Air Attack on Ships Part 5 - Early Dive Bombers

Throughout WWII, the single most effective way of delivering high explosives to ships was dive bombing. This is exactly what it sounds like, the aircraft diving towards the target before releasing the bomb, the performing a high-G pullout to avoid following the bomb into the target. It was originally developed during WWI to attack land targets, as pilots realized that it allowed them to place bombs far more accurately,¹ although the fragile aircraft of the time were limited to fairly shallow dives.²

Curtiss F8Cs, the first US dive bomber

Dive bombing nearly died off in the years immediately after WWI, abandoned in the enthusiasm of airpower advocates for level bombing, but it was adopted by the US Marines and Navy in the late 20s, using fighters equipped with light bombs for support in ground fighting. Initially, they used fairly shallow dives, but the target had a tendency to disappear under the nose of the aircraft, and it was soon realized that steeper dives not only solved this problem and improved accuracy, but also were far harder to defend against. The plunging dive bombers were generally difficult to see before they began their dives, and once in a dive, the rapid change in range meant that heavy AA guns were ineffective, while lighter AA weapons had only a narrow window to work against the diving aircraft.³ The basic technique they used would remain standard throughout the dive bomber's life: approach at 10,000' or so, then push over into a steep dive, 70° or more. As he dove, he would aim at the target attempting to compensate for wind and the movement of the ship. The bomb would be dropped during the dive, and the pilot would then pull out and fly away at high speed and relatively low altitude. What altitude the pull-out was done at varied, with early tests taking place at 1000' or so, although later aircraft had to do so at greater altitude to avoid plowing into the ground.⁴ The pullout itself would often subject pilot and plane to 6G or more, enough to make the pilot's vision gray out, which generally limited the tactic to smaller aircraft.⁵

A bomb is loaded on an SBD, with trapeze visible

But as dives got steeper and bombs got bigger for anti-shipping use, a new problem appeared. Bombs, particularly bigger ones, tended to fall faster than an airplane in a dive, which meant that in a steep enough dive, the bomb, released from under the fuselage, would hit the propeller. The solution was "displacement gear" or the "trapeze", a device under the fuselage of the aircraft that would swing the bomb clear of the propeller before release. A second unique feature of the dive bomber came a few years later, as biplanes gave way to streamlined monoplanes and designers realized that the planes would be too fast in a dive for safety. The answer was dive brakes, flaps that would be deployed in the dive to add drag on the way down. These were first seen on the Northrop BT, which was soon developed by its legendary designer, Ed Heinemann, into the most famous dive bomber of all, the Douglas SBD Dauntless. Also in this era, the concept of dive bombing spread outside of the US, propelled both by foreign observers reporting on American exercises and the 1932 film Hell Divers, starring Clark Gable, which probably did at least as much to get other nations actually interested.

Japanese Aichi D1A2 dive bomber

But even as Japan and Britain joined the US in the adoption of the naval dive bomber, they were also encountering the limitations imposed by the technology of the day. To achieve reasonable performance, a dive bomber could carry no more than a 1,000 lb bomb, and when combined with the relatively low impact velocities of dive-bombing⁶ tended to sharply limit armor penetration. The US 1000 lb AP bomb would have to be released from 6500' in a 300 kt 60° dive to pierce a 5" deck, and at that altitude, accuracy wouldn't be great, and even doubling bomb weight (which also limited the bomb to Helldivers and Avengers) only dropped the minimum release altitude to 4500'.⁷ But 5" decks were only found on battleships, so this wasn't much of a problem when attacking the vast majority of warships. Dive-bombers were accurate enough to be effective against destroyers and a 1,000 lb bomb would be enough against most cruisers, to say nothing of the effect on lightly-armored carriers. But it did mean that sinking a battleship would require torpedoes, although navies quickly realized that dive bombers could be quite effective at disrupting the ship's air defenses to open the way for their slower brethren if the two types could coordinate their attack. Ultimately, wartime experience would show that this was easier said than done.

Skuas in the Orkneys

Further limiting payload and performance was the fact that everyone had settled on using their dive-bombers as scouts as well, which required a second crewman to serve as navigator and radio operator. In case of fighter attack, the dive bomber was generally equipped with a rear-firing machine gun, although in the late 30s, the threat of fighter interception was generally considered fairly minimal. Without radar, there were decent odds that the dive-bombers would be overhead before they were spotted, and even if they were spotted earlier, vectoring the fighters out to intercept in time would be difficult. The British took this view the furthest, deciding that the best protection they could provide their carriers was extensive deck armor that would allow them to survive dive-bombing attacks, and making plans to use their new dive-bomber, the Blackburn Skua, as their primary fighter, too.⁸ The Skua proved a fairly effective dive-bomber, sinking the German cruiser Konigsberg off Bergen, the first major warship sunk by air attack, but it was a failure as a fighter, and the British soon had to scrounge up single-seat fighters for their carriers.

But this only takes us through the outbreak of war, and we'll have to look at the wartime performance of the dive bomber next time.