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Re: Engineering Newsletter

ReplyTo  : Keving
ReplyFrom: Timothy
Subject  : The bathtub is flowing over (EUREKA)


>> First convince me... then celebrate...

>First, there is _no_ way to reduce the momentum imparted by the photons.  
>However, we _can_ change the _direction_ of the thrust.

>> NO and yes, you cannot change the initial size and direction of the photon
>> thrust, but you can (of course) choose the direction of the thrust when
transmitting a photon.

>> In better words:
>> You should see the reflection of a photon as two steps, independant of
>> other.  The first step is receiving the photon where 
>> momenum to your mirror is added
>> in the same direction as the photon CAME FROM.
>> The second step is transmitting(reflecting) the photon, hereby is the
>> momentum of the photon added in the opposite direction the photon GOES TO.
>> ----- <snip>

Can't follow your argument, and the deleated graph seemed countradictory.  I
thing the fundemental part of kevins argument is correct.  That the thrust
vector on a photon sail is tangential to the surface of reflection, and the
amount is related to the angle of incidence of reflection.  (thats one of the
standard solar sailing tricks.)  

If of course you absorb the photons in a rectena array the result is a bit
less clear, but presumably you could reflect the energy off the sail to
converters, at an angle that would cut down the forward thrust.  The price of
this would be a lot of extra thrust, and stress, on the guy wires.

>> You tried to trick the photons and thereby violated the preservation of
>> momentum: If you receive an amount of photons, all their momentum is
>> transferred to you.

Yes, but if it is transfered at an angle; part the momentum will be disapated
in the structure, rather than acting to accelerate the ship.

>> Once more, whatever ingenious construction you can think of, to receive a
>> certain amount of photons and use their energy, you ALWAYS get ALL their
>> momentum in the same direction as they went to.

Define went to?

ReplyTo  : Kelly
ReplyFrom: Timothy
Subject  : Drawings

>>> (Tip: jpeg images would be about 4 times smaller, so would
>>>  mean a less bytes with almost the same image quality)
>Now if Rick would get a JPEG viewer.  ;)

>> I can't help with that, I don't know much about 
>> Mac-computers and their software.

Huh??  Well as a hint you generally need to have a copy before you can use

ReplyTo  : Kelly
ReplyFrom: Timothy
Subject  : Multi generation ship


>Building things takes skills, tools, and materials, as well as power.  Of
>course we didn't give the ship tremendous power reserves.

>> The power needed to build the ship can probably be 
>> neglected with respect to the energy needed for the 
>>  1000 year trip.  Materials should be mined at those 
>> asteroids. 

I wasn't talking about the energy to build the ship.  I was talking about the
energy, resources, and equipment needed to continually rebuild it in flight.
 Equipment has a life span too, and none if it will function for a thousand
years.  The ship will have to not only be equiped with its own internal ship
yard, but with all the manufacturing facilities needed to build every part,
and every tool in the complete ship.  Then you have to bring along all the
raw materials and fuel they'll need for the trip.  Then you have to staff it
with all the people with all the specialties needed to do all of that.  

The ship would need a degree of selfsuficency that no country, much less
industrial country, on Earth has had in generations.