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

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.
>KellySt@aol.com wrote:
>>In a message dated 11/14/97 11:00:59 AM, lparker@cacaphony.net wrote:
>>>Why was the sail made of lithium?
>>Lithium-6 and hydrogen can fuel a fusion reactor with out producing
>>or heat it noatable quantityies.  I.E. the microwave sail is collected and
>>burned as the deceleration fuel.  Hence the name Fuel/Sail.  ;)
>The basic cause of the problem with the fuel/sail concept is that it
>requires using an excellent fusion rocket to decelerate it.  If you've
>got such a good rocket for the deceleration run, you could have used
>it for _both_ the acceleration and deceleration run.  All the "sail"
>part of the fuel/sail really gets you is that it forces you to build
>this huge honking laser.
>Here's the proof:
>  Suppose you have a practical fuel/sail design capable of a one way
>  trip in the amount of time you desire using a laser with a power
>  level of X watts.  I will show you can modify the design so that
>  you will have a new design capable of satisfying all your mission
>  parameters, but with a laser of only X/100 watts.
>  The new design has a vehicle which is identical to the original
>  design, except it has no sail.  Instead it just has a fuel "tank"
>  which is a lump of lithium.  In addition, this vehicle has a
>  large scoop (magnetic and/or otherwise).  This will add some
>  weight, but only if the original design didn't already have
>  such a scoop for the fusion rocket nozzle.  I will assume
>  it _doubles_ the payload weight of the vehicle.
>  What is this scoop used for?  It's used to catch fuel, which consists
>  of lithium sails launched using a laser of X/100 watts.  Each of
>  these sails is merely 1/100th of the mass of the original starship,
>  but a total of 300 will be launched.
>  The new design has a fuel "tank" twice as big as the original
>  design.  It uses the fusion rocket for its acceleration run,
>  which uses most of its fuel.
>  Shortly before the acceleration run is finished, the launch of
>  the lithium sails starts, one at a time.  The velocity of each
>  of these sails is set to be a little bit faster than the
>  starship's cruise velocity.
>  While cruising, the lithium sails will catch up with the starship,
>  which uses rocket thrusts to maneuver into position in order to
>  catch the sails with its scoop.  Eventually the starship will be
>  refueled--theoretically this should take 200 sails, but I assume
>  an incredible amount of fuel will be wasted in maneuvering to
>  catch the sails.
>  After the ship is refueled, it cruises the rest of the way.
>  Near the target system, the starship uses its fusion drive to
>  decelerate.
>  This new design will work assuming the original design would have
>  worked, but it requires 1/100 the power in laser (which is easily
>  the biggest ticket item, in terms of how much power is needed and
>  probably cost).  More lithium is needed, but the lithium sails
>  can be manufactured while the ship is already underway.  Importantly,
>  the big honking laser of the original design will obviously be
>  much, much, heavier than the entire amount of lithium needed for
>  the latter design.  Even if the laser were as inexpensive per ton
>  as lithium sails, the latter design is much less expensive.
>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'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 that
design I laser launched fuel to the accelerating ship.  The problem with this
was geting the ships and fuel packets together.  Given the very high
velocities and distences involved, it seemed pretty unlikely you could boost
a fuel packet to the exact possition and speed of the receeding ship at
distences of a half light year or so (slight aim poroblems, or mis
calculations of the ships possition, would make it impossible for the ship to
catch the package).  That pushed the design to have extream acceleration at
the start (the farther you get from the sol launchers, the more extream the
drift), and heavy counter lasers to manuver the fuel to it at distences of
several light minuttes.  

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

>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.  So the 'fuel'
can be spread out into a mesh sail.  Given the decel fuel would need to weigh
about 400 times the dry weight of the ship, this dramatically simplifies the
sail carry structure and weight.

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, a straigh fusion rocket would need to carry 400 times as much fuel to
perform as well as a fuel sail configuration.  Actually more then that since
it would also need to carry engines 400 times heavyier, hence still more fuel
to drive the heavyier ship.

>    _____     Isaac Kuo