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

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

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 radiation
>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
lithium.

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).
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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