Cryogenic fuel 'boil off'

[Col_Klonk]

I've been thinking (miracles happen :rofl about the boil off rate of cryo-fuels in space..

Would this be at a rate dependent on the percentage of the tank fuel level (increasing as the tank empties) eventually 'stopping' under boil-off pressure, or doesn't it matter about the tank level in space and boils off at a constant rate ?

Just wondering as we could keep liquid nitrogen for months (6+) in an insulated container, with little/minimal loss.

 

[Urwumpe]

I've been thinking (miracles happen :rofl about the boil off rate of cryo-fuels in space..

Would this be at a rate dependent on the percentage of the tank fuel level (increasing as the tank empties) eventually 'stopping' under boil-off pressure, or doesn't it matter about the tank level in space and boils off at a constant rate ?

It depends on the surface area of the volume of fuel and the energy input, as well as the pressure (in comparison to the vapor pressure of the fuel). If you have a spherical tank, the surface area is minimal. If you put it behind a sun-shade, you have even less boil-off. Pressurize the tank, and it boils off slower, since a higher vapor pressure (=temperature) is needed to boil the fuel.

It does not boil off at constant speed. Even in the idealized case, the boil-off rate varies, as the surface area changes. For example, gaseous fuel has a lower heat transfer rate as liquid fuel or metal walls. the more gas is inside a tank in micro-gravity, the lower is the boil-off, as less energy reaches the liquid.

 

[Artlav]

If the tank was shaded from direct sunlight and pressurized, why would there be any boil-off at all?
I'd be more inclined to worry about the oxygen freezing solid.

 

[Thunder Chicken]

If the tank was shaded from direct sunlight and pressurized, why would there be any boil-off at all?
I'd be more inclined to worry about the oxygen freezing solid.

Even if the tank were shaded, the shade itself would slowly warm up and re-radiate heat to the tank. You can slow this down by maximizing the reflectivity of the shade (and the tank for that matter).

The trick is to minimize the absorption of incident radiation.

 

[Urwumpe]

Also conduction is another inevitable way to transport heat in space, even if you try to keep the connections between the tank and the rest of the spacecraft minimal, energy will flow.

 

[Thorsten]

To add to that, a fluid having a given temperature means that its constituent molecules have a certain distribution (which is characteristic for the temperature). While the mean energy of constituents is of order of the temperature (i.e. low if T is low), there are always those having an energy large compared to the mean - and they'll be able to escape.

Even a chunk of ice goes gradually that way with the temperature never even getting close to zero and solidly freezing.

 

[Notebook]

Everyone likes a graph, so here's an old one:

http://s89.photobucket.com/user/Notebook_04/media/img005.jpg.html

I'll have to dig around to see where it came from, I think this:
http://s89.photobucket.com/user/Notebook_04/media/img009.jpg.html

yes it does, so the graph relates to a tank standing around in Woomera, bit warm there!

N.

 

[Thunder Chicken]

Also conduction is another inevitable way to transport heat in space, even if you try to keep the connections between the tank and the rest of the spacecraft minimal, energy will flow.

Yes, and this can cause enormous thermal stresses in the connections as the temperature difference across them will be large.

I worked for a company that "designed" furnaces for solar cell manufacturing. They "designed" furnaces in the same sense that a million monkeys on typewriters will eventually "design" Hamlet. They seemed to be be perpetually re-learning heat transfer fundamentals, despite my efforts. They left a lot of burned wreckage in the prototype lab. Their favorite way to destroy prototypes was to rigidly support the CVD chambers away from the walls with thin support brackets with no means of expansion. They'd heat the chamber up to several hundred degrees, then wonder for 30 minutes what was making all the bending/breaking metal noises, shut down and find that the entire chamber separated and was sitting in a heap at the bottom of the unit.

I stayed on there just long enough to get my signing bonus, then I quickly found my way to the door. The Stupid was strong there.The company soon laid off the last of the competent people in a last gasp to stay alive, then was bought out.

 

[PhantomCruiser]

I feel your pain. I've a management team on which I'd like to use the monkeys at typewriter analog.

I's seen pictures of a proposed L1 refueling station with a large shade deployed to minimize boil-off. I'd tried to model something similar for my VSA use, but I'm not there yet.

 

[Urwumpe]

Yes, and this can cause enormous thermal stresses in the connections as the temperature difference across them will be large.

Yes - which is exactly why fueling a cryogen rocket for launch takes hours and why many rocket engines need priming (or "thermal conditioning").

if you pump the cryogen fuel too fast into the tank, it could simply implode. The bottom would contract so fast that the side walls would buckle and eventually fail. You have to start really slow and only increase the flow rate when the local temperature differences have been reduced.