Dear Evil Engineer: Could a cold bomb become the next hot trend in defence?

Dear Evil Engineer,

In May, I dropped out of my first year of business school. I don’t want to waste three of the best years of my life being lectured by fusty old academics who have never run real businesses of their own, especially when I can stream TED Talks and access Elon Musk’s Twitter feed without paying a penny.

No, instead I’m going to buy a black polo neck; launch a start-up of my own; disrupt an industry or two; take it public, and retire as a billionaire by 30.

I have spent the past few months thinking of ideas for my big product and I think I’ve got it at last. A bomb, except instead of blasting everything to pieces on detonation, it freezes everything in place – much less messy. I have already filed a trade mark application for its name (the ‘Carnot’) and tagline (‘Bombs without the Hot Air’) and secured an initial investment of £800,000 from a friend of my uncle’s. Now I just need to work out how to make it. Could you advise me on how to build my freeze bomb (Carnot™)?

Yours,

A cold-hearted villain.

Dear villain,

Well, as far as ideas go, it is an original one. Let’s see if it is feasible as well; there may be a good reason why freeze bombs do not already exist.

Explosions generally involve a sudden expansion in volume associated with a massive release of energy into the environment. Bombs are inherently associated with exothermic processes, so a freeze bomb would have to ‘explode’ in an unconventional sense to produce a cooling effect.

The simplest and most promising approach, I believe, would be to essentially scale up and weaponise a freeze spray. (These are the things used to harden dog poo to make it easier to pick up from the pavement, or in the construction of chocolate sculptures to solidify molten joints.) Freeze sprays are cans of compressed gas which release a stream of the gas from a nozzle. Depending on the gas, different temperatures can be achieved. A typical freeze spray might contain a mixture of fluorocarbons such as tetrafluoroethene.
As the valve is opened and liquefied gas is expelled, it immediately expands and falls to its boiling point, absorbing heat from the environment in the process and producing a cooling effect.

This, I suggest, is the principle on which you build your freeze bomb – or rather, the Carnot™. The bomb itself would be a highly pressurised vessel containing liquefied gas which, on ‘ignition’ depressurises very suddenly, causing a massive expansion of gas which lowers the temperature of the environment as it boils. This has the advantage of working on a very simple principle – in engineering it is critical to minimise the number of things that could go wrong. The innovation itself would be the development of a vessel capable of storing a very large quantity of liquefied gas which can be released with a simple mechanism. As for the choice of gas, the ideal gas (no pun intended) is one which is gaseous at ambient temperatures and which absorbs a lot of heat during boiling. Ammonia might be a good choice, being an easily accessible and inexpensive refrigerant with a boiling point of -33°C and large latent heat of evaporation of 1,369kJ/kg.

The most tedious and expensive part of preparing a Carnot™ will be compressing such a large quantity of gas (this tends to raise its temperature, so it calls for a lengthy multi-step process). That being said, the process is well understood, and infrastructure exists that you could use to carry it out.

Now, there are a couple of major shortcomings with this freeze bomb which I feel I must warn you of.

First, the freeze bomb must go out not with a bang, but a whimper. In a conventional explosion, thermal runaway conditions can be created. This occurs when, as temperature increases, the rate at which heat is generated rises exponentially while the rate at which heat is removed rises linearly, causing heating to accelerate and potentially triggering other processes. A freeze bomb, however, absorbs heat. This occurs at a rate which depends on factors including the temperature gradient between environment and gas. So, while with conventional explosives the rate of energy release is not constrained – and can fly out of control – the rate of energy absorption for a freeze bomb is constrained and must decrease as the gas boils and target cools.

The second shortcoming is that, in simple terms, extreme cold does not have such a destructive effect as extreme heat. Cryogenic fluids boil right off the human body and other objects, even when poured over them. In fact, you risk providing your victims with free cryotherapy rather than causing any lasting damage. At worst, you will cause cold burns – potentially suffocation if the freeze bomb can be ‘detonated’ in an enclosed space such as a lift or car, and displace enough oxygen.

So, could you build a freeze bomb? Yes! Scale up a freeze spray. You could probably kill the inhabitants of a garden pond with your Carnot™. However, it would not be nearly so explosive or destructive as a normal bomb.

Yours,

The Evil Engineer

P.S: Utility and technical feasibility have not stood in the path of all start-up founders in the recent past, so perhaps you still have a reasonable possibility of becoming a billionaire by 30 with this invention.

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