# Physics in Perspective 2013 An enrichment course for sixth-formers and college students Tuesday 19th February

Dambusters: the engineering behind the bouncing bomb

Dr Hugh Hunt /Hilary Costello, University of Cambridge

Notes by Pameer Saeed; form 12O; Rooks Heath College

In the above picture Dr Hugh Hunt is on the right and the yellow small cylinder in my hand, is the bouncing bomb which was used in cylinder test.

The last time that planes attempted to blow up a dam using a bouncing bomb was in May 1943 – until last year, that is. Dr Hugh Hunt and Hilary Costello were asked by Channel 4 to lead a team of engineers to re-create the Dambusters raid. Tracked by a film crew, their challenge was to design a rig to suspend a spinning bomb under a vintage aircraft and build a 10-metre-high dam especially for the purpose of blowing it up with a bouncing bomb. Their applied maths led to a dramatic conclusion.

Notes:

To the see what happens and how this bouncing and spinning theory works, Dr Hunt and Ms Costello started their bomb testing on a small scale.

• Testing of bouncing balls first by just throwing it into the water

• Cricket ball test in a bigger pool (They used cricket bowling machine and slow motion cameras to see all the motion spinning and bouncing)

• Cylinder test (large scale) they had to use all basic physics of projectiles and motion equations to find the angle and speed that the bomb should be fired.

They also had to use some force equations to find Magnus effect and at the end they figured out that backward spinning is needed for the Magnus effect. One of the equations that they used was (for centripetal force):

where F is the force, m is the mass of the spinning object, r represents the radius of the object and ω is the angular velocity of the spinning object.

http://en.wikipedia.org/wiki/Magnus_effect

The idea was that a bomb could skip over the surface of water (avoiding torpedo nets) and sink directly next to a battleship or dam wall as a depth charge, with the surrounding water concentrating the force of the explosion on the target. A crucial innovation was the addition of backspin, which caused the bomb to trail behind the dropping aircraft (decreasing the chance of the aircraft being downed by the force of the explosion below), increased the range of the bomb, and also prevented it from moving away from the target wall as it sank.

The group opted to set their bombs spinning before take-off. To keep them turning, Dr Hunt, who worked closely with his PhD student, Hilary Costello, designed a shield, rather like the windscreen on a vintage sports car. This was custom-designed to deflect air around one side of the device. The movement of the air kept the bomb spinning so effectively that it was still turning at 1,000RPM when it was dropped. The shield was developed and optimised with the aid of the Wind Tunnel in the aerodynamics laboratory in the Engineering Department in Cambridge, primarily with a view to spoiling the aerodynamic lift due to spin (Magnus effect) so that there was no risk of the bomb rising up and hitting the plane on release. During these tests, the team found that they could make the shield smaller. The back spin also stabilised the “bomb” as it bounced along the water.

The Magnus effect was therefore really important because at the end the bomb had to go to the bottom of the dam and blow it up there and not on the surface of the water (or blow up the plane).

The group also had to figure out how much energy is needed for a dam to blow up. This does not depend on other conditions such temperature or air resistance but it does depend on density, gravity and the slope angle of the dam.

E is the energy in the explosive, P is the density and H is the height of the dam (the number is 4).

Once the bomb hit the dam the spin would also make it run downwards along the dam wall until it reached the correct depth for exploding.

The reason for not exploding the bomb on the surface was that underwater explosions have a phenomenon, called bubble pulse, which enhances their effectiveness.

It was also essential that the bomb be dropped from the very specific height of 18 metres.

In the original bombs each carried nearly three tonnes of a high explosive called Torpex. Once the bomb had slammed into the dam wall (hopefully without destroying itself), it would sink and hydrostatic sensors would detonate the Torpex at a depth of nine metres.

In terms of safety Dr Hunt (and Barnes Wallace) had to make sure that the plane went up high immediately after releasing the bomb and graph analysis was used for this.

At the end of lecture Dr Hugh Hunt gave us has his website address, the website contains links to all the research he has done so far.

http://www2.eng.cam.ac.uk/~hemh/

http://www2.eng.cam.ac.uk/~hemh/dambusters/Dambusters.htm

http://en.wikipedia.org/wiki/Operation_Chastise

http://en.wikipedia.org/wiki/Bouncing_bomb

http://www2.eng.cam.ac.uk/~hemh/

http://www2.eng.cam.ac.uk/~hc360/

http://www.abc.net.au/science/articles/2010/10/27/3049583.htm

Bubbles in the ocean (and how a bubble can matter for a planet)

Dr Helen Czerski, University of Southampton

http://www.helenczerski.net/

The bubbles that are formed by waves breaking out in the open ocean illustrate a fascinating connection between the ocean and the atmosphere.

Dr Helen Czerski studies the physics of how these bubbles form and how they help gases and particles move between the water and the air. She discusses why it matters and how physicists approach understanding how some things as small as a bubble can affect something as large as a planet.

Unfortunately I wasn’t able to attend the lecture but you can get a lot of information by clicking on the above web address.

You can find out more about bubbles by watching “The sound of Bubbles” presented by Dr Czerski on the 9th April 2013 on BBC4 (repeated on the 10th and 11th of April).

http://www.iop.org/resources/videos/education/pip/page_60090.html