by Mark Lardas
Mitsubishi G3M (“Nell”) and G4M (“Betty”)
These were two twin-engine bombers developed as long-range bombers for the Imperial Japanese Navy. These land-based attack (rikko) bombers were intended to offset Japanese naval treaty tonnage limitations by substituting aircraft for warships. They were effective ship-killers early in the war, especially the G4M. Both carried one aerial torpedo or up to 800kg (1,760lb) of bombs. The G3M had a top speed of 233mph, a cruising speed of 174mph, and a range of 2,700 miles. The G4M had a top speed of 365mph, a cruising speed of 196mph, and a range of 1,770 miles. There were 87 G3Ms and G4Ms (mostly G4Ms) at Rabaul at the beginning of October. Many were destroyed on the ground at the outset of the campaign and they played a minor role thereafter.
G4M bombers in revetments at Vunakanau Airfield near Rabaul. The Mitsubishi G4M (Betty) formed the backbone of Japan’s long-range land-based rikko squadrons. This photo was taken by photo reconnaissance aircraft prior to the Fifth Air Force’s airstrikes in October, 1943. (USAAF)
Yokosuka D4Y1 (“Judy”)
The D4Y1 was intended as a dive bomber, a replacement for the Aichi D3A. The airplane proved to have structural problems. Unable to serve as a dive bomber, it was used for reconnaissance and as a level bomber. It was fast (a 342mph maximum speed) with a range of 910 miles and a 35,000ft service ceiling. It was armed with two forward-firing 7.7mm machine guns and one rearward-firing flexible 7.7mm machine gun, and could carry 1,000lb of bombs. One of the few Japanese aircraft with an inline liquid-cooled engine, it was often mistaken for the Kawasaki Ki-61 (“Tony”), an Army fighter. The confusion was compounded because D4Y1s were used for air defense at Rabaul. Armed with small time-fused bombs intended to explode mid-air, it flew over Allied bomber formations dropping air-to-air bombs in the hope of breaking up formations. One air group with 20 D4Y1s was stationed at Rabaul.
The Yokosuka D4Y1 (Judy) was intended to replace the Aichi D3A (Val) dive bomber. Structural issues prevented its use in dive bombing. At Rabaul its primary use was dropping air-burst phosphorous bombs on Allied bomber formations – an almost totally ineffective tactic. (AC)
Nakajima B5N (“Kate”)
The B5N was the standard carrier-based torpedo bomber for the Imperial Japanese Navy during World War II. It carried one 800kg (1,760lb) aerial torpedo, or up to 800kg in bombs, with a maximum speed of 235mph, a cruising speed of 161mph, a ceiling of 27,000ft, and a range of 1,200 miles. During the Rabaul campaign, the B5N was the primary threat to US Navy carriers attacking Rabaul. Japanese aerial torpedoes were deadly – faster, longer-ranged, with a larger warhead, and mechanically more reliable than Allied aerial torpedoes. The threat was mitigated because at the start of the campaign only 12 B5Ns were stationed at Rabaul, although Combined Fleet B5Ns could and were staged in to Rabaul at different times during the campaign.
Aichi D3A (“Val”)
A monoplane with fixed landing gear, the D3A was Japan’s standard dive bomber through most of the war. It was more a contemporary of the German Ju 87 Stuka than of the United States’ Dauntless and Helldiver dive bombers. It was to have been replaced by 1943, but problems with the D4Y1 kept the D3A in service. It had a top speed of 267mph, a cruising speed of 184mph, a ceiling of 34,500ft, and a range of 840 miles. It could carry one 250kg (551lb) bomb, making it significantly weaker than any other Pacific Theater dive bomber. Twenty-four D3As were stationed at Rabaul in the same air group as the B5Ns. As with the B5Ns, additional D3As could be staged to Rabaul, although with their shorter range, they generally staged through Kavieng.
All Japanese aircraft suffered an inability to take damage. In the search for the maximum possible range and attack capability anything viewed as unnecessary weight was omitted. This included armor protecting the crew and vital components, self-sealing gas tanks, and even radios. Only element leaders had radios. Japanese aircraft were vulnerable to the .50-caliber machine guns of US aircraft, and caught fire easily. (The Japanese called the G4M the “Type 1 Cigarette Lighter.”) The lack of radios made it difficult to control fighters in the air, as communications were limited to hand signals. By contrast, when US Navy aircraft attacked Simpson Harbor on November 5, 1943, the attack plan was developed in flight by discussion among the attacking aircrew.
Airfields and infrastructure
By October 1943 Japan had four operational airfields in the Gazelle Peninsula to protect Rabaul and Simpson Harbor, and a non-operational fifth airfield on the peninsula available if needed. Two airfields, Lakunai and Vunakanau, existed when the Japanese captured Rabaul. Both then had grass runways. The Japanese improved both airfields. Lakunai was given a 4,300ft by 650ft runway topped with sand and crushed coral. Two and a half miles of taxiways, 90 fighter revetments and ten bomber revetments, and support buildings were added. Vunakanau received even greater improvements. A graded landing strip 5,200ft by 720ft included a concrete-paved center section 4,200ft by 175ft. Vunakanau had 90 fighter and 60 bomber revetments linked by 5½ miles of taxiways.
Rapopo, 14 miles southeast of Rabaul, Keravat, 13 miles southwest of Rabaul, and Tobera, 20 miles south of Rabaul and deep in the jungle of the Gazelle Peninsula, were added in 1942–43. Rapopo and Tobera were given concrete runways – 4,600 by 630ft for Rapopo and 3,600 by 100ft for Tobera. (Tobera’s concrete strip lay in a 4,800 by 400ft graded surface.) Keravat had a 4,250 by 300ft graded surface, but was not paved or further improved due to drainage problems. It was used as a backup landing field. Rapopo was intended as an Army bomber base, with 90 bomber and ten fighter revetments. Tobera was a fighter strip, with revetments for 75 fighters and two bombers.
This constellation of airfields gave the Japanese strategic depth. If one were temporarily knocked out, aircraft could operate out of the remaining fields. To suppress Rabaul the Allies had to reduce all four operational airstrips, a challenging task, one the Japanese felt insurmountable.
The Japanese also had a network of airfields around New Britain, New Ireland, and the northern Solomons to provide defense in depth. Most were grass strips with light garrisons. As the campaign progressed some proved to be liabilities. They were seized by Allied forces, with the airfields turned against their original owners. The airfields on Bougainville and New Ireland were heavily garrisoned. Their units had to be subdued before Rabaul could be approached from that direction.
Antiaircraft defenses
Aircraft were not the only resource protecting Rabaul from air attack. Japan invested much of its available air warning radar defending Rabaul. The Imperial Japanese Navy built 30 fixed early warning radar sets during World War II. Eleven were sent to defend Rabaul. Seven were on New Britain, scattered around the Gazelle Peninsula. Four were on New Ireland. These could detect formations of aircraft 250km (150 miles) distant and single aircraft at up to 100km (62 miles) distance. Twenty-two Type 6 Aircraft radars were also sent to Rabaul, and removed for conversion to ground-based tracking units. Half were installed, seven on New Ireland and four on New Britain. These could detect formations at 100km (62 miles) and single aircraft at 70km (43 miles).
The Gazelle Peninsula was well equipped with antiaircraft artillery. The Army had 72 3in or 75mm guns and 120 20mm antiaircraft/antitank or 13.2mm machine guns stationed near Rabaul; the Navy had eight 12.7cm and 15 12cm dual-purpose guns, 23 3in or 75mm guns, 92 25mm antiaircraft guns and 37 machine guns ringing Blanche Bay and Simpson Harbor or protecting their airfields. Heavy concentrations of antiaircraft artillery protected Kavieng on the northern end of New Ireland and St George Point on its southern tip. While there was no airfield at St George Point, a radar complex there provided early warning of aircraft inbound from the Solomons.
Lakunai was one of two RAAF fields taken over by the Japanese when they occupied Rabaul. It had a crushed-coral runway. It was primarily used as a fighter strip by the Imperial Japanese Navy. (AC)
THE NORTHEASTERN GAZELLE PENINSULA
Japanese ground troops
A.Navy defense area
B.Part of Ba
se Corps – supply and motor vehicle
C.Army HQ, ordnance, supply and motor vehicle depots, sanitary corps, air defense corps
D.Signal Corps
E.Remainder of 65th Brigade
F.Main force of Borge Corps
G.4th Independent Heavy Artillery
H.Infantry Battalion
I.Part of Borge Corps
J.38th Division HQ
K.Tank Corps
L.Tank Regiment
M.Part of Base Corps, ordnance, supply and motor vehicle depots
N.3rd Infantry Battalion of 38th Division
O.Air Defense Batallion
P.75th Artillery
Q.Part of 38th Division
R.Army Veterinary Depot
S.Main force of 38th Division
Area covered by 25mm guns
Area covered by 75mm or 3 in guns
Area covered by 12.7 guns
Power plants
Supply depots
Fuel dumps
Ammunition dumps
Air bases
Ground troops
Keravat Airfield was never finished due to drainage problems. It was available as an emergency strip or as a backup strip when necessary. (AC)
Since Rabaul was the headquarters and main supply base for both the Southeastern Army Forces and the Southeastern Fleet, it had some of the largest supply bases in the Southwest Pacific. At the start of the Allied campaign against Rabaul it was well stocked with food, supplies, munitions, and fuel. The Gazelle Peninsula was filled with warehouses stuffed with supplies, linked by a road network with 500 miles of improved roads. Eighty percent were built after Japan occupied Rabaul.
The Army had 30,000 tons of ammunition stockpiled. Much was intended for the use of the 100,000-plus ground forces they had in the Gazelle Peninsula, but included generous stocks of antiaircraft artillery ammunition. The Japanese Navy had over 1.5 million rounds of antiaircraft ammunition, for everything from its 12.7cm heavy antiaircraft guns down to 25mm auto-cannon. This was sufficient to supply Japan’s air defense needs for many months.
Rabaul’s supply dumps held over 55,000 tons of food, 5,400 tons of clothing, 5,000 tons of canteen supplies, 3,300 tons of medical supplies, nearly 6 million gallons of aviation fuel, 1.5 million gallons of motor gasoline and diesel, and 12 million gallons of lubricants and other petroleum products. There were nearly 5,000 motor vehicles on the Gazelle Peninsula. Enough was stockpiled to feed the garrison for nearly a year at full rations, and allow every fighter stationed at Rabaul to fly at least 300 times. The Allies could not starve the Japanese out.
None of these stores were adequately protected in October 1943, however. Fuel was in above-ground bulk storage tanks. Ammunition was in open dumps protected only by earth revetments. Everything else was stored in lightly built wood warehouses, mostly scattered around Blanche Bay and Simpson Harbor. All of it was vulnerable to air attack. Not until 1944, well after the Allied air offensive began, did the Japanese disperse their stores into underground storage facilities.
Weapons and tactics
Japan was faced with a defensive struggle at Rabaul, something for which it was ill-equipped and doctrinally ill-prepared. The Japanese air forces, both naval and army, were instruments of attack, not defense. If you hit your foe first, hit your foe hard, you did not have to defend. Japanese aircraft were optimized for attack; built for long range, maximum bomb load, and maneuverability. It was a strategy which served the Japanese well during the first months of the war – until it did not. Losses at Coral Sea, Midway, and the struggle for Guadalcanal highlighted Japanese weaknesses: vulnerability to attack and inadequate reserves, particularly aircraft and aircrew.
When the Allies started their offensive against Rabaul Japanese weaknesses had become obvious. After their first operations against the encircling Allies, the Japanese on Rabaul ran out of bombers in the first half of 1943. With only a handful of replacements, counterstrikes against Allied airbases and naval vessels were thereafter rare. The last major Japanese raids against Allied airfields occurred in mid-October, while the last attempts to attack either an invasion fleet or a US Navy carrier task force were made in November. From then on, the Japanese defense of Rabaul depended solely on its fighter aircraft, bombers attacking aerial targets, and antiaircraft artillery. This ceded the initiative to the Allies, who thereafter set the tempo of their air offensive.
The Imperial Japanese Navy sent nearly one-third of its long-range land-based early warning radar units to defend Rabaul. This unit was one of two installed at Tomavatur Mission, 2 miles southeast of Vunakanau. (AC)
While Japan had radar at Rabaul, it never developed an integrated air defense system to take advantage of radar’s capabilities. Radar only provided an alert of impending raids. It was a glorified and long-range coast-watcher system, like having ground-based observers with 50-mile vision. When an incoming Allied raid was detected the radar sites notified the airfields of the direction and presumed size of the raid. Aircraft were launched and antiaircraft artillery manned based on the expected arrival time and direction. Beyond providing aircraft with a rough vector for the incoming foe, no further guidance was provided once Japanese aircraft were airborne. There was no vectoring or battle control. Fire-control radar did not exist. One attempt to direct fire from one twin-mount 12.7cm antiaircraft gun failed to produce satisfactory results, despite an extended trial period. The experiment was discontinued and the radar unit was dismantled.
Exacerbating the lack of radar vectoring was the inability to coordinate mass formations of fighters. As previously mentioned the aircraft of formation leaders had radios. Once airborne, the other aircraft depended upon hand signals and wing-waggling for communications. This made it difficult to arrange a massed attack on a bomber formation. Instead, individual elements attacked at the formation leader’s discretion. Pilots attacked the easiest target rather than the most significant. Stragglers were pursued while a tight formation was bypassed until easier opportunities were disposed of.
The pilots were not afraid to fight. They often displayed almost foolhardy aggressiveness. Some pilots even rammed enemy aircraft rather than let them escape. Rather the problem was that they fought as individual warriors, not as an integrated team. It reduced effectiveness when they could least afford it.
RABAUL AND SIMPSON HARBOR
AA batteries with heavy artillery (75mm or larger guns)
AA batteries with light artillery
Power plants
Supply depots
Fuel dumps
Ammunition dumps
Sea mines
Anchorage
< 10m sea depth
A final antiaircraft weapon wielded by the Japanese was the antiaircraft bomb. When warned of a raid, D4Ys were armed with 30kg and 60kg (66lb and 122lb) aerial phosphorus bombs. The D4Y would fly over a formation of bombers and drop the bomb on the formation. The bombs were time-fused, and – assuming the attacking D4Y was at the correct height above the formation – would explode in the middle of the formation. In theory, the exploding bombs would cause the formation to scatter, or even hit and set fire to a bomber. In practice, they had almost no effect. They rarely damaged Allied bombers, and the bombs’ spectacular pyrotechnics even more rarely caused any reaction in a high-altitude formation. The main result of these efforts was to give Allied fighter pilots an opportunity to shoot down a D4Y, thinking it the inline-engine Ki-61 fighter.
While Japan had a lot of antiaircraft artillery on Rabaul, flak never caused serious casualties to Allied attackers. The heaviest Japanese antiaircraft artillery, the 12.7cm and 12cm, had a theoretical effective ceiling of 25,000ft. During the siege of Rabaul they rarely achieved hits on aircraft higher than 20,000ft. The Model 88 75mm antiaircraft gun had an effective ceiling of 21,000ft, and the Model 10 3in gun was effective only up to 18,000ft.
Of the medium antiaircraft artillery, the 25mm machine cannon could hit targets at up to 6,000ft. These were effective guns against targets
within their range. The 20mm cannon and 13.2mm machine gun had virtually identical effective ranges, about 3,500ft, but the 20mm was more dangerous because its rounds were both heavier and explosive. These guns were effective if an enemy aircraft attacked at altitudes of between 1,000 and 4,000ft, but the Allies rarely obliged. Aircraft approached targets at medium to high altitudes of 10,000–25,000ft or at treetop level – between 100 and 300ft above the ground. Altitudes of 10,000–25,000ft were above the ceiling of this artillery. At treetop altitudes medium and light antiaircraft had only a very short time window in which to engage. An aircraft traveling at 300mph would fly through the effective range of a 25mm gun in 24 seconds or a 20mm cannon or 13.2mm machine gun in 15 seconds, during which the angle of fire changed rapidly.
One of the 92 25mm antiaircraft guns at Rabaul. This twin-mount 25mm was at Praed Point at the northern tip of the Crater Peninsula. A 25mm gun had an effective altitude of 6,000ft. (AC)
CAMPAIGN OBJECTIVES
Isolating a fortress
By late 1942 Rabaul had become Japan’s major air and naval base in the Allied Southwest Pacific Area and the Japanese Southeast Area. Rabaul as a center for Japanese naval and air activity threatened communications with Australia. Before the Allies could advance beyond Rabaul it had to be neutralized. The Allies began planning to retake Rabaul almost as soon as the Japanese captured it.