Gas Storage in Orbit

Thanks for the LOX vapor pressure graph, TangoMan.

Heh, heh --- take a look at the vapor pressure at 35K, and tell me what
kind of wall thickness you need !!

Ron

rmenich@...
01/03/03 03:28 PM

A shade can get you down to 30-35K. There's no need to store at 120K.

http://endo.sandia.gov/AIAA_MDOTC/sponsored/lillie_final_web.pdf
"
We allocated to thermal control the objective of passively cooling optics
to ~30 K
"

http://www.astro.utoronto.ca/~lilly/NGST/
"
NGST mission concept is for a large filled-aperture telescope (primary
mirror diameter of around 8m) located far from Earth (likely at the L2
Lagrange point). Shielded from the Sun and Earth by a large deployed
sun-shade, the entire telescope assembly will passively cool to about 35K
"

http://origins.jpl.nasa.gov/meetings/ulsoc/papers/labeyrie.pdf
"
... at cryogenic temperatures (30 degrees K achievable in space with an
umbrella)
"

Ron

"victoriatangoman victoriatangoman@... 01/03/03 03:18 PM

> Thanks for the information. I presume you mean sheet 'Sheet1' in
> 'TangoMan's Design Criteria for Space Station.xls'.
>
> But I think that you are ultimately still concluding that we
should store
> large amounts LOX at 10 bar ( ~150 psi ) at 120 K. I just
don't get
> it: why do that? That makes your wall thicknesses so excessive.
>
> Drop the temperature and pressure. Put a sun-shield in front of
the
> tank. LOX and liquid propane are "space-storable" propellants ---
- they
> require no active refrigeration at 1 bar in space. Your wall
thicknesses
> would go way down.
>

Your point is well made and I understand. I hope you're not taking
my observations from our last go 'round, and reaching your
conclusion that the wall thicknesses are excessive. I've conceded to
your argument. Using actual data for cryogenic gases, my
calculations are now more reasonable.

As for the sun shield, if the entire endeavor is as simple as your
statement indicates, then sure, no problem.

Here's why I favored a 120 K storage environment.

First I calculated the temperature in low earth orbit-cislunar space
using the Stefan-Boltzmann law for a black body in space:

S(sun) = xT^4W/m^2

1,360 = (5.67*10^-8W/(m^2K^4))T^4W/m^2

T = 394K

OK, that's kind of hot, but as you wrote all we have to do is keep
the sun from shining on the tanks and the temperature behind that
sunscreen will fall to any point above the 2.73 K microwave
background radiation.

Now I chose 120K for the following reasons:

1.) Steels and other metals start to lose some of their
specification properties at extremely low temperatures. I'm assuming
that the metal refinery in orbit won't be producing all sorts of
steels; rather there will be only a few steel specifications, and
those most likely won't be that complex. But, hey, they may very
well formulate steels for cryogenic use.

2.) I tried to find a temperature environment that would suit many
of the gases so that they could all be stored behind the same sun
shield. I thought that would make the orbital work environment a
little more efficient.

3.) If the tanks are fabricated to exist in an environment behind a
sun shield, then there is an engineering challenge to move the tank
so that no sunlight hits it. How long would it take for a tank to
rupture as the sun is shining on it? I thought better to over-
engineer a bit to a 120 K threshold so that the temperature could
rise to 120K before tank failure was immenent.

4.) With the temperature at 394K and the tanks engineered to 120K,
that means the sun shield has to radiate away 274 K/m^2. I haven't
looked into the issues of sun shield design but I wonder where does
that heat go? How much material and effort have to go into a fairly
large sun shield to protect the tanks of liquid gases? Now I know
that the dark side of the shield will have very low temperatures,
and I've seen comments that indicate the sun shield can be just a
sheet of mylar, but I haven't seen any explanations of how that
mylar radiates the heat. Maybe I'm missing something elementary.

5.) I thought that 120 K was a happy compromise with the O2 tank
having a wall thickness of 16 mm - or 5/8" That doesn't sound
onerous to me.

Feeling it's better to be safe than sorry in orbit, especially when
dealing with our precious and expensive gases. Where have I been too
cautious, or more likely, in error about my assumptions?

TangoMan