Re: Engineering Newsletter

[Kevin C Houston <hous0042@maroon.tc.umn.edu>]

On Tue, 7 Nov 1995, Timothy van der Linden wrote:

> ReplyTo: Kevin
> 
> >1) about the timing problem.  Since "Asimov" and the beam are traveling 
> >in the same direction, the beam starts at the same time that the "Asimov" 
> >does.  Also, the beam only needs to be on for about 1.5 years due to 
> >relativistic effects
> 
> The Asimov will take at least 1 year to accelerate.So the beam won't start
> at the same time but at least 1 year later. Also the beam of photons travels
> with the speed of light so it has be beamed another 5 years later, so that
> it does not catch up with the Asimov prematurely.
> Besides that problem, the beam then still has to be aimed exactly at the
> place where the Asimov will be after about 15 years, and it should be there
> at that time and not a month later.
> Maybe I haven't used the correct
> To me this still seems to be a timing problem. If I'm wrong, tell me where.

Be glad to.  First, the "Asimov" will need power from day one, so the beam 
will be on from day 1.  Toward the center of the trip, the "Asimov" will 
be moving close to C, and will travel with the beam.  if you look at the 
last day of the trip, you see that according to earth's clocks about 
13.25 years have elapsed, but since TC is only 12 LY away, that means 
that the beam of energy left earth only 1.25 years after we did.

Also, about "aiming",  the maser beam will be a straight line between the 
two stars,  we will follow the beam.  there isn't really any "aiming" 
problem, there _is_ a "jitter" problem ie.  how to keep the beam from 
pointing slightly away from TC and causing the poser to "wink" on and off 
years later at the "Asimov"

> 
> >2) this isn't a maser sail, the momentum imparted by the beam itself is 
> >negligible compared to the momentum generated by the engine exhaust.  and 
> >since the engine exhaust can be directed either foreward or backward, 
> >this is not a problem
> 
> What matters is the energy:momentum ratio of the beamed power. 
> For the photons in the beam that ratio is:
> 
>  E:p = c:1   where c=3E8 
> 
> For the Asimov moving at 0.7 c that ratio is:
> 
>  E:p = 0.4c:1
> 
> The beam needs to have about 1/0.4=2.5 times less momentum than the Asimov,
> but that is still a lot, so I can't agree with you that the momentum of the
> beam is neglectable.
> 
> Using a particle beam would make the problem worse because you would get the
> same energy:momentum ratio as the ship. Also a particle beam would mean that
> the amount of received particles increases as the Asimov slows down.

no, I am talking about the momentum of the photons as opposed to the 
momentum of the ions the "Asimov" will eject as exhaust.  Those will be 
Hydrogen ions or maybe Xenon moving at .9996 or (.99996, depending on how 
much energy you can invest) C  at these speeds, a small mass flow is 
sufficient to slow us down (or speed us up depending on which phase of 
the mission we are in) at a constant 1 G.

Now, about the energy of photons/area of collectors/mass of collectors 
problem.

Any of these so called problems can be solved by the same solution.  
namely increasing the number of collectors.  Timothy, you were talking 
about the problems associated with 100 collectors/transmitters beaming 
power to the "Asimov"  I agree with you, 100 transmitters is way too 
small.  I was initially planning on 720 (one every half degree) 
transmitters, but there is no reason (aside from cost) that we can't have 
7200, or 7.2 E 18 transmitters (each one sending a few watts of energy 
and costing ten dollars)

increasing the number of transmitters will:
reduce the amount of photon thrust that each transmitter is subjected to.
reduce the amount of solar array that each trnsmitter must have.
reduce the amount of beam jitter (by averaging the errors, they are reduced)
reduce the heat load of each transmitter.  (the non-Sol side of the solar 
                                panels will make an excellent heat radiator)
increase the total cost of the mission (hey, you don't get nothing for free)

Heat load on the asimov:

I do not understand why people think this is a problem, that must mean 
that I am not seeing something. so, I will tell you why I think this is a 
non-issue (or even a benefit) .

Heat generation will come from two sources.
1) (and the largest) conversion of microwaves to electricity.  This takes 
place with between 85 - 90 percent efficiency, and for our purposes, 15% 
heat load (from 1E18 Watts) is still a lot.

_BUT_
the conversion would take place on the antenna itself.  diodes wired 
directly onto the metal mesh would do the power converting and the mesh 
(of special radiator fins if need be) could radiate the heat.  we have 
thousands and thousands of square meters of antenna, it would serve as an 
exceptional radiator.  Also, if we used "memory metal" in the antenna, 
the heat of conversion could be directed to hold the antenna rigid.
_also_
Heat (for living quarters etc) radiates as the square of the size of the 
"Asimov", there will be little or no radiation coming into the living 
quarters, but a lot will be leaving.  We are going to have to do an 
energy balance on the heat so we can determine the equilibrium temp of 
the living quarters.  but my guess is that it will be on the chilly side, 
and so we may want all the heat we can get.

2) heat will be generated in the coils for the linear accelerator core.  
This can be minimized by using superconducting electromagnets, but there 
will always be inefficiencies.  Assuming that we can get 99% efficiency 
out of superconductors, (not unreasonable)  that means we will have to 
deal with 1 E16 watts of thermal energy.
_BUT_
This will be spread out over the length of the entire core.  also, we are 
going to have to raise the temp of the reaction mass from near Zero 
(kelvin) in the case of Hydrogen, to something approaching room temp 
(depends on which superconductors you use, we could use the old ceramic 
style ones, and limit ourselves to ~ 200 K I know a lot of people who'ld 
like to scrap their old ceramic superconductors for the newer room-temp 
ones, maybe we can save a buck or two and buy those up  ;p )
(the reaction mass would then also serve to cool the superconductors)

This heat, rather than the anntenna heat would probably be best suited to 
heating the living quarters.

Surprisingly enough, the exhaust itself will not be all that hot.  There 
is no chemical or fusion reaction going on in the exhaust, so the only 
heat is the heat we allow to go in from the coils on the magnets.

whatever heat is left over, can easily be pumped (using hot water) to the 
outside of the ship.  radiator fins along the length of the ship would 
radiate the heat into space.

Kevin