Engineering the Bouncing Bomb

Professor Hugh Hunt & Hilary Costello, University of Cambridge

Meera - Cambridge University’s Hugh Hunt is a multi-talented engineer and he’s recently been involved in a project that looked into the engineering behind the famous dam busters bouncing bomb used during World War II.

Dave - We joined him along by the River Cam in Cambridge to find out what's involved in designing something on a large scale to bounce on water.

Hugh - Barnes Wallis, in 1943, came up with this great scheme for blowing up German dams - having a bouncing bomb, bouncing on the water, nestling up against the dam, dropping down beside the dam to a depth of about 20 feet and then blowing up just at the right height. It was successful, although there were quite a lot of lives lost, but the technology is really quite fascinating and hasn’t been reproduced since 1943.

Meera - Well, to find out more about the workings of this bouncing bomb, we’ve come along to the River Cam. It’s a beautiful sunny day. Dave, why are we here?

Dave - One of the things which inspired Barnes Wallis to design the bouncing bomb was essentially just skipping stones on a river. Normally, when skipping stones, you get a nice flat stone, get very, very low towards the water, and you tend to throw it as hard as you can on a very, very flat trajectory, and with any luck, when it hits the water, it will skip.

Meera - Yep! So yours has just skipped twice over, which it’s actually quite hard to accomplish.

Dave - So what's happening is the stone hits the water at a slight angle. When it hits the water, it gains lots and lots of lift, and it bounces off, and it skips a few times.

Meera - Hugh, is this basically the principle behind the bouncing bomb?

Hugh - Barnes Wallis was sort of inspired by skipping stones, but actually, he figured out with his kids in his backyard and that he could actually get marbles – ordinary round glass marbles – to bounce. The great thing about something round is that it can contain a reasonable volume. It is predictable, but actually, there is an even better shape than being spherical. If you have something which is cylindrical then you can fit more explosive power into the given space in the bomb bay of your Lancaster bomber.

Meera - This bouncing bomb is something that you set about recreating recently. Firstly, what you even have to consider when designing or engineering a bouncing bomb?

Hugh - First thing I suppose you have to do is to decide what size you're going to do it. Barnes Wallis had one which was about a metre and a bit in diameter. We went for half the size. Then we had to design how to fit it underneath the plane. We had to design how to make it spin and that was one of the things that Barnes Wallis found that's really important - that this thing had to spin. We had to figure out how to release it at the right time, at the right height, and actually, we had to make ourselves a target, and that was quite interesting because Barnes Wallis had German Nazi dams as a target. We built our own dams!

Meera - To see how you set about coming up with your design, we’ve got a variety of bits and bobs around us. We’ve got cricket balls here, we’ve got cylinders representing different scales of your bouncing bombs. So firstly, these cricket balls, what have you used these for?

Hugh - We started off on a small scale just with cricket balls. We used a cricket ball bowling machine in the Jesus Green swimming pool and we wanted to figure out how we could make cricket balls bounce, and we worked out the height and the angle and the speed, and that was great.

Dave - So how fast are you actually having to fire these cricket balls to get them to bounce satisfactorily?

Hugh - Well the ordinary lightweight ones, we could get them to bounce at maybe 30 or 40 miles an hour, but the heavier ones, once we started putting weights in them, we had to get maybe up to 50 or 60 miles an hour, except if you put backspin on them. And this was a eureka moment for Barnes Wallis back in the 1940s.

Meera - Why is backspin so important and how does it have that beneficial effect to the movement?

Hugh - Well, backspin is really important when the bomb goes through the air. Now if you think of Roger Federer and his topspin, topspin makes the ball dip over the net really fast. So you can imagine if you put topspin on a spinning bomb, it would just dive into the water and just sink. But backspin sort of hovers over the net and so, you can make it skim at a much shallower angle onto the water.

Dave - So the important thing is the angle that which the bomb hits the water and if that's too steep, it just digs straight in, but if you can get that shallow enough, it skips.

Hugh - That's absolutely right Dave, and if you have this thing called ‘magnus lift’ - the magnus effect is what it called - you can get a really nice sort of shallow angle under the water and we’ve got a really good demo how that works.

Meera - So here to actually show us just how important this is is Hillary Costello, one of Hugh’s PhD students. Now Hillary, you've got two polystyrene cups there that have both been taped together at their bottom ends and then you have an elastic band which has been cut open.

Hilary - Well basically, this our cylinder, or bomb. It’s a cylindrical polystyrene cup thing that we stuck together and then if you just wrap the string around, you can give it some either backspin or topspin, depending on how you throw it. So if I just wrap the elastic band around the cylinder or the polystyrene cups...

Meera - Okay , so you've wrapped that round now and so there's not much elastic band left…

Hilary - Yup! And then if I just pull the elastic band with my thumb so that the cups go forward when I release it, it gives it some backspin and you could see that it sort of hovers in the air before coming down.

Meera - So that actually hovered about a good meter away from you.

Hilary - Yeah.

Meera - Now you're just wrapping the elastic around again but in the opposite direction so that you'll have topspin.

Hilary - I actually just hold it from the top so that it has top spin and then when I release it...

Meera - That pretty much went straight down to the floor.

Hilary - Yeah, exactly. So, the backspin gives it a little bit of lift and the topspin gives it a downwards force.

Meera - And Hugh, so why do we need this backspin and for it to hover, I guess at a quite constant height?

Hugh - Well the idea really is to get the angle as shallow as possible. When you're skimming stones, we all know that you need to get it down as low as you can close to the water, but one of the problems that flying at night with gunfire, and all sorts of this stuff, you really don't want to be flying very, very close to the water. So, Barnes Wallis and his pilots figured they couldn’t fly lower than 60 feet, the backspin enabled them to get a shallow angle of bounce on to the water.

Dave - So this basically stabilises it because otherwise, after the first bounce, it would start to tumble and it would go anywhere and stop skipping.

Meera - You've got a scale model here. It’s a cylinder with a 180 millimetres or 18-centimetre diameter.

Hugh - This is what we used to do some tests. In fact, it was in the Cotswolds. We got a gas cannon to fire this cylinder at a high speed across the lake and we had to make it spin. And you can sort of see that – if I toss this up in the air with no spin...

Meera - It’s turned over. It’s rotated.

Hugh - It turned over very quickly, but once I put spin onto it...

Meera - It smoothly just come straight back down to your hand.

Hugh - The spin stabilised and that was very important.

Meera - So what scales were you working to when you recreated your bouncing bomb?

Hugh - We’ve got the cricket balls here. We’ve got our 20-centimetre diameter scale over here. We’ve also got our 60-centimetre diameter bomb. It’s actually a plastic replica because the real one is really heavy.

Dave - So, you were working with lots of different scales. It’s not entirely obvious that you can transfer things you've learned at the size of a marble to something of a size of a person. Was it easy to take information you learned on the small scale to the largest scale?

Hugh - It’s very interesting doing scale model tests. People designing aircraft do this all the time. You put a small scale model of an airplane in a wind tunnel and they’ll learn how the aircraft works at that scale and then they'll scale it up to the big Airbus or Boeing full size aircraft. We did the same thing. We were able to use what's called dimensional analysis to scale up the effects we saw at the small scale cricket balls up to the bigger scale.

Meera - So we’ve established that scaling is important and backspin is crucial, but what about delivery? There are planes delivering these bombs so there must be something about the speed they're moving, the height that the bomb has actually dropped at, and so on?

Hugh - The critical parameters are speed; speed of the plane, height; the height that it’s flying at, and how much you spin the bomb. And so, for a given diameter, it turns out that if you do dimensional analysis, the speed squared divided by the diameter and G – the acceleration of gravity - is dimensionless. That means that if you were to have a bomb which was twice as big, you would have to fly 40% faster - the square root of 2. It's important to know that scaling factor.

Meera - And so, moving back to you Hillary, just to conclude, having done all these tests with you, what were your final parameters to actually drop the bomb and explode the dam?

Hilary - From our testing, we determined that we needed to drop it 350 metres away from the dam, going 180 miles per hour, flying at 60 feet, and spinning the bomb at 800 rpm. It turned out that on our final run, the pilot decided to fly a little bit lower at 30 feet, so the bomb did hit a bit harder than anticipated, but it was a successful run. So, like Barnes Wallis, we bounced a bomb and blew up a dam.

Dave - Hillary Costello and before her, Hugh Hunt from the University of Cambridge, taking us through the process of designing a large scale bouncing bomb.