The Forgotten Giant: A Deep Dive into the Allison V-3420 Engine

To realize its grand vision of a new type of warfare, strategic bombing, in the early to mid-1930s, the Army Air Corps granted Boeing a contract to create a bomber aircraft initially christened the XB-LR-1, denoting its role as an experimental bomber long-range one. Over time, it acquired a more intimidating name, the XB-15. As 1935 rolled around, the Army Air Corps recognized that their aspirations for a sizable aircraft demanded a degree of power that the existing engines and those in development As the Army Air Corps pursued its next long-range experimental bomber, the Douglas XB-LR-2, later known as the XB-19, they engaged Allison Engineering Company to build a potent 1,600 horsepower engine. At that time, Allison was crafting its 1,000-horsepower V-1710 engine, but was ready for anything and soon began work on a 1,600-horsepower class engine. However, the AAC had a unique requirement for the new engine. It had to incorporate a single crankshaft and maximize the use of V-1710 components After several evaluations, Allison chose to double the V-1710, creating a 24-cylinder engine configured in an X formation. This birthed the X-3420. The X-3420, with its innovative crankcase, crankshaft, gear reduction, supercharger, and accessories section, while reusing the basic V-1710 cylinder and head, promised to generate 1,600 horsepower at 2,400 RPM for takeoff and a feasible 1,000 horsepower at 1,800 RPM for efficient cruising. Despite utilizing existing V-1710 components, the single crankshaft and master and articulating rods of the X-3420 demanded significant design work, and the fuel injection system introduced its own challenges. Strained resources and technical issues with the V-1710 led Allison to sideline the X-3420 project in 1936. However, adversity often paves the way for innovation. Allison's chief engineer, Ron Hazen, proposed a new 2,000-horsepower engine based heavily on the V-1710, but featuring two crankshafts. With more power than the X-3420 and a shorter development timeline, the AAC approved this new engine and the V-3420, otherwise known as the W24 or Double V, was born. The design of the V-3420 was a feat of engineering requiring more than just coupling two V-1710 engines together. It shared some similarities with the shelved X-3420, necessitating a new crankcase, gear reduction, supercharger, and accessory section, all while maintaining the use of a significant number of V-1710 components. The idea to include two V-1710 crankshafts and connecting rods streamlined the design and development process of the V-3420. The final design comprised two 60-degree V-12 engine sections attached to a common crankcase and separated by 90 degrees, resulting in a 30-degree separation for the inner cylinder banks. Simultaneously working on the V-1710 allowed Allison to present the V-3420 with an impressive 2,300 horsepower for takeoff. This powerhouse of an engine weighing in at 2,300 pounds was only 140 pounds heavier than the single crankshaft X-3420, but delivered an additional 700 horsepower. By May 1937, the AAC commissioned Allison to build the V-3420 engine prototype. The 24-cylinder V-3420 engine comprised a large aluminum crankcase with four cylinder banks attached. Each of these banks housed six steel cylinder barrels fitted into a one-piece aluminum cylinder head enclosed by an aluminum water jacket. A single overhead camshaft controlled two intake and two exhaust valves per cylinder. Each cylinder of the V-3420 engine boasted a 5.5-inch bore and a 6-inch stroke. The engine's total displacement was 3,421 cubic inches with a compression ratio of 6.65 to 1. The United States Navy had taken note of the V-3420 engine and reached out to Allison inquiring about the possibility of modifying it for marine applications. Allison in turn put forth the necessary blueprints for the conversion. The Navy, convinced of its potential, placed an order for two marine versions of the V-3420 in December of 1939. These were intended for incorporation to a state-of-the-art aluminum-hulled patrol torpedo boat dubbed the PT-8. The two maritime-adapted V-3420s were delivered to the Navy and the PT-8, outfitted with the powerful engines, began its initial trials in November 1940. A series of exhaustive tests were conducted throughout the course of 1941. However, despite the engine's performance, the Navy did not proceed with additional orders for the V-3420 marine engines or further PT-8 boats. Continued development of the V-3420 for aircraft usage was slowed by Allison's commitment to the V-1710 engine and the Army Air Corps' focus on expanding resources for the looming war. Yet, the fate of the V-3420 engine was not sealed. The ongoing issues with the Wright R-3350 engine for the B-29 Superfortress raised questions, and in February 1941, the Army Air Corps requested Allison to resume the V-3420-A development, aiming for an output of 3,000 horsepower. This move emphasized the B-29 bomber's significance. It was too crucial to rely on a single engine type. In October 1941, a V-3420 engine was supplied to Wright Field. Yet, following the Pearl Harbor bombing in December, the V-3420 project was again put on the back burner as Allison refocused its efforts on the B-1710 engine. However, the R-3350's engine suitability was questioned again by mid-1942, prompting the Army Air Forces, formerly the Army Air Corps, since June 1941, to instruct Allison to prepare the V-3420 for installation in a B-29, now renamed the XB-39. By October 1942, nine such engines were assembled and delivered. On October 1, 1942, the Army Air Forces commissioned two Fischer XP-75 Eagle fighter prototypes powered by the V-3420-B engine. This was followed by an order for 500 V-3420-A engines to be fitted into 100 production V-39 aircraft later in the month. The V-3420 engine saw further enhancements as the aircraft project progressed, eventually achieving a takeoff rating of 2,600 horsepower at 3,000 rpm with 8 psi of boost, a standard landing gear rating of 2,100 horsepower at 2,600 rpm at 25,000 feet, and a cruise rating of 1,575 horsepower at 2,300 rpm at 25,000 feet. Under emergency conditions, the engine could be pushed to deliver 3,000 horsepower at 3,000 rpm with 10.2 psi of boost. The first aircraft powered by the V-3420 was the Fischer XP-75. This aircraft, developed by General Motors' Fischer Body Division, was designed as a long-range escort fighter. As 1943 progressed, the Army Air Forces urgently needed such an aircraft and ordered six more XP-75 prototypes. However, due to various factors, including the aircraft's failure to meet expectations and the looming end of the war, the entire P-75 program was canceled by October 1944. Only one P-75A survived and is now on display in the National Museum of the U.S. Air Force. The first application of the V-3420-A engine in the XP-19 commenced in November 1942 at Fischer. The aircraft was renamed the XP-19A and took its maiden flight with its V-3420 engine in January of 1944. Despite the successful installation and test flights, the XP-19A remained an experimental aircraft and never entered production. In February 1943, the V-3420-A engines were selected to power the Lockheed XP-58 Chain Lightning, albeit not being the first or even third choice. The XP-58 heavy fighter program had been plagued by constant design changes and engine development issues since its inception in 1940. The end product was an oversized, overweight, underpowered aircraft that was essentially redundant. Consequently, the XP-58 program was terminated in May 1945 after just one prototype was built. Despite being the initial motivation behind the V-3420 project, the V-3420-powered V-29 aircraft was the last aircraft to fly with the engine. A B-29, technically a YB-29, was transferred to Fischer for conversion into an XB-39 equipped with V-3420-A engines. The conversion work was completed in 1944, and the XB-39 made its first flight in December of the same year. Unlike the Wright R-3350, the Allison V-3420 performed exceptionally well. However, by the time the XB-39 took to the skies, the Wright R-3350 had overcome its initial developmental issues and was now performing satisfactorily. Although it's worth noting that the Allison-powered V-29 was much more powerful than the R-3350's that the cost of converting to V-3420 power was deemed unnecessary. The V-3420's fate was seemingly sealed in 1945. By this time, the jet age was emerging, and the piston engines were being pushed to the sidelines. The development of the V-3420 had been slow and haphazard due to the prioritization of the V-1710 and the complex issues presented by war production. Therefore, the Army Air Force's focus shifted towards jet engine development. In order to cater to the requirements of gargantuan aircraft designs during the Second World War, Allison put forth an innovative proposal, the DV-6840. This powerful engine was essentially an amalgamation of two V-3420 engines working in harmony to power a centralized remote gearbox designed for contra-rotating propellers. The construction or gearbox specifically for the DV-6840 was finished in 1946. However, no records have been discovered that indicate this gearbox ever went through testing. In addition to this, Allison had sketched out plans for an evolved variant of the V-3420. This advanced model, christened the V-3420-C, was to be fuel injection and boasted an estimated emergency output of a staggering 4,800 horsepower with a takeoff rating of 4,000 horsepower. All these figures at a rotational speed of 3,200 RPM and with the use of water injection. However, the ambitious V-3420-C remained a concept on paper and was never brought to life. Ultimately, while the V-3420 was an engineering marvel, its full potential was never realized. It was developed and refined during the war when resources were focused on proven technology rather than innovative experimentation. Despite the setbacks and the ultimate fate of the V-3420, its development represented a significant achievement in the realm of piston engine design. It showcased the ambitious and innovative thinking of the engineers at Allison. In total, only 150 Allison V-3420 engines were built. Several engines and parts are preserved in museums, including the National Museum of the U.S. Air Force and the Allison Engine Company Collection at the Rolls-Royce Heritage Trust Indianapolis. Though it didn't serve as widely or famously as some other engines, the V-3420 remains a remarkable footnote in the history of aircraft engine development and deserves to be remembered.