9 January 1941: Test pilot Captain Harry Albert (“Sam”) Brown, O.B.E., (1896–1953) makes the first flight of the Avro Lancaster prototype, BT308, at RAF Ringway, Cheshire, England, south of Manchester.
Throughout World War II, 7,377 of these long range heavy bombers were produced for the Royal Air Force. The majority were powered by Rolls-Royce or Packard Merlin V-12 engines—the same engines that powered the Supermarine Spitfire and North American P-51 Mustang fighters.
The bomber was designed by Roy Chadwick, F.R.S.A., F.R.Ae.S., the Chief Designer and Engineer of A. V. Roe & Company Limited, based on the earlier twin-engine Avro Manchester Mk.I. Because of this, it was originally designated as the Manchester Mk.III, before being re-named Lancaster. Chadwick was appointed Commander of the Most Excellent Order of the British Empire, 2 June 1943, for his work.
The first prototype, BT308, was unarmed and had three small vertical fins.
With the second prototype, DG595, the small center vertical fin was deleted and two larger fins were used at the outboard ends of a longer horizontal tailplane. DG595 was also equipped with power gun turrets at the nose, dorsal and ventral positions, and at the tail.
The first production model, Lancaster Mk.I, was operated by a crew of seven: pilot, flight engineer, navigator/bombardier, radio operator and three gunners. It was a large, all-metal, mid-wing monoplane with retractable landing gear. It was 68 feet, 11 inches (21.001 meters) long with a wingspan of 102 feet, 0 inches (31.090) meters and an overall height of 19 feet, 6 inches (5.944 meters). The Mk.I had an empty weight of 36,900 pounds (16,738 kilograms) and its maximum takeoff weight was 68,000 pounds (30,909 kilograms).
BT308 and early production Lancasters were equipped with four liquid-cooled, supercharged, 1,648.96-cubic-inch-displacement (27.01 liter), Roll-Royce Merlin XX single overhead camshaft (SOHC) 60° V-12 engines, which were rated at 1,480 horsepower at 3,000 r.p.m. to 6,000 feet (1,829 meters). The Merlins drove three-bladed de Havilland Hydromatic quick-feathering, constant-speed airscrews (propellers), which had a diameter of 13 feet, 0 inches (3.962 meters), through a 0.420:1 gear reduction.
DG595 was used for performance testing at the Aeroplane and Armament Experimental Establishment (A&AEE) at Boscombe Down. The Mark I had a maximum economic cruise speed of 267 miles per hour (430 kilometers per hour) at 20,800 feet (6,340 meters), and a maximum speed of 286 miles per hour (460 kilometers per hour) at 20,000 feet (6,096 meters) at a gross weight of 45,300 pounds (20,548 kilograms).¹ Its service ceiling was 20,000 feet (6,096 meters) at 64,500 pounds (29,257 kilograms). It had a range of 2,530 miles (4,072 kilometers) with a 7,000 pound (3,175 kilogram) bomb load.
The Lancaster was designed to carry a 14,000 pound (6,350 kilogram) bomb load, but modified bombers carried the 22,000 pound (9,979 kilogram) Grand Slam bomb. For defense, the standard Lancaster had eight Browning .303-caliber Mark II machine guns in three power-operated turrets, with a total of 14,000 rounds of ammunition.
According to the Royal Air Force, “Almost half all Lancasters delivered during the war (3,345 of 7,373) were lost on operations with the loss of over 21,000 crew members.”
Only two airworthy Avro Lancasters are in existence.
7 January 1931: Guy Lambton Menzies flew an Avro 616 Sports Avian IV-A named Southern Cross Junior, solo across the Tasman Sea from Mascot Aerodrome, Sydney, New South Wales, Australia, to New Zealand.
Concerned that aviation authorities would prevent his flight, Menzies had said that his destination was Perth, Western Australia.
While en route, severe weather blew the Avian off course. Seeing an area that appeared to be level ground, he landed at the La Fontaine Swamp, Hari Hari, Westland, on New Zealand’s South Island. The airplane flipped over.
Guy Menzies was unhurt. His flight had taken 11 hours, 45 minutes.
According to Terry Mace’s website, A Fleeting Peace: Golden-Age Aviation in the British Empire (afleetingpeace.org), G-ABCF was repaired, but crashed again 21 April 1931 at Sydney, New South Wales, Australia. (Another source states 12 April.) The Australian registration VH-UPT had been reserved for the airplane, but because of the crash, the registration was cancelled in June 1931.
The Avro 616 Sports Avian was a two-place, single-engine, single-bay biplane, produced by A.V. Roe from 1926 to 1928. 405 were built. Guy Menzies’ airplane was a specially constructed variant, the Avro 616 Sports Avian IV-A, serial number 467, which had been ordered by Sir Charles Edward Kingsford Smith, M.C., A.F.C., and named Southern Cross Junior. Sir Charles had flown the airplane from England to Australia in 1930. (He had previously flown across the Pacific in 1928 with a Fokker F.VIIb/3m three-engine monoplane named Southern Cross). The Avian was registered to Sir Charles, 20 June 1930, identified as G-ABCF.
The Avian was constructed of wood-braced plywood panels, forming a tapered box. The sides and bottom were flat and the upper deck arched. The wings were built of two wooden spars with wood ribs, covered in doped fabric. The standard airplane was designed so that the wings could be folded alongside the fuselage.
G-ABCF was 24 feet, 5 inches (7.442 meters) long. Its wing span had been extended from the standard Avian’s 28 feet to 30 feet, 0 inches (9.144 meters). The overall height was 9 feet, 3 inches (2.819 meters). The wings had a total area of 262.0 square feet (24.34 square meters), an increase of 18.0 square feet (1.67 square meters) over that of the standard Avian. The wings are slightly staggered. Both upper and lower wings had a chord of 4 feet, 9 inches (1.448 meters), with a vertical gap of 5 feet, 0 inches (1.524 meters).
It weighed 1,100 pounds (499 kilograms) empty and had a gross weight of 2,225 pounds (1,009 kilograms).
The Avian’s standard 105 horsepower A.D.C. Cirrus Hermes engine was replaced with an air-cooled, normally-aspirated, 349.89-cubic-inch-displacement (5.734 liter) de Havilland Gipsy II inline 4-cylinder direct-drive engine, rated at 112.5 horsepower at 2,000 r.p.m., and122.5 horsepower at 2,200 r.p.m. The engine turned a two-bladed, fixed-pitch propeller with a diameter of 6 feet, 2 inches (1.880 meters). The Gipsy II weighed 295 pounds (134 kilograms).
In addition to the standard 24 Imperial gallon (109 liter) fuel tank, a welded-aluminum tank with a capacity of 91 gallons (414 liters) was installed in the forward cockpit. The airplane carried 3.5 gallons (15.9 liters) of lubricating oil, with 2 gallons in the engine, and 1½ gallons in a reserve tank. The airplane was also equipped with a 2 gallon (9.1 liters) tank for drinking water.
The Sports Avian IV-A had a cruising speed of 92 miles per hour (148 kilometers per hour) with the engine turning 1,900 r.p.m., and maximum speed of 115 miles per hour (185 kilometers per hour) at 2,100 r.p.m., at ground level, and 98 miles per hour (158 kilometers per hour) at 10,000 feet (3,048 meters), its service ceiling. It could climb to 10,000 feet in 41.5 minutes. The airplane’s absolute ceiling was 12,500 feet (3,810 meters). The range was calculated at 1,842 miles (2,964 kilometers) at 1,900 r.p.m.
Guy Lambton Menzies was born 20 August 1909 at Drummoyne (a suburb of Sydney), New South Wales, Australia. He was the the second of four children of Dr. Guy Dixon Menzies, a physician and founder of Seacombe Hospital, and Ida Mabel Lambton Menzies.
Shortly after his flight across the Tasman Sea, Menzies traveled to England where he joined the Royal Air Force. He was granted a short service commission as a Pilot Officer on probation, with effect 11 July 1931. He was confirmed in that rank one year later, 11 July 1932. Pilot Officer Menzies was promoted to Flying Officer 11 January 1933.
Menzies’ short term commission was soon to come to an end, which would have resulted in his being transferred to the Reserve. However, he was selected for appointment to a permanent commission as a Flying Officer in April 1936, and promoted to Flight Lieutenant, effective 1 April 1936. Flight Lieutenant Menzies was granted a permanent commission in that rank with effect from 11 July 1936. He was promoted to the rank of Squadron Leader, 1 December 1938.
On 1 November 1940, Squadron Leader Menzies, No. 228 Squadron, Coastal Command, was flying a Short Sunderland Mk.I four-engine flying boat, N9020, from RAF Kalafrana, Malta, patrolling near Sicily. The airplane was attacked by Italian Air Force Macchi C.200 Saettta fighters ¹ and “was observed falling onto the sea. There were no reported survivors.” ²
Squadron Leader Menzies’ younger brother, Flying Officer Ian Lambton Menzies, No. 24 Squadron, Royal Australian Air Force, was also killed in an airplane crash, 18 April 1941, near Ravenswood, North Queensland, Australia. The airplane, a Commonwealth Aircraft Corporation Wirraway, A20-117, a development of North American Aviation’s NA-16 trainer, stalled during a steep turn.
Their mother, Mrs. Menzies, said,
“I have given my two sons to the Empire.”
—TheDaily Telegraph, 19 April 1941.
¹ Internet sources identify the Regia Aeronautica fighter pilots who shot down N9020 as Tenente Luigi Armanino and Sergent Maggiore Natalino Stabile of 88° Squadriglia, VI Gruppo.
² The members of Menzies’ crew: Flying Officer Stuart Maxwell Farries, 40098; Sergeant Elias Dawes, 568257; Sergeant Frederick Harris, 563782; Sergeant Edward Louis Setterfield, 543241; Sergeant George Arthur Stamp, 580074; Leading Aircraftman Leslie Charles Major Hale, 522295; Leading Aircraftman Ronald Fletcher, 535135; and Leading Aircraftman Benjamin Edwin Nicholas, 526309.
17 November 1946: A modified Avro 691 Lancastrian C.1, VH742, under the command of Rolls-Royce’s chief test pilot, Captain Ronald Thomas Shepherd, O.B.E., flew from London Heathrow Airport (LHR) to Aéroport de Paris – Le Bourget (LBG) for 17th Salon de Aviation (Paris Air Show) with two Rolls-Royce RB.41 Nene Mk.I turbojet engines for propulsion. The airplane’s two Rolls-Royce Merlin V-12 piston engines were shut down, except for takeoff and landing, and their three-bladed propellers were feathered to reduce drag. It was the first-jet-powered passenger transport to fly from one country to another.
A contemporary aviation industry news article described the event:
The Nene-Lanc, Flies to Paris
THE flight of the Nene Lancaster from London to Paris last Monday, to play its part in connection with the exhibition, may be said to have marked a historic part in British aircraft development, for it constituted the first time that any jet-powered airliner had flown from one country to another. Moreover, since this particular aircraft has been flying fairly regularly since round about the time of the Radlett exhibition, the flight to Paris was no special performance, but merely one more public demonstration of its inherent reliability.
In the hands of Capt. R. T. Shepherd, chief test pilot for Rolls-Royce, the “Nene-Lanc” landed at Le Bourget at 10.58 a.m., G.M.T., after a 50-minute flight from London Airport, giving an average speed of 247.5 m.p.h. [398.3 kilometers per hour] Two passengers were carried in addition to the crew; they were Mr. Roy Chadwick, the Avro designer, and Mr. R. B. William Thompson, Chief Information Officer of the Ministry of Supply.
Capt. Shepherd said that he was very pleased with the aircraft’s performance and added that, but for having to circle Le Bourget Airport Twice before landing, the flight would have been completed in 43 minutes.
— FLIGHT and AIRCRAFT ENGINEER, No. 1978. Vol. L., Thursday, November 21st, 1946 at Page 561, Column 2.
Five days later, VH742 flew back to England:
THE return of the Nene Lancastrian on Nov. 22nd, direct from Le Bourget to Heathrow, was made in only 49 min, including landing, actual flying time from point to point being 41 min—an average speed of 322 mp.h. [518.2 kilometers per hour] This remarkable performance was in spite a beam wind and the dead weight and drag of the two inboard Merlins, which are only used for takeoff and landing.
Passengers of the return trip included Mr. Roy Chadwick, chief designer and a director of A. V. Roe and Co., Air Comdre. Kirk and Air Comdre. Pike.
—FLIGHT and AIRCRAFT ENGINEER, No. 1979., Vol. L., Thursday, November 28th, 1946 at Page 588, Column 1.
The Rolls-Royce RB.41 Nene engine first been run in October 1944. It installed in a Lockheed YP-80A Shooting Star, 44-83027, and the engine was first flown 18 July 1945 with Rolls-Royce test pilot Wing Commander John Harvey Heyworth, A.F.C., in the cockpit. The Nene-powered P-80 had made approximately 30 test flights when it was damaged beyond repair at RAF Syerston, 6 December 1945. With test pilot Andy McDowall flying, a fractured fuel pipe caused the engine to flame out from fuel starvation. McDowall tried to glide to a landing but another airplane was on the runway. He touched down on the grass but the landing gears were pushed up through the Shooting Star’s wings.
The jet fighter had been too small to allow for adequate test equipment. A larger aircraft was needed. The R.A.F. assigned VH742 the role of test aircraft.
The new Lancastrian arrived at the Rolls-Royce Flight Test Establishment at Hucknall Aerodrome, Nottinghamshire, 30 October 1945. The modification was engineered and the airplane was modified. The Lanc’s two outboard Rolls-Royce Merlin V-12 engines were removed and two Nene Mk.I engines were installed in underslung nacelles. The wing flaps were shortened by 3 feet, 4 inches (1.016 meters) and the ailerons by 10 inches (0.254 meters) to provide clearance from the jet engines’ exhaust. Sheet steel was installed on the lower surfaces of the wings as protect against the heat.
Three fuel tanks were installed in each of the Lancastrian’s wings. The center tank contained gasoline for the Merlin engines, while the inner and outer tanks, plus two auxiliary tanks in the fuselage, carried kerosene for the jet engines. Fuel capacity was 760 gallons (2,877 liters) of gasoline and 2,420 gallons (9,161 liters) of kerosene.
In the Lancastrian’s cockpit, additional instruments were installed for the turbojets: tachometers reading from 0–20,000 r.p.m.; oil pressure gauges, 0–80 p.s.i.; exhaust gas temperature, 400˚–750 ˚C., and exhaust gas pressure.
The first flight of the modified VH742 took place 14 August 1946, with Ronnie Shepherd in the cockpit. Running on the jet engines alone, the airplane was extraordinarily quiet and vibration free. Like all early turbojets, the Nenes were slow to accelerate from low r.p.m. Test pilots had to use caution. Jim and Harvey Heyworth also flew VH742 during the last half of August.
The Rolls-Royce RB.41 Nene Mk.I was developed from the earlier RB.40 Derwent.¹ It was considerably larger and produced nearly double the thrust. It was a single-stage centrifugal-flow compressor/single-stage axial-flow turbine, rated at 5,000 pounds of thrust (22.24 kilonewtons) at 12,400 r.p.m. for takeoff.
A second Nene-powered Lancastrian was added to the test fleet at Hucknall the following year. Last Nene flight took place in August 1949.
VH742 had been ordered by the Royal Air Force during World War II as an Avro Type 683 Lancaster B. Mk.III, a very long range heavy bomber, and assigned identity markings PD194. With the end of World War II in Europe, orders for hundreds of Lancaster bombers were cancelled. The partially completed PD194 was modified on the assembly line as a Lancastrian C. Mk.I passenger transport and renumbered as VH742.
The Avro Type 691 Lancastrian was a four-engine civil transport based on the World War II very long range heavy bomber, the Avro Lancaster. The airliner was operated by a flight crew of four and carried one flight attendant. It could carry up to thirteen passengers. The Lancastrian was 76 feet, 10 inches (23.419 meters) long with a wingspan of 102 feet (31.090 meters) and overall height of 19 feet, 6 inches (5.944 meters). The empty weight was 30,220 pounds (13,707.6 kilograms) and gross weight was 65,000 pounds (29,483.5 kilograms).
The Lancastrian Mk.III was powered by four 1,648.9-cubic-inch-displacement (27.04 liter) liquid-cooled, supercharged, Rolls-Royce Merlin T24/2 single overhead camshaft (SOHC) 60° V-12 engines producing 1,650 horsepower and turning three bladed propellers.
The airplane a cruise speed of 245 miles per hour (394.3 kilometers per hour) and a maximum speed of 315 miles per hour (506.9 kilometers per hour). The service ceiling was 25,500 feet (7,772 meters) and the range was 4,150 miles (6,679 kilometers).
91 Avro Lancastrians were built, including modified Lancaster bombers. The transport variant first flew in 1943. In addition to the Royal Air Force, commercial Lancastrians were operated by British European Airways, British Overseas Airways Corporation and British South American Airways. The last one was retired in 1960.
Rolls-Royce built more than 1,100 RB.41 Nene engines. It was licensed for production by Pratt & Whitney as the J42. Forty Nenes were sold to the Soviet Union under the condition that they would not be used for military purposes. These were reverse-engineered and produced as the Klimov RD-45 which powered the Mikoyan-Gurevich MiG-15 fighter.
¹ While Rolls-Royce named its piston-driven aircraft engines after birds of prey, the turbojet engines were named for rivers.
16 November 1920: Queensland and Northern Territories Aerial Services, Ltd., today known as Qantas, is one of the oldest airlines in the world. It was founded at Winton, Queensland, Australia, on 16 November 1920 as Queensland and Northern Territory Aerial Services Limited by Paul Joseph McGinness, D.F.C., D.C.M., and Wilmot Hudson Fysh, D.F.C., both World War I fighter aces; Fergus McMaster, a local businessman; and Wilfred Arthur Baird, an aircraft mechanic who had served in Egypt with McGinness and Fysh.
Initially the airline operated air mail services which were subsidized by the Australian government, linking railheads in western Queensland. It flew its first commercial passenger, Alexander Kennedy, on 2 November 1922.
The airline’s first airplane was a two-place, single-engine A. V. Roe & Co., Ltd., Avro 504, c/n D1, G-AUBG. D1 was one of nine Avro 504-series airplanes which were assembled by the Australian Aircraft and Engineering Company, A. V. Roe’s licensee at Sydney, New South Wales. A replica of the airplane is on display at the Kingsford Smith Memorial, Mascot, New South Wales.
Qantas took delivery of D1 on 30 January 1921. It was assigned a registration mark of G-AUBG, 28 June 1921. The airplane was involved in a serious accident at Ingham, North Queensland, 2 August 1921. It was repaired and returned to service three months later.
Qantas operated it until 6 November 1926, when it was sold to H.J. Taylor, of Hawthorn, Victoria. The airplane was later owned by Matthews Aviation, and finally, by Newcastle Air Service, as Newcastle’s Own. The Avro’s registration was changed to VH-UBG, 28 March 1929. The registration was cancelled 14 April 1932.
Between 1913 and 1932, nearly 9,000 Avro 504-series airplanes were built by more than twenty manufacturers. The Avro 504K was 29 feet, 5 inches (8.996 meters) long with a wingspan of 36 feet (10.973 meters) and height of 10 feet, 5 inches (3.175 meters). Its empty weight was 1,231 pounds (558 kilograms) and the maximum takeoff weight was 1,829 pounds (830 kilograms).
The Avro 504 had been designed to accept installation of several different engines. D1 had been assembled using a water-cooled, normally-aspirated, 8.822 liter (538.351 cubic inches) Sunbeam-Coatalen Aircraft Engines Dyak single overhead camshaft (SOHC) inline 6-cylinder engine. (Sunbeam named its aircraft engines after ethnic groups. The Dyak are an indigenous people of Borneo.) The Sunbeam Dyak was a left-hand tractor, direct-drive engine with two valves per cylinder and a compression ratio of 5:1. It used two Claudel-Hobson B.Z.S. 38 updraft carburetors, two magnetos, and was rated at 100 horsepower at 1,200 r.p.m. The Dyak weighed 399 pounds (190 kilograms).
In 1931, the Avro was re-engined with a right-hand tractor, water-cooled, normally-aspirated, 9.005 liter (549.519 cubic inches) A.D.C. (Aircraft Disposal Company, or “Airdisco”) overhead valve (OHV) V-8 engine. The A.D.C. V-8 had a compression ratio of 4.6:1 and was rated at rated at 120 horsepower at 1,800 r.p.m. It had 2:1 propeller gear reduction.
The 504K had a cruise speed of 75 miles per hour (121 kilometers per hour), maximum speed of 90 miles per hour (145 kilometers per hour), service ceiling of 16,000 feet (4,877 meters) and range of 250 miles (402 kilometers).
As of October 2017, Qantas had 29,596 employees. After-tax profit for 2016 was A$1,029,000,000. Qantas currently operates a fleet of 118 aircraft, which includes 28 Airbus A330s and 12 A380s, 10 Boeing 747-400s, 67 B737-800s and 1 787-9. The airline has 45 Boeing 787-9 Dreamliners on order, with confirmed delivery dates for the first 15.
12 November 1944: No. 9 Squadron and No. 617 Squadron (Dambusters), Royal Air Force, sent a force of 32 Avro Lancaster long range heavy bombers to attack the 49,948 metric-ton-displacement Kriegsmarine battleship KMS Tirpitz at Tromsø Fjord, Norway. The attack was filmed by a photo aircraft of No. 463 Squadron, Royal Australian Air Force.
The Lancasters were armed with 12,030 pound (5,457 kilogram) Tallboy bombs. They bombed from altitudes from 12,000 to 16,000 feet (3,658–4,877 meters). Two of the bombs hit the battleship, one was a very near miss and another three also were close enough that they probably contributed to the overall damage. Many other Tallboys landed within the torpedo nets that surrounded the ship and cratered the seabed, removing the sandy bottom which had been built up under Tirpitz‘ hull to prevent her from sinking. Tirpitz immediately began to list and was then rocked by an internal explosion. It capsized and sank to the sea bed. As many as 1,204 sailors were killed.
Tirpitz was a Bismarck-class battleship armed with a main battery of eight 38-centimeter (15-inch/52-caliber) guns in four turrets. These guns had a maximum range of 22.7 miles (36.5 kilometers) when firing a 1,800 pound (816 kilogram) projectile. The German Navy did not use its heavy warships to directly engage the British fleet, but instead to raid the Atlantic convoys. The merchant ships with their destroyer escorts were defenseless against a battleship or battle cruiser. Allied forces expended tremendous effort and resources to contain or destroy Tirpitz throughout the war.
The Avro Lancaster was a four-engine long range heavy bomber. It wasn’t as fast as the American B-17 Flying Fortress, but was capable of flying longer distances with a heavier bomb load. It was operated by a crew of seven: Pilot, flight engineer, navigator, radio operator, bomb aimer/nose gunner, top gunner and tail gunner. The “Lanc” was 69 feet, 4 inches (21.133 meters) long, with a wingspan of 102 feet (31.090 meters) and had an overall height of 20 feet, 6 inches (6.248 meters). It had a maximum takeoff weight (MTOW) of 72,000 pounds (32,657 kilograms) when carrying a 22,000 pound (9,979 kilogram) Grand Slam bomb.
The Lancaster was powered by four liquid-cooled, supercharged, 1,648.96-cubic-inch-displacement (27.01 liter), Rolls Royce Merlin XX or Packard V-1650 single overhead camshaft (SOHC) 60° V-12 engines, which were rated at 1,480 horsepower at 3,000 r.p.m. to 6,000 feet (1,829 meters). They turned three-bladed de Havilland Hydromatic constant-speed propellers which had a diameter of 13 feet (3.962 meters) through a 0.420:1 gear reduction.
These Merlin engines, the same as those powering Supermarine Spitfire, Hawker Hurricane and North American P-51 Mustang fighters, gave the Lancaster a maximum speed of 282 miles per hour (456 kilometers per hour) at 13,000 feet (3,962 meters) at a weight of 63,000 pounds (28,576 kilometers per hour). The service ceiling was 21,400 feet (6,523 meters) and maximum range was 2,530 miles (4,073 kilometers).
Defensive armament for a standard Lancaster consisted of eight Browning Mark II .303-caliber machine guns in three power turrets, nose, dorsal and tail. Modified bombers deleted various combinations of guns to reduce weight.
The Tallboy (Bomb, Medium Capacity, 12,000 lb) was a special demolition bomb designed to be dropped from high altitude, reach supersonic speeds, then penetrate as far as 90 feet (27 meters) into the ground before detonating. It was built of a specially hardened steel casing filled with 5,200 pounds (2,358 kilograms) of Torpex explosive. The bomb was designed by Barnes Wallis, who had also designed the special bomb used by the Dambusters in their famous 1943 attack on the Ruhr Valley hydroelectric dams, as well as the Grand Slam, a 22,000-pound (10,000 kilogram) scaled-up version of the Tallboy. The Tallboy and Grand Slam bombs were very successfully used against U-boat pens and heavily fortified underground rocket facilities.