The Dam Busters

THE
DAM
BUSTERS

PAUL BRICKHILL

To the men living and dead
who did these things

LIST OF ILLUSTRATIONS

CHAPTER I A WEAPON IS CONCEIVED

THE day before the war started Barnes Wallis drove for five hours back to Vickers’ works at Weybridge, leaving his wife and family in the quiet Dorset bay where they had pitched tents for a holiday. He had that morning reluctantly decided that war was not only inevitable but imminent, and he was going to be needed at his drawing-board.

Wallis did not look like a man who was going to have much influence on the war. At 53 his face was unlined and composed, the skin smooth and pink and the eyes behind the hornrimmed glasses mild and grey; crisp white hair like a woolly cap enhanced the effect of benevolence. Many people who stood in his way in the next three years were deceived by this.

He spent the last night of peace alone in his house near Effingham, and in the morning, like most people, listened to the oddly inspiring speech of Chamberlain’s. Afterwards he sat in silence and misery.

One thought had been haunting him since the previous morning’s decision: what could he, as an aircraft designer and engineer, do to shorten the war? The thought stayed with him for a long time and through remarkable events before it was honourably discharged.

He had been designing for Vickers since before the first world war. In the twenties he designed the R.ioo, the most successful British dirigible. In the thirties he invented the geodetic form of aircraft construction and, using this, designed the Wellesley which captured the world’s non-stop distance record, and the Wellington, which was the mainstay of Bomber Command for the first three years of the war.

Vickers’ works, nestled in the baked perimeter of the old Brooklands motor-racing track, was turning out Wellingtons as fast as it could, and Wallis was designing its proposed successor, the Warwick. At this time he was on the design of the Warwick’s tailplane, which was being troublesome. Clearly any additional work would have to be done in his own spare time and there was, also quite clearly, not going to be much spare time.

Bombers and bombs were the directions in which he was most qualified to help. Bombs, particularly, seemed a fruitful field. He knew something about R.A.F. bombs, their size, shape, weight and so on; the knowledge had been essential when he was designing the Wellington, so that it could carry the required bombs over the required distance. It was not knowledge which, in Wallis, inspired complacency. The heaviest bomb was only 500 Ib., and aiming was so unpredictable that the Air Force was forced to indulge in stick bombing—you dropped them one after another in the pious hope that one would hit the target. One hoped then that it would go off. Too many didn’t.

R.A.F. bombs, too, were old, very old. Nearly all were stocks hoarded from 1919. There had been an attempt in 1921 to design a better bomb, and in 1938 they actually started to produce them, but in 1940 there were still very few of them. Both new bombs and old were filled with a mediocre explosive called amatol (and only 25 per cent of the weight consisted of explosive). There
was
a far better explosive called RDX, but production of that had been stopped in 1937. (It was not till 1942 that the R.A.F. was able to use RDX-filled bombs.) Meantime Luftwaffe bombs contained a much more powerful explosive than amatol—and half the weight of the German bomb was explosive.

Wallis knew there had been an attempt in 1926 to make 1,000-lb. bombs for the R.A.F. but they never even got to the testing stage. The Treasury was against them; the Air Staff thought they would never need a bomb larger than 500 Ib., and anyway Air Force planes were designed to carry 500-pounders. Not till 1939 (did the Air Staff begin to think seriously again of the 1,000-pounder, and six months
after
the war started they placed an order for some.

These shortcomings were not so obvious then, particularly as all air forces favoured small bombs designed to attack surface targets. The blast of bigger bombs was curiously local against buildings, and a lot of little bombs seemed better than an equal weight of larger ones. Even larger bombs needed a direct hit to cause much damage and there was more chance of a direct hit with a lot of little bombs.

To Wallis’s methodically logical mind there was a serious flaw to all this. Factories and transport could be dispersed; in fact
were
dispersed all over Germany. Bombing (vintage 1939) would not damage enough factories to make much difference.

He started wondering
where
and
how
bombing could hurt Germany most. If one could not hit the dispersed war effort perhaps there were key points. Perhaps the sources of the effort. And here the probing mind was fastening on a new principle.

The sources of Germany’s effort, in war or peace, lay in power. Not political power, but physical power! Great sources of energy too massive to move or hide — coal mines, oil dumps and wells, and “white coal” — hydro-electric power from dams. Without them there could be no production and no transport. No weapons. No war.

But they were too massive to dent by existing bombs. One might as well kick them with a dancing pump! The next step— in theory anyway — was easy. Bigger bombs. Much bigger!

But that meant bigger aircraft; much bigger than existing ones. All right then — bigger aircraft too.

That was the start of it. It sounds simple but it was against the tenets of the experts of every air force in the world.

Wallis started calculating and found the blast of bigger bombs
was
puny against steadfast targets like coal mines, buried oil and dams. Particularly dams, ramparts of ferroconcrete anchored in the earth.

Then perhaps a new
type
of bomb. But there Wallis did not know enough about bombs and the logic stopped short.

The war was a few weeks old when the dogged scientist dived into engineering and scientific libraries and at lunch-times, when he pushed the problem of the Warwick’s tail-plane aside for an hour or so, he sent out for sandwiches, stayed at his desk and started to learn about bombs. At night at home he did the same, absorbed and lost to his family for hours. As the hard winter of 1939 arrived he progressed to the study of the sources of power.

Coal mines! Impossible to collapse the galleries and tunnels hundreds of feet underground. Possible, he decided, that a heavy bomb might collapse the winding shaft so that the lift would not work. No lift. No work. No coal. But that could soon be repaired.

Oil! Rumanian oil fields were too far for existing bombers, but a possibility for a future bomber. Germany’s synthetic refineries were massive and well defended; perhaps a target for bigger bombers.

Dams! Three German dams stood out—the Moehne, the Eder and the Sorpe. All in the Ruhr, they accounted for nearly all the water supply to that monstrous arsenal. Wallis knew that the German method needed eight tons of water to produce a ton of steel. The possibilities were intriguing.

The Moehne dammed Moehne Lake where the Heve flowed into the Ruhr River, maintaining the level so that barges with coal and steel and tanks could go to and from the foundries. Moehne Lake held 134 million tons of water. The Eder dammed the Eder River in Eder Lake, 212 million tons of water. It controlled the level of Germany’s second most important waterway, the Mittelland Canal. Even Kassel, forty miles away, got its water from the Eder. The Sorpe dammed another tributary of the Ruhr River in Sorpe Lake.

The Moehne was 112 feet thick at the base, 130 feet high and 25 feet thick at the top where a roadway ran; the Eder was even bigger. Wallis acknowledged that they were formidable. A 500-lb. bomb would hardly scratch the concrete. No less formidable the Sorpe, an earth dam, two sloping mounds of earth sealed and buttressed in the centre by a core of concrete.

In an engineering library Wallis unearthed accounts of their construction compiled by the proud engineers who had built them and found it hard to discipline his excitement as he read what the effects of breaching the dams could be.

It would not merely destroy hydro-electric power and deprive foundries of essential water, but affect other war factories which needed water for their processes. Disrupting them might cause a dozen critical bottlenecks in the completion of tanks, locomotives, guns, aircraft—almost anything one cared to name. It would deprive the populace of water too, which was no cause for joy in a gentle soul like Wallis but would at least induce in them a lessening of zest for the war.

There was still more to it. Breaches in the dams would send enormous floods ripping down the valleys, tearing away roads, bridges and railway lines, smashing factories and houses, so that some factories, rather than be deprived of water, would receive somewhat too much.

All this was fine, Wallis thought… logical ideas; but again one big flaw. The dams were so colossal that bombs twenty times bigger than existing ones were not going to hurt them.

His figures showed that when a 1,000-pounder exploded the charge expanded as a gas bubble, but at the end the bubble was only 20 feet across. A lot of damage was done beyond this 10-foot radius, however, by flying fragments, by blast and by the pressure pulse, or “shock wave.” Wallis well remembered the pedantic description of Shockwaves… “there is no motion of the transmitting medium other than the usual oscillation of particles to and fro about their position of rest as the wave passes through them.” Thin air gave scope to flying fragments and blast but the shock wave soon dissipated.

It would vibrate a structure, but not enough. To be destructive, shock waves had to travel through a more solid medium than air. And somewhere in Wallis’s brain a little cell awoke and stirred restlessly, an old memory, locked up and almost forgotten. He felt there was something he knew about shock waves that he should remember, tried to think what it was—it was a long time ago—but the harder he tried the farther it receded. It was only when he put it out of his mind that it sneaked insidiously back to him again.

It was something he had read, something about concrete. And then it hit him. Waterloo Bridge! Concrete piles being driven into the bed of the Thames ! That was years ago. The piles had kept shattering mysteriously and there had been an investigation. He started searching his bookcases and in a quarter of an hour had found it, an article in a 1935 journal of the Institution of Civil Engineers. The great drop-hammers had been slamming the piles into the river-bed and the tops of the piles had been exploding upwards.

Investigation narrowed the cause to the shock waves. The sudden blows sent shock waves shivering down the piles; at the bottom they met the blunt resistance of the clay and bounced back up the pile at something like 15,000 feet a second, reaching the top just after the hammer had bounced off, so there was nothing to rebound from again and they passed out and away, and in their wake you got a tension after the compression. A sort of crush and then a sharp stretch, almost in the same moment; enough to make a structure split—to shatter it.

Concrete, the article concluded sagely, well resisted compression but poorly withstood tension. Wallis docketed the fact in his mind, thinking of dams.

You needed a solid medium to get destructive shock waves !

Of course, if you could bury a bomb
deep
in… But you couldn’t slice a big bomb into ferro-concrete. No, but you might be able to inject it deep into some less solid medium before it exploded. You’d get the shock waves then. The expanding gas effects would be greater too; tamped by the encircling solids they would have to burst their way out.

He was aware that bombs and shells often buried themselves 3 or 4 feet in the ground before exploding, but that was so shallow the explosion forced its way easily to the top, causing a small crater, and the shock waves dissipated into the air. It was less effective than a surface explosion because the blast and shock waves went straight up instead of outwards.

But if you could
lock
the explosion underground so it could not break out you would get a sort of seismic disturbance… an earthquake! An earthquake bomb!

The idea shaped in his mind while he was sitting in a deep chair in his home at Emngham, an unspectacular setting for the birth of something so powerful.

But how to sink a bomb deeply into a resisting medium? You could not put one deep into a concrete dam. But a dam is set in water!

Water! It might not transmit a shock wave as well as earth but it would do so better than air. The tamping effect of water would produce a concentrated explosion and carry the “shock “punch. Wallis was starting to feel he might be getting somewhere.

And how about sinking the bomb in earth? A schoolboy knew the two principles. The heavier the bomb, the more power and speed it developed in falling. Wallis had learned the classic example in school. Drop a mouse down a well and at the bottom it will be able to get up and run. Drop a horse down and the horse will, probably burst. Because it was heavier it would hit
harder.
And the
farther
it fell, the
faster
it would fall!

So there it was : a bomb as heavy as possible (and as slim as possible) dropped from as high as possible.

Wallis looked up more books, studied the propagation of shock waves in soil, the effects of underground explosions at depth, and even found pages on the penetrative powers into soils of shells and light bombs. There was a piece about an enormous land mine exploded under a German-held hill at Messines Ridge in World War I. A colossal charge sent shock waves ripping into the earth, the hill was destroyed and the shock was felt in Cassell—30 miles away.

Wallis pulled out a pad and pencil and worked for a week, covering sheets with calculations, equations, formulas—and came up with a preliminary theoretical answer. A 10-ton bomb, with 7 tons of explosive in an aerodynamically-designed case of special steel, dropped from 40,000 feet, would reach a speed of 1,440 feet per second, or 982 m.p.h.—well over the speed of sound. At that rate it should penetrate an average soil to a depth of 135 feet.

A charge of that size should theoretically “camouflet” (not break the surface) at a depth of 130 feet. What it
would
do was cause a violent earthquake movement on the surface resulting in a hump forming.