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

[KellySt@aol.com]

In a message dated 11/17/97 6:53:05 AM, kuo@bit.csc.lsu.edu wrote:

>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.

I'm not clear how many fuel packets you expect to launch.  Presumably
hundreds.  If each on average requires a 1% delta-v (and the worst delta-vs
will likely be the last ones launched to catch the ship at is max intercept
range)  you could be talking about acceling and deceling the ship more then ne
ssisary to reach cruse speed.  The alternat is an extensive beaming systems
(and support power systems) to steer the packets to the main ship.  fuel
consuption to beam the fuel ships to a docking could take as much as the
acceleration fuel for the ship, and be more complicated.


>>Two: the power savings of the laser array might be minimal.
>
>Wrong.  The power required is proportional to the mass of the sailships.

You have to send the same mass of sail ships, since it still must total up to
the same mass of fuel.  More if your assuming loses, fuel consuption for
catching the fuel by the ship, and extra guidence and structure for the
separte sailships as apposed to the major fuel sail.

>>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.

Since the ship has to get up to speed, and do all its intercepts pretty close
to sol, it has to boost harder to do all that in the shorter distence.  I.E.
it has to launch under pure fusion thrust while the maser arrays are
launching hundreds of sailships to intercept.  Since the intercepts have to
be pretty precise, you want to do them close to the systems where accuracy
limits of the systems and time delays for corrects are minimized.

Defraction limits of the maser (not laser) arry should be a factor for a
light year or two given I was assuming a ring of maser transmitters 2 AU
across.  Given that fuel sail just needs to find the beam, rather then keep
precise speeds and positions, it should be less sensative to fluctuations in
beam density.

>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.

How do you figure?  The ship is boosting under fusion power, while the maser
arrays are launching the fuel in the sailships.  Since it has to launch the
sailships (which weight at least 400 times as much as the ship) at higher
accelerations then fuel sail would (Explorer assumed a 1 G boost to shorten
the distences) why would it need less power?

>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.

I don't have the mass numbers with me and don't have time to check the nubers
on the web page ( I'm rushed to answer this before leaving town today), but
as I remember fuel/sail would only boost at a 1/10th g at full fuel load,
compared to Explorer.  The engines were much lighter (but still were about as
heavy as the unfueld ship, far more then the cargo), so the high fuel ratios
and 30% higher cruse speeds were possible.


>>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.

200 weeks!?  4 years!  That would spreed the fuel out over more then 1.5
light years.   It would take the ship years to catch up to all that, during
which time they could drift light months laterally in all all directions, and
you might not ever be able to find the fuel packats over that much range
unless they had huge becons and stayed almost perfectly on course. 

Since the ships max speed is about .4c.  Unless the fuel is launched in a
close group at nearly the same time as the ship, they would drift too far
apart for the ship to find and pick up.  Also since they need to intercept at
near identical speeds, and distences, they have to start out at about the
same time.  (I'm not clear what your assuming for the acceleration rates for
the two types of ships).  This would require everthing be launched at about
the same time.  Probably about the same total maser time as fuel sail
launching about the same mass.  

I was assuming the total boost time for an explorer would be limited to 3-4
month to keep the distences down, and fuel sail could take a year or two to
boost.   If you assume the maser time is 4 years to boost the sailships
(which have to weight about the same amount as the fuel sail ship) they would
need about half to 1/4th the total power per month of beam time (i.e. same
total mass launched to about the same speed but spread over twice to max 4
times as long), not 1/100th.
  


>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.

Numbers aer fairly meaningless if you don't explain the concept they relate
to.  I still have no idea why you thing maser launching the same mass over
2-4 times the amount of time (and I debate that assumption hotly) will only
take 1/100th the power?


>>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.

No just dificulting figureing out what your talking about.  Judgeing from
your comments above, you are similarly confused about my concepts.


>>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.

The maser array is about 2 AU in diameter.  I.E. its a ring around the sun
about the same diameter as earth orbit.  Closer in gives lighter power sats,
but worse resulotion.


>>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.

See above.


>>>>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.

??!  The fusion motor has to decelerat the loaded ship at a g load similar to
the acceleration rate leaving the system (a bit less given you can spread
decel over a couple more years, but not much more or it would take years more
ship time).  So the power levels are similar.

Opps, forgot the fusion motor is more efficent since all its thrust would be
applied to the ship.  Most (probably up to 90%) of the maser beam would pass
to the sides of the ship sail.  (I assume a beam diameter 5 to 10 times that
of the sail to give a clean even hot spot for the ship to stay in.)



>>> 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?

Posibly a factor of ten.  I remenber fuel sail could only do about a 1/10th g
when fully fueled.  Explorer needed to do 1 g.

>>>>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.

Were not talking about science, were talking about engineering.  I.E. the
same kind of fusion systems, just higher power levels.  I'ld be more worried
about scaling up the reactors to 'burn' about 40, million tons of fuel in few
months/years.


>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.

Orion has a poor spec imp and mass ratio, but your right it, or laser fusion,
or a couple other designs are within present tech.  (Give or take a decade to
develop and test the systems.)


>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?

Assuming a few hundred million a year (oh say a couple billion to be really
safe) probably 20 years.  Also note the scale of the engines required for
such a ship would make them vstly larger and more stable, then the ground
based power systems were studing now.

>>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.

Depends on the manufacturing tech.  I roughly remember the O'Neil Maser
platforms were expected to produce10^11 maybe 10^12 watts and cost about $10
billion a peice.  I remember we were expecting to need 10^16 to 10^18th in
total beamed power (real rough memory) sou that would cost so the cost would
be $100 trillion to $10,000 trillion (compared to $tens of billions for the
litium fuel).  Obviously impossible, it would be cheaper to just use 400
times as much fusion fuel and boost pure fusion.  But automated mass
production of standard sateligths, consuming near earthy asteroids for ore,
should (hopefully) bring the cost of the sats down by a couple orders of
magnitude or the projects obviousl imposible with current tech.

Kelly