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(Re:)^4 starship-design: The Size of the Problem
At 1:53 AM 8/20/96, DotarSojat@aol.com wrote:
>At 3:08 PM 8/4/96, I sent the memo "The Size of the Problem."
>
>At 2:47 PM 8/8/96, I wrote:
>
>>Note that 1 cubic mile of (sea) water contains enough deuterium
>>to provide about 5,400 USEs from the fusion reaction(s)--
>>
>> D + D + D --> He4 + n + p + 21.6 MeV .
>
>At 9:23 AM 8/12/96, Kelly Starks wrote:
>
>>The problem you listed was the difficulty in manufacturing (not
>>to mention storing) the "fuel". Given that mining and using more
>>conventional fuels (like Lithium, duterium, etc..) don't have the
>>heavy power costs and system complexity problems.
>
>Actually it wasn't the "difficulty in manufacturing" the antimat-
>ter I listed, it was merely the equivalent energy content in USEs
>(units equal to the total production of U.S. Electricity in 1987 =
>51.47 kg antimatter). I assumed the manufacturing method (unspec-
>ified) of antimatter was 100 percent efficient. My 8/8 note gave
>the volume of water (1/5400 of a cu mi) that would have to be pro-
>cessed to extract the deuterium to give 1 USE via the specified
>fusion reaction.
May I inquire what unspecified technology you expected to be able to
generate anti-mater with 100% efficency?
I wasn't particularly interested in the energy costs. Assuming high
efficency systems that wouldn't melt the nieghborhood, power generation in
a fixed installation is mainly a cost issue. We could build power plant
hundreds thousands of times the size and capacity of the current ones, but
what would be the point? MOre importantly to this project, what would be
the cost!?
>>Hell yes I'ld rather have hundreds of times the fuel weight!
>>(Obviously due to the weight of the fuel you'ld need more than
>>250 times as much, but it would be a lot easier to carry!)
>
>Using antimatter fuel, the example mission to 8 lt-yr at 1-g con-
>tinuous acceleration/deceleration is calculated (see my 8/12 note)
>to require a mass ratio of at least 15.11 for the acceleration
>phase alone. Using deuterium fuel in the above fusion reaction
>(with a Timothy "f" factor of 261) for the same mission, the cal-
>culated required mass ratio for the acceleration phase is at least
>3.84E12 kg D for each kg delivered to the peak velocity half way
>to the destination. (Note to Timothy: This mass ratio is for an
>"optimum" exhaust velocity of 0.09580 c. One of the reaction
>products is a neutron, which can't be used as reaction mass.)
>With 1.44E8 kg D per cubic mile of ocean, this mass ratio says you
>have to extract all the deuterium from more than 26,000 cubic
>miles of ocean to provide the fusion fuel to accelerate 1 kg at
>1 g over 4 lt-yr, half way to the destination.
>
>But there is a dramatic reduction in required mass ratio as the
>g-level is reduced (and the trip time is increased). The follow-
>ing table shows that the required mass ratio is reduced by about
>9 orders of magnitude while the trip time is increased by about a
>factor of five:
>
> accel/decel Uend trip time mass ratio(accel)
> (g) (lt-yr/yr) (yr)
> 1.0 5.000 4.480 3.842E12
> 0.5 2.881 6.897 4.869E09
> 0.2 1.520 11.692 3.659E06
> 0.1 0.994 16.991 5.929E04
> 0.05 0.672 24.401 2.675E03
> 0.0132 0.333 48.064 6.057E01
>
>For the last line, the mass ratio for the full trip would only
>be about 3,600 kg D for each kg of final mass, so one would
>only have to process about 2.5E-5 cubic miles of water for each
>kg delivered to 8 lt-yr in a continuous-g trip time of about
>48 yr (24 yr with 1-g accel/coast/1-g decel; see below). To de-
>liver Kelly's 500,000-ton-dry-weight Explorer-class starship on
>this mission would require extracting all the deuterium from about
>12,500 cubic miles of water.
>
>This may be a more "down-to-Earth" measure of the size of the
>problem. Now we're addressing the "difficulty in manufacturing."
You and I are starting to get into and apples vrs oranges argument here.
I've pretty well droped considering continuous G missions, or a Tau Ceti
Mission, due to the extream power/fuel requirements. As your table above
shows, you eiather wind up with unusably long flight times or rediculas
fuel mass ratios.
Note. I'm not clear if the fule ratios listed above are for
acceleration/deceleration, or even if they are the fule weight on the ship
vrs fuel needed to fuel the anti-mater generators.
>>Thought. What is the relative weight of an Anti-matter tank to
>>the weight of the anti-matter? Would its weight added to the
>>ship start outweighing the advantages of the lighter fuel?
>
>If the deuterium for the fusion engine is carried in a tank made
>of an alloy of lithium and aluminum, the anti-hydrogen could be
>carried in a tank made of an alloy of anti-lithium and anti-
>aluminum, with a mass fraction similar to that of the deuterium
>tank. ;) ["mass fraction" = (mass of contents)/(sum of masses
>of tank and contents)]
I might have some technical issues with Lithium and aluminum tanks, I flat
cant take seriously anti-Lithium and anti-aluminum! How does one attach
and anti-matter tank to a ship? How do you manufacture it? And most
importantly do you seriously expect to see any of this technology by 2050?!
>>Also you would need to carry (and store) a full round trip worth
>>of anti-matter since you couldn't refuel in the target system.
>
>Kelly beware! I'm on Zenon's side regarding "kamikaze"
>missions. ;)
A bizzar and politically unastute mindset to put it mildly!
Oddly this issue may get public debate soon. I ran into someone on the
sci.space.tech (or something board) that is working up a paper for
presentation at the next Case for mars conference. He is going to propose
that NASA consider one way missions. His analysis was that it was cheaper
to send one exploration crew and yearly supply flights 30 years, than to
cycle back crews every year or two.
>>You'ld probably get shorter trip times if you had used the same
>>power in higher boosts at the start and end of the trips, with
>>a coast phase in the middle. Same power consumption, but higher
>>average speed.
>
>Kelly, go to the blackboard and write 100 times "Power is the
>rate of use of energy." Whap, whap, whap. ;)
>If you replace the word "power" with "energy" you're right on.
>The table below shows the comparative trip times (to alpha Cen-
>tauri: distance = 4.35 lt-yr) for continuous accel/decel at g-
>levels below 1, with trip times for 1-g accel/coast/1-g decel
>with the same peak velocity (same energy requirement). The
>values of peak velocity, Energy/Mbo and mass ratio (accel alone)
>for each entry are given in the table in my note of 8/12 (and
>Kelly's response of 8/12).
>
> Accel/decel trip time | 1-g accel/decel w/coast
> (g) (yr) | trip time (yr) coast time (yr)
> 1.0 3.576 | 3.576 0.000
> 0.9 3.810 | 3.582 0.152
> 0.8 4.087 | 3.603 0.334
> 0.7 4.421 | 3.646 0.551
> 0.6 4.835 | 3.720 0.819
> 0.5 5.366 | 3.845 1.162
> 0.4 6.083 | 4.051 1.619
> 0.3 7.128 | 4.407 2.269
> 0.2 8.867 | 5.091 3.317
> 0.1 12.751 | 6.813 5.537
>
>For acceleration periods that become a smaller fraction of the
>trip time, the average speed tends toward twice the average speed
>for continuous accel/decel, leading to cutting the trip time just
>in half.
>
>Note: For a reduction in energy requirement by a factor of 10
>from the 1-g-all-the-way trip (an accel/decel g of about 0.17,
>from the 8/12 table), the trip time for 1 g with coast for the
>same energy is increased by less than a factor of 2.
>A factor of 10 reduction in energy requirement (for antimatter
>fuel) for less than a factor of 2 increase in time says the coast
>phase is worth considering. On the down side, however, are the
>requirements for heavier thruster (and power-system) weight for
>1 g vs. reduced g, and increased complexity for artificial-gravity
>provisions during the coast time (the last column).
Your right about the heavyier drive systems, but your stuck with the heavy
artificial G systems regardless. As you've shown you cant sustain a
continuous 1g during flight, and obviously need to stop once you reach the
starsystem. So the only way to give the crew a 1 G environment is to make
your own.
Actually it doesn't really cost any extra mass to provide the 1G
environment. The only exception might be if you shielding mass is spread
out due to the larger centrafuge tourus. But give the fuel masses we'ld
need anyway. Shielding may just be a repackaging of the fuel mass.
>>?? You still use goto's in your code! Bad Rex, Bad. Whap,
>>whap, whap. ;)
>
>I can't imagine how to avoid them. Also, my Fortran compiler is
>dated May 1988 and doesn't allow ENDDO's, so it may not allow
>whatever gets around GOTO's.
Strange. Structured extensions were added to FORTRAN by the late '70's and
structured code was the standard I was using when I stoped programing
profesionally in '86. (Did you get a really cheap compiler? ;) )
>BTW, the code I provided in my 8/12 note had to be modified to
>cover fusion energy. But the size of the numbers above indicates
>to me that possibly nobody would be interested in using the modi-
>fied code (except to show I'm wrong). ;)
>
>Rex
Kelly
----------------------------------------------------------------------
Kelly Starks Phone: (219) 429-7066 Fax: (219) 429-6859
Sr. Systems Engineer Mail Stop: 10-39
Hughes defense Communications
1010 Production Road, Fort Wayne, IN 46808-4106
Email: kgstar@most.fw.hac.com
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