Pluto-Charon downlift

As recently remarked elsewhere ...

For getting very much larger amounts of energy --
not necessarily in the form of heat,
although that's certainly an option --
than U and Th can give, finding high-up mass
and lifting it down is interesting.

Robot spaceships built at Pluto-Charon out of local materials
that boost themselves by IIRC 4.7 km/s
to cancel the orbital speed of their construction site
can then drop Sunward; as they pass the Earth's orbit
they have ~880 MJ/kg of kinetic energy,
about 88 times the chemical potential energy of oil-plus-O2.

If they hit a lava lake on the Moon
that has been built for the purpose, it gets hotter.
(The lava should be fountained upwards just before they
hit so their disintegration happens along a tunnel
through a droplet cloud, and the bright light of it
is absorbed within the cloud.)

But maybe only 100-fold oil-plus-O2 equivalency isn't
enough; maybe a pound of robot costs more than 100 pounds
of oil-plus-O2. Probably.

So let them drop further down. *Way* down.
Let them drop down to skim the Sun's surface at, IIRC,
620 km/s if they don't fire their engines. But they do:
while Sun-skimming they keep cool by circulating
Pluto- or Charon-derived hydrogen through their ventral skin,
then letting it out rocket nozzles aft. So they accelerate
to 630 km/s.

And then they swing back up out, and this time, on the upstroke,
they don't miss the moon and the lava fountain.
They hit it at 120 km/s, four times faster than they would have
earlier. But they're lighter, having jetted off that hydrogen,
so their energy is, I guess, only about three times what it
would have been without the Oberth maneuver.

Still, a whole binary planet that no-one would have missed
before 1931 is energetically worth, like, a thousand times
its mass in oil. Lots of uranium ores are better, but
a thousand is pretty good.