Could liquid airlocks work?

Question

I was looking through an old copy of Moon Miner's Manifesto a while ago and found the concept of a liquid airlock in an early issue.

It is on page 32 of this issue from 1988. It posits this as an option in places with gravity, or simulated gravity on rotating space stations, and suggested 3 possible liquids that could be used: mercury, gallium, or a sodium-potassium alloy known as NaK.

At first glance it seems like it could be really useful in certain situations. I wonder what the likely problems and limitations would be.

In a pinch it even seems like it would be a faster way to get to safety - if your space suit could deal with immersion in liquid. I suppose it would be a good idea to put it inside an alcove or shed, and maybe have a lid on it for when it isn't being used. What else needs to be considered?

Answer 1

The diagram you show would work only for specific instances. A few things that it relies on:

1. Gravity is required for this to work, or else all of the liquid will escape. The diagram you show works because of pressure differential, basically the pressure due to gravity counteracts the pressure from the air. Without gravity, the air would simply push the liquid completely out.
2. The "Vacuum" must be an enclosed surface, or else the liquid will all boil away. The enclosed surface will then become composed of a thin (at best) atmosphere of the liquid. In a pure vacuum, such as space, the pressure won't be high enough to keep the liquid in its liquid state. See this question on Physics.SE. One could theoretically keep the a lid on the liquid, which would be sufficient to keep it from boiling, until one actually wishes to use the airlock. This would allow for an airlock of sorts.

Thinking about this, the only places I could really see this potentially working is on Mars or Titan, both of which have a thin atmosphere. Titan in particular might be of interest as its atmosphere could be explosive if combined with oxygen, and this would be an effective way to keep the oxygen away from Titan's atmosphere when performing an EVA.

There is an option that could work. Gallium is a relatively toxic free material. Its melting point is near room temperature, so it would be reasonable to survive going through it, although a human could very well have parts solidified to one's suit after passing through it, as they would freeze on it (Unless the suit was pre-warmed). It is a fairly expensive material, and the amount required to use for the space application would be quite large.

Answer 2

The only reason to have an airlock is to move through it: if you're not going to move through it, you'd just replace the whole thing with a steel plate.

• NaK is a total non-starter for this. It is extremely reactive so anything passing through it would need to be extremely well protected. It tends to catch fire when exposed to air, so the surfacing pool needs to be carefully designed to avoid this. If you're going to pass through another liquid first, you need to make sure there are no air bubbles.

• Mercury is a non-starter because of its extremely high density: you'd have to transport a huge mass of it to the Moon and moving things through the mercury would require you to deal with huge buoyancy forces.

• Gallium seems like a non-starter because of its rarity. Wikipedia quotes annual world production at a few hundred tons a year and you're going to need tens of cubic metres of the stuff, at a density of 5.9 tons/m³ and a price of a few hundred dollars a kilogram in today's market, but a whole lot more than that if NASA decides to buy up half the world's production. It would also need to be kept warm: its melting point is just under 30 °C.

That seems to rule out all the options that are currently on the table.

Answer 3

In order for a liquid airlock to work, some points must be considered; they have been mentioned in some of the other answers, but I will try to combine them and suggest some new implementations.

• Gravity:

  As stated, there must be some gravity (or constant acceleration) present for this to work in principle. The required column height (see sketch in the question) of the pressure head is $h = p/(\rho \cdot g)$.

  To compensate for a 1 atm habitat pressure on the Moon, a water column would need to be about 60 m high, and a mercury column 4.5 m. On Earth, 1 atm corresponds to only 760 mmHg (millimeters of mercury).

• Vapour pressure of the liquid vs. pressure of the atmosphere (if any):

  The rate of evaporation depends on the external pressure as well as the intrinsic vapour pressure of the liquid.

  Liquid metals have low vapour pressures, which is why they were suggested in the original article. Water, on the other hand, has a much higher vapour pressure and will evaporate / freeze and sublimate very quickly in space or on a body without a significant atmosphere, e.g. the Moon.

  There are some other possibilities for liquids from this stand point; silicone oils, for example, are used in vacuum applications because they can have low vapour pressure. Quite recently (compared to the article's publication date) room temperature ionic liquids have been studied which have very low vapour pressures and have also been used in vacuum applications, e.g. in this article.

  In any case there will always be a small rate of evaporation on a body without any atmosphere, which means you should top up your air lock once in a while.

• Practicalities of passing through the liquid airlock, buoyancy forces and reactivity of the liquid:

  Liquid metals like mercury and gallium tend to be very reactive and will form solutions with most other metals. NaK is very corrosive as well. This could presumably be solved by coating exposed portions of cargo with inert polymers like PTFE. Silicone oils, on the other hand, are very inert and should pose no problem at all.

  David Richerby mentioned that buoyancy can also be a problem, especially with the high density of metals. Buoyancy is proportional to gravitational acceleration (see xkcd's What If) but diving through roughly 5 m of mercury in your non-metallic space suit on the Moon would be next to impossible unless you had big tungsten or platinum ankle weights).

So, to sum up, it would be pretty difficult to make as well as use a liquid airlock. The volumes and masses of the liquid alone would be way too great to carry even to the Moon. Maybe you could extract some of the resources in-situ, however, to me it seems this type of an airlock is totally impractical, unless you already have a well developed industry on another planet. Although, it is a nice physics and chemistry problem to think about.

Answer 4

In terms of protecting some kind of habitat viable for humans you can eliminate NaK and Hg.

It is incorrect to say NaK is pyrophoric or reacts violently with air (I have worked with NaK). But it does react very violently with water. It also would consume the oxygen of the habitat over time (and you would form salts from these reactions which would not be fluid in nature). Basically, you'd have a zero humidity habitat on the other side of the airlock and over time the oxygen would be consumed as well. NaK is basically a substance that only behaves nicely in very controlled and isolated environments.

A human habitat sealed in by liquid mercury would also be a habitat where everyone lost their hair, fingernails, went insane, and died prematurely... Mercury has a low vapor pressure but that vapor pressure is vastly greater than what is required to slow poison most mammals.

I don't know what the health factors with Gallium are, but out of the three it would be the only one that you could maintain a viable habitat on the other side with.

Answer 5

There are several answers now that examine different aspects of this. I wanted to add the matter of waterglass, (sodium silicate Na₂SiO₃, or potassium silicate K₂O₃Si) as a potential liquid for use in such a design. A liquid solution is roughly half water. Though water is volatile, the vapor pressure (page 10 here) of waterglass is very low. With a cover over the exit of the airlock, there would be no vapor loss issue other than a scum developing on the surface as the water there evaporated. It is non-toxic, non-reactive, and the elements for it are all found on the Moon. (Sodium is much more widely available and plentiful than potassium, oxygen and silicon bound in minerals make up about 2/3 of the Moon's surface.)

It is a clear, somewhat viscous liquid with a density of about 1.3 g/cm³ in a half-water mix. Thus a person in a space suit would have little trouble moving through it. The column needed to balance a full atmosphere of pressure would be about 8 m.

Answer 6

A potential solution would be an heterogeneous solution. A denser solution (e.g. water) covered with a sufficiently deep layer of a less dense solution. Under a gravity field such as the Moon's this would separate out into two dissimilar layers (assuming there was no significant solubility of one solution in the other). The lighter layer would have to have a low vapour pressure. An oil such as (honestly) olive oil or kerosene would have much lower vapour pressures than water and as such would evaporate significantly more slowly.

In fact when there were believed to be canals on Mars several authors suggested that the canals would be protected from the smaller atmospheric pressure (assumed due to the lower mass of Mars) causing excessive water loss by floating oils on the surface.

Whilst this would be insufficient in itself to stop vapour loss, a thermal condenser could be placed above the solution which would cool vapour losses and condense them back to liquid which would dribble back into the original lock. This could be powered on the moon by simply keeping it in the shade — no Maxwell's Demon would be required!

The actual exit in the low pressure region would be effected by opening the pressure equilibrated cap, leaving, and closing it. Why a cap? Because pressure equilibration could occur through a small diameter tube which would allow a small thermal condenser.