Strategic Bombardment Help for WWII Series


#21

I apologize for being a little tardy in writing this post - things have been hectic.

Enjoyed your post, Alex!

Before I begin, I would like to let everyone at TimeGhost know that you are doing a great job on the Battle of Britain thus far!

Some thoughts:

  • Germany’s lack of a long-range, heavy bomber was a serious handicap and several Luftwaffe commanders noted this. The fact that Britain’s aircraft factories were generally out of range of Germany’s medium bombers meant that the best they could hope for was ‘local air superiority.’

  • The problem with this approach was that it forced attacks against airfields – a tactic derogatively known as ‘potholing.’ Airfields were/are easily repaired, especially grass fields.

  • As such, the only realistic way to achieve ‘local air superiority’ would have been to wage a campaign of attrition – something no-one was willing to do

As you have noted, Luftwaffe high-command was not approaching the Battle of Britain with any clarity of mind. I am particularly pleased that you acknowledged their lack of a well-defined strategy.

Anyways, on to my regular post:

Now that I’ve given a basic overview of strategic airpower theory in the Interwar years, I think I might now cover some aspects of technological development.

Aircraft Armament:

While aircraft design evolved rapidly in the years following the Great War, it took some time for aircraft armament to catch up. For much of the world, the Great War tradition of twin .30/.303 caliber MGs, mounted above the engine with an interrupter gear, remained the dominant arrangement. The problem was that, with the ever increasing speeds of modern aircraft, the amount of time an airman had to hit his opponent was steadily decreasing – something not helped by the constraints of an interrupter gear.

The weapons themselves were changing at a very slow pace. As late as 1944, many RAF aircraft were still being outfitted with Great War-surplus Vickers MGs. This is not to say there was not any improvement, primarily in rate-of-fire. Legendary US gun designer John M. Browning perfected his M1917/M1919 .30 caliber MG in the Interwar years to serve in the aircraft role, increasing the rate-of-fire from the standard 450 rpm of his original design to a blistering 1,200 rpm (the cold temperatures at altitude allowing for an increased rate-of-fire).

Great Britain was the first to break from the interrupter trend. In 1931, AVM Hugh C. T. Dowding – CINC Air Defense of Great Britain – called for a series of trials to determine fighter armament needs regarding advances in bomber design. The solution was determined by Capt Frederick William Hill but was most loudly championed by Sqn Ldr Ralph S. Sorley:

AM Sir Ralph S. Sorley in 1944.

Armorers load the eight Browning .303 MGs of an early-war Hawker Hurricane. When armed, the gun ports were covered with red duct tape to smooth airflow. After missions, ground crews knew to look for aircraft with ripped duct tape - these were aircraft which had seen combat.

Like so many others, I had spent many years trying to hit targets with one, two, or even four machine guns, and I confess, with singularly poor results. Others were so much better, but I guessed that if one could hold the sight on for longer than two seconds, that was better than average. We were now going to have to hold it on at appreciably higher speeds so the average sight might be even less than two seconds. . . . By dint of much blotting paper, arithmetic and burning of midnight oil, I reached the answer of eight guns being the number required to give a lethal dose in two seconds of fire. I reckoned that the bomber’s speed would probably be such as to allow the pursuing fighter only one chance of attack so it must be destroyed in that vital two-second burst.

Papers of Air Marshal Sir Ralph S. Sorley. As quoted in: Bungay, Stephen. The Most Dangerous Enemy. London, UK: Aurum Press, 2000.

Sorley’s figures were included in the Air Ministry’s requirements for a new fighter in 1934, locking the design into a thick-wing monoplane holding the eight Browning .30/.303 MGs. The first of these modern, wing-mounted gun fighters, the Hawker Hurricane, entered service in 1937.

Debates still rage over the practicality of wing-mounted MGs versus those mounted in the fuselage. While wing-mounted weaponry guaranteed a higher rate-of-fire and greater amounts of ammunition, this set-up also required the guns be converged some distance ahead. At the war’s opening, the standard RAF distance was at 1,200 ft, though critiques by pilots in the field saw this number later shorten to 750 ft – the idea being the closer to the target, the more accurate the attack. Conversely, fuselage-mounted weapons did not suffer from convergence requirements, and it is worth noting that some of the more successful fighters of the war (in terms of ace pilots) were designs with fuselage-mounted MGs.

The alternative to the high-volume approach was to increase the size of the cartridge itself. As discussed earlier, the rise of cantilever aircraft like the Junkers J.1 and Fokker D.VII at the end of the Great War proved the need for heavier-hitting weaponry. Capitaine Georges M. L. J. Guynemer (Escadrille 3), for example, became the first airman to shoot down a heavy bomber when he downed a Gotha G.III on 8 February 1917. Upon Guynemer’s recommendation, his SPAD VII was armed with a 37mm to make future attempts easier – even though it greatly affected the aircraft’s balance and had to be manually reloaded in flight. Guynemer’s experience was typical and all nations worked toward a solution.

Armorers work to set the gun convergence of an early-war Supermarine Spitfire.

An armorer loads the Hispano 20mm autocannon of a mid-war Hawker Hurricane. Note the cans which hold the weapon’s ammunition.

Here Germany took the lead, with Reinhold Becker introducing the 20mm autocannon in 1916. Intended to serve in an aeronautical capacity, the Becker M2 was too clunky to be fitted with an interrupter gear. Instead, Becker’s work found its way to Switzerland, where it was perfected by Oerlikon as an anti-aircraft gun in 1927. The first aircraft mounted variant, the Oelikon FF (FlügelFest or “wing-mount”), entered production in 1935. This weapon set off a firestorm of interest, with derivative designs being produced all over the world (the most successful of which was the Hispano HS.404). The 20mm autocannon, however, did not find acceptance overnight. There were several issues:

  • The guns were very large and very heavy, meaning that not only did they affect weight, but adapting the airframe to hold autocannon often meant large bulges that increased drag.

  • While devastatingly powerful, the ballistics of autocannon shells differed sharply from that of typical MGs, typically having shorter range and a heavy ballistic drop

  • The design of these weapons was a bit more complex than a typical MG, and as such, tended to jam rather easily.

  • This last point was often worsened by many designs not being belt-fed but fed through interchangeable ammunition cans.

In fact, when one RAF unit – 19 Squadron – was assigned 20mm-equipped Spitfires in June 1940, reliability was so poor that they requested the .303-equipped variants be returned.

Easily history’s most accomplished gunsmith, John Browning was responsible for designing, among others: the M1887 (the first lever-action shotgun), the M1893/1897 (the first pump-action shotgun), the Auto-5 (the first semi-auto shotgun), the M1911 .45 pistol, the M1917/1919 .30 caliber MG, the M1918 B.A.R., and the M2 .50 caliber MG.

Armorers load the AN-M2 .50 caliber MGs of LTC Francis S. Gabreski’s Republic P-47 in mid-1944. The son of Polish immigrants, Gabreski served as an unofficial liaison to Polish units in the Battle of Britain, earning valuable experience. By the end of the war Gabreski was the highest scoring US ace in Europe with 28 aerial victories.

While the US had experimented with autocannon (most notably, the 37mm on the Bell P-39), the US opted toward a heavier MG: the .50 caliber. Once again, Browning led the way. The death of 2LT Quentin Roosevelt in 1918, son of former US president Theodore Roosevelt, Jr. (R-NY), had given rise to rumors of ‘armored’ German aircraft (in fact, Roosevelt had been killed by a Fokker D.VII). As a result, CINC AEF GEN John J. Pershing called for a heavier cartridge MG, preferably along the lines of the Mauser 13 mm antitank rifle. The resultant MG, the Browning M2 .50 caliber, featured a slower rate-of-fire than its .30 caliber cousin, but boasted superb ballistics – so much so that a similar cartridge is today used in sniper rifles. The first fighter to use this weapon, the Curtiss P-36, entered frontline service in 1938 with one .30 caliber MG and one .50 caliber MG, both fitted atop the nose.

The Battle of Britain would have a great influence on aircraft armament. Not only did it prove the wisdom of wing-mounted MGs, but it also proved the superiority of heavier cartridges like the .50 caliber MG and 20mm autocannon in comparison to.30/.303 caliber MGs. With their copious armor-plating and self-sealing fuel tanks, many German bombers were able to return despite being riddled with bullets – the .303 simply did not have the stopping power. And, despite the offer of US .50 caliber MGs, the RAF ultimately rejected the concept, arguing that perfecting the 20mm was preferable simply because the .50 MG lacked high-explosive cartridges.

For comparison’s sake:

  • Vickers .303 MG (7.7 mm)
    Rate of Fire: 450 rpm
    Muzzle Velocity: 2,440 ft/s
    Effective Range: 2,525 yd

  • Browning AN-M2 .30 caliber MG (7.62 mm)
    Rate of Fire: 1,200 rpm
    Muzzle Velocity: 2,800 ft/s
    Effective Range: 1,500 yd

  • Browning AN-M2 .50 caliber MG (12.7 mm)
    Rate of Fire: 750 rpm
    Muzzle Velocity: 2,910 ft/s
    Effective Range: 2,000 yd

  • Hispano-Suiza HS.404 Autocannon (20 mm)
    Rate of Fire: 650 rpm
    Muzzle Velocity: 2,850 ft/s
    Effective Range: 2,000 yd

In regard to flexible gunnery, change came even slower.

From the beginning of military aviation it was understood that the slipstream would eventually become a problem for aerial gunners. With air combat becoming a regular occurrence in 1915, a realistic solution to flexible aerial gunnery became urgent. Early ‘gun-bus’ designs solved the problem of forward-facing armament by using pusher-propellers, having a flexible gunner at the front with a pintle-mounted MG. Little difference existed between aerial gunners and those on the ground, and it should be noted that some of the more successful gunners – 2Lt Frederick Libby, RCAF, for example – showed a marked preference for MGs with stock attachments.

Flexible aerial gunnery ostensibly came of age with the introduction of the Scarff ring in July 1915. Designed by WO Frederick W. Scarff, RN, the Scarff ring mounted the MG (sometimes in pairs) on a metal ring, using springs or bungee cords to hold the guns in the slipstream.

The nose gunner of a Handley Page Type O poses with his Scarff ring.

The top turret of a Douglas B-18 - little more than a Scarff ring with a Plexiglas dome.

The Scarff ring (and it foreign derivatives) set the standard for defensive armament in the Interwar years. Still, aircraft were reaching speeds too fast for the gunner to withstand in the open.

  • 200 mph – France’s Joseph Sadi-Lecointe, 1922
  • 300 mph – Italy’s Mario de Bernardi, 1928
  • 400 mph – Great Britain’s George H. Stainforth, 1931.

As such, Scarff rings throughout the 1930s were often paired with a variety of Plexiglas molds in attempt to shield the gunner from the increasingly harsh winds. These crude turrets did not provide any support for the gunner, so it remained difficult for him to actually aim and fire his weapons.

Several nations experimented with the concept of a powered-turret and the first to put one into mass-production was Great Britain, introducing the Boulton Paul Overstrand – a development of the 1926 Sidestrand – in 1935. The Sidestrand/Overstrand was Britain’s last biplane bomber and was surprisingly agile for its size, making it a natural testbed. Designed by John Dudley North, the turret took up the entirety of the aircraft’s nose, being pneumatically powered and carrying a single Lewis .303 MG. The change was dramatic. Overnight, the accuracy of RAF gunners in mock attacks increased from 15% to 55%.

The pneumatic nose turret of the Boulton Paul Overstrand - the first powered gun turret on an aircraft.

A de Boysson turret is installed on an Overstrand for testing. Though I have not found any confirmation, British sources claim that de Boysson sold his turret out of disgust with French officials who expected to be bribed to accept the design.

Unfortunately, using compressed air (charged at 120 lbs/sq in) meant that there was a possibility that the turret might lose pressure in midst of combat, having to recharge before the turret could move again. The solution came from Archibald Frazer-Nash of Great Britain and J. B. Antoine de Boysson of France. Both engineers had experimented with hydraulic systems, with Frazer-Nash producing a hydraulic-shell to protect the gunners of Hawker Demons in 1935. De Boysson’s turret was far more advanced. An engineer noted for his consultant work in Japan, de Boysson had grappled with these issues while designing French ‘combat planes’ (detailed earlier). While working for the Société d’Appareils de Transmission in 1932, he patented a hydraulically-powered turret where the unit controlled not only the rotation of the guns, but the elevation as well. Setting up his own business (the Societe d’Applications des Machines Motrices), de Boysson failed to arouse political interest, eventually licensing the turret to Boulton Paul of Great Britain in 1935. While Frazer-Nash’s design powered its hydraulics from the aircraft’s engines, de Boysson’s design was self-contained, using electric motors to power the turret’s hydraulic pumps. Both variants saw extensive use with the RAF, initially with Frazer-Nash developing turrets for the Armstrong Whitworth Whitley and Vickers Wellington bombers and Boulton Paul developing turrets for the Defiant turret fighter.

These early powered turrets performed remarkably well. On 3 December 1939, LCpl John J. Copley – a tail gunner on a Vickers Wellington – managed to shoot down a Bf-110C (_ZG_26) piloted by future ace Lt Günther Specht (who lost an eye). This was the first aerial victory made by a powered turret and none of the 24 Wellingtons launched this day were lost in combat. Unfortunately, RAF BC was slow to recognize what this meant for operational tactics. RAF doctrine called for bombers to fly in close formations while at the same time warning against ‘unwieldy’ groups larger than 12 aircraft. The reason for this Catch-22 was ground fire, and many airmen consciously flew loose formations thinking (wrongfully) that AAA was the greater threat. As a result, RAF formations were often lacking, and it should be noted that the one unit which held their formation on the infamous 18 December 1939 raid over Heligoland Bight (149 Squadron) escaped with comparatively few losses (losing two while the other groups lost five each).

The world took strong notice of these early successes, with aviation journals particularly lauding where 264 Squadron – a Defiant unit – claimed 37 aerial victories on 29 May alone. While these figures were obviously exaggerated, they still contained a kernel of truth: powered-gun turrets worked. An USAAC report noted:

An Australian airman of 37 Squadron leans out the tail turret of a Vickers Wellington.

Wellingtons of 9 Squadron, circa early-1939.

The installation of these turrets on some of their bombers had caused such losses to the enemy attacking airplanes that the British now claim that the attacks directly aft have been discontinued on these installations. . . . It is imperative that our heavy and long-range bombers be equipped with at least four turrets. . . The British have shown the way and the method together with the results to be obtained in the installation of machine gun turrets. . . no effort must be spared to apply this type of mechanism to our own advantage.

Gardner, United States Army Air Corps, Lieutenant Colonel Grandison, and Major Franklin O. Carroll, United States Army Air Corps. “Report on Trip Abroad.” 1 July 1940. Wright Field Library.

The US Navy attaché concurred in its report dated 29 November 1940.

For their part, the Germans learned to avoid the nose and tail turrets of British bombers by approaching ‘on the beam’ (sideways). This proved frightfully effective as many older aircraft lacked self-sealing fuel tanks, making them extremely flammable. The solution was obvious: a powered gun turret for all four approaches. Unfortunately, fitting a powered turret in the ventral position proved difficult. Old style ‘dustbin’ retractable turrets caused excessive drag and powered-remotes had a disturbing tendency to cause vertigo. Regardless, by the time the US introduced the Sperry Ball Turret (April 1941), RAF BC was already committed to a night war. As such, the British approach toward defensive gunnery changed somewhat, with aerial gunners serving more as lookouts than as protectors. In fact, by 1942 the standard procedure when spotting an enemy aircraft was for the gunner to call for evasive action, taking care to fire only if they had already been spotted.

As for the US…

As I noted before, strategic histories of the US tend to accuse Army Air Forces leaders of being overconfident in the ability of bombers to defend themselves. The actual history is not so simple. In regard to powered gun turrets, the US did not actually have a powered gun turret until after the start of the war.

Ever since the end of the Great War, calls for powered gun turrets had been constant, but it was not until 31 July 1936 that the Army Materiel Division first called for development in that area (a reaction to the British Overstrand). Little effort was actually made. The futuristic Bell YFM-1 – first flown on 1 September 1937 - offered some promise. Forward of each engine, electrical systems controlled a pair of 37mm autocannon, reloaded by an airman inside the pod but moved and sighted via a remote in the cockpit. On 20 June 1939, the Materiel Division called on several US companies to develop a tail gun for the Douglas B-18 along these same lines, of which General Electric led the way. Rather than redesign the airplane itself to allow a gunner to fit in the tail, the idea was to mount a remote tail unit, sighted by a gunner located somewhere amidships. Progress toward turret remotes like this (known in the US as “fire control”) was shelved following the noted success of manned British power turrets overseas.

Immediately the USAAC put into place a crash program to develop powered gun turrets based on the British model, assigning each turret manufacturer a specific airframe to arm. Calls were made for sample British turrets and the RAF happily complied, though their request for a sample Norden Bombsight in trade was denied (they were property of the US Navy, not the USAAC). By the time representatives from Frazer-Nash and Boulton Paul arrived with sample turrets at Wright Field (22 November 1940), several US prototypes were already undergoing testing.

The Bell YFM-1. The “Fire Control” concept eventually came into being with the Boeing B-29 in 1944. Multiple sighting stations, scattered throughout the bomber, remotely controlled a series of external turrets overseen by a Central Fire Controller.

As the most successful of the British turret manufacturers, Boulton Paul’s designs - like the one seen here on the Defiant - particularly influenced Sperry and Emerson in designing turrets for the Boeing B-17 and Consolidated B-24 (respectively).

Despite its horrific reputation, the Sperry Ball Turret was statistically the safest position on an American heavy bomber. Suspended below the aircraft, the ball turret gunner effectively blocked potential attacks from below.

The most successful US turret: the Martin 250CE. This turret was most notably used on Consolidated B-24s, Martin B-26s, and Canadian-built Avro Lancasters (among many others).

US turrets proved far more successful than their British counterparts. A big reason for their success lay in the Browning AN-M2 MG, whose performance far outpaced that of the British .303. Many RAF turrets went so far as to mount .303 MGs in quads rather than pairs, and even still, RAF BC saw it necessary to formally request .50 caliber MGs for its turrets on 11 January 1943. Likewise, with the notable exception of the in-house Consolidated A-6 Tail Turret, virtually all US turrets were electrically-powered using a variant of GE’s ‘amplidyne’ system. Doing so eliminated the fire-hazard of hydraulic lines as well as the tendency of hydraulic turrets to ‘creep’ when fired.

I should note a couple things before wrapping up:

First, the late-arrival of powered turrets on US aircraft meant that there was little real understanding of what their introduction meant. When the US 8AF began operations against occupied-Europe in August 1942, the US had still not yet developed formations of interlocking fire.

Second, tests at Wright Field using a Curtiss P-36 making head-on passes against a Martin B-10 – combined with the relatively few head-on attacks made by Luftwaffe against RAF BC – suggested to the USAAC that forward-facing armament was not a necessity. MAJ Richard C. Coupland concluded:

It is theoretically possible to attack a bombardment airplane from any angle . . ., [but] the practical difficulties involved and the loss of effectiveness in any zone, except that of circumscribed by a 20° to 30° angle from the line of flight of the objective airplane, place such attacks in the category of a threat rather than an effective method of employing fire power.

Coupland, United States Army Air Corps, Major Richard C. “Ordnance Department Memo to Chief of Air Corps Major General Henry H. Arnold, United States Army Air Corps.” 9 February 1940. Armament Laboratory File, Airplanes Miscellaneous, 1940.

For this reason, Boeing B-17s and Consolidated B-24s would suffer heavily from head-on attacks throughout late-1942/early-1943. When solutions to this problem ‘plugged the gap,’ Luftwaffe interceptors would begin experimenting with ways to spread apart US formations, which included the first air-to-air missiles.

Thirdly, it should be noted that, regardless of whether one is researching fighters or aerial gunners, kill claims are really of little consequence. While victory claims were usually exaggerated, this fact should never be used to suggest a fighter/gun turret was ineffective. In the case of gun turrets, for example, the number of German fighters brought down by RAF/US gun turrets may never be known. It is far better to quantify their effectiveness by looking at survival statistics. The interceptor does not have to be shot down for the turret to do its job – being scared away is equally as effective.

And lastly, I should probably make a special note on the Boulton Paul Defiant. The idea was to create a modern version of the Great War Bristol F.2. To quote a 1938 Air Ministry memo:

Boulton Paul Defiant

The speed of modern bombers is so great that it is not only worthwhile to attack them under conditions which allow no relative motion between the fighter and its target. The fixed gun fighter with guns firing ahead can only realize these conditions by attacking the bomber from dead astern. The duties of a fighter engaged in ‘air superiority’ fighting will be the destruction of opposing fighters. . . . For these purposes, it requires an armament that can be used defensively as well as offensively in order to enable it to penetrate into enemy territory and withdraw at will. The fixed-gun fighter cannot do this.

As quoted in: Bungay, Stephen. The Most Dangerous Enemy. London, UK: Aurum Press, 2000.

The failure of the Defiant is legendary in military history circles. Bismark has a video regarding the Defiant that I recommend if you wish to learn more about it. I will only add this:

The Defiant was part of a trend in the late-1930s which, acknowledging that increased range meant increased size (prior to droptanks), flirted with the idea of large, ‘heavy fighters.’ While not actually well-suited for the air superiority role, heavy fighters proved adept at intercepting heavy bombers. In fact, this was precisely how the RAF had intended to use the Defiant, the fighter performing well in mock interceptions of bombers equipped with tail turrets. And, I must stress that later in the war, the Luftwaffe would have considerable success using heavy fighters against RAF BC – particularly with schrage musik, a tactic the Defiants had actually been trained to practice.

The failure of the Defiant primarily came from misuse. 264 Squadron had captured the world’s attention over France, pouncing on unescorted Stukas and, when intercepted by Bf-109s, using Great-War Lufbery circles to defend themselves. During the Battle of Britain, not only would the second Defiant unit, 141 Squadron, deliberately ignore tactics developed by 264 over France, but the Defiants would be suicidally tasked with intercepting fully-escorted German formations. While the Defiant was by no means a perfect fighter, it certainly did not help that it was so poorly used.

Cheers!

-G

G. A. Blume
Historian

If you would like to learn more about strategic airpower you can check out my Timeline of Strategic Aviation. Likewise, if you would like to see a write-up I did on a turret restoration specialist assisting in our Boeing B-17F rebuild, check out this article.