THE DAM BUSTERS
U.S. Gold


HISTORICAL NOTES


CERTAIN ASPECTS OF HARD-CASING EXPLOSIVE BEHAVIOUR AT DEPTH
by B.N. Wallis, M.S.E., F.R.S.


PREFACE:

Current strategic theory holds that the bombing of enemy factories and
centres of population beyond the battlefield will cause a collapse of
production capacity and severe deterioration in civilian morale. The
Air Targets Sub-Committee has identified three targets of special
strategic significance: the Moehne, the Eder and the Sorpe Dams. All
are in the Ruhr Valley and account for the bulk of water supply to the
monstrous German arsenal. For example, the German method of iron
production needs between 100 and 150 tons of water to produce a ton of
steel. These dams also provide domestic water to the Ruhr district.

The Moehne creates Moehne Lake. The level of this lake is maintained
so that barges with coal and steel and tanks can travel to and from
the foundries. If the dam were to be breached, the reservoir would
empty 134 million tons of water in approximately ten hours into the
lower Ruhr, causing widespread disaster. There would be a serious
shortage of water for drinking purposes and industrial supplies.

The Eder dams the Eder River to form Eder Lake - 212 million tons of
water. It controls the level of Germany's second most important
waterway, the Mittelland Canal, and prevents flooding of surrounding
agricultural land and towns. Several power stations lying along the
river would be damaged or destroyed by a breach in the dam, and
transportation on the Mittelland would be seriously hampered to the
point of a virtual cessation of traffic. The Sorpe holds a similar
position of importance.

A psychological as well as physical effect will be felt, should the
dams be burst. Rumours will circulate regarding disease, water
shortage, and loss of fire- fighting capabilities.

Countervailing arguments were submitted by high-ranking officers of
Bomber Command, who drew the Sub- Committee's attention to the massive
construction of the German dams, against which existing weaponry would
be useless. There was considerable doubt as to whether the structure
could be breached even if fissures were made in a gravity- type dam
(the Moehne). These dams are also protected by nets against torpedoes.

The Moehne is 112 feet thick at the base, 130 feet high and 25 feet
thick at the top. The Eder, also a gravity dam, is even bigger.

It is calculated that the bomb will not ricochet if the angle of
impact exceeds 30 degrees, and therefore the best height is 10-15,000
feet. At this height the average error was 102-113 yards (if a
50-yard-long portion of the dam was attacked, only a 6% chance existed
of hitting it - this is reduced to 2% during war-time).

Nonetheless, air attacks on reservoirs and dams have been deemed so
important that the Air Targets Sub-Committee desires that the issue be
"treated as urgent and of pressing importance:'


DEVELOPMENT

It was clear that conventional techniques were unsuitable to the
destruction of these very solid objects, and that an unusual approach
would be required to solve the problem. Obviously, a kind of
"explosive judo" would be needed, to use the vast weight of water
behind the dam to assist in its own destruction.

An underwater bomb exploded on the upstream side of the dam would use
the water pressure to magnify the shockwave against the dam. Such a
bomb would produce a shockwave that would travel through the side of
the dam, smashing a hole through the masonry. However, experimentation
revealed that if the bomb was even slightly too far upstream from the
dam face when detonated, the surrounding water would damp and absorb
the shockwave, making the explosion useless. A new delivery system,
incorporating both weapons and techniques, was called for.

Early in 1942, I had the idea of a missile, which if dropped on the
water at a considerable distance upstream of the dam would reach the
dam in a series of ricochets, and after impact against the crest of
the dam, would sink in close contact with the upstream face of the
masonry. The germ of this idea came from a technique used by one of
the greatest naval strategists of all time, Horatio Nelson, who
discovered that by skipping cannon shot across the surface of the
water it would gain distance and hit the target vessel just above the
water line.

The bomb uses some of the same principles as a rock skipping across
the water, but differs in that a rock skipping rotates along its
vertical axis while the bomb rotates counter-clockwise along its
horizontal axis. The essential parameters in delivering such a bomb
are airspeed and initial approach angle. In theory, an appropriately
constructed bomb capable of being carried by a heavy bomber could be
delivered using this principle. Extensive testing has proven this to
be correct.

I had projected a near-spherical steel weapon seven and a half feet in
diameter. But the Ministry of Supply predicted a two- year wait for
steel to make the case, so we settled on a smaller cylinder. The final
version of the bomb is approximately 60in. long and 50in. in diameter;
made of 3/8in. thick steel, weighing 2,650 lbs., and containing 6,600
lbs. of Torpex underwater explosive compound. There are three pistols,
armed with the powerful initiating explosive Tetryl, set to explode at
30 ft., and a fourth self-destructive pistol set to go off 90 seconds
after release. Total weight of the weapon is 9,250 lbs.

Bomber Command, in the person of Air Marshal Harris, assures me that
its personnel and equipment can deliver the weapon on target within
the specified parameters. To that end, a special squadron, number 617,
has been formed and is currently undergoing intensive training for the
exclusive purpose of conducting this single mission. Equipped with
modified Lancaster III bombers and carefully selected on the basis of
their low- level expertise, the men of 617 Squadron should have an
excellent chance of success. Time, however, is of the essence, since
the dams are now filling with water; and will be at the ideal highest
level for only a few days in mid-April. I pray the indulgence of
Cabinet to expedite this matter with all its powers, as the successful
completion of this mission, will, in all likelihood, be the greatest
strategic blow for freedom in the entire conduct of the war to date.




DAMBUSTING BOMB DETAIL


ED825/G carried out test dropping of the cylindrical mines, but was
not selected to be one of the attacking aircraft. Underneath can be
seen the mine-support pylons and belt drive mechanism for spinning the
mine prior to release.


LANCASTER B. MK I/III (DAM BUSTER)

This modified version of the Mark III has been especially adapted for
this mission. The original Mark I/III had H2S radar, a downward
looking radar, used to obtain directional bearings from the local
landscape. This has been removed in the Dam Buster Lancaster to
increase the bomb load capacity. The bomb bay doors were removed and
faired in to allow for two v-shaped caliper arms which protrude from
the front of the bomb bay. These calipers hold the mine between their
points and a 20-inch diameter disk mounted on the inside of these
extremities engages a track at the end of the cylinder. A hydraulic
motor attached to the track (used for steering the hydroplane operator
in submarines) is mounted on the floor of the fuselage. This motor is
used to spin the mine backwards at the required 500rpm.


LANCASTER BOMBER

Because of the difficulty and importance of the mission the latest and
most sophisticated bomber, the Lancaster Mark III was chosen. Although
it was a bomber, according to Gibson it could manoeuvre as well as
most German fighters.




REPORT ON THE FORMATION AND TRAINING OF 617 SQN.
WITH NOTES ON THE LOW-LEVEL PERFORMANCE OF THE 
MODIFIED LANCASTER BOMBER
by Guy Gibson, W/C, RAF


Wing Commander G. Gibson, DSO, DFC, was chosen by Air Marshal Harris
as Commanding Officer of "Squadron X." He began assembling the
hand-picked crew, some of whom were chosen by Gibson himself, at RAF
Scampton on March 21, 1943. The crews range in age from 20 to 32.
There are currently 21 pilots serving with 617 - three from the Royal
Australian Air Force, five from the Royal Canadian Air Force, one from
the Royal New Zealand Air Force, and twelve from the Royal Air Force.
This last figure includes two U.S.-born Squadron Leaders, Young and
McCarthy.

The speed with which the squadron had been formed presented various
difficulties. Initial facilities at Scampton were quite limited, and
indeed, until the arrival of the Type 464 Provisioning Lancasters,
only ten aircraft on loan from other squadrons were available to the
men. Their accommodations were less than luxurious, being a group of
condemned wooden billets of First World War vintage. Each hut housed
24 men. In the interests of bringing the motley group together, it was
suggested that each night they do PT before retiring. A newcomer to
the crew who arrived one evening in the midst of these exercises was
convinced that he had "stumbled on an annex of the local mental
institution".

The 700 men of the squadron raided other squadrons for furniture -
beds and chairs. With Air Vice Marshal Cochrane's intervention,
supplies such as uniforms and blankets for 617 were given top
priority. Official pressure resulted in the appearance of spark plugs,
tools, starter motors, bomb trolleys and winches.

On March 27, 1943, I was issued with "most secret" written orders,
which outlined the plan of attack without naming the targets:

	"No. 617 Squadron will be required to attack a number of
	lightly defended special low level targets over enemy territory in
	moonlight with a final approach to the target at 60 ft. at a precise
	speed, which will be about 240 mph."

It was noted that the exact speed would be determined later and
visibility might well "not exceed one mile". It was assumed that
aircraft would be despatched at ten-minute intervals to attack the
first target. When this was destroyed, subsequent aircraft would be
diverted in the air to the next target and so we had to ensure that
navigation was accurate in moonlight, at a height which would be as
secure as possible against fighter attack. Air position indicators
would be available, but training was to proceed without them.
Accordingly, the squadron has been performing low-level night flying
exercises almost non-stop to date. The efficiency attained in these
areas has been most gratifying.

According to Barnes Wallis's specifications of the delivery of the
bomb, each Lancaster must release the bomb at 240 mph, 60ft above
water and exactly 800 yards away from the dam.

Visual sighting at night is difficult to impossible because of the
existence of a sort of a grey no-man's land between the surface of the
water and the aircraft flying so close at high speed. Several
different techniques were tried and all were rejected due to measuring
error or impracticality. Finally, a simple solution was found, using
two spotlights, one at either end of the aircraft. As the aircraft
flies over the water the spots shine down upon the surface of the
water. The spotlights are angled such that when the two spotlights
touch, the aircraft is flying at 60 ft. with virtually no error.

The distancing problem had a similar trivial solution involving
angles. The front gunner, using the bomb aimer's bubble, will sight on
the twin towers of the dam, through a Y shaped distancing sight. When
the twin towers of the dam align with the markers on the end of the
sight, the aircraft is exactly 800 yards from the dam, again with
virtually no error.

The conventional airspeed indicator used by the Lancaster is accurate
enough to render an airspeed reading within acceptable tolerances.

Therefore, I have the honour to report that 617 Squadron is, in all
respects, ready for battle.




INTELLIGENCE - TACTICAL REVIEW
prepared by J.A. Franklyn-Smith, Sqn. Ldr. (Int)


GERMAN DEFENCE SYSTEMS

Light anti-aircraft pose formidable problems for low flying aircraft.
The basis for nearly all German 20MM models is the FLAK 30, capable of
120 rounds per minute with a ceiling of around 6,630 feet. A wide
range of heavier flak guns, including those mounted on concrete towers
or formed into mobile railway batteries will also prove dangerous if
the crews stray too close to military or industrial centres.

An integrated defence system manned by the Luftwaffe has been
developed in Northern Europe. Co-ordination from local radar operators
directs night fighters, anti- aircraft guns and searchlights.

Two types of German radar can detect incoming Allied aircraft and
co-ordinate flak, searchlight and interceptor aircraft.

A)	Freya stations on the coast give the direction and range of attackers up
	to 100 miles but are unable to determine altitude.
B)	Mobile Wurzburg sets with a range of 45 miles are used by ground
	controllers inland and many fighters have airborne Lichtenstein sets
	accurate up to two miles.

So the 617 Squadron Lancasters involved in Operation Chastise must
contend with this defensive organisation: fighters with airborne
radars, and a strong array of flak weapons assisted by search- lights
and radar and often grouped around vulnerable targets.

One advantage to the 617 Squadron is that the German ground radar is
ineffective at tracking aircraft below 1,000 feet, especially in the
relative haven of valleys near the targets. Also, airborne night
fighters equipped with Lichtenstein radar sets are not effective in
scanning downwards from their regular operating attitude. Thus, flying
as low as possible at night offers the Lancaster the optimal chance of
survival.

From the inception of the operation, a Mosquito reconnaissance
aircraft has flown daily at 25,000 feet over the dams, taking
photographs of rising water and the defences. The Mosquitoes are flown
in such a manner that to the Germans it appears they are crossing the
dams by accident.

During the afternoon of May 14, a photo-reconnaissance operation was
flown at 30,000 feet over targets "in the Soest area" and on the
morning of May 15 the Dortmund, Duisburg and Soest regions were
photographed so as not to draw attention to the dams. This information
was combined with other results to show that there was no unusual
defensive activity in the target area.




CATHODE-RAY TUBE SIMULATION SYSTEMS - THEIR ROLE IN AIRCREW TRAINING
by Sydney H. Prendergast, Ph.D.

The cost of aircrew training in a wartime economy is unacceptably
high, in terms of equipment, fuel and risk to personnel who are, as
they say, new at the game. We at the Shipton-on-Stowe Research
Establishment propose to reduce these costs and improve efficiency
through the use of an image-projection device, the Cathode- Ray Tube.

The trainee will sit at a console in front of a phosphor screen on
which will be projected tactical situations, as realistically
presented as possible. He will respond to these through a "joystick",
not unlike an aircraft control yoke, and his responses will be
transmitted back to a panel of expert "analysts" who will assess his
response, determine its effects in a "real-life" situation, and alter
the project image he sees accordingly. I foresee various rooms set up
with "television" cameras (a distasteful American term, but their
research is ahead of us in this area) pointed at simulation boards,
instrument panels, or rear-projection screens, all linked up to the
trainee's console, and have performed the "thought experiment" of
running through an entire simulation of an imaginary low- level raid
on some large, strategic German site, for example, one of the dams
along the Eder River. I have taken the liberty of appending a copy of
the instructions for a "game" or simulation of this nature, and am
confident you of the War Cabinet will find it worthy of increased
funding.
