On Jul 24, 2:18 pm, Peter Fairbrother wrote:
Keith Henson wrote:
On Sat, Jul 23, 2011 at 10:41 AM, Peter Fairbrother
Ok, the problem is that the spreadsheet ignores the mirror.

You have the collection and radiating areas at approximately the
same mass per unit area. This is wrong. The collector is very much
lighter
than the radiator per unit area, at least 20 times lighter and
maybe 100 times lighter.

You have calculated the minimum total area of collector and
radiator, but not the minimum mass.

Your collector is collecting at 1.33 kW/m2, and weighing 1 kg/m2,
but that's ridiculously heavy. For a start, the collector cannot
collect at 1400 K without a concentrating mirror, it's thermodynamical
ly
impossible.

The mirror is presumably thin aluminium or metallised mylar, and
weighs in at about 0.005 kg/m2.

I agree with you *if* you can tell me how to support accurate
pointing mirrors over km scales without structure. Virtually all of
the collector mass is structure,

Agreed. I allowed 0.005 kg/m2 for the mirror itself, the same for the
high temperature bits, and 0.15 kg/m2 for structure.

The obvious ways to do this include gas-filled tubes, spinning a round
mirror, and a double very low pressure envelope, but I don't have a
specific design in mind.

Gas filled tubes . . . . how do you keep the from being punctured?

Spinning round inflated mirror, same problem. Plus you need to
precess the mirror over a year to keep it pointed at the sun. Now we
are talking bearings.

Anyway, no matter what the structure is, it isn't going to weigh 1
kg/m2, or anything like that much.

The pointing doesn't have to be that accurate - I have the pointing
ratio [1] at 1 in 60, so a fairly easy pointing accuracy of 1 in 600
would give 90% efficiency.

[1] the distance between mirror and pickup, divided by the width of the
pickup.

It's an optics problem.

The high temperature part of the collector can be very much smaller
than sunlight collecting area, and thus the overall mass of the
collector can be very much less than 1 kg/m2, or the mass of the ra
diator.

I'd use something like 0.025 g/m2 for the collector mass, and 1
kg/m2 for the radiator mass.

This gives a minimum mass at about 720K, see:

http://www.zenadsl6186.zen.co.uk/minimum_mass.xls
I *think* I can make a 1kg/m2 self sealing, radiator surface at ~130
deg C. I don't know how at 450 C. Any ideas? Also are you coun
ting
the heat transfer fluid?,

Two sheets of thin alloy, about 1 meter by 2. say 0.2mm Ti alloy, that's
1.8 kg/m2, and the radiator is double sided.

I am not sure you grok the scope of a power sat. For 2.45 GHz, the
smallest practical size is 5 GW on the ground, 10 GW into the
transmitter, even for 60% efficient, 16.7 GW sunlight in and 6.7 GW
waste heat. 12-13 square km of reflectors into the heat cavities.
It's worth working out the flow of heat sink fluid.

High surface area on the insides for good transfer between the fluid and
the metal. The sheets are roughly roller-welded in lines at say 2cm
intervals along the 2m axis. The welds do not have to be leak-free, they
are only there to keep the sheets from moving apart under pressure
(0.4MPa). This gives a relatively low initial leakage.

There are two cutoff valves so that if punctured the section is
isolated. Larger sub-sections of the whole also have cutoffs. The cutoff
valves could be pyro, pyro melting, chemical or other things.

I am ignoring the mass of the transfer fluid, it's a couple of litres of
argon at 0.4MPa, weighing 7 grams per square meter, or 0.7% of the
radiator mass.

And how fast does the argon need to be moving to transfer the waste
heat?

Keith

You may also notice that the total mass is now about 0.07 kg/kW,
rather than 1.5 kg/kW.

Actually, my estimate of the total mass was 5 km/kW, but that was
after taking a 50% transmission loss, so the power at the satellite
including transmitter and the structure that keeps the antenna flat
to 1/4 wave is 2.5 kg/kW.

Agreed the transmission loss to Earth is about 50%.

However trying to keep the huge main power Tx antenna flat to 1/4 wave
sounds like a .... bad ... idea.

It makes the electronics a little easier, but the penalty in structure
mass is so huge that it isn't really even worth considering.

-- Peter F





You should perhaps talk to the Solaren people, they are down in that
region.

I hope you are right.

Keith

-- Peter Fairbrother

Keith Henson wrote:
Here you go.

Keith

On Sat, Jul 23, 2011 at 6:46 AM, Peter Fairbrother
wrote:
If you could send me ac opy of the spreadsheet please?

-- Peter F

Keith Henson wrote:
On Jul 21, 10:47 am, Peter Fairbrother
wrote:
Keith Henson wrote:

[...]

Assuming the radiator and collector mass per square meter
is
about the
same, then you can see from the graph that the minimum
occurs a bit above 100 deg C, which is far below the
370-650 deg C quoted in
an old
paper he

http://contrails.iit.edu/DigitalColl...2article42.pdf

I'd use something like 1,000 K as Tl. High efficiency and
high rate heat radiation in space is problematic unless the
temp is high. Radiative heat dispersal is about 100 kW/m2
for the low temp radiator.
That's not what the minimum mass calculation show, at least
for the assumption that collector surface and radiator
surface have about the same mass per unit area. I am
assuming about a kg/m2 for both, taking into account the
supporting structure.

What you want is for the sum of mass for the collector and
radiator per kW, and taking into consideration the Carnot
efficiency to be at a minimum.

Here is the graph. http://www.htyp.org/Space_radiator

The minimum came out 130 C with not much penalty between 75 C
and 200 C.

Of course, there could be an error in the spread sheet. If
you can find one, please let me know.

Keith