11 June 2013

The porous plate sublimator


Surviving the harsh lunar environment is apparently one of mankind's greatest engineering achievements. 

The space suit worn by the Apollo astronauts on the surface of the moon had to protect them from UV, X, and gamma-rays, heat, and solar flare radiation from the sun; cosmic rays; micro meteors; regulate humidity, carbon dioxide and oxygen; and maintain a pressurised layer of air.

Amazing life-saving suit technology: the state of the art in 1969.  The Apollo 11 suit.

Without an atmosphere to keep them cool, the astronauts could get as hot as 107C in the sunlight on the surface of the moon.  The astronauts were kept cool by a liquid cooling garment worn under the space suit. 

Remarkably, there's no record of a test of one of these suits, with their cooling system, working on an astronaut in full vacuum conditions on earth before their use in space.


Astronaut wearing cooling garment under quartz heating tubes in normal atmospheric pressure

The astronauts in training on earth were in full earth atmospheric pressure, using umbilical tethers connected to standard air conditioners.


Not having an atmosphere to exchange heat with, air conditioners of the regular type would not work on the moon.  To overcome this problem NASA commissioned Hamilton Standard (HS) to supply them with the porous plate sublimator.  

I've been told by a reliable source (a blogger on the internet actually) that the porous plate sublimator is a standard technology used extensively by the Americans, Russians and other countries in space, and is practically available off the shelf.  Really?  Well, my investigations painted a somewhat different picture.  

For example, there is no Wikipedia page on the porous plate sublimator.  There are no technical details to be found in any engineering reference on it, only the most general of descriptions. 

There are no data sheets, pictures, charts, graphs, or specifications to be found on the internet.   NASA has never demonstrated one working in a vacuum chamber here on earth.  And you certainly can't buy one, not even second hand.  

The only information available on the internet I could find were a few old patents and two documents concerning the actual working of it.  The first document is a final report by Hamilton Standard published in 1965[1].  The second is a thesis for a PhD received in 1970 by James Shero of Rice University[2].  


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A sintered nickel porous plate consists of small balls of nickel that are brought up to a temperature just below the melting point.  By staying below the melting point, the balls fuse together while not melting, thus retaining the gaps in between.

Bronze sintered metal plate


Nickel sintered metal plate.  The 2 - 4 micrometre particle size demonstrated in the plate above is similar to the pore size in plates used by HS and Shero.

HS and Shero mention several possible modes of operation: 

A) Sublimation
B) Glacial
C) Boiling
D) Mixed (a combination of sublimation and boiling)


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A) Sublimation mode

In sublimation mode water freezes in the pores of the plate and heat is removed from the system by sublimation to the vacuum of space.

Ice will generate some heat as it freezes from liquid to solid form due to the heat of fusion.  Water has an unusually high latent heat of evaporation/sublimation which can more than compensate for the heat generated upon fusion (freezing), as well as any heat that might be generated by friction as the water moves through the plate.  

In order for the water to get cold enough to freeze, it must first lose heat by the water boiling from a liquid, rather than a solid, state (see boiling mode below).  

Some suggest that boiling isn't necessary to produce ice at the outer pores of the plate -- ice will freeze because space is cold.  In fact, unlike earth which has an atmosphere with some heat capacity, space doesn't have a temperature and is neither hot nor cold (more here).  

HS and Shero both mention that without freezing, breakthrough of the water to the vacuum was an ever-present problem.  Ice in pores is useful for blocking water flow, but the expansion of the ice would shatter the plate.

HS found the unreinforced sintered plates to be brittle.  They could fracture easily (pg 20 - 21 [1]).  HS also found that brazed versions of the porous plates weren't much stronger.

The expansion of ice in the pores has the effect of ripping the plate apart.  HS said such a fracture would result in a thin sliver of ice emanating from the porous plate.  In reality such a rupture would let liquid water leak to space, ruining the cooling effect of the sublimator.

In order to continue their experiments, HS used another unspecified bonding technique for the spheres composing the porous plate.  It's odd for them not to test the material that was supposed to be used in the final product, especially as this was HS's final report.


B) Glacial mode

The above scenario of ice freezing in the pores of the plate has been suggested by some, but is not how HS and Shero actually portray the sublimation mode.  HS and Shero claim another type of sublimation mode exists, which I have dubbed the glacial mode, because Shero describes the ice sheet as advancing like a glacier (pg 5 of [2]).

In glacial mode a layer of ice forms behind the plate, rather than inside it.  

Figure "HS":  HS's depiction of the sublimation mode

This ice sheet was claimed to have been observed by HS and Shero in their lab tests.  By having such a layer of ice inside the surface of the plate, the mode solves a couple of problems:  1) Because the ice is behind rather than in the pores of the plate, cracking due to the expansion of ice within the pores is avoided.  2) The ice layer also stops breakthrough of the water to the vacuum by forming a seal that the water cannot get past. 

But the layer of ice introduces other problems.  With no ice or liquid in the plate, just water vapour, it's not clear how the porous plate now sustains a pressure drop across it.  Even with steam going through the plate, with steam's low viscosity, the outside face of the ice sheet, just inside the plate, will be near 0 psi, just like the vacuum is on the outside of the plate.  

This means that from the inside to the outside of the ice sheet there is difference of about 5 psi (33kPa or 1/3 atmosphere).  That's a lot of pressure for the ice sheet to sustain.  

It would be unlikely for the ice sheet to move forward in a uniform manner; for it not to crack and let water slip past; and for it to retain a constant thickness.  

And imagine how easy would it be for the whole chamber to freeze solid, thus altering the conduction properties of it, and changing the whole heat rejection capability of the sublimator.  All of these uncertainties would endanger the life of the astronaut.

Both HS and Shero depict the water feedpipe as being on one side of the liquid chamber.  That would make the side of the chamber, with fresh feedwater coming in, have a much thinner ice sheet:


Surely an uneven ice sheet would be created: thinner where the water comes in where there would be turbulent flow, and thicker in the far corner where the water stagnates.  This would affect the performance of the sublimator.  Yet, such an effect was never mentioned by HS or Shero.

Another problem with the scenario is that the ice would expand by 10% and press against the sides of the sublimator walls.  This expansion would press against the walls of the chamber possibly damaging it, like water pipes bursting in the winter; and the increased friction with the walls would stop it from moving forward, especially at the edges.  The expansion could also make the ice crack and buckle especially toward the middle of the sheet.  


Glacial mode: Not all it's cracked up to be.  Ice would expand, crack and damage the equipment, and let water leak out to space.

Below is HS's diagram (pg 52 [1]) of the heat flow across the system depicted above (Fig "HS" above).  The gradient of temperature drop across the water is roughly 5.5 times that through the ice.  

HS's temperature diagram referring to HS's diagram, (Fig "HS" above)

This would be okay if both substances were solid and ice had roughly 5.5 times the thermal conductivity of water.  As it happens, ice's thermal conductivity is about 4 times that of water.  But that's only in terms of conduction -- as though water had no ability for convection.

By the time convection is factored in, water could be many times a better conveyor of heat than ice  (depending on the size of the volume water can convect in).  

In the graph above, considering the steep gradient of the line from 0 to L, with convection factored in, heat is arriving at a much faster rate at L than it is being removed across the ice layer from L to S.  This will have the effect of building up heat at the ice-liquid interface L, gradually melting it and reducing the ice layer until it's all gone; thus maximizing heat transfer across the gap.

This is therefore clearly not a mode that is sustainable or realistic.  The only real way to maintain an ice layer in such a fashion would be to insert refrigeration coils into it, in order to extract enough heat.  (Obviously that would be cheating -- you can't have such refrigeration coils on the moon).


C) Boiling mode

Like most metals, nickel is hydrophilic, meaning easily wet by water.  Therefore, water will be readily drawn into the pores of the plate, such that when the water is static, the porous plate provides no barrier to the water, and will not constrain it from escaping to the vacuum of space.  

HS and Shero contradict this claim.  HS makes the following statement on page 11 [1]:

The absence of a solid ice layer to prevent liquid from passing through the porous plate requires that some other mechanism retain the liquid....It was found that surface tension supplies the necessary restraining force...The head of liquid which a pore can retain by surface tension is inversely proportional to the equivalent size of the restriction.  Thus, in sintered metal powder plates, liquid enters the pores until the integrated restrictive force balances the pressure differential.

Even if surface tension were to increase with decreased pore size, the number of pores goes up in equal measure.  The greater number of pores counters the alleged inverse proportionality of the "restrictive" force.  

Anyway, the claim is nonsense because, while the capillary force does decrease as the radius of the pore, the surface tension increases as the square of decrease in pore size.  In combination with capillary force, the surface tension has the effect of drawing the liquid into the plate, like water moves up in these straws:


On page 20 HS admits it didn't test the effect of plate wettability on water retention stating: 

These hypotheses were not examined experimentally.  

They could apparently afford to be confident in this regard because the pressure of the steam generated by the boiling was supposed to restrain the water, no matter the plate's wettability.  It's true, steam expansion does create pressure, but only if it is constrained. 

Being unbounded on one side, the steam will escape to space, providing little barrier to the water from creeping forward.  

The steam would have to create a considerable dynamic force if it was to have any back pressure to stop the water.  But steam has 50 times less viscosity than water, so it's hard to see how this vapour can have enough back force to prevent water breakthrough.  

And if the steam really were to stop the flow of water, such a drop in water flow would immediately eliminate the dynamic pressure drop in the water, such that the water would creep forward again until it escaped to space.

Even if the nickel plate were hydrophobic (water rejecting) instead of hydrophilic, the breakthrough would be an all or nothing proposition.  Once the pressure pushed the water through the inside face of the plate it would go all the way through to the outside face.  

Whatever the case may be, any static force, such as surface tension, or capillary force etc, is not one that can create a pressure drop, like a gradient, across the plate (see diagram below).   

A dynamically created pressure drop could do this by resisting flow in one direction.  As water moves through the plate, it collides with the walls of the pores, and loses kinetic energy and pressure.

Pressure drops as a fluid flows across a perforated pressure plate

Example of how pressure drop should look across the porous plate

For water to boil at room temperature the pressure must reduce to 0.0886 psi.  This pressure must be reached inside the plate or the water will not boil inside it, water will escape to space unboiled, and heat will not be extracted from the system.  

The problem with this method of dynamic pressure drop is that the amount of water flow necessary to create a 5 to 0 psi pressure drop is so large that the system would be drained of its 5.2 litres of water in a matter of seconds, not the hours that were required to sustain the astronauts during their lunar EVAs.

Using data from the Shero document, for example, the following calculations can be made using an online pressure drop calculator:


Typical results using pressure drop calculator of water flowing through a perforated orifice plate.  You need many litres per second to get a 5 psi pressure drop.  (Parameters from Shero plates 2 and 10, Table 1 pg 58 of [2])

Another consideration is that, the pressure drop wouldn't be linear across the plate.  

Because the pressure drop is proportional to the flow of water, the flow and the resistive force, would be greater at the entrance of the plate and less at the exit.  And as the pressure tends to zero, so does the velocity of the fluid through the plate -- a self-neutralising effect.

From all of the above it can be seen that the porous plate is not a device that can create the necessary pressure drop from 5 psi on the inside of the plate to 0 psi on the outside. 

The porous plate is therefore an open valve to space.  All the water would leak out in a matter of seconds or minutes and the cooling system would fail.


D) Mixed mode

Given the different pore sizes in the plate, different pores would be at different pressures or temperatures, resulting in ice in some areas and water in others.  This was called mixed mode by HS and Shero.

Given the high conductivity of heat through the nickel of the plate (compared to water and ice), it's hard to see how different parts of it can be at significantly different temperatures.  

Furthermore, if different parts of the plate have differing phases of water, heat flow through the plate would be different in different places.  This could affect the porous plate's heat rejection operation, yet such considerations were never raised by HS or Shero.

It's basically a bet each way: HS and Shero claim that if it doesn't work in the sublimation mode it works in the evaporation mode and vice versa.  It's like: it works one way or the other, and it'll all be OK on the day, trust us.

If it really were true that the machine operated in evaporative mode, sublimation mode or a combination of the two, consider that although the small pore sizes restrict convection, there would still be quite a difference in rate of mass loss between a pore that was boiling and a pore that was sublimating.  Imagine how different the heat rejection rates would be in modes a), b) or c) above. 

Yet this difference was not considered by HS or Shero.



In a boiling volume of water steam comes from any place within the material; and the action of convection supercharges the release.  By contrast, sublimation is a much slower process, occurring only on the surface, and is more akin to the process of evaporation in water, than boiling.

The different rate would make the sublimator behave in very different ways depending on if it was in sublimation mode or boiling mode, making the sublimator unreliable as a rejector of heat.


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As well as the above considerations consider what would happen every time an astronaut jumped up and down to do a jump salute; fell down; or swung around rapidly.  And consider all of those bumps that would be endured when the astronauts were driving in their lunar buggy. 



All these movements would put huge forces and strains on the plate, and would greatly vary the pressure through it.  This could cause the operation of the plate to change, or for the water to leak out.

Finally, it was noticed by HS that the nickel plates would oxidise with time; and that the water would become contaminated with time.  Both these things would affect the plate's wettability and change the sublimator's heat rejection capability.  That seems like a dangerous aspect of the device to leave untested, especially at the time of HS's final report in 1965.

Conclusion:

One could argue that all these problems were worked out later than HS's report, yet there's is no indication that was the case in any of the literature. Shero was still investigating fundamental aspects, such as plate wettability, when the moon landings were supposedly already underway in July of 1969.

Other than Hamilton Standard, the only apparently independent testing of the device was by Shero of Rice University; and yet Rice University, and Shero's project, was a recipient of funding from NASA, so it wasn't really that independent.

NASA is, after all, just another branch of the government, like the Department of Defense, and would have been used any way the American government saw fit in its cold war effort against the Soviet Union. 

HS and Shero fulfilled their role as best they could: to provide a plausible theoretical basis for something that might work...maybe.  HS also provided some hardware.  Beyond that NASA was solely responsible for the final assembly and use of the device. 

Whether or not it ultimately worked at the time of the moon landing only NASA knows.


References:

[1]  Hamilton Standard (HS),  Sangiovanni and Hepner.  Porous Plate Water Boiler Design Study, Final Report, May 20, 1965. Hamilton Standard Report: HSER 3509.  (Link)

[2]  Shero, James Philip, Porous Plate Sublimator Analysis, November 1969, Rice University.  (Link)

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