[ExI] Written for another list

[Adrian Tymes]

On Sat, Aug 4, 2012 at 10:30 AM, Keith Henson <hkeithhenson at gmail.com> wrote:
> You asked how I calculated what the first power sat was worth powering
> propulsion lasers vs selling the power.

Yes.  And as noted, I see that you calculated this
using false assumptions.

>>> You need a minimum induced voltage on the rectenna
>>> diodes for them to forward conduct.
>>
>> Watts are not a measure of voltage.  Volts are voltage.
>> The minimum voltage can be induced for a power less
>> than MW, possibly sub-W.
>
> Schottky diodes have a forward voltage drop between approximately
> 0.15–0.45 volts.  For voltage levels below where they start to
> conduct, you don't get output at all from a rectenna.  Normal
> operation of a rectenna is around 1/4 kW/m^2  At 25 diodes per m^2,
> 250 W/25 is ten W/diode.  Assuming an antenna equivalent of 50 ohms,
> the induced voltage across a diode would be V^2 =10*50.  Or 22.4
> volts, plenty to put the diode in forward conduction.  Keep the
> geometry the same and drop the power to 0.5 MW.  Now we are down to 1
> mW/diode and V^2 = 0.01*50, or 0.22 volts.  No output.

Use less diodes per m^2, or assume more ohms.  Or just use
less m^2: at 1 m^2, that's 1/4 kW, which is less than MW.
Even 1,000 m^2 is only 250 kW.  But less diodes per m^2 is
probably the easiest solution, if you're designing for low power.

>>> 50 MW is 1/100 of 5000 MW.  So
>>> the ground antenna would need to be one km across and the antenna in
>>> space 10 km in diameter and would cost 100 times as much as a
>>> transmitter for a 5 GW unit.
>>
>> How does the fact that you're transmitting less power
>> translate into requiring a larger antenna?  Unless
>> you're basing off the minimum - in which case, you
>> need to show why the minimum is so gigantic.
>
> Microwave optics.  It's what drives power sats to such large sizes.

So again: use visible wavelengths.  Or perhaps masers,
to reduce divergence.

But how does transmitting *less* power require a *larger*
antenna?  Microwave optics does not require this.

> Other than this as yet unvetted proposal, I don't recall any such case
> to solve energy problems being made.  And you have yet to point to a
> specific past discussion.

Would you call your proposal vetted?

I do not have the complete archive of this chat on hand.
Rather, it is the nature of this chat that such discussions
happen.  If you want a specific one, would some list
member who is more familiar with the archives care to
point out other energy discussions that have happened
on this list?

>> You need to be clearer to yourself, first and foremost,
>> as to why you're doing this.  That will help you make
>> your case better.
>
> I thought I stated it clearly.

Are you just trying to make a theoretical case, or is this
something you want to actually make happen?  There is
a big difference between the two.

>>> OK.  The rough cost breakdown is in the paper.
>>
>> That's the one you haven't sent?
>
> I don't recall your asking for it.  Taking this as a request, will
> send the current draft.

Yes, please.

>> Or did you mean the
>> Boeing one, which I reject as an unreliable data source
>> since they have a history of invalid (and often
>> cost-inflated) data for this sort of thing.
>
> Boeing is well aware, and has been since the 70s, that for them to get
> any income from such a project they have to make an economic case.  I
> would expect underestimation rather than cost-inflated from them.

That's not how they work government space contracts.
Most such contracts are one-off, so they try to jam in
any cost-inflators they can get away with.  The whole
concept of off-the-shelf, truly large numbers of flights,
and such tricks that even airplane manufacture (let
alone autos & other consumer goods) have used to
lower costs is anathema to their approach.

> The existence of communication satellites shows that a lot of the
> physics questions have been answered.

Ah.  Then you are just making the theoretical case,
and to hell with the economics of how you would
actually get it working.

Okay, case made.  Now what?

> Ah, even if I wanted to be in charge of this, how many years would it take?

Something like this is going to take decades to
put into operation no matter who is in charge of it.
The question is, how many?

To answer that, you need to actually get into the
economics of how to bootstrap it, and the step by
step details of how the project could be realized.
You can't just assume $B of funding from the
get-go, because no funding agency - not even the
US government - works that way.  You start with
small scale technical demonstrators, then work
your way up.

If you really think the US government would just
pony up lots of money for this tomorrow, I advise
you to read up on Technology Readiness Levels
and how technologies go from low TRLs to high
TRLs.  All this stuff I've been saying about
bootstrapping and demonstrating?  That is in
fact a necessary prerequisite to getting the
money to build and launch the first multi-GW
powersat.  Just because these details are not
physical or technical does not make them any
less real, nor does it mean they can be
dismissed if you truly wish to make the case
that this can be done - because if there is, in
fact, no path to the destination, then the
destination can not be reached, whether or not
the destination exists.