Re: starship-design: The Size of the Problem

["Kevin 'Tex' Houston" <hous0042@maroon.tc.umn.edu>]

DotarSojat@aol.com wrote:
> 
> MEMORANDUM
> TO:      The Starship Design Group
> FROM:    Rex Finke
> SUBJECT: The Size of the Problem
> 
> INTRODUCTION AND SUMMARY
> 
> A challenge has been identified to convey to the SD group the
> size of the problem of providing the energy required for inter-
> stellar flight.
> 
> Let us consider an example mission to deliver 100 tonnes to a
> distance of 8 light-years at a continuous acceleration/
> deceleration of 1 g, a mission for which we have some numbers.
> 
> Calculations below indicate that this mission, if powered by
> antimatter, requires a mass of antimatter that represents an
> investment in energy equivalent to at least 105,500 years of
> the rate of production of electric energy in the entire US in
> 1987.
> 
> CALCULATIONS

<snipped>
because I do not have the tools to argue them. I will accept them as givens

> the minimum energy requirement to create the antimatter to deliver
> 1 kg to a distance of 8 lt-yr at 1-g continuous acceleration/
> deceleration becomes
> 
>          54.3 kg am / 51.47 kg am/USE = 1.055 USE   .

So at about 91 Kg, I'm looking at 95 USE's

> IMPLICATIONS
> 
> Similar considerations could be applied to relate to the USE the
> fusion-powered-rocket energy requirement, the sail/beam energy
> requirement, etc.  All will certainly be about as much beyond
> current power-generation capabilities as antimatter creation is.
> Human interstellar flight in a human lifetime is even further
> beyond current physics/economics than I had imagined.  Orders of
> magnitude reduction in payload and increase in transit time are
> required to reduce the energy problem to a "manageable" size, to
> only a few USEs, say.
> 
> What to do about the energy problem for human starflight?
> 
> A few plausible ways around the problem (requiring extensions of
> physics, however) come to mind:
> 
>      1. Find some process to make antimatter which does not
>         require creation energy, such as changing matter to anti-
>         matter through some kind of quantum manipulation (trans-
>         mutation).  On 3/27/96 at 9:06 p EST, Lee Parker quoted
>         Kelly Starks (private email of 3/22 at 8:40 a EST), para-
>         phrased, "So physicists are talking about the possibility
>         of rotating the quantum particles to convert a particle
>         of matter to antimatter."


My (limited) understanding is that unless Quarks are made up of even smaller
particles (hints of which have been bandied about) this is not possible.  The 
quarks in matter are not enough to make the quarks in antimatter

>      2. Discover some ultra-cheap source of energy (cold fusion?).

Most likely case.  Consider that for the last 200 years, the amount of energy
commanded by each person (in US since you use USE's) has steadily increased, and 
that the rate of increase is also increasing.  There will come a time when each
of us commands the energy to cover the cost of interstellar travel.  I don't know 
when it will happen, but the day will come.

>      3. Tunnel through space ("warp drive"?).
> 
> The group might think of others.


The best possibility I can think of is self-replicating robots.  Let's look at
using solar panels for the energy for the ship.


> Let us call 293,156 Mw-yr 1 "USE."   Well, Ok.  But I do prefer round numbers.


Now you (Rex) say that it's gonna take 105,500 years to fuel the ship, and in the 
meantime we all shiver in the dark.  Kelly says: (to paraphrase) "given that length 
of time, and that mass of anti-matter, the chance of an accident is near 100%"
so let's try to fuel that ship in one year OK?

So a simple calculation yeilds a total of 30,927,958,000,000,000 Watts per year
for the whole ship.

At the earth's orbit, the solar flux is 1 kilowatt/m^2 second.  however, the 
best soalr cells can only achieve .10% conversion, so we are left with 
100 watts/m^2 second.  But there are 60*60*24*365=31,536,000 seconds in a year
so we can get 100*31,536,000=3,153,600,000 watts per m^2 per year.

So, the solar collecting area "only" needs to be 30,927,958,000,000,000/3,153,600,000 
=9,807,191 sq meters (at earth's orbit remember)

Now let's suppose a "magical" robot with the following abilities:

1) Able to gather and refine lunar materials into stock elements (Si, Al, Ti, O ...)
2) Able to build a copy of itself from such stock materials in one week's time.
3) Able to make one square meter of solar cell in one week's time
    (for the sake of argument, I'll assume that it can only do one or the other at a
     given time, not both)

Now the problemn gets tricky, do we have the robots reproduce until we have 
9,807,190 of them and then set them to make the solar cells, or do we stop 
after some amount of time and start making solar cells and produce at a steady rate.

Let's look at option a: it will take 23.225 weeks to replicate 9,807,190 Robots
at which time they can make the required number of solar cells one week later.

If they stop after only 22 weeks, there will only be 4,903,595 (half as many) so they 
will need to work two weeks to make the required amount of solar panels.

But if they stop even one week earlier, there will be half again as many, and then it 
will require four weeks to make the solar panels, which would be one week later than
any senario mentioned thus far.  

So we can seee that 22.225 weeks of reproduction followed by 2 weeks of production 
will give us all the power needed.  the best part is that by picking up a few 
hundred of these robots prior to lift-off, we can be assured of being able to 
make the return trip just as painless.

Now you can question the reasonableness of these robots, but I really think
that they will be built soon.  
Ability #1 could be done now.  There are prototype models that can pick up 
sand and deliver it to a central point.
Ability #2 is tricky, but not unbearably so (say ten to twenty years from now)
Ability #3 is subsumed under #2, because each of these robots will itself
run on solar cells.

In fact, if you gave each robot a solar "footprint" of 1 m^2, then they would 
become the collection grid.  no doubt, three similar "colonies" would need to
be deployed around the moon (or other body) to ensure that daylight was always 
on the grid.


Some general notes, I would suggest mercury as a better collection site, due to
its proximity to our star.

-- 
Kevin "Tex" Houston 	http://umn.edu/~hous0042/index.html