AIRCRAFT SURVIVABILITY: NEW CHALLENGES FOR A NEW GLOBAL CONFLICT (WORLD WAR II)
By: David Legg
[EDITOR’S NOTE: This article is the second in a series of historical aircraft survivability articles. The first, which covered pre-World War I through World War I, was published in the spring 2017 issue of Aircraft Survivability.]
On 4 September 1939, the Royal Air Force (RAF) recorded its first losses of World War II when five Wellingtons of No. 9 Squadron were shot down during a raid on German warships in the Elbe estuary . Three months later, a force of 24 RAF Wellington Mk 1a medium bombers (such as those shown in Figure 1) attempted a daylight raid against the German port of Wilhemshaven. These bombers flew in a spread formation to avoid anti-aircraft fire. However, German Ground Control Intercept (GCI) radars tracked the bombers and vectored a formation of single-engine BF-109 and twin-engine Bf-110 fighters onto the bombers. A total of 12 RAF Wellington bombers were shot down for the loss of 2 Luftwaffe fighters . In addition, 56 of the 65 bomber aircrew who set off on this mission were killed.
The post-mission debriefs uncovered that several of the aircraft caught fire when hit, and the recommendation was made that the aircraft be equipped with self-sealing fuel tanks. The RAF also noted, based upon these and other missions, that the bombers could not defend themselves against fighters during daylight missions. This conclusion eventually led the RAF to embark upon its night-time bombing campaign against Germany.
RELEARNING THE LESSONS OF WORLD WAR I
During the years between Word War I and World War II, the air forces of the world soon forgot the costly lessons learned over the trenches of France in World War I. Unfortunately, these lessons would have to be quickly relearned (by the previously mentioned RAF Wellington bombers, for example) in the early years of World War II, prior to the involvement of the United States. During this time, Britain freely transferred the lessons learned to the U.S. government and aircraft manufacturers. And in relatively quick order, U.S. manufacturers were including survivability features due the requirements stipulated by the RAF.
For example, when the U.S.-produced Curtiss P-40A Tomahawk fighters arrived in England, the RAF restricted its use to units deployed in England until these aircraft were equipped with armor and self-sealing fuel tanks. These deficiencies were corrected in the P-40B, which the RAF used to good effect in North Africa. In contrast, the North American P-51 Mustang was developed based upon RAF requirements, which included armor and self-sealing fuel tanks. Later, this U.S.-manufactured aircraft would become arguably the best escort fighter aircraft of the war—once the British replaced its Allison engine with a Rolls-Royce Merlin engine.
While attempts were being made to reduce the vulnerability of their respective aircraft, the Allied and Axis air forces soon learned that their aircrafts’ armaments required improved lethality. Early versions of the RAF’s Supermarine Spitfire Mk I and II and Hawker Hurricane fighters were each equipped with eight .303-caliber machine guns. As aircraft equipped with self-sealing fuel tanks and armor became much more common during 1940, it proved necessary to concentrate machine gun fire at much closer ranges to the target aircraft. The RAF quickly discovered that these .303-caliber rifle bullets could not carry enough incendiary or explosive to guarantee success and had insufficient penetration capability to defeat armor reliably.
During one documented encounter between a Luftwaffe Dornier DO-17Z bomber and an RAF Spitfire, approximately 200 rounds were required to down the bomber. This encounter also highlights the robust design (excluding fire prevention) of military aircraft of this era. Later versions of the Spitfire Mk II and subsequent Mk’s included a mixture of 20-mm cannon and .303- or .5-inch machine guns. Early versions of the Luftwaffe’s Bf-109, including those involved in the raid on Wilhemshaven, already included a mixture of 20-mm cannon and 7.62-mm machine guns. Like the Spitfire, later versions of the Bf-109 also included heavier armament, including a mixture of 12.7-mm machine guns and 30-mm cannon.
Likewise, the Allied and Axis tactical air-defense artillery units soon learned that they also required improved lethality. In the early phases of World War II, German ground forces were typically protected against ground attack aircraft by either fixed or mobile 20-mm anti-aircraft cannon. In the mid-to-late years of the war, these would be replaced or augmented by more lethal single- or multi-barrel 30-mm and 37-mm anti-aircraft cannon and single-barrel 40-mm anti-aircraft cannon.
This increase in threat caliber was primary driven by the increased robustness of the U.S. Army Air Force (USAAF) P-47 Thunderbolt, the RAF Typhoon, and the Soviet Air Force IL-2 ground attack aircraft designs. However, it was not uncommon for these aircraft to return to base after being impacted by even these rounds. These were the types of lethal defenses with which Allied fighter-bomber pilots had to contend throughout most of the war.
The Republic P-47 is representative of U.S. aircraft manufacturers’ attempts to keep pace with evolving wartime requirements for performance, lethality, and survivability. The P-47 would evolve to become arguably the best close air support fighter-bomber and one of the best fighters of World War II.
In 1939, the XP-47A aircraft was originally conceived as a lightweight, high-altitude interceptor. It was powered by a 1,150-hp liquid-cooled inline engine, was armed with two .50-caliber machine guns, had a top speed of 415 mph, and had a gross weight of 4,900 lbs. However, reports from Europe indicated that more firepower, armor plate, and self-sealing fuel tanks were required. But the XP-47A had insufficient power to bear the additional weight, which resulted in a significant redesign of the aircraft.
The resultant XP-47B design included a 2,800-hp air-cooled turbo-supercharged radial for additional power, an armament of eight .50-machine guns for increased lethality, a top speed of 400 mph at 25,000 ft and 340 mph at 5,000 ft, self-sealing fuselage fuel tanks for survivability, cockpit armor for aircrew protection/survivability, and a 11,500-lb gross weight. Although the engine change was made to improve performance, it would also later prove to be a significant vulnerability reduction feature.
Due to the additional weight, one of the main P-47B shortcomings was that it had insufficient range to permit deep penetration and bomber escort into Germany. This deficiency, however, was addressed by mid-1943 with the introduction of the P-47C, which included the capability to mount external fuel tanks for increased range and a longer fuselage to improve maneuverability.
The follow-on P-47D was the first version of the Thunderbolt to undergo large-scale production and would undergo many block improvements. The D-25-RE Block included a “bubble top” canopy with improved all-around visibility, and the P-47D-40-RA Block was the first P-47 to include tail warning radar equipment. Subsequently, the P-47M, based on the P-47 D-27 to D-30 series, was a pure fighter version with increased power.
Vulnerability features of the P-47 included:
- Self-sealing internal fuel tanks, which shielded the pilot from below and the front
- 9-mm-thick armor plate to protect the pilot’s back
- A separate plate of “bullet-resistant glass” installed behind the windscreen for pilot protection on the P-47D Razorback
- An integral bullet-resistant glass incorporated with the windscreen for pilot protection on the P-47D Bubbletop
- The air-cooled engine, which eliminated the vulnerable liquid cooling system and shielded the pilot from the front.
Susceptibility reduction features of the P-47 included:
- Incorporation of the bubbletop canopy for increase threat awareness
- Tail warning radar equipment
- Increased power.
The P-47N was the last version of the P-47 to be produced. It was a long-range (2,200 miles) version designed specifically for service in the Pacific theater. The improved range permitted the aircraft to escort B-29 bombers from the island of Saipan to Japan. The P-47N was powered by the same R-2800-57 engine as the P-47M. While the fuselage was essentially unchanged from the D series, the wing was completely redesigned to include four self-sealing fuel tanks per side.
On 26 June 1943, LT Robert S. Johnson would push limits of the P-47C’s survivability. While on patrol, his squadron was jumped by a formation of FW-190A fighters. His aircraft (which he called Half Pint) was hit by 20-mm cannon and 7.9-mm machine gun projectiles, which resulted in a fire and an uncontrolled spin. After the aircraft lost several thousand feet in altitude, the fire self-extinguished, and LT Johnson regained control of the aircraft. His vision was impaired by a hydraulic fluid leak, he had two bullet fragments in his right leg, another bullet had nicked his nose, and part of the windscreen had been shattered. He tried to bail out, but his cockpit was jammed (due to a 20-mm projectile impact).
Under these dire circumstances, LT Johnson headed back toward England. Somewhere over France, unable to maneuver his damaged aircraft, he was attacked by another FW-190A. Luckily, this aircraft had expended all of its 20-mm cannon rounds in an earlier engagement. The FW-190A pilot, Luftwaffe ace MAJ Egon Mayer, made several attacks and fired every remaining 7.9-mm machine gun round into LT Johnson’s aircraft. After exhausting all of his aircraft’s machine gun rounds, MAJ Mayer then flew alongside LT Johnson for almost 30 minutes, amazingly escorting him to the English Channel to make sure he made it. LT Johnson continued on and landed safely at his home airfield in England. Upon inspection, it was determined that the aircraft (shown in Figures 2 and 3) had more than 210 holes in it, with at least 20 being inflicted by 20-mm cannon rounds from the initial attack. LT Johnson would survive to become one of the top American aces in World War II, with 27 aerial victories . In addition, his plane would later be repaired and reissued to the 9th Air Force’s 36th Fighter Group, before being permanently destroyed in August 1944.
Ironically, MAJ Mayer was shot down and killed in action by a P-47 pilot near Montmédy, France, on 2 March 1944. He was officially credited with 102 aircraft victories in more than 353 combat missions.
The P-47 Thunderbolt and P-51 Mustang were both robust designs capable of absorbing significant structural damage, and they both included self-sealing fuel tanks and armor for pilot protection. However, the P-51 had one significant vulnerability not shared by the P-47— the P-51’s liquid-cooled engine. And this vulnerability became more critical as the aircraft was used in the close air support and airfield strafing role toward the end of the war.
Edgar Schmued, chief designer of the P-51, explained that using the Mustang for ground attack was “. . . absolutely hopeless, because a .30-caliber bullet can rip a hole in the radiator and you fly two more minutes before your engine freezes up .” The Army Air Forces Proving Ground Command at Eglin Field, FL, also documented this vulnerability in its December 1942 “Final Report on Tactical Suitability of the P-51 Type Airplane,” which states :
The coolant and oil radiators are combined into one (1) assembled unit and are located in the belly of the aircraft just behind the cockpit. For the reason that most hits on an airplane in combat are to the rear of the cockpit, it is believed that this radiator installation may prove to be quite vulnerable. It is recommended that the designer of the subject aircraft make a study of the possibilities of incorporating a sheet of armor plate to protect the radiator from fire from the rear.
The recommended armor plate was never incorporated in the P-51 design.
During World War II, the combat loss rate per sortie for the P-51 was 1.2% (vs. 0.7% for the P-47). Another World War II study indicated the P-51 was three times more vulnerable to ground fire than the P-47.
In addition to the incorporation of armor and self-sealing fuel tanks typical of USAAF, U.S. Navy (USN), and RAF fighter aircraft, several Soviet fighter aircraft designs included a form of fuel tank ullage inerting. Portions of the inert gases that formed the engine exhaust were collected, cooled, and pumped into the fuel tanks of the Yakolev-1 and 9, Mig-3, and Lavochkin (LaG)-3, 5, and 7 fighter aircraft.
Soviet fighter ace Ivan Kozhedub stated :
Later on, I had many occasions to admire the strength and staying power of this plane (LaG-5). It had excellent structural mounting points and an ingenious fire-fighting system, which diverted the exhaust gases into the fuel tanks, and once saved me from what seemed certain death.”
Ivan Kozhedub would later become the top scoring Allied ace of WWII with 62 victories, including an Me-262 jet fighter.
Although not commonly known, the Japanese also included vulnerability reduction in their aircraft designs. The Mitsubishi A6M2 Zero/Zeke entered service in 1940 and quickly became known by the Allies for its inherent overall vulnerability. This vulnerability was corrected much later in the war with the introduction of the A6M5b Model 52 version in 1944. This version includes a CO2-based dry bay fire extinguishing system and windscreen armor .
Additional vulnerability reduction improvements followed. The A6M5c Model 52 added armor plate behind the pilot and a fuselage self-sealing fuel tank, and the A6M6c Model 53c added wing self-sealing fuel tanks .
In another case, the Japanese observer reports from the European conflict were acted upon by the designers of the Kawasaki Ki-61 Tony fighter. The Ki-61-I Ko (1942) included self-sealing fuel tanks as well as pilot seat and headrest armor. The Ki-61-I Otsu (1943) added thicker self-sealing fuel tanks, thicker pilot seat and headrest armor, and radiator armor (the Ki-61 was equipped with a liquid-cooled engine). The Ki-61-I Hei (1943) and Tei (1944) versions added a CO2-based dry bay fire extinguishing system .
The heavy bombers of the USAAF (e.g., the B-17 and B-24) and RAF Bomber Command (e.g., Lancaster, Halifax, and Stirling) were, like their fighter counterparts, of robust design and incorporated armor protection for the aircrew and generally self-sealing fuel tanks. Thus, they could, on occasion, survive the severe damage inflicted by near-by bursts of German 88-mm, 105-mm, and 128-mm anti-aircraft artillery (AAA) projectiles (provided that no fire was initiated). However, a direct hit by any of these projectiles would mean almost certain catastrophic loss.
Because the RAF bombers flew at night without escort fighter protection, they were particularly susceptible to fighter attack. These radar-equipped night fighters could inflict severe damage in a matter of seconds due to their heavy armament of multiple 20-mm and 30-mm cannons. Some of these cannon were mounted upward in the night fighter’s fuselage to enable attacks unobserved from beneath the bomber. In an attempt to stem the losses, some versions of the Lancaster, Halifax, and Stirling were equipped with fuel tank ullage inerting systems. The engine exhausts of these bombers were also equipped with flame dampers to hide the glowing exhaust from night fighter crew observation.
ELECTRONIC WARFARE: A NEW TYPE OF BATTLE
The aforementioned survivability features could be considered a natural progression of the improvements implemented in World War I, but World War II became the catalyst for the development a whole new kind of battle—electronic warfare.
During the closing stages of World War I, and in response to German Zeppelin and bomber raids, the British implemented the first Integrated Air Defense System (IADS). Designated the London Air Defense Area (LADA), this IADS brought together units composed of coastal and inland observation posts, sound locators, searchlight and AAA stations, balloon aprons, and fighter aircraft. This IADS was reconstituted during the late 1930s with the addition of the “Chain Home” early warning radar system and was key to the RAF’s Fighter Command success over the Luftwaffe during the Battle of Britain.
In parallel, the Germans also developed a similar IADS to defend Germany against aerial attacks. This system included Ground Controlled Intercept (GCI) directed night fighters and radar-directed 88-mm, 105-mm, and 128-mm AAA. Freya radar, operating at 120–166-MHz and with a range of ~200 km, provided early warning of incoming Allied aircraft.
Würzburg-Riese radar, operating at 560 MHz and with a range of ~70 km, would be used to direct the AAA and/or night fighter aircraft. Early Luftwaffe night fighter aircraft were equipped with a Lichtenstein FuG 220 radar operating at 33/82/91/118 MHz with a detection range of ~4 km. Later aircraft were equipped with FuG 240 “Berlin” radar operating at 3.250–3.330 MHz with a maximum detection range of ~ 9 km. In the case of night fighter aircraft, GCI would typically guide them to within a few kilometers of the target aircraft. At this point, the radar operator onboard the night fighter would guide the pilot to within visual acquisition or firing range. Understandably, these developing capabilities led to an ongoing game of “cat and mouse” over the night skies of Germany and occupied Europe.
To counter these radar-directed threats, British scientists developed a variety of countermeasures, which were employed selectively by RAF Bomber Command aircraft. For example, the Airborne Cigar (ABC) radar jammers jammed the German VHF communications and navigation aid frequencies. A German-speaking operator on board the bomber would monitor the German communications to make sure all frequencies were jammed and could also provide false commands to German night fighter crews.
The Mandrel jammers (such as those shown in Figure 4) jammed the Freya early warning radar. These jammers periodically stopped transmitting for 2 min to prevent night fighters from homing in on the Mandrel transmission.
Window strips (the invention of which is credited to Joan Elizabeth Curran of the Telecommunications Research Establishment) were strips of coarse, black paper with thin aluminum foil stuck to one of the sides. These strips (known in the United States as chaff) were cut to be either a 1/2 or 1/4 wavelength of the frequency of the target radar. As the bundles of Window were dropped from an aircraft (such as shown Figure 5), they separated to form a vast cloud of metallic strips. These clouds would send back an exceptionally strong echo, and when dropped in great numbers, they swamped the enemy’s radar. One of the main targets of Window was the Würzburg-Riese anti-aircraft gun laying radar.Monica was a tail-mounted radar that entered RAF service in early 1943. It provided audible bleeps as a warning of an aircraft approaching from the rear. However, a German night fighter could “hide” among the bleeps generated by reflections from other bombers flying in trail. By March 1943, the Germans had examples of Monica from shot-down aircraft. So, in the spring of 1944, they German night fighters were equipped with the FuG 227 passive radar receiver to detect and track the Monica emissions. The FuG 227 range was ~62 miles. Luckily for the British, on the morning of 13 July 1944, a FuG 227-equipped German Ju 88G-1 night fighter mistakenly landed at the RAF base Woodbridge. After examining the FuG 227, the RAF ordered Monica to be withdrawn from all RAF Bomber Command aircraft.
Monica was replaced by the Boozer passive radar warning receiver (RWR). This RWR would provide bomber crews warning when their aircraft were detected by German Wurzburg GCI or FuG 202/212 Lichtenstein airborne interception (AI) radars. It was introduced into service in the spring of 1943. A display was mounted on the pilot’s instrument panel and the radio operator’s position, and lights warned if the aircraft was being tracked. A yellow light indicated the aircraft was illuminated by a FuG 202/212 AI radar, and a red light (hence the name “Boozer”) indicated that the aircraft was illuminated by a Wurburg GCI radar.
The Handley Page Halifax (which produced the “Jane” bomber in Figures 4 and 6) is representative of British aircraft manufacturers’ attempts to keep pace with evolving wartime requirements for bomber survivability. Notable vulnerability features of the Halifax included the following:
- 12 self-sealing fuel tanks (fuel containment and fire reduction)
- Fuel pumps inside fuel tanks (component shielding)
- Fuel tank cross-feed (component redundancy)
- 2 self-sealing oil tanks (containment and fire reduction)
- Fuel tank nitrogen inerting for all fuel tanks (fire/explosion reduction)
- Air-cooled engines (component elimination – liquid coolant)
- Engine fire extinguishing system (fire reduction)
- General rugged construction (critical component redundancy).
Likewise, notable susceptibility reduction features of the Halifax included the following:
- Monica active tail warning radar or Boozer passive RWR
- ABC, Mandrel, and Window countermeasures incorporated on special-mission aircraft
- Engine exhaust dampers.
STRATEGIC BOMBER LOSSES IN THE WAR OF WESTERN EUROPE
Despite the RAF Bomber Command’s decision to fly in what was originally calculated to be the relative safety of night and the implementation of the previously mentioned (and other) electronic warfare countermeasures, the Bomber Command’s losses ultimately surpassed those sustained by the USAAF during daylight operations. According to the Bomber Command Museum of Canada :
The successes of Bomber Command were purchased at terrible cost. Of every 100 airmen who joined Bomber Command, 45 were killed, 6 were seriously wounded, 8 became Prisoners of War, and only 41 escaped unscathed (at least physically). Of the 120,000 who served, 55,573 were killed . . . . Of those who were flying at the beginning of the war, only ten percent survived. It is a loss rate comparable only to the worst slaughter of the First World War trenches. Only the Nazi U-Boat force suffered a higher casualty rate.
And the USAAF Eight Air Force did not fare much better. Reportedly :
During World War II, one in three airmen survived the air battle over Europe. The losses were extraordinary. The casualties suffered by the Eighth Air Force were about half of the USAAF’s casualties (47,483 out of 115,332), including more than 26,000 dead.
USAAF battle casualties in all overseas theater of operations include 40,061 killed and 18,238 wounded .
LESSONS TO BE RELEARNED
Immediately following World War II, the emphasis for airpower was placed on the delivery of nuclear weapons. The unprecedented demonstration of the uranium bomb “Little Boy” on Hiroshima on 6 August 1945 and then the plutonium bomb “Fat Man” 3 days later had instantly changed how the military (and the world) viewed warfare. Such weapons seemed to make major conventional conflicts unthinkable, at least for a time. Unfortunately, this situation would not last long. And as with the survivability lessons learned during and forgotten after World War I, the same cycle would be repeated in subsequent conflicts in Korea and Vietnam.
ABOUT THE AUTHOR
Mr. David Legg is currently the Fixed- Wing Aircraft Branch Head of the Naval Air Warfare Center – Aircraft Division. With 33 years of experience in the aircraft survivability discipline, he has also served as the Survivability Team Lead for many U.S. Navy aircraft and weapons programs, and he assisted in the rapid development and implementation of tactical paint schemes for in-theater U.S. Marine Corps helicopters during Operation Desert Shield/Storm. Mr. Legg was named a NAVAIR Associate Fellow in 2011 and holds bachelor’s degrees in mathematics and mechanical engineering from Saint Vincent College and the University of Pittsburgh, respectively.
[EDITOR’S NOTE: The on-again-off-again effect described herein caused most of these survivability efforts to dry up between wars, which was the reason we established the Joint Technical Coordinating Group on Aircraft Survivability (JTCG/AS) in 1971. One of the main missions of the JTCG/AS was to establish survivability as a design discipline so that it would continue as a major player when no one was shooting at our aircraft. The JTCG/AS (now the Joint Aircraft Survivability Program [JASP]) was successful in doing that, and survivability programs have continued over the years. Look for an article in a future issue, on how this mission was accomplished.]
References  Royal Air Force. “RAF Timeline 1939.” https:// www.raf.mod.uk/history/rafhistorytimeline1939.cfm, accessed May 2017.  Wagner, Ray. Mustang Designer: Edgar Schmued and the P-51. Washington, DC: Smithsonian Institution Press, 1990.  Army Air Forces Proving Ground Command. “Final Report on Tactical Suitability of the P-51 Type Airplane.” Eglin Field, FL, 30 December 1942.  HistoryNet. “Aviation History: Interview With World War II Soviet Ace Ivan Kozhedub.” http:// www.historynet.com/aviation-history-interview-with-world-war-ii-soviet-ace-ivan-kozhedub.htm, accessed May 2017.  Juszczak, Artur. Mitsubishi A6M Zero. MMP Books, 2015.  Wieliczko, Leszek. Kawasaki Ki-61 / Hein Ki-100. Kagero, 2014.  Bomber Command Museum of Canada. “Bomber Command’s Losses.” http://www.bombercommandmuseum.ca/commandlosses.html, accessed May 2017.  “Eighth Air Force Combat Losses.” http:// personal.psu.edu/kbf107/Losses.html, accessed May 2017.  “Army Air Forces Statistical Digest: World War II.” Second Printing, Office of Statistical Control, December 1945.