Sukhoi's First Jet Bomber

The Sukhoi Su-10 in its final configuration

Following the end of the Second World War, both the Soviet Union and the West aggressively pursued jet bomber designs after the Luftwaffe had successfully introduced the Arado Ar 234 to combat in the waning months of the war. In the West, many early designs were based on layouts of high-mounted wings with wing-mounted nacelles to allow for a reasonably-sized bomb bay. Similar approaches were taken in the Soviet Union with the design bureaus (called OKBs) of Illyushin, Tupolev, and Sukhoi tapped to develop jet bomber designs to succeed the Soviet Air Force's standard bomber, the piston-powered Tupolev Tu-2. Last summer I had posted about an early Tupolev design that actually did fly, the Tu-12, that was based on the Tu-2 as a matter of expediency pending the arrival of the Tu-14 bomber. While Illyushin and Tupolev both had large aircraft design experience from their own work on twin-engine bombers during the war, Pavel Sukhoi's experience was limited to his prewar tenure at OKB Tupolev. But, given the pace of technological progress and the urgency of rearmament in competition with the West, Sukhoi was ordered on 26 February 1946 to develop a jet bomber powered by four Junkers Jumo 004B turbojets, the same jet engines that powered the Arado Ar 234. Work on Sukhoi's first bomber design began in earnest in April of that year and the aircraft received the official designation of Su-10. 

Several powerplant arrangements were considered along with the use of six engines instead of the four as originally specified. In the Soviet Union, the Central Aerohydrodynamic Institute in Moscow, or TsAGI by its Russian name, did a lot of wind tunnel and theoretical research work to support design efforts at each of the OKBs. At the time that OKB Sukhoi was working on the Su-10, TsAGI had lagged behind Germany and the West in high speed research and as a result, they lacked a significant amount of information on evaluating jet aircraft designs. As a result, much of what laid the basis of TsAGI's high speed aircraft research came about during their work in support of the development of the Su-10 in addition to what had been obtained of German design work following the end of the war. Two leading design variants were evaluated in TsAGI's wind tunnels- one design had four of the jet engines clustered in the mid fuselage and exhausting out the rear with two jet engines under the nose- this was felt to be advantageous design as it left the wings clean. The other leading variant had wing mounted nacelles with a high unswept wing with three engines mounted in clusters on each wing. 

To meet the specified target speed of 528 mph at 26,000 feet, Pavel Sukhoi decided that four engines were insufficient and that six were necessary. Since the Junkers Jumo 004 engine was the only jet engine available to the Soviet Union at the time that had reached production status, the Soviet Politburo placed high priority in reverse-engineering the engine for production- Vladimir Klimov and his OKB were already known for their piston engine designs during the war and he was put in charge of getting the German engine into production as the Klimov RD-10. Klimov's closest aide, Nikolay Kuznetsov, headed the actual reverse-engineering effort- Kuznetsov would go on to form his own engine OKB several years later. 

Inboard layout of the Su-10

With approval from the state authorities to use six engines, design work had settled on a cluster of three engines on each wing as the most efficient layout- the nacelle had two engines side by side with the third RD-10 engine below and slightly ahead of the pair. On 6 May of that year, a full scale mockup was built that was tested in TsAGI's largest wind tunnel with real RD-10 jet engines. Within two months, refinements to the design based on TsAGI's evaluation were in place as full-scale engineering began on the prototype. While two months sounds rapid, development of the Su-10 hit repeated technical hurdles, the biggest of which was that TsAGI lacked a significant portfolio of well-studied high speed airfoils. As a result, while supporting development work on the Su-10, assimilating German design work, TsAGI was also hurriedly developing its on portfolio of high speed airfoils. As a result, Sukhoi's team was constantly having to revise the Su-10 design based on developments from TsAGI. 

By October the full scale mockup had been approved by Soviet Air Force authorities and metal was finally cut for the prototype on 14 October 1946. In the first week of December, the Soviet government commission in charge of aircraft production decreed that the Su-10 would no longer use the Klimov RD-10 engine but instead use the TR-1 engine from the Lyulka OKB, the first indigenous Soviet jet engine design. Since the TR-1 was more powerful than the RD-10, the Su-10 could revert back to a four-engined design and once again Sukhoi and his team had to revise the bomber's design to accommodate the new Lyulka engines. Working at a frantic pace to meet state-decreed deadlines, Sukhoi managed to have a full set of production drawings ready by 23 December 1946 and three days later the OKB's own workshops had completed a static test airframe and production jigs and tooling for the prototype. If things weren't frantic enough as it was, the Minister of Aircraft Industry wanted the Su-10 flying for participation in the air show at Moscow on 18 August 1947! Common sense prevailed and that was one deadline Sukhoi was allowed to ignore. 

Three-view showing the layout of the Su-10 medium bomber

By 15 December 1947 the hydraulic system had been fully tested on a special ground rig (similar to today's "iron bird"), but construction of the prototype was hampered by slow progress from the various subcontractors that were responsible for some of the Su-10's systems. For example, the defensive armament system (which consisted of a manned tail turret, a remotely-operated dorsal turret and forward-firing cannon), the autopilot, the navigation suite and even the Lyulka TR-1 engines were to have all been delivered to Sukhoi's workshops for the prototype but by the end of 1947 none of those items were ready yet. During the delay, studies looked at alternative powerplant options and it was decided that the initial flight tests of the Su-10 would use the TR-1 engine but as soon as the more powerful TR-2 engine developed from the TR-1 became available, the Su-10 prototype would have its engines swapped out and then continue with the flight test program. 

 

These persistent delays led to the Su-10 prototype to sit in the OKB workshops missing various components- by 4 June 1948 the Soviet Council of Ministers ordered that spending had to be reduced on aircraft development programs that year and one of the unlucky programs to get canceled was the Su-10. By that summer OKB Ilyushin had already made the first flight of its Il-28 medium bomber and its performance outstripped what was projected for the Su-10. Not even rolled out, the Su-10 prototype was donated to the Moscow Aviation Institute where it was slowly reduced to parts over time as an instructional airframe. Sukhoi in the years to come devoted its efforts at interceptor, fighter, and ground attack aircraft and it wasn't until the arrival of the Su-24 Fencer in the late 1970s that Sukhoi finally had a production jet bomber. 

Source/Images: OKB Sukhoi: A History of the Design Bureau and Its Aircraft by Yefim Gordon and Dmitriy Komissarov. Midland Publishing, 2010, p93-101.

The Mikoyan I-320 Interceptor

One of the best aspects of being an avgeek comes when you find out about an aircraft you never knew even flew. I recently got the vastly upgraded edition of OKB Mikoyan and learned about the I-320 two-seat all-weather interceptor. From several different angles it looks a lot like the MiG-17 and it's clearly derived from that aircraft. In the late 1940s the Soviet Ministry of Defense issued a specification for a long-range interceptor- three bids were submitted from the OKBs of Lavochkin, Sukhoi, and Mikoyan. Late to the party but accepted for consideration was also a design from OKB Yakovlev. All of the designs were twin-jet aircraft.

Mikoyan's submission had the internal designation I-320/R-1 and it was designed and built in 1949- in tandem with the development of the famous MiG-15/17 family of jet fighters, accounting for its similarities in general layout. The main difference, however, was that the I-320 was a much larger and heavier aircraft with two crew and two engines. The engines were uniquely mounted in a stepped-tandem arrangement. The forward jet engine was mounted low in the forward fuselage with its exhaust semi-recessed underneath the fuselage just behind the cockpit. The aft engine was mounted in the aft fuselage just behind the wings much in the same way as the single engine of the MiG-15/17 with its exhaust at the end of the fuselage under the cruciform tail. The large nose intake had two splitters instead of the one intake splitter of the smaller MiG-15/17. The central portion of the nose intake fed the forward engine and the two lateral portions fed ducts that ran on each side of the cockpit to the aft engine.

The pilot and radar operator sat side-by-side under a broad cockpit canopy. This way both crew could scan the radar scopes and the radar operator had full dual controls and could relieve the pilot on long air patrols. The Toriy-A radar was mounted in a bullet-shaped projection above the nose intake and two 37mm cannons flanked the intake on each side of the nose.

Powered by two Klimov RD-45 engines of 5,000 lbs thrust, the I-320 was rolled out in April 1949 and made its first flight that month. Mikoyan's flight tests continued through 1950 and the aircraft was then delivered to the Soviet air defense forces, the PVO, for state acceptance and combat testing. The first version of the I-320, the R-1, didn't get accepted due to some stability issues at high speeds. Following the R-1 version in flight testing was the R-2 version which had more powerful Klimov VK-1 engines (the VK-1 was an upgraded and more reliable version of the earlier RD-45 engine used on the R-1 variant), aerodynamic refinements, and a third 37mm cannon mounted under the nose. With better performance in testing with the PVO, the R-2 got a more advanced Korshun radar to replace the earlier Toriy-A unit. During operational testing in 1951, a 37mm shell exploded in the feed belt and damaged the R-2's nose.

Mikoyan took the opportunity to try and rectify some of the stability issues dogging the I-320 as the R-2 was being repaired. As a result, it was redesignated as the R-3 version. The repaired and upgraded aircraft made its first flight on 31 March 1951 and it would make 60 flights during state acceptance trials for a total of nearly 46 flight hours. However, the I-320 never went into production as its performance was deemed inadequate. With the cancellation of Lavochkin's design, the La-200, and the crash of the Sukhoi entry, the Su-15 (the first use of the designation, not to be confused with the later and totally different Su-15 "Flagon" interceptor), Yakovlev's late entry won the competition and went into production as the Yak-25. For a while, though, Mikoyan retained the R-1 and the R-3 versions of the I-320 interceptor to develop instrument landing systems for the Soviet Air Force.

Source: OKB Mikoyan: A History of the Design Bureau and its Aircraft by Yefim Gordon and Dmitriy Komissarov. Midland Publishing, 2009, p127-130.

The VDRK Motor Compressor Engine: The Sort-of-a-Jet

As 1944 began, Soviet aeronautical experts were already filtering through information of successful flight tests of British and American jet aircraft and the impending service introduction of rocket- and jet-powered aircraft in the Luftwaffe. The Soviet GKO (State Defense Committee- a wartime organization that ran the Soviet Union during the Second World War), headed by Stalin, quickly concluded that the different efforts by engineers doing research on jet propulsion at the time needed to be consolidated under one organization, the NKAP (People's Commissariat for Aviation Industry). By March of 1944 officials of the NKAP were ready to present Stalin proposals to accelerate Soviet aeronautical technology to match that of the Luftwaffe, but most importantly to keep up with the British and the Americans. Semyon Lavochkin, Artyom Mikoyan, and Pavel Sukhoi of their respective design bureaus (called OKBs) were instructed to develop jet fighters as a matter of national security. At the time, though, jet engine technology in the Soviet Union wasn't as far advanced as that of either the Germans or the British and American teams. Both Sukhoi and Mikoyan went looking for an alternative with more technical maturity to speed the development process of their candidate designs. 

If we roll back the clock a bit to 1942, a special section within the famous aeronautical and hydrodynamics institute, TsAGI, in Moscow, was set up to explore jet propulsion. One TsAGI engineer, Ghenrikh Abramovich, had devoted time to study the use of a piston engine to drive a separate compressor that would force air into a combustion chamber like a jet engine, only the thrust from the combustion chamber was used purely for propulsion and would not be used to drive a turbine as was the case with a jet engine. Piston engine supercharger technology at the time was mature enough that work on what was called a "motor compressor engine" was simply a derivation of that work as many Allied engines used exhaust driven turbines to compress air for the supercharger to allow piston engines to operate at high power levels at higher altitudes. A motor compressor engine simply used a drive shaft to drive the compressor instead of exhaust gases. 

(I should note here that some versions of the Pratt & Whitney R-4360 Wasp Major radial engine actually had a shaft-driven supercharger inside the engine. This would later lead to the VDT (Variable Discharge Turbine) which will be the subject of a future entry on this blog.)

The intended production version of the motor compressor engine was designated the VDRK. The piston engine drove an axial compressor in the VDRK with then not only drove compressed air like a supercharger to the engine, but also served the radiators. But the air was then passed into a combustion chamber mixed with fuel and ignited, allowing the VDRK's exhaust to act as a rudimentary jet booster. Both Sukhoi and MiG design bureaus selected the Klimov VK-107 liquid-cooled inline engine which via an auxiliary gearbox, drove the VDRK booster. At 23,000 feet the VDRK not only boosted the speed of the aircraft, it also boosted the power of the Klimov engine from 1,650 hp to 2,500 hp, making it an effective supercharger as well. An air inlet below the Klimov inline engine fed the VDRK. On takeoff, 95% of the piston engine's power drive the propeller. But once at altitude, a clutch was engaged which shifted some of the power to a step-up gearbox to drive the VDRK. The main engine's radiator was directly behind the axial compressor with a small diverter duct to feed compressed air to the engine like a conventional supercharger. The air that wasn't fed into the diverter was then mixed with fuel sprayed from seven nozzles and ignited by sparkplugs. At this point in the engine, the duct expanded to form a combustion chamber made of stainless steel to withstand the heat. At the very end of the fuselage a two part eyelid-like nozzle modulated the exhaust flow like a rudimentary adjustable engine nozzle. 

In May 1944 the MiG bureau began work on their design using the Klimov/VDRK engine, the aircraft being designated I-250. The NKAP instructed MiG to have two prototypes ready for testing as high-altitude interceptors by late winter 1945. The specification called for the I-250 to be able to reach 16,400 feet in 4.5 minutes (less if not using the VDRK), a maximum speed of 502 mph (again, less if the VDRK was not engaged) and a service ceiling of 39,000 feet with the VDRK engaged. The armament was one 23mm cannon and two 12.7mm machine guns. By October the mockups were being reviewed by the NKAP and engine testing at TsAGI showed that the combustion chamber of the VDRK needed to strengthened. Given the urgencies of war, prototype construction had already started with the first flight of the I-250 on 4 April 1945 and on its third test flight, the VDRK was engaged for the first time. The second I-250 prototype was completed the following month, flying for the first time on 25 May 1945. 

However, technical issues with the VDRK booster kept popping up and in July 1945 the first I-250 was lost with its test pilot when G-limits were exceeded and the tailplane failed. By this point the NKAP had concluded that the future of fighter aircraft lay with pure jet engines, but that the I-250 project was to continue to provide an aircraft that would give pilots experience in high speed flight. Sukhoi's Su-5 design was canceled as inferior to the MiG I-250 and a pre-production batch was ordered at the end of July for ten aircraft to be delivered at end of December. Power failures, winter fuel rationing and mounting technical challenges with the construction of both the aircraft (sometimes designated MiG-13) and the Klimov/VDRK engine meant that only one aircraft was built by the deadline. As a result, Stalin had the head of the NKAP arrested and a committee formed to investigate the reasons for the delays. Numerous directors, from the aircraft factory to the engine factories involved, were severely reprimanded (I cringe to think what a Stalinist reprimand involved) and the plant manager where the VDRK was built and his quality control manager were arrested and charged with industrial sabotage. 

Despite this, another FIFTY aircraft were ordered in February 1946 and the first pre-production batch had yet to be completed let alone start State certification trials! A new deadline for July 1946 was set and when this deadline failed to be met, 24-hour shifts were imposed on all the manufacturing facilities involved in MiG-13 production. Seven airframes were completed, but the VDRK engines continued to be source of technical obstacles. It was already August 1946 when the eighth aircraft from the original ten-aircraft order first flew and it promptly suffered an engine fire. It wasn't until September of that year that the first aircraft were delivered to the Air Force for acceptance trials. 

Oddly enough, at a meeting on 29 November 1946 chaired by Stalin himself, the pure jet MiG-9 and Yak-15 would be the main jet fighters of the Soviet Air Force and with the astounding prospect of getting superior jet engines and technical assistance from Britain, there would be no future for motor compressor engines. Despite this, Stalin instructed that acceptance trials for the I-250/MiG-13 continue and that production capacity be retained. By May 1947 the acceptance trials were in full swing and the Klimov/VDRK combination was FINALLY operating at specification. Since the Soviet Air Force was no longer interested in the aircraft, the naval air arm, AVMF, was convinced to accept the aircraft as an escort fighter for torpedo bombers. State acceptance trials were to continue under AVMF auspices at Riga on the Baltic coast, but continuing reliability problems and bad weather meant that only six flights had been completed by January 1948 and the VDRK had only been engaged once, and that one time was only 1.5 minute ground run. On 3 April 1948 the I-250/MiG-13 was declared as having failed its State acceptance trials and the aircraft faded into historical obscurity. By that time, the prototype of the MiG-15 jet fighter had already taken place in December 1947, powered by a license-produced Klimov version of the Rolls-Royce Nene engine obtained openly and with the approval of the British government!

Source: OKB Mikoyan: A History of the Design Bureau and its Aircraft by Yefim Gordon and Dmitriy Komissarov. Midland Publishing, 2009, p35-39.