- I thought that scientists understood how explosions work.
Now, we're not going to see big orange fireballs
going up into the sky.
I've done that before, I've filmed with a pyrotechnician.
We added a load of fuel
to make the blast look like a Hollywood explosion.
But that is not realistic.
Besides, these days Hollywood, and scientists,
can simulate those sort of effects near-perfectly in a computer.
I looked it up.
There are industry standard ways of predicting blast waves.
They are very accurate... once you're about half a meter away
and there's clear air in between,
which is good enough for most people, 'cause let's face it,
if you're five centimeters away from an explosion,
you've probably got bigger problems.
But it turns out that's big picture stuff.
Right at the start, in the detonation,
in the first few instants of an explosion as all that energy is released,
that's where the simulations break down,
and solving that problem could save lives.
This bleak old World War II munitions site is now a university test laboratory
and the research is close up.
So let's see a test.
- Explosions are very difficult to measure.
We need something that's going to be rugged
and strong enough to survive an explosion,
but we also need something that's sensitive enough
to measure things that are changing
over millimeter length scales and sub-microsecond timescales.
The rig that we're going to be using today is called MaCE,
which stands for "Mechanisms and Characterization of Explosions".
It's a bit of world-leading kit
where we can directly measure much closer than anybody worldwide can do,
gigapascals worth of pressures,
and that's a very, very complicated situation
which we can't necessarily model or predict currently.
And we're going to use some high speed video cameras
to film what's going on as well.
- If you hit a metal bar the right way,
it'll make a ringing sound, basically a musical note.
That ringing is made by the bar itself vibrating
as the force from that initial hit
bounces back and forth within the metal.
And the pitch of that note is really easy to measure.
You just listen.
The work the researchers are doing here
is much more technical and much more precise,
but it's kind of the same process.
- It's based on theory by a British physicist
called Bertram Hopkinson, 109 years ago,
and it's only now that the technology is catching up
where we can affix very, very small strain gauges
and we've got data acquisition systems that can sample it
at the required rates and required frequencies.
The strain gauge is a tiny dot
that we mount on the outside of Hopkinson pressure bars,
solid steel, one meter long, four millimeter diameter.
And we fix this strain gauge 40mm from the loaded face of the bar,
We sample it 5 million times per second.
We're going to set the explosive
a little bit further away from the surface,
so hopefully we can see the fireball
and the detonation products start to expand.
That's going to set up a blast wave
that's going to propagate through the bar as a stress wave.
So the strain gauge records a change in voltage.
We can work out stress, which we can work out the pressure
and the load on the end of the bar from.
And there's a whacking great big steel plate
that just acts as a shield and just deflects everything away.
So the pressure bars trap that pressure
and send it downwards, and then the arrangement
of the MaCE rig that we're building towards
is 37 of these measurement locations.
But today we've just got one! Just to focus on what's happening
right in the most severe part.
- So I asked the researchers, what's the use for this?
Their first answer was, well, for science,
and that's fair enough,
but there are practical applications.
To start with, if this works,
building things that resist explosions will become a lot easier and cheaper.
- In the 80s and the 90s,
the big challenge was big truck bombs with tonnes of explosives.
Nowadays, explosive attacks are much more targeted.
They tend to be smaller devices, and as a result,
they're located closer to the thing that they want to damage.
So that's why we've looked in this very extreme near-field setting.
We're going to be doing some tests today with 100 grams of high explosive.
It's around about tennis ball sized.
There are a lot of secondary reactions that go on.
It takes a while to develop, at least on our time scales,
and there are ongoing chemical reactions.
And that offers up the really interesting prospect of,
can we do something that's going to mitigate them
or even turn them off?
Can we surround explosives in a particular type of material
to manage those reactions?
Bombproof luggage containers or storage containers on transport?
That's a potential thing that could be done with this research.
- If we can understand those first instants of an explosion,
then maybe we can help keep it under control.
After all, this munitions site is built to do that.
It was designed to contain anything that goes wrong by brute force,
just using huge amounts of earth and concrete bunkers.
So can that be done with less materials and less cost?
We're about to take just one step towards finding out.
[horn blaring]
- We've never been able to observe these features before,
so we kind of don't know what we don't know.
That's what the group looks at,
uncovering what we don't know about explosions yet.
- All right, what have we got?
- So this is filmed at about a quarter of a million frames per second.
- Wow, okay.
Wow.
You can see it bounce back!
- So it hits the surface.
And then as it goes back up, see it reigniting.
All that hot gas is getting churned back up
and it's mixing with the oxygen.
And that's giving that secondary kick
that we see at these sorts of scale distances.
It's continually pushing the end of that bar
and it's sending that information down
- Because it's not one hit.
It's a push that-
It feels like one hit. It feels like one hit.
- Exactly.
- But you are recording that so quickly
that what you're actually getting is a push.
- Yep. - Wow.
- Cool, isn't it?
- That's pretty cool.