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RE: starship-design: Timothy's beamed power paper

Hello Lee,

You wrote me:
>I'm not sure where you got your calculations on the magnetic braking, 
>but everything presented to NASA seems to work. It is a viable concept, 
>although unproven. I think I would consider the fact that it is unproven 
>before your objections regarding the strength of the field or the mass of 
>the wires. (Of course, sail technology in general is unproven <G>.)

I don't know about NASA's reports, but I'm quite sure they weren't thinking
about velocities more than 0.001c (1,000,000 km/h)
The main problem is that we don't know anything about interstellar magnetic
and electric fields.
Can you show me some webpages or numbers about it. I might be able to
extrapolate some of these numbers.

BTW with 0.5c it is only 11 hours from Plute to Sol. Even if we could
decelerate that fast, we would not want to.

>If worse comes to worse, let's try this: 
>1) 	POWERED gravity assist launch
>2) 	Deploy solar sail while DEEP within sun's solar wind (where it is 
>	most effective)

Solarwind? A sail reflects solarlight, not solarparticles.
The amount of Sunlight that will be reflected by the sail compared to the
amount it receives from the beaming station is neglectable. Step 2 can be
left out.

>3) 	Additional boost from beamed power arrays until turnover
>4) 	Deployment of retromirror at turnover

The main disadvantage of the retromirror was that it had to be
selfregulating and very precise. Selfregulating in the sense that there will
be no crew that does repairs or alike. Precise means that it's surface
should be real smooth, otherwise the reflected beam will soon deminish in
all directions.

>5) 	Solar braking assist from target star

If you already slowed down to 0.001c, you can just as well use some engine
to move around. When we are in the destination system we would like to
quickly fly back and forth anyhow (not necessary with the mothership).
Soaring for a year to reach a planet sounds unacceptable (and unrealistic).
Really for the speeds where solarsails are usable, their use is ridiculous
compared to the interstellar travel.

>6) 	Use sail to "tack" into orbit around target star (This is a wide 
>	orbit that 	gradually spirals inward)
>7)	Construction of power arrays in target system

Only possible if you've nanobots and AI. We're talking about an equivalent
of building 1E9(?) nuclear powerplants with a relative small crew.

>8)	Repeat from 1) but now use beamed power to brake directly into orbit 
>	around target star.

>As far as terminal velocity goes, I don't know the relevant equations and 
>I suspect their derivation will be messy, but I'm fairly sure that if we 
>want to get much above .9 c it is going to be a factor. Once you start 
>factoring in time dilation, the RELATIVE density is going to go up in 
>DIRECT proportion to the time dilation. Something else to consider, the 
>calculations so far on sail material have simply factored in heating from 
>solar light and beamed power, I didn't see any calculations of heating on 
>BOTH sides of the sail. Forget terminal velocity, at what speed does the 
>sail melt?

I did do some calculations some time ago. (Rex corrected me) Look for the
subject "close but no cigar" (around may 12 1996)

P_absorped = s * (T_eq)^4  .

Where T_eq is the equilibrium temperature in Kelvin which should be below
the melting temperature.
Bolzmann's constant s=5.67 E-8 W/(m^2 K^4)
P_absorped is the absorped power per square meter

So suppose we've
- a sail of 1000 square km (1E12 m^2)
- a total power input of 1E19 Watt
- 1% absorption in the sail
That means we have an absorption of (1E19/1E12)*0.01 = 1E5 Watt/m^2

1E5 = 5.67E-8 * (T_eq)^4

--> T_eq = 1152 Kelvin

Well below the melting temp of a lot of metals.

>Tungsten has the highest melting point that I can think of. How much power 
>can we beam at a sail 4 microns thick made out of vacuum deposited 
>tungsten before we reach 3,410 degrees C? What is the impact density at 
>various speed regimes given the interstellar density in our region (thin). 
>How much heat will the sail absorb from impact with the interstellar 
>medium at these velocities?

Hard to say, likely such a thin foil will not absorb much. Dust will just
evaporate a small hole in it.
This may indeed be another disadvantage of a sail, you just can't shield it,
unless you want to make it extremely heavy.

>Can we develop a coefficient of heat (similar to a coefficient of friction)
>that can be input into a relativistic rocket equation to yield an upper limit
>on velocity? This is applicable to ANY starship, not just sail powered ships
>unless we are going to start speculating about non-material shielding.

This is beyond my knowledge. Impact studies seem a specialized subject.