How to make a 'freeze' weapon work?

In a project of mine, I've contemplated an alien species that uses a weapon that shoots liquid nitrogen (or something similar) that makes enemy armour brittle by freezing it. Is there a way they could avoid damaging their own weapon with the freezing liquid?

• Are you talking about personal body armour or armoured vehicles? Commented Sep 11, 2015 at 23:53
• Personal armour, I would imagine them throwing something else at vehicles. Commented Sep 11, 2015 at 23:58
• Worth noting: you can actually stick your hand in liquid nitrogen without it freezing. It's pretty cool to watch on youtube (don't try it unless you know the correct way to do it... no point in getting frostbite unnecessarily) Commented Sep 12, 2015 at 0:07
• For a weapon that shoots liquid nitrogen, I would assume that it looks a lot like a squirt gun. What part of the weapon is in danger of being frozen by liquid nitrogen that would cause it to malfunction?
– Aify
Commented Sep 12, 2015 at 0:22
• @Aify, I meant if the liquid can damage enemy armour what would stop said liquid from damaging the device containing it in the same way? Commented Sep 12, 2015 at 1:17

While this is not exactly a response to the question you asked, it is too long for a comment, so I'll do it this way.

Weakening armor by spraying very cold liquid on it is not particularly practical. The problem is that even liquid nitrogen doesn't cool stuff very quickly, especially if the LN2 is sprayed on (as opposed to being an immersion bath). Consider that a reasonable goal for making steel brittle is -46 C, or just about 70 C below ambient.

From this link it seems reasonable to assume plate armor has a nominal thickness of about 2 mm. Let's take an area of 0.1 square meters as the target. How much liquid nitrogen is needed? The volume of the armor affected is $$V = A \times T = .01 m^2 \times 0.0002 m = .0002 m^3$$ which has a mass of $$m=V \times \rho = .0002 m^3 \times {8000kg/m^3} = 1.6 kg$$

Steel has a specific heat of ~.5 kJ/kg deg, so the total energy required to drop the temperature of the armor by 70 deg is $$E= 0.5 \times 1.6 \times 70 = 56 kJ$$.

For any armor temperature above 77 K, the interface between the LN2 and the armor will consist of boiling LN2, so the energy transferred will be dominated by the latent heat of vaporization, which for LN2 is (about) 26 kJ/kg. Therefor, in order to cool the armor to -46 C will require $$M = \frac {56}{26} = 2.15 kg$$ of LN2.

Of course, most of the LN2 sprayed on the armor will never contact the armor - it will be forced away by the gaseous nitrogen created by what vaporization does occur, just as most of the surface of a very hot object is protected in water by evolved steam. Worse, a suit of armor is overwhelmingly vertical, in that liquids drop away from it due to gravity, so very little spray will actually make thermal contact after it is repelled by vaporized nitrogen. Even assuming (and I consider this a very optimistic number) 10% of the spray does its job, this is going to require delivery of about 20 kg of cold liquid to each armor surface in order to get it cold enough. In the case of LN2, with a specific gravity of 0.8, this amounts to about 25 liters of liquid. Assuming this has to be delivered in 2 seconds, this is a flow rate of about 25 cubic feet per minute, or about 187 gallons per minute, or 11,000 gallons per hour. For contrast, a fire hydrant typically produces a pressure of ~50 psi and a standard requirement for hydrant flow is 1000 gpm. In other words, your cold liquid sprayers will need to be roughly on a par with a fire hose.

Frankly, I'd be inclined to give it up as an idea that sounds neat but doesn't seem particularly practical.

• Would a colder liquid help? Commented Sep 12, 2015 at 4:25
• @Shalvenay - Well, some. Neon, for instance, has a latent heat of evaporation of ~86 kJ/kg-deg, and a density of ~1.2, so it would be about 4 to 5 times as effective (all else being equal). Problem is, neon is pretty rare, and it's hard to imagine a society able to handle the stuff which can't do more straightforward weapons. Commented Sep 12, 2015 at 5:06
• @WhatRoughBeast I like your analysis, and it may be worth adding that the armor is surrounding a heat producing body (i.e. a person) so it's being warmed at the same time. Also the stream of LN2 would probably have a limited effective range, as it would tend to break up into droplets, becoming diffuse (and so warming quicker) as it travels further. Commented Sep 12, 2015 at 9:48
• @KillingTime - thanks for the good words, but you should be aware that armor, particularly plate, was worn with fairly heavy padding underneath, which would insulate the armor from body heat (it also made battle a really hot, miserable experience in summer). Commented Sep 12, 2015 at 16:18
• How about a liquid nitrogen in gel form? Kind of a "freezing napalm"? Commented Oct 3, 2015 at 12:47

WhatRoughBeast does an excellent job of expounding on the issues of a liquid nitrogen weapon. It is rather impractical, as it takes a lot of work to get the "amunition" prepared, and continued effort to keep it in that state, and then immediate loss of effectiveness when shot over any distance.

However, the concept of a freezing weapon is interesting. The bombardier beetle uses a rapid oxidation agent to shoot an exothermic reaction that exceeds the boiling point of water. It has a complicated system with the reagents, an inhibitor, and an inhibitor-remover. Adapting this design to your idea, but using a violently endothermic reaction to cause rapid drop in temperature, I think you could do it. Now, google did not provide me with a good enough endothermic reaction to use as an example, but your aliens should be able to find one. Also, if your aliens coat whatever their armor is with a reaction inhibitor, or a method to neutralize one of the reagents, they would be immune to their own weapon. This would still be able to freeze the ground and other objects around the aliens, causing terrain hazards.

Having the "freeze" action occur closer to the target by aiming the two sprays to mix in front of, or immediately on, the target would reduce the efficiency issues brought up with the liquid Nitrogen design.

Now you say this is for use against a personal armor, so there are joints and flex points to consider as well. Perhaps this endothermic compound sucks the heat from what it touches as it grows and hardens. If its stable form is a crystaline structure, it could cause armor to turn brittle by cold while also applying pressure from its growth.

Maybe it immobilizes instead. Think of the Kenbishi AMPT Sticky Gun from Ghost in the Shell

Yes they can avoid the damage of their own weapons by:

1- Using different type of armor. Metal armor would indeed lose tensile strength and break easier when at extremely low temperatures. But an armor made of nylon would retain it's characteristics and not become brittle. Of course you don't expect a 100% nylon armor for space battles, but you can have an armor that has nylon on the outside layers and metal on the inside. When hit with the liquid nitrogen, it will hit the nylon layer. Nylon is a bad conductor of heat so the metal layer will not drop to lower temperatures and keep working effectively.

2- By using quick heaters. When hit by a jet of their own deadly liquid nitrogen, the soldiers would quickly turn on the internal heating for their armor plates which would return them to normal temperatures quickly. Notice that these heating systems do not heat the army directly by flame, but through a very hot blow of gas, like a hair dryer.

This weapon could consist of a medium that gets really cold on contact with air, thus freezing itself and super-cooling what it hits.