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From: (Henry Spencer)
Subject: Re: Temperature control in spacecraft.....
Date: Fri, 12 Mar 1999 15:11:14 GMT

In article <>,
Don Schmitz  <> wrote:
>Unlike many of the mockups and illustrations of Apollo spacecraft, the
>actual CSM had a chrome-like reflective finish to reflect sunlight.

One reason why mockups and illustrations often get this wrong is that the
Block 1 CSM did *not* have this reflective finish... because it didn't
have a Boost Protective Cover to shield the surface against aerodynamic
heating on the way up.  The BPC was originally added to the Block 2 design
to keep the windows clean during ascent (in reaction to problems in this
area on Gemini), but it didn't take long for people to notice that it
could also handle ascent thermal protection.  That permitted a number of
useful small changes, including a CSM surface optimized for temperature
control in space.

>...Heat is transfered both by conduction
>and *radiation*.  Don't believe me - touch the outside of a lit
>lightbulb - even though there is a good vacuum inside, heat is still
>transfered from the filament to the glass...

While the principle is right, the example is misleading -- modern light
bulbs are filled with argon (or for expensive ones, krypton), not vacuum.
The filament lasts longer if there is gas in the bulb, because that slows
down the evaporation of the filament material.  Heavier gases do it
better, which is why expensive bulbs use krypton, either for longer life
or to permit running the filament hotter with the same life.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: light bulbs (!) (was Re: Temperature control in spacecraft.....)
Date: Sat, 13 Mar 1999 01:13:13 GMT

In article <7cblcf$906$>,
Christopher Michael Jones <> wrote:
>> While the principle is right, the example is misleading -- modern light
>> bulbs are filled with argon (or for expensive ones, krypton), not vacuum.
>There may be some sort of light "bulb" that uses a vacuum, but
>I can't think of any (unfortunately, I do not have an
>encyclopedic knowledge of lighting devices :)

Edison's original designs did use vacuum, which may be where the
misunderstanding came from.

>...Other common gases are Xenon (for movie projector lamps)...

That's a slightly different story, because those are xenon *arc* lamps,
not just incandescents with a different filler gas.

>halogens (typically Iodine) for high intensity
>"halogen" lights ... the current through the filament
>is so great that it actually breaks it down, however, through
>a reaction with the halogen gas, the filament is regenerated.

Basically correct, but perhaps not perfectly explained.  The main filler
gas is still something inert -- I'm not sure exactly what they use, my
guess would be krypton -- but there is a little bit of halogen (iodine, I
think) added.

Tungsten atoms evaporate off the filament just like in a normal
incandescent bulb, but where in a normal bulb they end up mostly on the
glass (which is where the dark discoloration inside an old bulb comes
from), in a halogen bulb they react with the iodine to produce tungsten
iodide.  The bulb is run hot enough that the iodide cannot condense on the
glass, so it wanders around until it encounters the filament instead...
and the heat there breaks it down, redepositing the tungsten on the
filament. As a bonus, the filament is hottest at its thinnest point, so
that's where the tungsten tends to redeposit.

The result is a bulb which recycles its evaporated filament material quite
effectively, so its filament can be run much hotter than an ordinary bulb,
giving much more light.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Why Depleted Uranium for High Density?
Date: Tue, 6 Feb 2001 23:47:47 GMT

In article <viMf6.283015$>,
Ray Drouillard <> wrote:
>> the region of US$ 200 per kg. Depending on the form it comes in.
>Do you have any old light bulbs?  How much W is in a typical light bulb?

About a milligram.  It's very fine wire; not only is the thing obviously
coiled, but when you look closely, the "wire" that the coil itself is made
of is actually a much finer coil -- there's circa half a meter of wire in
the filament of a 15W bulb (the one I happen to have a number for).

>For that matter, if it's so tough to machine, how do they manage to draw it
>into thin wires and coil the wires?  How do they do it so cheaply?

The stuff is reasonably workable if it's processed either hot (nowhere
near its melting point, but still far above room temperature) or under
very high pressure.  Wire drawing uses high pressure, many thousands of
atmospheres.  The drawing process itself tends to make the stuff more
ductile, which simplifies coiling.  (It goes brittle again when heated and
cooled, which is why light-bulb filaments are quite fragile when cold.)
It's cheap because of very highly developed automation and very sparing
use of tungsten.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

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