By David Hambling, technology expert
While the progress in military technology over the last few decades has certainly been explosive, one aspect which has barely changed is the explosives themselves.
That may change as scientists push at the boundaries of chemistry in their quest for more powerful materials.
For centuries, the only available explosive for gunpowder. New compounds only became available in the nineteenth centuries, but chemists found that many of the new explosives were highly unstable.
Nitroglycerine, a liquid which can explode at the slightest shock or temperature change, could only be used by those with strong nerves or a death wish.
Alfred Nobel, founder of the Nobel prize, built an industrial empire by taming nitroglycerine, capturing it in a stable form he called dynamite.
Modern munitions use explosives like RDX, short for Research Department Explosive, given by the Royal Arsenal in Woolwich in the 1930s.
The American C-4 explosive, and its UK equivalent PE-4, are both 90% RDX, the rest being materials give it a soft, workable texture. Improving on explosives like RDX has proven difficult, and a 500-pound bomb today is no more powerful than similar weapon from the Second World War.
Institutions like the Centre for Excellence in Energetic Materials - the UK government’s source for all things explosive - have continued to make advances in this area.
But progress tends to me in areas like manufacture, disposal, storage, environmental contamination, detection and of course safety.
For example, in 2016 BAE Systems upgraded the British Army’s Python mine-clearing system with a new ‘insensitive’ explosive called Rowanex 4400M.
This means it cannot be set off accidentally by bullets or shell fragments, which is just as well given that the charge is powerful enough to blast a 200-metre path through a minefield.
This is still potential for new explosives which pack more punch.
One of the more promising candidates is a substance known as bis-oxadiazole. This is about 50% more powerful than TNT, and crucially, bis-oxadiazole has a low melting point so that it can be easily melted and cast into shape to fill bombs or shell casings.
However, discovering the new material is not enough; researchers also have to develop a way to mass-produce bis-oxadiazole cheaply and that’s a work in progress.
This is by no means the limit of what can be achieved.
Chemists know that they can get more energy out of explosives by adding more nitrogen atoms the molecular formula.
The power of a molecule can be predicted in advance, but it may be difficult to make, and its other properties can be unpredictable.
In 2011, German scientists succeeded in producing aziroazide azide, a new ‘high-nitrogen’ explosive. This was tremendously powerful - as anticipated - but was unexpectedly unstable.
The samples the scientists created blew up every time they were moved or touched. It practically blew up when they looked at it, making it highly unsafe for real-world use.
The ultimate high-nitrogen explosive would be a ‘polymeric nitrogen’, a substance made of nothing but nitrogen atoms strung together.
Researchers at the US Army Research Laboratory in Maryland have succeeded in producing polymeric nitrogen under extreme pressure in an instrument which crushes material together between two diamonds.
The result is a blue liquid, three times as dense as water. According to theory, polymeric nitrogen ought to be five times as powerful as TNT.
However, as only a few thousandths of a gram can be made at a time, there is not yet enough to confirm its explosive properties.
Even polynitrogen is not the most powerful possible explosive. That honour would go to another extreme material: metallic hydrogen.
Under normal conditions, hydrogen is a very light gas. However, when it is compressed with sufficient force – the sort of force you normally only find at the heart of a star – it can be converted into a metallic solid.
As with polymeric nitrogen, making metallic hydrogen involved gigantic pressures and is hard to make even in literally microscopic quantities.
Nobody has yet succeeded in making even a millionth of a gram. Hence, its exact properties are not known, but one thing we do know from physics is that it stores fifty times as much energy as TNT.
That sort of power would turn hand grenades into heavy artillery, and artillery rounds into blockbusters.
Metallic hydrogen would be a game-changer, not just for weapons but also for propulsion, enabling small missiles with vastly higher speeds and longer ranges. It might even be an expensive, high-octane aviation fuel giving aircraft greatly extended endurance.
Progress in this field has been slow for the past seventy years.
We have plenty of theory, but end-products take much longer. Explosives are likely to remain close to the Second World War level for some time yet.