The bomb was always intended to have a backspin, as that would enable it to
bounce over the torpedo nets. When it hit the wall of the dam, the spin
would ensure that it remained there as it sank to it's detonation depth,
rather than bounce away from it.

Wallis intended that the bomb would be a sphere, but a cylinder was the
easiest shape to manufacture out of sheet metal. To maintain the spherical
shape, they initally surrounded the cylinder with a wooden sphere, but this
disintergrated the instant the first test bomb hit the water. No matter ...
the cylinder bounced just as well!

"John Halliwell" wrote in message
news
In article ,
BackToNormal writes
Phew John. I opened a can of worms here by mentioning the program I saw.
It said the bomb was designed to hit the dam wall, bounce away, and then
the gyroscopic motion would claw the bomb back to and down against the
wall to detonate at predetermined depth.

It's hard to be sure exactly what led to what (cause & effect) of the
various techniques used in designing the bomb. My understanding (mostly
gained from Brickhill), is that originally Barnes used a sphere to
ensure each bounce would present the same surface to the water. He found
it would be too big and stretched it to a cylinder, adding spin to
stabilise it. Not sure if he tried forward spin or not, but he found
back spin allowed it to 'skip' off the water. The crawling down the dam
wall was discovered during testing as an additional benefit.

Highball, the smaller anti-shipping version which didn't go into
service, was pretty much spherical (only the very ends were flat to
allow it to be attached to the spinning gear).

It sounds like there are different accounts, and the whole thing was
probably muddied by wartime secrecy (the 1954 film was not allowed to
mention anything of the back-spin as that was still secret). It is also
possible that later editions of The Dam Busters might have more
information as more was released.

The spin was primarily designed to work the same as spin put on a golf
ball to keep the bomb against the wall, and not as a trajectory aid.
No?

Different accounts suggest different developments, but at the end of the
day, they're all correct when it comes to the operation of the bomb.
There is also the possibility of each account being tailored to a given
audience or compiled to fit the 'established history'. Brickhill's
account is probably incomplete or inaccurate in many ways.

A good, more recent account is given in 'The Dambusters Raid', one of
the 'Cassell Military Paperbacks' series (sorry haven't got more details
to hand). It clears up a number of points and tries to identify where
many of the 'missing' aircraft were shot down.

Just out of interest, what was the angle Nat Geo used in the programme,
seems a bit out of character (I only read the magazine, haven't seen any
of their TV progs)?

--
John