And, yes, this thread is eerily reminiscent of when Isaac and I threadjacked Camillas posting of the article suggesting advanced ETs might "punish" us for Global Warming (frankly, that sounds like someone who watched the remake of "The Day the Earth Stood Still" too many times. )

Anyway, like I say, energy density seems like the biggest problem, in terms of mass as well as volume. As Isaac pointed out, burning the Amazon would provide enough energy to get Apollo 11s Lunar Lander to the Moon many times over, and whether it would give an object enough energy to escape the suns gravity well is as much a function of the objects mass as the energy available. The real questions are twofold:

1) Can the more than adequate energy be supplied at a great enough RATE to overcome the diminishing but constant pull of planetary and stellar gravity and

2) Is the energy source itself so massive that IT makes the answer to the first question "no"?

The Apollo vessel itself weighed a little over 43 tons (including fuel for use to and from the lunar service and back to Earth.) The Saturn V that launched it weighed about that much more than 3000 tons, and was near the limit of its lunar capacity of a 45 ton payload. It took that much just to get to the Moon and back, which is not even (technically) out of the Earths gravity well (else the Moon would not stay in orbit.) All 3039 tons of it was burned in just under 19 minutes. Wood could also do it--but it would take ~11,000 tons of wood burned in that same 19 minutes and focused in one direction on an object about the size of a bus, preferably without disintegrating it and any passengers. Just to get to the Moon, not Mars, Jupiter, Centauri Prime or Gliese 581g.

It gets a little complicated because the Saturn Vs first stage burned highly refined kerosene while the later ones burned liquid hydrogen. The reason is the problem I alluded to earlier: Liquid hydrogen has a much lower density than other conventional fuels, despite having a much higher energy density per unit mass. At low altitudes, the kerosene is more efficient, even though it has less energy per unit mass, because it takes up less space (among other reasons; it is also more stable and requires less energy to pressurize and cool.)

The problem is not getting the energy from the fuel, but doing so quickly enough to launch the craft and without the sheer mass and size of the fuel itself making any launch impossible. That is the kind of problem that led us to fossil fuels at the dawn of industrialization in the first place. As Isaac notes, a steam engine can be run as easily with wood as with anything else (the first ones did, after all.) Make it large enough and magnetic, then wrap a coil of wire around it, and it generates electricity. Unfortunately, it is impractical to run a large city power grid that way, so we generally use more compact energy sources like coal, oil and nuclear, though even garbage (which has less than half woods energy density) is used to a limited degree in some places. As Isaac noted in our earlier discussion, cars can run on wood, if one does not mind filling their back seat and trunk with logs.

The irony here is that not only does space travel require more compact energy sources like fossil fuels, the industrial era gave humanity the incentive to discover and develop them as well as the means to do so. There are others as good as and better than fossil fuels; fission, fusion and anti-matter come to mind, in ascending order of energy density. Yet the fact humanity has only harnessed the least energetic one of those testifies to how critical industrialization is for that purpose. Advanced extra-terrestrials do not need fossil fuels for interstellar travel, but DO need an equivalent to fuel the industrial and technological development that opens the door to interstellar travel. If an equivalent can be found and harnessed fossil fuels become irrelevant, but that seems likely to be a very big "if."