: Jim Davis
: Firstly, those aren't the all the alternatives. There is the "have
: lots of energy, use a large fraction" option (gaseous fission for
: example).

They are, however, the alternatives that were under discussion.
Your claim was that it is, for some reason you haven't shared,
more impotant to be in the one category than the other. If that
wasn't your claim, what high-thrust, near-future technologies
do you have in mind, exactly?

: Secondly, the ones that have "lots of energy" are extremely expensive
: compared to the ones that "have very little energy".

Near as I can tell, that turns out not to be the case.
Why would nuclear pulse propulsion be more expensive in terms of delta-v
per given mass? Indeed the reverse seems to be the case; chemical
rockets and their infrastructure would be many times as expensive, to
deliver a given payload to, say mars. Such as an exploratory
expedition. Do you have references to the costs you are concerned with?
How many dollars per ton are we talking, exactly?

: Not a good analogy.

Shrug. It illustrates the folly of an inappropriate definition
of "efficiency". And you still haven't explained why delta-v per
reacion mass is the wrong one (though see below).

: Do you imagine IC engines would be used for any application if its
: efficiency was 3 or 4 orders of magnitude less than that of Stirling
: engines?

If you really really wanted to fly a helecopter, yes.
But more to the point, recall that the comparison is to chemical fuels,
which are six orders of magnitude less energy per fuel mass. So, four
orders of magnitude less energy-efficient, times six orders of magnitude
more energy, still yields a hundred times more efective.

: The ratio of useful work done to available energy expended (the usual
: measure of efficiency) is very low for Orion and this is unfortunately
: coupled with the fact that the energy source for Orion (fissile
: material) is very expensive.

Which still doesn't explain it, since you've pretty much merely
substituted dollars for reaction mass, but haven't said why the million
times more mass per unit energy is cheaper. Again, how many dollars
per ton of payload are we talking, exactly? Or even approximately.


Wayne Throop http://sheol.org/throopw