The Yokosuka D4Y Suisei (彗星, "comet") was a dive bomber of the Imperial Japanese Navy. Its Allied codename was "Judy". The D4Y was one of the fastest dive-bombers in the entire war, and only the delays in its development hindered its service, while its predecessor, the more robust but slower Aichi D3A remained in service for years. Despite this limited use, the speed and the range of the D4Y was nevertheless valuable, and the type was used with success as reconnaissance aircraft as well as in kamikaze missions.
The aircraft was a single engined all-metal low winged monoplane, with a wide-track retractable undercarriage and wing mounted dive brakes. It had a crew of two: a pilot and a navigator/radio-operator/gunner, seated under a long glazed canopy which provided good all-round visibility. The pilot of bomber versions was provided with a telescopic bomb-sight. The aircraft was powered by an Aichi Atsuta liquid-cooled inverted V-12 in-line engine, a licenced copy of the German DB 601, a rated at 1,200 hp. The radiator was behind and below the three-bladed propeller, as in the P-40.
It had a slim, elegant fuselage that enabled it to reach high speeds in horizontal flight and in dives, while low wing-loading gave excellent maneuverability, with the Suisei having superior performance than contemporary dive-bombers such as the SB2C Helldiver. In order to conform with the Japanese Navy's doctrine of ensuring that its aircraft could outrange potential enemies, weight had to be minimized with the result that the D4Y was not fitted with self-sealing fuel tanks and armor. In consequence, the D4Y was extremely vulnerable and tended to catch fire when hit. This made the D4Y extremely vulnerable to any enemy aircraft that could catch it.
Bombs were fitted under the wings and in an internal bomb bay, something rare in a single-engine aircraft. It carried one 500 kg bomb, but there were reports that the D4Y sometimes carried two 250 kg bombs, for example during the attack on USS Princeton. Only 30 kg bombs were carried externally. The aircraft was armed with two 7.7mm machine guns in the nose, and one 7.92mm Type 1 machine gun in the rear of the cockpit. (The 7.92mm was carried because of its higher rate of fire.) Later the 7.92mm was replaced by a 13mm Type 2 machine gun. This light gun armament was typical for a Japanese carrier bomber. The forward machine guns were retained in the kamikaze version.
The first D4Y1 prototype made its maiden flight in December 1940. After the prototype's successful trials, development continued, and the first problems appeared. During dive-bombing trials the wings of the D4Y started to flutter, a fatal flaw for an airframe subject to the stresses of dive bombing. Because of this, initial production aircraft were used as reconnaissance aircraft, as the D4Y1-C, which took advantage of its high speed and long range, while not over-stressing the airframe. Production of the D4Y1-C continued in small numbers until March 1943, when the increasing losses incurred by the D3A resulted in production switching to the D4Y1 dive-bomber, the aircraft's structural problems finally being solved. Although the D4Y could operate successfully from the large and fast Fleet Carriers that formed the core of the Combined Fleet at the start of the war, it had problems operating from the smaller and slower carriers such as the Hiyō class which formed a large proportion of Japan's carrier fleet after the losses received in the Battle of Midway. Catapult equipment was therefore fitted, giving rise to the D4Y-1 Kai (or improved) model.
These early versions of the D4Y were difficult to keep in service because the Atsuta engines were unreliable and difficult to maintain in front line service. From the beginning some had argued that the D4Y should be powered by an air-cooled radial engine, a type Japanese engineers had experience with and trusted. The aircraft was therefore fitted with the reliable Mitsubishi MK8P Kinsei 62, a fourteen-cylinder two-row radial engine. This version was the Yokosuka D4Y3 Model 33.
Although the new engine improved ceiling and rate of climb (over 10,000 m, and climb to 3,000 m in 4.5 minutes, instead of 9,400 m and 5 minutes), the higher fuel consumption resulted in shorter range and a slower cruise speed, while the bulky engine obstructed the forward and downward view of the pilot, hampering carrier operations. These problems were tolerated because of the increased availability of the new variant.
The last version was the D4Y4 Special Strike Bomber. This one-seat kamikaze aircraft, capable of carrying one 800 kg bomb, was put into production in February 1945. It was equipped with three RATO boosters for terminal dive acceleration. This aircraft was an almost ideal kamikaze model: it had a combination of speed (560 km/h), range (2,500 km) and payload (800 kg) probably not matched by any other Japanese aircraft.
The D4Y5 Model 54 was a planned version designed in 1945. It was to be powered by the Nakajima JK9C Homare 12 radial engine rated at 1,825 hp (1,361 kW), would have a new four-blade metal propeller of the constant-speed type, and would have more armor protection for the crew and fuel tanks.
Ultimately 2,038 of all variants were produced, mostly by Aichi.
Lacking armor and self-sealing fuel tanks, the Suiseis did not do well against Allied fighters. They did, however, cause considerable damage to ships, including the carrier USS Franklin, which was nearly sunk by a single D4Y.
The D4Y1-C reconnaissance aircraft entered service in mid-1942, when two of these aircraft were deployed aboard the Sōryū at the Battle of Midway, where both were lost when Sōryū was sunk.
The U.S. Task Force 58 struck the Philippine airfields and destroyed the land air forces first, before engaging the Japanese naval aircraft. The result was what the Americans called "The Great Marianas Turkey Shoot", with 400 Japanese aircraft shot down in a single day. A single Hellcat pilot, Lt. Alexander Vraciu, shot down six D4Ys within a few minutes.
The D4Y was relegated to land operations where both the liquid-cooled engine D4Y2, and the radial engine D4Y3 fought against the U.S. fleet, scoring some successes. An unseen D4Y bombed and sank the light carrier USS Princeton on 24 October 1944. D4Ys hit other carriers as well, by both conventional attacks and kamikaze actions. In the Philippines air battles, the Japanese used kamikazes for the first time, and they scored heavily. D4Ys from 761 Kokutai may have hit USS Kalinin Bay on 25 October 1944, and the next day the USS Suwannee. Both were badly damaged, especially Suwannee, with heavy casualties and many aircraft destroyed. A month later on 25 November, USS Essex, USS Hancock, USS Intrepid and USS Cabot were hit by kamikazes, almost exclusively A6M Zero fighters and D4Ys, with much more damage. D4Ys also made conventional attacks. All these D4Ys were from 601 and 653 Kokutai.
Carriers USS Enterprise and USS Yorktown were damaged by D4Ys of 701 Wing on 18 March. On 19 March, carrier USS Franklin was hit by another D4Y, despite heavy anti-aircraft fire. Franklin was so heavily damaged that she was retired until the end of the war. Another D4Y hit the carrier USS Wasp.
During Kikusui N.6, on 11 May 1945, USS Bunker Hill was hit and put out of action by two kamikazes that some sources identify as D4Ys. This was the third Essex-class carrier forced to retire to the States to repair.