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Re: Engineering Newsletter
>> A reflected photon transfers momentum to a reflector normal to
>> the reflecting surface. An absorbed photon transfers
>> momentum in the direction and magnitude of the the
>> photon's original momentum. This is the only consistent
>> way to preserve conservation of momentum in both cases.
>> If an absorbed photon only transferred momentum normal
>> to the surface of the absorber, then the final momentum
>> total for the system (photon and absorber) would
>> _decrease_. This can't be true. You can't even weasel
>> out of it by saying the momentum goes into extra heat
>> or energy or something; momentum and energy
>> are tallied in separate components. The photon energy
>> goes into raising the heat of the absorber. The momentum
>> goes into changing the velocity of the absorber.
The papers I've seen on on solar sails seem to disagree with you. RThey
propose angling the sial to alter the thrust vector. Which would be
impossible if the momenum transfer was always in the direction of the photons
origional vector. Come to think of it all solar sailing would be impossible,
since the purpose is alway to add or subtract velocity perpendicular to the
solar light vector. Also it would be rather strange given thatthe photon has
changed its course and mometum vector to one crossing the ship and beam
vectors. Since its new course has an added lateral vector, there must have
been a coresponding lateral vector componenect in the reflection of the sail.
I have a lot of problem bying the idea that the light bouncing off a sail
doesn't lose energy proportional to the kinetic or heat energy gain of the
ship. Th power has to be coming from somewhere.
> > On Wed, 15 Nov 1995 KellySt@aol.com wrote:
> > > I still have questions about momentum transfer vs power absorbsion.
> > > neat conversion of absorbed power and momentum bothers me. I would
> > > that converting the power would use up the energy that would generate
> > > thrust. Maybe I'm confused?
> > >
> > Not at all, if you reflect, you get twice the momentum that you'd get if
> > you absorbed. so there is no violation of energy conservation.
This implies that the power of the beam is half the momentum of the beam.
But if the beam is reflected, and gives up twice the momenum, it would have
exausted all its power. If its then reflected again, it would lose more.
This doesn't work.
What percentage of the beam energy is lost in a reflection?
(If this % is small, the the remainder might be enough to power the ships
KS >> Durring the decel phase. The outer sails are trimed to reflect their
KS >> energy inward toward frount of the inner sails. The inner sails are
KS >> to reflect the energy hiting their back inward toward other sails or
KS >> dumping it. But this foward and outward thrust, serves to support them
KS >> against backward and inward thrurst from the energy pouring inward and
KS >> back from the outer sails onto the frount of the inner sails. The
KS >> of the inner sails have the rectenna arrays. Which absorb and convert
KS >> microwaves.
KS >> Good news.
KS >> The sails have max reflectivity durring accel phase.
KS >> Only the inner sail array needs the more complecated rectenna and power
KS >> conversion/transmition/cooling equipment. (The outer segments are just
KS >> wire mesh.) So servicing and costs can be minimized.
KS >> Bad news
KS >> The expensive power converters are going to be face first into the
KS >> stellar medium, and they are going to be operating at much higher power
KS >> densities. Cooling is going to be much harder.
>1) about power converters (schottsky <sp?> diodes) being
> face first into the "wind" we are going to need a lot of
> tension wires to keep the antenna from blowing apart, so
>if they were on the front, they would effectivly shield the diodes.
Not unless the wires became a wall.
> 2) about cooling, angling the antennas requires more surface
> area, allowing for _smaller_ power densities, and more
> surface to radiate the heat.
But if you reflect the beam inward, it will be concentrated to far higher
power levels. The angle and demensions of the antenna isn't important. only
the power density on the converter array.
KS >> Weird thought. Am I right that microwaves can impart thier momentum on
KS >> ions directly? Could you channel the power via waveguides, and feed
KS >> reaction mass into the power stream and get it to accelerate the plasma
KS >> directly? I can't remenber it clearly, but I think their is some such
KS >> mechanism. If so, you could avoid the heating problems of the power
KS >> conversion step.
> Agreed, this should be looked into. Not sure if this is possible with
> microwaves, I'm having difficulty seeing how the microwaves could be
> reflected into the reaction chamber without the inner sail elements
> getting in the way of the outer ones. but if it could be made to work,
> it would solve the heat problem neatly.
The sails could be kept out of the way. Also the use of waveguides would be
usefull for the inner systems.