible once you add in the actual real-world constraints. As said in Terr_'s link: "Rockets at 85% propellant and 15% structure and payload are on the extreme edge of our engineering ability to even fabricate (and to pay for!). They require constant engineering to keep flying."

Thank you for your reply - and thank all the others who replied.

This is a link on a NASA website :) and that organization is known to err in questions related to space. Seriously, guys - Don Pettit wrote it for general audience, and skimming the article I spotted this -

> Hydrogen-oxygen is the most energetic chemical reaction known for use in a human rated rocket. Chemistry is unable to give us any more.

This can be understood in a certain way - but also can be argued, as here - http://yarchive.net/space/rocket/fuels/hydrogen.html

> The highest measured Isp ever achieved with chemical propellants was with LH2/LF2/LLi (yes, liquid lithium), at 542s in vacuum.

Next, 15% structure certainly doesn't apply to some first stages - particularly recent Falcons from SpaceX.

The point which I was trying to make is I doubt that big of a difference. Sure, it's pretty hard to get to orbit from, say, Jupiter-like planet. However orbital speed for a planet with constant density is linear by the planet radius, and to get to New Horizons' ~16 km/s delta V as orbital speed the Earth would have to be 2 times bigger - 8 times heavier. And New Horizon's ain't cubesat either.

And, on top of that, we still have other readily available tricks up our sleeve. So I think even with rather large variations of Earth size we'd get in time to the stars.

Here are the numbers: http://www.nasa.gov/mission_pages/station/expeditions/expedi...

Note that the 50% figure refers to escaping fat-Earth's gravity well with a single launch, as opposed to putting things into orbit.

Its not about the payload. Its about the geometric growth in fuel required to lift - the fuel.

In theory you could always add more, ever larger stages to the bottom to keep increasing delta-V. But in practice there's a limit to how big we can make something. Weight increases as l^3 but strength as l^2 so at some point you just won't be able to create a large enough stage with existing materials.

How about putting two of the same stages alongside? No difference in mass ratio whatsoever.

Besides, we aren't actually at the limit of even modern materials. Rockets were built decades ago - and comparatively slowly evolved ever since. SpaceX demonstrates the hidden opportunities.

Staging isn't magic. It just lets you get rid of some dead weight. The fundamental problem is that the "future" fuel you need to carry with you gets too heavy to lift.