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

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

>>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
>> (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.

Two assumptions I'm not comfortable with.  One: given the major delta V
requirements for manuvers to intercept fuel ships, assuming they are close
enough together that the mainship could intercept them (were talking about
fractions of light speed and potentially light minutes of lateral drift.

Two: the power savings of the laser array might be minimal.  The need to
intercept with the followon fuel 'ships' kinda forces you to use far higher
boost rates so you get up to speed closer to sol, and fusion motors with
higher thrust to boost faster.  Also since the fuel and ship have to meet at
nearly the same speed and position (at least within 1% or less) the fuel will
need to be launched at the same time the ship boosts out.  Since your talking
about the same total weight, 'sailing' out at about the same time as
fuel/sail (but with far greater delta-V needs and dry weight for the ship)
your likely to have similar power requirements for the launcher masers as a
fuel sail boosting at the same rate.  But fuel/sail doesn't need to boost out
at the same rate since it doesn't need to do its bosting (and the fuel
intercepts) close to the sol system.

All in all, I'ld expect fuel sail to use less maser power to boost out.

> ...
>>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.

That weight and power savings seems unlikely.

>>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
>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?

I was reffering to the power needs of the maser launcher platforms, and those
of the fusion boost motor.

> 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.

The higher acceleration rates needed by the main ship would probably require
the motor be scaled up more than that.

>>>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.
>>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.

Possibly,  but the development time can be accelerated considferably given a
R&D budget.  Given the needs of this project would require a huge budget, the
funds to develop a fusion system to our needs would be trivial.  Given the
ship would require tens of millions of tons of decel fuel, the fuel cost
savings of Lithium-6 + P, over D-D or D-T (not to mention the relative
unavaliblity of the later fuels) could easily pay for the development of the
better reactors.  Hence my comment:

>>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.

Oh I agree by the way that the cost of the maser stations to boost the ship
out would be the dominent cost factor of a maser sail systems, but don't
think you variation would reduce that.

    _____     Isaac Kuo