Science Interviews

Interview

Sun, 24th Jan 2010

Insensitive Munitions

Professor Jackie Akhavan and Dr James Padfield, Cranfield University

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Chris -   The field of munitions, in other words, weaponry like shells, mortars, torpedoes, and so on; in this particular domain - safety is crucial.  If you're handling something thatís highly explosive, the last thing you want is for it to go off in your own hands or the hands of those people who are transporting it for you.  So this week, Meera Senthilingam has been to find out how scientists are trying to ensure that the munitions weíre creating now and are being used in warfare, only go off when they're supposed to.

Meera -   This week, I've come along to the Defence Academy of United Kingdom, located in Shrivenham in Swindon, to find out about how the munitions used today in warfare are made as safe as possible and basically, made sure that they won't go off when they shouldnít.  With me is Jacky Akhavan, head of the Centre for Defence Chemistry at Cranfield University.  Weíve got a range of mortars in front of usmortar, how do these work?

Jackie -   You have, at the very bottom, a propellant.  Before that, you have to ignite the propellant, so you would have an augmenting charge and then that would send the mortar up.  When the mortar hits its target, the top of it will be squashed and then a pin will go through your explosive which then will send a shockwave into your main charge which does all the damage.

Meera -   Traditionally, how do these types of munitions work?  So, what explosives do they contain?

Jackie -   Okay.  Well, if you just go into the top bit, which is your fusing system, you would have a prime explosive in there which, generally speaking, would contain lead azide which is very sensitive.  And then your main charge will be a combination of TNT, tri-nitrotoluene, with RDX.  RDX stands for Research Development-X.  It is an explosive.  Itís proper name is cyclotrimethylene-trinitramine.

Meera -   And these materials arenít really in use as much anymore.  So, why arenít they around?  What were the flaws with these original designs?

Jackie -   We do still use TNT and RDX, but weíre trying to move away from TNT.  And the reason is that first of all when you fill a munition with TNT, it shrinks when it cools down.  You heat TNT up, it melts at 81 degrees centigrade and then you put in your other explosive, and then you cool it down, but you have to be very careful of the shrinkage.  And also, over time, it tends to move around.  So, it often comes out of  the fusing system where you screw it in, you see these yellow crystals.  And by moving around, you can sensitize the actual explosive.  You might leave cracks and voids in the explosive, and also, around the top where itís coming out, itís quite sensitive.

Meera -   So now, one way your team here are trying to get around some of these problems is by designing insensitive munitions?

Jackie -   Yes, thatís right.  Insensitive munitions is a term to describe munition that in an accident, it won't detonate.  The first thing is, we've got to take the TNT out.  We have RDX, which looks like sugar, so we need to hold it in something else, but not TNT.  The most common thing now is using a plastic, a polymer, and these new ones are called polymer bonded explosives.  But polymers are inert whereas TNT is energetic, so we can't put too much in.  So we tend to put about 95% of our high explosive with a small amount of polymer.  Itís like baking a cake where the polymer is your egg and the rest of the dry ingredients like the flour and sugar, is your explosive.  When you mix it as we do, and then you put it into the oven, and like baking a cake, the actual composition goes solid.  But it does form a very brittle type material.  So, what we tend to do is add one more ingredient, like an oil, called a plasticizer, and that just gives ductility to the polymer.  So if you drop it and it did crack, it wouldnít go off.

Meera -   Having put all these materials together to make an insensitive munition, how do you go about making sure that this thing won't detonate under the wrong circumstances?

Jackie -   What we tend to do is to do some preliminary tests, small scale powder tests.  This is when we manufacture a very small quantity, then go and drop a weight on it, hit it with a hammer, or heat it up.  There are standard tests weíll actually do.

Meera -   And you're now going to take me along to your testing hall to see how some of these munitions are tested.

Jackie -   Yes.  Weíre going to go to the test house next...

Jackie -   ...Weíre now in our test house and here, we have James Padfield whoís a research fellow at the university and heís going to explain everything about the testing here.

rotter_testJames -   Hi, Meera.  We have several tests here, Iíll run you through a few of them.  We have the Rotter test machine that measures a materialís sensitiveness to impact when it gets dropped or something hits it.  We have a friction test, an electric spark test, and we also have tests that measure the effect of heating and spark from a flame.  Weíre looking for a very benign reaction, a sort of partial burn perhaps rather than a full explosion from an insensitive munition.

Meera -   In front of us now, we have the Rotter test which tests to see how impact will affect a munition.  Thereís a large tower, about 4 meters in height in front of me.  What are the various components of this experiment, James?

James -   We put a small sample of the explosive into the chamber at the bottom of the test and thereís a 5-kilogram weight that we winch up the tower to a pre-determined height, and we drop the weight on the explosive.

Meera -   Now, thereís a small cap here that you would actually put the explosive into.  The cap has a diameter of less than a centimetre?

James -   About that, yes and the sample size, we use around about 30 milligrams.  So yes, a very small amount.

Meera -   What explosive are we going to test now?

James -   This is going to be a sample of RDX.

Meera -   What height are you going to drop the weight from?

James -   I'm going to drop it from around about a metre and a half.  Okay, so I've loaded sample into the brass cap.

Meera -   Okay, so your just putting that at the bottom of the tower from which the weight will be dropped.  So, the weight is being lifted up to a height of 150 centimetres.  Okay.  Letís go.

James -   So now, we examined the sound bolt.

Meera -   Thereís smoke coming out.

James -   Yes, thereís smoke and you can see the brass cap is being shattered by theRotter_test_cap_blast RDX going off when the weight hits it.

Meera -   So, this was a definite explosion then?

James -   Yes.

Meera -   So that was a height of 150 centimetres.  And so, how this works is the higher the height at which it doesnít go off, the better?

James -   Yes, thatís right.

Meera -   And now, coming back to you, Jackie.  Having done these various experiments on a small scale, I imagine you must have to test these using much greater quantities in order to see if they're usable in actual munitions?

Jackie -   Yes.  I mean, once it passes all our tests, and it will now go into an actual warhead, they're made into a munition.  The whole munition, plus the packaging, plus the container will then need to be tested on a large scale, and this then will be taken to maybe Salisbury Plain or somewhere where this can be done.  A few tests might involve dropping it from a height, maybe impact via a fragment, heating it up, put it on a bonfire.  The response of these tests must be burning, not detonation.

Meera -   Are these insensitive munitions in use now then?

Jackie -   There arenít that many actually in use.  Stormshadow is certainly one thatís actually been classed as an insensitive munition.  In the future, yes.  Definitely all of them will be classed as insensitive.  No new munitions will be allowed to be used particularly for the MOD, unless they are insensitive.

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Yeah..
What we need is more research on how to build better bombs.
Although, I guess there are many uses for explosives other than bombs.

The best way to make an "insensitive" explosive would be to make one that either requires an exceptionally high activation energy, or one that requires the mixing of two parts. 

I was reading about Thermite recently.
Perhaps not a true "explosive", although it could certainly be used as part of an explosive fuse.
It is made of two relatively inert materials, plain iron oxide (rust) and aluminum.  Transported separately, they should be relatively safe.  Combined and heated, they can burn very very hot.

Black Powder is also just a mixture, although apparently the ingredients are typically mixed wet and allowed to dry.

Even still, one generally needs some kind of primer to set off gunpowder.  CliffordK, Mon, 29th Nov 2010

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