In 1925, the Soviet Air Force (Voyenno-Vozdushnye Sily or VVS) approached the TsAGI (Tsentral’nyy Aerogidrodinamicheskiy Institut, the Central Aerohydrodynamic Institute) and requested proposals for a large, heavy bomber. Under the direction of Andrei Nikolayevich Tupolev, the Tupolev OKB (Opytno-Konstruktorskoye Byuro, the Experimental Design Bureau) started design work on the aircraft in 1926, and the government finalized the aircraft’s operational requirements in 1929. The aircraft created from this program was the Tupolev ANT-6, which was given the military designation TB-3.
Model of the Tupolev TB-6 6M-44 with its six M-44 engines. Gunner stations are seen outside of the outer engines and in the wing’s trailing edge.
The large, four-engine TB-3 lifted its 137 ft 2 in (41.80 m) wingspan from earth for the first time on 22 December 1930, but plans for even larger and more ambitious aircraft were underway. In October 1929, the Scientific and Technical Committee of the Air Force (Nauchno-tekhnicheskiy komitet upravleniya Voyenno-Vozdushnye Sily or NTK UVVS) instructed Tupolev to design bombers capable of carrying a 10-tonne (22,046 lb) and a 25-tonne (55,116 lb) payload. With a 177 ft 2 in (54 m) wingspan, the 10-tonne bomber became the ANT-16, which was given the military designation TB-4. The 25-tonne bomber had a 311 ft 8 in (95 m) wingspan and became the ANT-26, which was given the military designation TB-6. However, this line of developing very large aircraft, the TB-6 in particular, quickly illustrated that there was a lack of powerful engines and that numerous smaller engines were required for the aircraft. The TB-4 required six 800 hp (597 kW) engines, and the TB-6 required twelve 830 hp (619 kW) engines. If an engine with a 2,000 hp (1,491 kW) output could be built, not only could it power these large aircraft, but it would also simplify their construction, maintenance, and control.
Back in 1928, the TsAGI had realized the need for more powerful engines and initiated work on a single-cylinder test engine to precede the design of a large, high-power bomber engine. This test engine was designated M-170; “170” was the anticipated horsepower (127 kW) output of the cylinder. The results were encouraging, and in 1930, the Institute of Aviation Motors (Institut aviatsionnogo motorostroyeniya or IAM) was tasked with the construction of a V-12 engine based on the M-170 cylinder. The 12-cylinder engine was designated M-44, and the single-cylinder test engine was renamed M-170/44.
The design of the M-44 was initiated in February 1931 under the supervision of N. P. Serdyukov. The design progressed rapidly and was completed in May. The M-44 was a four-stroke, water-cooled, 60-degree V-12. Based on a sectional drawing, the crankcase was split horizontally with main bearing caps for the crankshaft machined integral into the lower half of the case. The main bearings were secured by long bolts that passed through the lower crankcase half and screwed into the upper half. The crankshaft accommodated side-by-side connecting rods with flat-top aluminum pistons.
Sectional drawing of the IAM M-44 reveals some of the engine’s inner workings. The design was fairly conventional, just extremely large. Unfortunately, no images or other drawings of the engine have been found.
The individual steel cylinders were secured to the crankcase via hold down studs. A steel water jacket surrounded the cylinder barrel. The cylinder had a flat-roof combustion chamber, and four spark plugs were positioned horizontally at its top, just below the valves. Two spark plugs were on the outer side of the cylinder and the other two on the Vee side. Each cylinder bank was capped by a monobloc cylinder head with dual overhead camshafts. One camshaft operated the two intake valves for each cylinder, and the other camshaft operated the two exhaust valves for each cylinder. An intake manifold was attached to the Vee side of the cylinder head, and individual exhaust stacks were attached to the outer side of the cylinder head.
The normally aspirated M-44 had a compression ratio of 6 to 1 (some sources state 5 to 1). A propeller gear reduction (most likely using spur gears) was incorporated onto the front of the engine. The IAM M-44 had an 8.74 in (222 mm) bore and a 11.26 in (286 mm) stroke. Each cylinder displaced 675.6 cu in (11.07 L), and the engine’s total displacement was 8,107 cu in (132.9 L). The M-44 was the largest V-12 aircraft engine ever built. The engine produced 2,000 hp (1,491 kW) for takeoff and 1,700 hp (1,268 kW) for continuous operation. Some sources indicate that 2,400 hp (1,790 kW) was expected out of the engine after it was fully developed. The M-44 was approximately 118 in (3.00 m) long, 46 in (1.16 m) wide, and 65 in (1.66 m) tall. The engine weighed around 3,858 lb (1,750 kg).
With development of the 2,000 hp (1,491 kW) M-44 engine underway, studies were started to incorporate the engine into the ANT-16 (TB-4) and ANT-26 (TB-6) aircraft designs. Proposals to re-engine the ANT-16 with four M-44s were quickly abandoned so that work could focus on using six M-44 engines to power the ANT-26. This version of the aircraft is often cited as TB-6 6M-44. The ANT-26 design was ordered in July 1932, with construction starting soon after. Delivery of the ANT-26 prototype was expected in December 1935. Some sources state that an even larger, 30-tonne (66,139 lb) bomber with a 656 ft (200 m) wingspan and powered by eight M-44 engines was conceived, but it appears this aircraft never progressed beyond the rough design phase.
The Tupolev TB-6 6M-44 had two engines installed in each wing and two engines positioned back-to-back and mounted above the aircraft’s fuselage. The aircraft had a 311 ft 8 in (95 m) wingspan and was 127 ft 11 in (39 m) long. The TB-6 6M-44’s top speed was 155 mph (250 km/h), and it had a ceiling of 22,966 ft (7,000 m). The aircraft had a maximum bomb load of 48,502 lb (22,000 kg) and could carry a 33,069 lb (15,000 kg) bomb load 2,051 miles (3,300 km). Its maximum range was 2,983 miles (4,800 km).
This rear view of the TB-6 6M-44 illustrates the tandem engines mounted above the fuselage.
The construction of three M-44 prototypes was planned, but the first engine was delayed by continued trials of the M-170/44 test engine, which was given a higher priority. The manufacture of the first M-44 engine began in early 1933, and the engine was first run later that year. The second engine was built and run in 1934. Plans to build the third M-44 engine were suspended on account of issues with the first two engines. The M-44 test engines had trouble producing the desired power and suffered from reliability issues. It became clear that the engine was not going to be successful, and the program was cancelled in 1934.
A supercharged version of the engine, known as the M-44H, had undergone preliminary design work in 1932. However, performance specifications for this engine have not been found, and it is doubtful that detailed design work was completed. In 1935, a decision was made to build the third M-44 engine, modified for marine use. This engine was designated GM-44 and incorporated a reversing gearbox. The GM-44 produced 1,870 hp (1,394 kW), but it was no more reliable than the M-44 aircraft engine. The GM-44 engine was cancelled in 1936.
With the M-44 engine program dead, the ANT-26 design reverted back to using 12 engines (1,200 hp / 895 kW Mikulin M-34FRN). However, studies concluded that the multitude of engines created additional drag that impacted the aircraft’s performance, and the engines added so much complexity that the ANT-26 would be difficult to fly and very difficult to maintain. Simply put, the giant aircraft was impractical, and it was subsequently cancelled in July 1934. A transport/commercial version of the aircraft, designated ANT-28, was also cancelled. The ANT-26’s airframe was 75 percent complete at the time of cancellation.
With the M-44 cancelled, the 12-engine TB-6 12M-34FRN was designed to preserve the aircraft’s capabilities with reliable engines. However, one would question the practicality of such an aircraft. Note the set of tandem engines that was placed above each wing.
