Owing to severe bombing of the railways, not all of the trains carrying equipment and Su-2 parts from the evacuated Kharkov plant reached Molotov. Consequently only 40 Su-2 airframes were assembled at the new location at the beginning of 1942. Most or these aircraft were fitted with the 1,540hp (1.148 kW) Shvetsov M-82 14-cylinder air-cooled radial engine produced by Plant No.19, also in Molotov.

The aircraft designated Su-4 successfully underwent its state acceptance trials between 25th February and 23rd April 1942, flown by test pilot Korobov. It featured enhanced armour protection for the navigator: the TSS-I turret was Titled with the 3/8 in (10mm) armour plates to protect his breast, two 3/8 in (10mm) armour plates were installed vertically outside the TSS-1 turret, and one similar inclined plate was provided to protect the hatch gun position. The floor of the navigator's compartment was fitted with 3/16 (4mm) armour plate. The M-82 engine, which could run even with several cylinders damaged, provided effective forward protection for the pilot, whose cockpit was fitted with 5/16 in (8.5mm) back armour.

The Su-4 seemed to be a promising aircraft with potential for development and improvement owing to its high powered air-cooled engine. Indeed, the evolutionary line of Sukhoi attack aircraft, starting with the "Ivanov" and passing through the Shb prototype and the Su-2 (Su-4) production aircraft, was crowned hy the Su-6 prototype of 1942, for wich Pavel Sukhoi received the coveted title of Stalin Prize Laureate.

A total or 889 production Su-2s and Su-4s had been delivered by March 1942. when the plant in Molotov was disbanded and its personnel distributed among other factories. So the line of Sukhoi production aircraft was discontinued, to be re-established only after Stalin's death.

Like all or the aircraft created by the Sukhoi Design Bureau, the Su-2 (Su-4) was notable for its designer's consideration for the crew-members. Besides the external radio communication provided by the KSB radio in navigator's compartment, Ihe aircraft was equipped with an SPU-2 internal communication system. Indirect lighting of the instrument panel and warm air heating of the crew positions were innovations in Soviet production aircraft of the time. In case of failure of the electro-hydraulic system, the undercarriage could be extended manually by the navigator, using an emergency cable transmission. Moreover, this dual control aircraft was extremely airworthy, its high survivability being revealed when it was operated under conditions of Luftwaffe supremacy.

By the beginning of the German assault on the Soviet Union 13 bomber regiments had been fully or partly equipped with Su-2s. In the first month of the war, the most dramatic period for the VVS, these regiments were assigned to the western, south western and southern sectors of the front, and their losses were therefore not as great as those of the bomber regiments equipped with the Tupolev SB and llyushin DB-3. Af the beginning of the war Su-2's comprised 14% of the total bomber force deployed on the Soviet Army's western sector of the front, while by 10th Jury 1941 this had increased to 21%. By 4th October 1941, 116 of the 174 operational daylight bombers on the South Western Front were Su-2s, The 135th Bomber Air Regiment operated successfully in that area, completing 630 sorties without loss and destroying 217 tanks and up to 400 infantry vehicles. In May-June 1942 5u-2s comprised 55% or the daylighl bombers in service on the South Western Front. On 12th September 1941 female pilot Yekaterina Zelenko rammed a Messerschmitt Bf 109 near the town of Sumy while flying a burning Su-2. This was the only ramming attack performed by a woman in the history of air combat.

Sukhoi Su-2s were flown by pilots of 15 VVS regiments and two separate squadrons. The aircraft participated in combat operations near Lvov, Kiev, Moscow and Stalingrad, and also in the battles of Orel and Kursk. The bomber played a significant role in the fight for Stalingrad. During ihe Stalingrad defensive operations in 1942 the 8th Air Army lost eight Su-2s in air combats. On average, an Su-2 of the 270th Bomber Air Regiment made 30 sorties before it was shot down, white the 221st Bomber Air Regiment, operating Douglas Bostons, lost one aircraft for every 19 sorties. A similar figure, 20 sorties, was typical for yhe Pe-2 until its turret mounted 7.6mm ShkAS was replaced by the 12.7mm UB gun, which increased the average number of sorties before being shot down to 54. The I1-2, proclaimed by the Soviet propaganda machine as an outstanding attack aircraft while it was being operated as a single-seater in 1941 and most of 1942, completed an average of 13 sorties before being shot down, this figure being improved two-fold after it was converted to a two-seater. However, because of the poor protection afforded the gunner and his lack of firm support while firing the 12.7mm gun (he sat on a canvas belt tn the rear cockpit), gunners had to be changed, on average, every seven sorties. Whether there was any justification in pulling the I1-2 on a pedestal in the light of such costs awaits investigation.

Indeed, the 270th Bomber Air Regiment, was formed with highly experienced crews, but its ability to retain its personnel and thus accumulate combat experience can be traced to the exceptional survivability of the Su-2. Sometimes, to enhance the aircraft's defensive capabilities on daylight missions, its crew was increased to three.

The pilots of the 288th Bomber Air Regiament, commanded by I Gotokhov, managed to fly their Su-2s overloaded with two externally mounted 551lb (250kg) bombs, which significantly enhanced the effectiveness of bombing attacks. The rear defensive armament of the DB-3 and Pe-2 had proved inadequate, and this was also the case with the Su-2. Replacement of the turret mounted 7.6 mm Shkas with the larger calibre UB gun seemed inevitable.

Photo	Description
	Drawing Su-2 with M-82 engine
	The Sу-2 No 15116 powerd by M-82 with the ski landing gear during trials in the Research institute of Air Force at March 1942.

Su-4 (BB-3)

Su-4 (BB-3) - modification of the Su-2 with the proposed M-90 EV engine. Urmine's 2100 hp, radial, two-row, 18-cylinder, significantly smaller diameter than other radial engines. Unlike the Su-2, they had to have not only a wooden fuselage, but also a wooden wing with metal spars. It was planned that the Taubin heavy machine guns would replace the ShKAS in the wings and on the upper turret.

But the M-90 engine did not come out of the experimental stage, and the serial M-82 engine was installed on the plane, with which in April 1942 the plane passed state tests. The Su-4 was serially built and took part in the Great Patriotic War. Due to the lack of aluminum with the same metal spars, the wing was manufactured with wooden ribs and plywood sheathing. The armament was more powerful - two large-caliber BS machine guns in the center section with the same two ShKAS machine guns at the shooter and 400 kg of bombs on the internal sling. The speed at the ground was 450 km/h and the time to climb 6000 m was 10.5 minutes.

Bibliography

• "The history of designs of planes in USSR 1938-1950" /Vadim Shavrov/
• "Soviet Combat Aircrafts" /Yefim Gordon and Dmitri Khazanov with Alexander Medved`/
• "Attack aircrafts Su-2. " /Dmitriy Khazanov, Nikolay Gordyukov/