Re: The fuelsail is stupid (was starship-design: Hull Materials)

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

KellySt@aol.com wrote:
>In a message dated 11/15/97 5:31:48 AM, kuo@bit.csc.lsu.edu wrote:

>>The fuelsail concept is stupid.  The reason is that if it is possible to
>>build such a thing with one's technology, it is much easier to build
>>a variant of it--and probably a straight fusion rocket would _greatly_
>>outperform it.

[...]

>>So there you have it.  If the fuel/sail concept is even possible,
>>there is no good reason to do it.  Even if the scoop I mentionned
>>weighed 10 _times_ as much as the payload, you'll probably save
>>in the cost of the laser more than you'll save in the cost of the
>>lithium.

>The concept you outlined is a mild variation of my Explorer class.

I'm aware of this.  However, I explained in detail why this specific
modification of a theoretical fuel/sail design would be better.

>I'E. the
>ship accelerates out using onboard fuel or fuel delivered to it as is needed.
> (for safty carrying the full decel fuel load from the start.)

In this design I assume that the ship itself has to maneuver in order
to catch the fuel packages.  I assume this requires 50% more fuel
just to catch the fuel packages!  And I assume the scoop used weighs
as much as the rest of the payload!

This increases the fuel requirements by a factor of 6.

However, considering the laser's cost is reduced by a factor of 100,
it's easily worth it.  Consider that the laser will weigh many
magnitudes more than the fueled starship in the fuel/sail design.

For simplicity's sake, lets assume the laser is merely 1000 times as
heavy as the starship.  The new design will require 6 times as much
fuel, but only 1% as much laser.  Overall, the total mass of the
system is reduced by 99%.

Kilogram for kilogram, the laser is going to cost at least as much
as the fuel packages.  Probably it will cost a _lot_ more, pound
for pound.  Thus, the new system will cost about 1% as much as
the fuel/sail design.

>Given all that it seemed far simpler and safer to use the fuel as the
>acceleration sail for the ship.

It costs and weighs 100 times as much as this alternative.  And that's
assuming the scoop needed weighs an incredible amount and an incredible
amount of fuel is wasted making fuel intercepts by moving the entire
ship.

>The expence of the launching maser platforms
>is considerable, but the power levels and fusion motor needed are less,
>acceleration can take place over longer times, and you don't need any precise
>intercepts.

The power level of the maser platform is _much_ higher than anything
else in the entire system.  What in the world in the entire starship
system has comparable power levels?

Assuming the new design requires a scoop which weighs as much as the
entire payload, the fusion motor only needs to be twice as powerful.
Even if this resulted in spiraling costs making 10 times as much
fuel required, the overal cost of 60 times as much fuel is dwarfed
by the savings in laser.

>>That said, the new design is pretty stupid as well.  Why bother
>>using lithium at all?  It's going to be a lot harder to develop
>>a lithium/hydrogen fusion reactor than a D-D or D-T reactor.
>>Why wait an extra hundred years for a lithium/hydrogen fusion
>>reactor when you can use D-D or D-T today?  At worst, you can
>>get interstellar capable Isp levels using MagOrion (H-bombs
>>pushing a huge superconducting loop).

>D-D or D-T need a tank (one likely to outweigh the unfueled starship), will
>boil off into space over the years, and are very rare and expensive.  Lithium
>is extreamly common and cheap (well under a dollar a pound assuming you
>refine medical grade Lithium to Lithum-6), can be chemically bonded with
>hydrogen to carry it, and can be used as a structural metal.

However the simple fact is that if you can't achieve Lithium-hydrogen
fusion, you can't use it.  Period.  D-D or D-T fusion will be acheived
much sooner, probably.

>The difficulty of constructing a lithium/hydrogen fusion reactor is
>comparativly simple compared to our other technical problems.  So its
>unlikely to be a major cost or schedule driver.

Only if you include the massive cost of that incredible laser.  Which
is, in the fuel/sail design, THE overriding cost.  Nothing else comes
close.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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