Re: 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:

>>>>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.

>>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.

So what?  The amount of fuel expense I assume would be enough to handle
light _days_ of lateral maneuvering.  Remember that this ship is
using an entire 1/3 of its fuel capacity in order to make the 200
intercepts.  To a rough degree, this allows 4/400 light years of
maneuvering (assuming the design target system is Alpha Centauri,
4 light years away).

I am assuming what I consider to be absolutely obviously worse than
would ever be the case.

>Two: the power savings of the laser array might be minimal.

Wrong.  The power required is proportional to the mass of the sailships.

>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.

Why are higher boost rates required?  Because of diffraction limits,
you need to boost with the laser near sol to begin with.

Besides, even if we assume the boost rate needs to be 10 _times_ as
much, the new design is _still_ only about 10% of the cost of the
original.

The fusion motors already need to be relatively high thrust in order to
complete the deceleration run in a reasonable amount of time.  I doubt
any increased thrust requirement is going to increase the payload mass
by more than 100% more (the assumed increased payload mass including
the scoop).

You have to show that these factors some how add up to making it cost
at least 50 times as much before the original fuel/sail design becomes
competitive.

>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.

In this design I do assume the fuel and ship need to meet at around
the same velocity.  I also assume the intercept is accomplished entirely
using the ship's maneuvering (which is ridiculous, of course).  That
is why I assume the ship blows such outrageous amounts of fuel making
those intercepts.

However, the fuel does _not_ need to be launched at exactly the same
time as the ship.  If we assume Starwisp like acceleration runs with the
laser, 200 shots will take about 400 weeks.  Since the laser isn't used
to power the ship, these can start before and during the ship's
acceleration run.  (The ship catches up with the shots fired
before/during the acceleration run; the shots fired afterward catch up
with the ship.)  If half of the shots are shot before and half
afterward, they'll only be about 200 weeks off.

I would continue with back-of-the-envelope calculations to show you
that these fuel shots could be intercepted for far less than the
amount of fuel I describe, but now that I think about it it isn't
worth it, because you probably will simply dismiss it without even
conjuring up a single rough number estimate to back your claims up.

Kelly, if you want to talk numbers, then this argument can go further.
It's all about overall cost.  I have made a bold claim--that the
modified design is about 1% the cost of the original fuel/sail design.
I've given at least rough calculations to show why.  If you want
to dismiss my argument, do it with NUMBERS.

>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)

No, the total weight in the modified design is 6 times as much as
the weight before.  Pay attention.  As I recall from the discussion
of the acceleration/deceleration track ramjet design, you show
difficulty paying attention.

>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.

Actually it does require acceleration close to sol because of
diffraction limits.  How close?  You tell me how big you want
the huge honking laser's lens to be, and what frequency it's
using.

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

Back it up with some numbers.  I don't ask for precise ones, just
rough guestimates.

>>>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?

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

Okay.  But if you really did _any_ numbers on this design, you'd see
that the power levels of the masers were _much_ higher than that of
the fusion motor.

Really, this should be quite obvious.  If the on board fusion reactor
produced about the same power level as the masers, then it could
be used directly to power a laser in order to make the deceleration
run (and/or the acceleration run).  But that's completely ludicrous.

>> 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.

How much more?

>>>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.

>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.

That's not the way science works.  You don't just funnel in money, and
get results proportional to your money.  No matter how much you spend,
R&D requires experiments which take a certain amount of time to
accomplish.

And even then there's no guarantee of success.

The fact is that we could build a vehicle powered primarily by D-D
reactions _today_ (a crude Orion rocket).  Within 10 or 20 years,
we could even do a pretty good job of it (a refined MagOrion).
By the time we can even consider a manned interstellar mission,
it should be easy technology.

The fact is that there is no telling how long it would take to power
something with lithium fusion, no matter how much money is spent
researching it.

How long do you think it will take, assuming we spend 100 billion
dollars a year on fusion research specifically aimed at acheiving
lithium fusion?  Rough estimate?  Ten years?  100 years?  500 years?

>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.

Back it up with some numbers.  Just rough guestimates.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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