Dam busters – the bomb?

Here are few more useful points

Yet the basic physics of the operation is pretty straight forward, as the basic principle of the bouncing bomb is similar to that of a skimming stone. You can try this yourself anywhere such as a lake, river or calm ocean.

“The bomb characteristics – Upkeep
Cylindrical in shape, the bouncing bomb, also known as Highball or Upkeep, measured 60 inches (1,5 meter) long and 50 inches (1,3 meter) in diameter. It contained 3 hydrostatic pistols, which measured the hydrostatic pressure of the water as the bomb sank, until it was equal to the pressure corresponding to 30 feet (10 meters) depth. The bomb was also fitted with a 90 second time fuse so that if the hydrostatic mechanism failed, the bomb would still detonate. The bomb weighed a total 9250 lb (4200 kg), 6000 lb (2700 kg) of this weight being explosive charge of TNT and RDX and possibly other materials to improve the efficiency of the explosion.“

“…Gibson made a dummy run over the dam and then circled round and went in for his real attack. The German gunners were now ready for him and were probably amazed to see the Lancaster turning on its lamps (when beams merged the Lancaster was at the perilous height of 60 ft) as it approached the dam.
With the Upkeep now being revolved backwards at 500 rpm in the belly of the aircraft , Taerum talked Gibson down to the required 60 feet. At 0028 hours Spafford the bomb aimer pressed the release button and Upkeep was away for real.
Just as it did in testing, the Upkeep bounced across the lake three times before sinking down the centre of the dam. Moments later a huge column of water was thrown up as it exploded, momentarily obscuring Trevor-Roper’s view of the dam from the rear turret of AJ-G.

Dinghy Young was the fourth plane to attack in AJ-A. Despite the damage sustained during his attack Martin in AJ-P joined Gibson in trying to distract the gunners on the dam while Young made his attack. This time Gibson turned on his navigation lights to draw the gunners attention while MacCausland, Young’s bomb aimer released the Upkeep. Their attack was spot on, the bomb bounced three times, sank down next to the dam wall and exploded throwing up another huge column of water. Again the crews must have thought that the dam had gone but when the water subsided they found to their dismay that the structure was still intact. Goner 78A was sent to Grantham – Weapon released at Mohne dam, exploded on contact with the dam but no breach made.

There was an air of gloom descending over Grantham and frustration with the crews at the dam. There had been one perfect hit and another close one yet still the dam stood firm. After all, Wallis believed only one Upkeep would be required to punch a hole through the dam.

Undeterred Gibson radioed Dave Maltby into attack in AJ-J. With Gibson and Martin drawing fire on either side Maltby went in for his run. Just as he was about to release his bomb, Maltby noticed that the crown of the dam had began to crumble and the centre had opened up slightly. Young’s Upkeep had worked! AJ-J continued the attack never the less and with another perfect hit the dam gave way with spectacular effect. For some reason another ‘no breach’ message was sent back to Grantham after the attack, probably because the scene was still obscured. Gibson however soon saw a river of water rushing through the dam wall sweeping it away down the valley. He called off Shannon who was about to make his attack and ordered his wireless operator Hutchinson to send *igger back to Grantham – the codeword for the successful breach of the Mohne dam….”

http://www.thedambusters.org.uk/mohne_attack.html

http://www.1001crash.com/index-page-bomb-lg-2.html

Although the bombs would damage the dams it was probably water pressure that created the real breach and the parapet would almost certainly fall into the breach as the escaping water enlarged it.

Exactly so, and witnessed by Les Knight, whose bomb finally breached the Eder:
“Large breach in wall of dam almost 30ft below top of dam, leaving top of dam intact”, and Robert Kellow, his wireless operator “It was still intact for a short while, then as if some huge fist had been jabbed at the wall a large almost round black hole appeared and water gushed as if from a large hose”.

Quotes from John Sweetman’s book on the raid.

That illustration works well. For some reason (and I have no idea why) I had always presumed that the bomb was released and struck the down steam side of the dam.

The image of the breach now makes alot more sense.

Thanks guys

From memory I think the fuses were designed to detonate the bomb at thirty feet; that never sounded that deep to me but then the ‘water’ side of the dam isn’t vertical. I believe there was a large embankment of clay up against the dam itself and if the bomb had rolled down that away from the dam the explosion may not have breached the dam. Although the bombs would damage the dams it was probably water pressure that created the real breach and the parapet would almost certainly fall into the breach as the escaping water enlarged it.

Proportionally speaking the top half of the dam will retain much more water than the bottom half of the dam anyway.

Good diagram illustrating the dam profile here:

http://www.rafmuseum.org.uk/online-exhibitions/dambusters/transcripts/dam_diagram.cfm

Unless the bouncing bomb went over the top they caused weakening

Hi Rick

Unless the bouncing bomb went over the top they caused weakening due to impact explosion.

On the raid on the Mohne, Lancaster 2’s (M Mother in film) bomb went over the top, the others hit the dam.

The dam in virtue of design is thicker at the base.

The theory of the bouncing bomb was impact as low near the water line as possible, as you rightly confer to weaken the base.

The picture shows that this had the desired effect and the thinner top gave way finally.