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RE: starship-design: Timothy's beamed power paper
Well, I will grant that the first trip will be the hardest ;-) but, there
are a multitude of deceleration schemes available. It may take longer to
slow down than it did to accelerate, but once we can get there we can use
beamed power at both ends of the journey. A straight solar sail
deceleration at the end is not out of the question, it just means that
turn over will be earlier.
Or perhaps we can use one of Forward's ideas for a two stage mirror/sail
where the larger sail continues to accelerate while a smaller central
sail is decelerated by the reflection from the main sail.
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>.)
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
3) Additional boost from beamed power arrays until turnover
4) Deployment of retromirror at turnover
5) Solar braking assist from target star
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
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
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?
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.
Long experience has taught me not to believe in the limitations indicated by
purely theoretical considerations. These - as we well know - are based on
insufficient knowledge of all the relevant factors."