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

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 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.